Report for Floodway Research Project – Central Local Government Region of South Australia (TPD1210028)

Background

Many of the floodways have been damaged on a number of occasions over the past five years, resulting in significant costs to each Council to reinstate the floodways. The Councils have limited budgets and personnel to properly design and reinstate the floodways after a significant storm event. The Council’s main objective is to reinstate the floodways as quickly as possible and to make the floodways trafficable. The necessity to patch repair floodways results in a high risk of failure in subsequent storm events.
Several different methodologies have been implemented by the Councils with varying levels of success, to counter this problem. The challenge has always been for the Councils to identify the most cost effective solution when each of the crossings is only used by a limited number of vehicles.

 

Design Guides and Alternate Technology
The standard erosion control material used in floodway design guides includes:

•  Reinforced concrete
•  Rock rip-rap and rock rip-rap with concrete infill
•  Rock filled mattresses and gabions
•  Flexible mats
•  Vegetation cover

All of these erosion control materials are effective erosion control materials, however the remoteness of a large number of the 21 floodway sites mean that it may not be cost effective to utilise these materials.
An investigation was undertaken to identify cost effective erosion control materials that could be transported to site and installed using resources and equipment readily available to the Councils. The alternate technology erosion control materials identified include:

t Concrete Canvas®

t Polycom® and Soiltac®

t Concrete blocks (1 m3)

t Cement treated product

Failure Mechanisms
Floodways commonly fail as a result of three main causes, namely erosion, deposition and failure of infrastructure.
Floodway failure typically occurs at any or all of three zones across a floodway, including: the upstream, roadway and downstream zones. An additional zone referred to as the peripheral zone was also identified to contribute to the failure of a floodway. No significant erosion was identified at the upstream end of any of the 21 floodways sites assessed. The most common erosion was observed on the roadway and downstream zones.
The most common form of failure at the 21 floodway sites occurred in the downstream zone, largely due to the presence of an erosion head advancing upstream and undermining the downstream end of floodways. Other factors that contributed to the failure of the 21 floodway sites included:
t Floodway alignment with respect to the watercourse. Floodways cutting through a watercourse on the outside of a bend were at a high risk of erosion. Creeks within the study area are characterised by steep or vertical side walls on the outside of a bend.
t Sediment deposition as a result of widening of the watercourse at the floodway and resultant drop in flow velocity. Floodways at larger watercourses (typically greater than 50 m wide) are often closed following a significant flood event, as a result of debris in the form of sediment, gravel and uprooted gum trees.
t Low flow channel erosion. Larger watercourses have mobile beds and it is not uncommon for a low flow channel to form and result in erosion of the floodway.
t Erosion around infrastructure including end walls and culvert headwalls.
t End of design life of infrastructure.

 

t Flood events in excess of the original design intent. The five Councils have experienced flood events in the last 5 to 10 years that have not been witnessed before by the Works Managers. This has resulted in erosion around the end of protective measures.
Hydrology and Hydraulics
A lack of continuous rainfall and flow gauging stations in the study area, combined with the isolated nature of the storm events in this region make it very difficult to calculate peak flows for a particular level of service. The rational method with a modified time of concentration was used for Flinders Ranges Council and the District Council of Orroroo and Carrieton. No modification was made to the time of concentration calculated for the District Council of Peterborough. Regional flood frequency analysis for the Mount Lofty Ranges was used to calculate peak flows for the catchments within Northern Areas Council and the Regional Council of Goyder.
An analysis of 23 daily rainfall gauging stations between Leigh Creek in the north and Tanunda in the south was undertaken to assess the spatial coverage of typical storm events and the frequency and magnitude of storm events in the study area. The analysis showed that there is an increase in storm activity every 30 years and the storm event rainfall totals in the north of the study area are higher than in the south. There was no conclusive evidence that more frequent and intense storm events were occurring in the southern areas of the study area. A more detailed analysis of rainfall data (pluviographs) would be required to confirm this.
Manning’s equation and HEC-RAS were used to calculate peak flow velocities across three zones of a selection of floodways. This analysis showed that the flow velocity across a floodway is typically between 2.5 to 3 m/s and the flow velocity, depending on the height of the drop, at the downstream end of a floodway can be as high as 4 to 7 m/s.
Climate Change
The Central Local Government Region of SA commissioned a report titled “Central Local Government Region Integrated Climate Change Vulnerability Assessment – 2030”, November 2011. This report discusses the impact of climate change on rainfall intensity. For South Australia the suggested increase in rainfall intensity is likely to be only minor (perhaps only about 2% above current levels, Darren Ray, BOM, Head Climatologist, South Australia, pers comms.)
Classification of Sites and Serviceability Level
The level of importance of a floodway was based on the road classification with the option to apply adjustment factors to account for traffic volume and designated tourist routes, population serviced, local economy and alternate route availability. A high, medium or low classification was assigned to each floodway.
Three serviceability levels were defined and floodway protection measures assigned to each level to suit the level of service to be provided at the three floodway zones and the peripheral zone. Level 3 (highest) floodway protection measures include reinforced concrete and are expected to have a design life of greater than 25 years. Level 2 floodway protection measures include Concrete Canvas®, polymers, rock rip-rap, precast concrete blocks and cement treated material. Level 1 (lowest) floodway protection measures can be implemented with minimum cost until more permanent measures can be constructed.

 

The “Do Nothing” option was included in each of the three levels to account for sites that did not experience any damage in a particular zone and to account for sites where the primary failure mechanism is sediment deposition.
Cost Matrix
The cost per metre for various floodway protection measures were developed so that each Council can undertake a cost analysis of the various protection measures. Councils are encouraged to develop their own costs to suit their circumstances.
A cost matrix was developed for the various floodway protection measures at the roadway and downstream zones of the floodway. The cost matrix includes the year 1 capital cost and an estimate of the operation and maintenance costs over a 25 year period with an 8% discount factor.
Option Selection
A floodway option selection flowchart was developed to assist the Councils in implementing the most appropriate floodway protection measures for a particular site. The floodway option selection flowchart is shown on Page vii. The floodway option selection flowchart utilises a level of serviceability matrix, as shown in Table i, to select the level of flood protection measure to be provided. The level of flood protection measure is based on the route importance and the consequence of failure.

Table i Level of Serviceability Matrix

Consequence of Failure
Route importance Catastrophic Limited Maintenance Deposition

High
Level 3
Level 2
Level 1 No works recommended

Medium
Level 2
Level 2
Level 1 No works recommended

Low
Level 2
Level 1 No works recommended No works recommended

Site specific recommendations are shown in Table G1, Appendix G. Site specific recommendations are provided for three floodway zones and three different levels of service to provide the Councils with options, dependent on the serviceability level required. The peripheral zone has also been included to identify any specific works surrounding the floodway.
Product Trials and Knowledge Sharing
There are thousands of floodways within the five Councils. Significant cost savings could be achieved by utilising products that provide a reasonable design life (25 years) and require minimal maintenance after flood events. The five Councils are planning to undertake trials of the various erosion control measures in this report, to assess their performance. The results of these trials will be shared amongst the five Councils.

 

Management of Floodway Assets
It is recommended that the Councils initiate a system to assess each of the floodways in their Council area to allow for a more economical integration of floodway maintenance into their Road Maintenance Program and/or Asset Management Register. The use of tablets to record GPS co-ordinates, take photos and note observations for each site should be investigated. This would enable works managers to more easily document claims due to flood damage and track the condition of floodway assets.
Conclusion
This floodway research project has highlighted a number of issues surrounding the ongoing maintenance and repair of the thousands of floodways within the five Council boundaries. The issues surrounding the ongoing maintenance and repair of the floodways include:
t More frequent and intense storm events over the last five years have resulted in significant damage to floodway infrastructure.
t The Councils do not have the capital and resources to provide the level of service at floodways that is consistent with the road category.
t A lack of resources results in a reactionary response to floodway maintenance and repair, which ultimately impacts on the level of service provided.
t Uncertainty with respect to identifying the extent of floodway erosion control works to mitigate large flood events.
t Insurance claims as a result of flood damage to floodways are only sufficient to reinstate damaged infrastructure and not replace the damaged floodway with an erosion control measure that is less at risk of failure.
The Works Managers within each Council have a wealth of experience in maintaining and protecting floodways. Their efforts to maintain and repair floodways with insufficient capital are to be commended. This floodway research project has built upon the works managers knowledge and ideas to develop cost effective floodway erosion control measures. A process has been developed to assist in recommending the most appropriate floodway erosion control measures that considers the:
t Primary and any secondary floodway failure mechanisms.
t Existing floodway infrastructure.
t Level of service to be provided by a floodway.
t Construction and maintenance costs and availability of materials.
Floodway erosion control measures or enhancement works at each of the 21 sites have been recommended for each floodway zone and for three levels of serviceability. This information provides the Councils with options depending on their funding constraints and the localised issues at each site.
Alternative erosion control products such as Concrete Canvas®, Polycom®, Soiltac®, concrete blocks (1 m3) and cement treated product have been identified in this report. The Councils will be undertaking trials using these products and will report back to the other Councils on the outcomes of the product trials.
Significant cost savings can be realised by the Councils with a pro-active monitoring and maintenance regime of floodway assets. It is recognised that the Councils have limited resources and funds available in order to maintain such a programme but these challenges must be overcome to realise long term cost savings.

 

33/16230/50302 Floodway Research Project
Final Report

 

 

Contents
Executive Summary i
1. Introduction 1
1.1 Floodway Sites 1
1.2 Scope of Services 2
1.3 Background 3
1.4 Funding 4
2. Project Intent 6

2.1 Aim 6
2.2 Methodology 6
2.3 Limitations 7

3. Study Area 9

3.1 Selected Sites 9
3.2 Site Descriptions 9
3.3 Study Area Description 9
4. Review of Floodway Design Guides 11
4.1 Design Guides 11
4.2 Hydraulic Design 11
4.3 Erosion Protection 13
5. Current Practice 19
5.1 Variation in Approaches 19
5.2 Repairing Damaged Floodways 19
5.3 Crossing Geometry 20
5.4 Erosion Protection 21
6. Alternate Technology 22
6.1 Concrete Canvas® 22
6.2 Soil Stabilisation 22
6.3 Non-engineered Solutions 23
7. Typical Failure Mechanisms 24
7.1 Failure Zones 24
7.2 Failure Modes 24
8. Hydrology 30
8.1 Regional Precipitation 30
8.2 Peak Runoff Analysis 32
8.3 Design Flows 34
9. Hydraulics 38
9.1 General 38
9.2 Hydraulic Modelling 40
9.3 Results of Modelling 40
9.4 Treatment Options 42
10. Climate Change 43
10.1 Published Information 43
10.2 Impact on Mid North & Northern Districts 47
11. Classification of Sites 48
11.1 Council Road Classification 48
11.2 Definitions 48
11.3 Adjustment Factors 50
11.4 Serviceability Level 51
12. Cost Matrix 54
12.1 Construction Zones 54
12.2 Costs of Various Floodway Protection Measures 54
12.3 Costs Matrix 55
12.4 Limitations 55
13. Option Selection 62
13.1 Floodway Options Selection Flow Chart 62
13.2 Worked Examples 65
14. Trials 70
14.1 Manufacturers 70
14.2 Economy of scale 70
14.3 Whole of Life Costs 70
14.4 Site Selection 70
15. Site Specific Recommendations 72
16. Management of Floodway Assets 73
16.1 Asset Tracking 73
16.2 Monitoring Program 73
16.3 Shared Knowledge 74
17. Conclusions and Recommendations 75
17.1 General 75
17.2 Floodway Sites and Failure Mechanisms 75
17.3 Hydrology and Hydraulics 76
17.4 Floodway Designs and Cost Matrix 76
17.5 Trials 77
17.6 Site Specific Recommendations 77
17.7 Shared Knowledge and Asset Management 77
17.8 Recommendations 77
18. Bibliography 79

Table Index

Table 1 21 Floodway Sites 1
Table 2 Design of Rock Rip-Rap Slope Protection
(Austroads Waterway Design Guide) 16
Table 3 Standard Classes of Rock Slope Protection
(Austroads Waterway Design Guide) 17
Table 4 Indicative Thickness of Rock Mattresses 17
Table 5 Highest Recorded Rainfall at Selected Sites Within
the Study Area 31
Table 6 Rainfall Totals Recorded During March 1989 Storm
Event 31
Table 7 Rainfall Recorded During January 2007 Storm 32
Table 8 Equations to Calculate Peak Flow Rates for the
Regional Council of Goyder and Northern Areas
Council 34
Table 9 10, 20, 50 and 100 year ARI Flow Rates (m3/s) for
the Flinders Ranges Council, District Council of
Orroroo and Carrieton and District Council of
Peterborough 35
Table 10 Regional Flood Frequency Analysis – Eusuff High Runoff Area Flow m3/s
36
Table 11 200 and 500 Year ARI Flows (m3/s) 37
Table 12 At Grade Floodway Crossings Within the Study
Area 38

 

Table 13 Floodways with Erosion Heads 39
Table 14 Floodways with Culvert Crossings 39
Table 15 Results of the Hydraulic Analysis using Manning’s Equation
41
Table 16 Results of HEC RAS Modelling of Erosion Head and Culvert Crossing Sites
42
Table 17 Road Category versus Importance Rating 48
Table 18 Floodway Sites Road Classifications 49
Table 19 Floodway Protection Measures for Level 3 Serviceability
52
Table 20 Floodway Protection Measures for Level 2 Serviceability
52
Table 21 Floodway Protection Measures for Level 1 Serviceability
53
Table 22 Floodway Protection Measures Assumptions 54
Table 23 Costs (per metre) of Various Floodway Protection Measures
57
Table 24 Cost Matrix 60
Table 25 Level of Serviceability Matrix 63
Table 26 Site G1 Flow Chart Summary 65
Table 27 Site F4 Flow Chart Summary 66
Table 28 Site 03 Flow Chart Summary 67
Table 29 Site P1 Flow Chart Summary 68
Table 30 Site N1 Flow Chart Summary 69

Figure Index

Figure 1
Figure 2 Study Area
Velocity Conditions for Plunging Free Flow over a Typical Floodway (Austroads Waterway Design 5
Guide) 12
Figure 3 Velocity Conditions for Submerged Hydraulic Jump
over a Typical Floodway (Austroads Water Design Guide)
12
Figure 4 Velocity Conditions for Submerged Flow over a Typical Floodway (Austroads Waterway Design Guide)

12
Figure 5 Rock Rip-Rap Protection (Dumped Rock) (Department of Transport and Main Roads, QLD)
14

 

Figure 6 Rock Mattress Protection (Department of Transport and Main Roads, QLD)
14
Figure 7 Concrete Protection (Department of Transport and Main Roads, QLD)
15
Figure 8 Channel Long Section for O3 – Johnburgh Road Floodway
41
Figure 9 Autumn Winter Rainfall Deciles from 1997 – 2011 45
Figure 10 Trend in Annual Total Rainfall 1970 – 2011 46
Figure 11 Trend in Annual Total Rainfall 1900 – 2011 46

Appendices
A Site Information
B Hydrological Data
C Rational Method Calculation
D Catchment Plans
E Alternate Floodway Design Sketches
F Alternate Technology Brochures
G Site Specific Recommendations

 

 

1. Introduction

Floodways are sections of road which have been designed to be overtopped by floodwater during relatively low average recurrence interval (ARI) floods (Department of Transport and Main Roads – Road and Drainage Manual, QLD, 2010).
They are assessed in principle as being a more cost effective solution than significant infrastructure such as a large culvert crossing or bridge because flood durations are generally short, and the route does not service sufficient people to warrant a larger structure. Floodways may [also] offer environmental advantages over culverts and bridges, since they will tend to spread flows more widely. This means the risk of erosion to surrounding land is generally reduced because flow is less concentrated (Department of Transport and Main Roads-Road and Drainage Manual, QLD, 2010).
In July 2011 the Central Local Government Region of South Australia (CLGRSA) submitted an application for Local Government Association of South Australia (LGASA) funding to investigate floodway damage and to develop floodway designs to reduce the risk of future floodway damage. The five Councils that were invited by the CLGRSA to submit 21 floodway sites for investigation by GHD were:
t Flinders Ranges Council
t District Council of Carrieton and Orroroo
t District Council of Peterborough
t Northern Areas Council
t Regional Council of Goyder

1.1 Floodway Sites
The 21 floodway sites identified by the five Councils for further investigation are shown in Table 1. The location of each floodway is shown in Figure 1.

Table 1 21 Floodway Sites

Location Creek
Flinders Ranges Council
F1 Carrieton – Quorn Road Unknown Creek
F2 Yednalue Road Wirreanda Creek
F3 Yednalue Road Yednalue Creek
F4 Warcowie Road Wonoka Creek
District Council of Orroroo – Carrieton
O1 Belton Road Unknown Creek
O2 Belton Road Unknown Creek
O3 Johnburgh Road Unknown Creek
O3A Johnburgh Road Unknown Creek
O4 Orroroo Road Bambricks Creek

 

Location Creek
District Council of Peterborough
P1 Gumbowie Road Unknown Creek
P2 Bullyaninnie Road Black Fella Creek
P3 Bullyaninnie Road Little Black Fella Creek
Northern Areas Council
N1 Hornsdale – Tarcowie Road Unknown Creek
N2 West Terrace Pine Creek
N3 Georgetown – Huddleston Road Rocky River
N4 Broughton Valley Road Hutt River
Regional Council of Goyder
G1 Bower Road Unknown Creek
G2 Caroona Road Unknown Creek
G3 Angle Road Unknown Creek
G4 Belalie Road Unknown Creek
G5 Ketchowla Road Unknown Creek

1.2 Scope of Services
The CLGRSA commissioned GHD to undertake the following scope of services:
1. Identify floodway sites in conjunction with each of the five Councils.
2. Attend a project inception meeting with the CLGRSA and representatives from each of the five Councils to discuss the project objectives, history of floodway damage and obtain copies of any relevant information.
3. Undertake site visits, with the Works Managers, to assess each of the floodway sites.
4. Review climate change projections for South Australia and assess any impact climate change may have on the frequency of significant storm events and increased intensity of storm events.
5. Undertake a review of standard floodway design guides around Australia.
6. Map the catchment area for each of the floodway sites and undertake a hydrological analysis to calculate the 10, 20, 50, 100, 200 and 500-year ARI flow rates at each floodway crossing.
7. Estimate peak flow velocities across a range of typical floodways.
8. Develop a range of floodway designs to cater for the various levels of service to be provided by the floodways.
9. Establish the level of service to be provided by the floodways and develop a cost matrix that includes various floodway designs that would be expected to provide three levels of service at each floodway. The cost per metre length of the various floodway treatments will also be included.
10. Recommendation of floodway design options at each site to provide three levels of service.
11. Preparation of a report documenting the investigation and project findings.

 

1.3 Background
The study area, with the boundaries of each of the five Council areas, is shown in Figure 1. There are thousands of creeks crossing primarily unsealed roadways within the study area. These roadways are not travelled by significant numbers of vehicles but remain crucial to the local population and tourists.
Many of the floodways have been damaged on a number of occasions over the past five years, resulting in significant costs to each Council to reinstate the floodways. The Councils have limited budgets and personnel to properly design and reinstate the floodways after a significant storm event. The Council’s main objective is to reinstate the floodways as quickly as possible and to make the floodways trafficable. The necessity to patch repair floodways results in a high risk of failure in subsequent storm events.
Several different methodologies have been implemented by the Councils with varying levels of success, to counter this problem. The challenge has always been for the Councils to identify the most cost effective solution when each of the crossings is only used by a limited number of vehicles. With sufficient funding the Councils would ideally replace all of the floodways with reinforced concrete decks and batter slopes, however it is often difficult to justify the expense of a reinforced concrete structure that is infrequently used. This report aims to identify suitable replacement options that are cost effective and would reduce the risk of future damage and ongoing maintenance costs, as a result of potentially more frequent and intense storm events.
In 2007 major flooding in the five Councils resulted in significant damage to floodway assets owned by the Councils and the Department of Planning, Transport and Infrastructure (DPTI). A report was commissioned by DPTI in 2007 (Assessment of Stream Crossings on State Roads, the Flinders Ranges Region, Earthtech, 2007) to investigate floodway damage and to assist in the design of floodway reinstatement works. At significant expense, DPTI replaced the damaged floodways with reinforced concrete protection on the upstream and downstream ends of the floodway.
During the drier months of the year (October to March) the catchments can have relatively little vegetation and low infiltration rates. A combination of high runoff potential, due to limited rainfall/runoff losses, and high intensity storm events results in flash flooding. The resulting damage of flash flooding can be significant, as shown in Photo 1. The creeks within the study area are susceptible to erosion. Photo 1, shows the highly erodible vertical side walls of an existing creek on the downstream side of a floodway. This type of erosion is typical throughout the study area. Many of the floodways within the study area have failed as a result of erosion heads advancing upstream, ultimately reaching the location of a floodway crossing, resulting in partial or complete failure of the floodway. The costs to replace and/or protect the floodway increase significantly when an erosion head is present at a floodway site.
The five Councils have raised concerns about the increased frequency and intensity of storm events witnessed throughout the study area over the last five years. The resultant damage to floodways has had an adverse impact on the design life of floodways in the study area. If the trend of more frequent and intense storm events continues then the Councils could be faced with significant ongoing costs to maintain its existing floodways or replace the floodways to provide an increased level of service and reduced risk of damage.

 

Photo 1 – Failure of a 100-year-old floodway, on the downstream side, as a result of flash flooding.

1.4 Funding
After a major flood event each of the Councils submits flood damage appraisals to the Local Government Association Mutual Liability Scheme and receives commensurate funding to reinstate damaged infrastructure. The key issue is that funds are only sufficient to reinstate damaged infrastructure and not replace the damaged floodway with a design that is less at risk of failure.
It is hoped that the Councils are able to use this report to assist in their decision making processes to maintain or replace damaged floodways using funds received from the Mutual Liability Scheme and other funding sources to achieve immediate and long term savings.

Figure 1 Study Area

 

2. Project Intent
2.1 Aim
The fundamental challenge with this project is that the Councils simply do not have sufficient funding to implement significant infrastructure projects at each site. A measure of the success of this project will be the development of alternative technologies to mitigate the costs of installing significant infrastructure across a wide and remote area.
The aim of this report is to provide the Councils with:
t An understanding of the typical methods of floodway failure.
t Information to identify future threats to existing floodways.
t Floodway designs and the principles behind these designs.
t Alternate floodway designs that are less expensive than traditional floodway design techniques.
t Information to assist in identifying the level of importance of a floodway.
t Typical costs associated with identified floodway designs.
t Recommended works at 21 typical floodway sites across the study area, to assist in identifying floodway works at other sites.
t Organisation of products to trial at three of the 21 floodway sites.

2.2 Methodology
The methodology adopted for this project is based on utilising the experience of all stakeholders involved in this project. The Works Managers at each of the Councils have a wealth of experience in maintaining existing floodways, along with methods to reduce the risk of damage to floodways, with limited available funds. The key component of this project was the site visits with each of the Council Works Managers. The Works Managers and GHD visited not only the 21 floodway sites, but many other floodways that were operating effectively under high flow conditions.
Following the identification and site visits to each of the 21 floodway sites, the catchments for each of the floodway sites were calculated and a hydrological assessment was undertaken. A selection of the 21 floodway sites were analysed using the hydraulic modelling software, HEC-RAS and Manning’s Equation to calculate flow velocities across the floodways.
A review of the latest climate change predictions was undertaken, along with an assessment of the historical rainfall figures for a number of rainfall gauging stations between Leigh Creek and Kapunda. This was undertaken to determine if there has been any increase in the intensity of storm events in the last 10 years and whether there has been an increase in the number and severity of storm events in the southern region of the study area.
A review of current and alternate floodway designs was undertaken to develop a range of floodway design options to meet various levels of service and budget constraints. A cost matrix was developed that included the per metre cost of installing the various floodway treatment options. The installation and maintenance costs were also considered to factor in the “Do Nothing” option for comparative purposes. The level of importance of each floodway was determined based on the road classification and other key considerations.

 

Preferred floodway design options, for three levels of service, were recommended for each of the 21 floodway sites. The design options considered the availability of resources, the cost of resources and remoteness of the sites.

2.3 Limitations
The floodway design options in this report are intended as general ideas and should in no way be treated as construction drawings or complete design solutions. All ideas presented in this report are conceptual options, as they provide a solution to a broad range of floodway configurations. Further engineering design will be necessary to further develop these conceptual options before construction.

2.3.1 Site Data
Investigation of the 21 floodway sites showed that the method of floodway failure can be dependent on local site issues. Every floodway site should be assessed individually before considering implementing remedial works. Site specific work will need to be conducted to supplement the floodway designs.
The lack of site specific flow data provides a real challenge to this project and significantly increases the risk to any floodway design. It is impractical to try and obtain this data and therefore sizing of infrastructure should be undertaken using hydrologic and hydraulic analysis discussed in this report in conjunction with the use of both anecdotal evidence and physical site evidence such as erosion lines on channel banks, debris in fences or trees and scarring on trees.
The need to consider the local issues at every site is evident in Photo 2, which shows an erosion head forming perpendicular to the creek flow direction. This erosion head is being formed by high flows directed to the north of the floodway. If not addressed in the floodway design, there is a high risk that the roadway will be damaged to the north of the floodway.

2.3.2 Trials
Trials at subject sites using alternate technologies are being encouraged. The use of alternate technologies and the level of service that they provide will be based on the results of these trials. The timeframe to the next significant storm event is unknown, which makes it difficult to assess the performance of alternate technologies. The performance of alternate technologies will also need to be assessed against similar existing installations in other states around Australia. There is a risk that the use of stabilising compounds and Concrete Canvas® may not provide a sufficiently robust solution, particularly if they are not installed correctly. This risk has been reduced by involving the manufacturers in the trials.

2.3.3 Whole of Life Costing
It should be noted that the use of alternate technologies and trials is likely to create solutions with a lifespan significantly less than a full reinforced concrete solution. Each of the solutions implemented should be measured across its entire lifespan in order to evaluate its cost effectiveness.

 

Photo 2 – Peripheral downstream damage surrounding a floodway crossing

 

3. Study Area
3.1 Selected Sites
The study area is focused within the Mid North and Flinders agricultural districts. Figure 1 shows the Study Area and floodway sites selected by each Council. The number of floodway sites selected by each Council, include:
t Flinders Ranges Council 4 sites t District Council of Carrieton and Orroroo 5 sites t District Council of Peterborough 3 sites
t Northern Areas Council 4 sites
t The Regional Council of Goyder 5 sites

3.2 Site Descriptions
Each Council has identified floodway sites for consideration. These are by no means the full extent of problems found at floodway sites within each Council, but show sites the Councils consider to be the most import or those that highlight a particular set of circumstances recurring at numerous sites.
Appendix A contains a description of each site and the initial observations regarding current and future failure mechanisms.
The challenges faced by each of the Councils were not the same, even for sites with similar physical attributes. The road grade of the approach, topography and geological conditions are important aspects in how the damage presented at each of the floodway sites. These factors varied from Council area to Council area.

3.3 Study Area Description
The upper reaches of the Flinders Ranges Council catchments are characterised by steep ranges with rock outcrops and sparse vegetation coverage, particularly during periods of drought. The floodplains are characterised by a relatively high concentration of watercourses that flow into larger creeks, some with a base width in excess of 70 m and vertical side walls in excess of 3 m high. The watercourses typically have vertical side walls on the outside of each bend and shallow sparsely vegetated batters on the inside of each bend. Creek beds are typically lined with gravel and rock up to 300 mm in size or have been eroded down to bedrock in more undulating areas. The creek beds have a high degree of mobility and there was evidence of large uprooted gum trees scattered along creeks and floodplains.
There is very little cropping area in the Flinders Ranges Council area.
The District Council of Orroroo and Carrieton area is similar to the Flinders Ranges with the notable exception of the vast open, gently grading area of the Walloway Plain. The Walloway Plain has a high concentration of shallow watercourses that have been carved out of the plain. The watercourses typically have vertical side walls on the outside of each bend and shallow vegetated batters on the inside of each bend. The creek beds have a high degree of mobility and there was evidence of large uprooted gum trees scattered along the creeks and floodplains. The northern area of the District Council of Orroroo and Carrieton is dominated by salt bush and the southern area is dominated by cropping and grazing.

 

Creeks within the District Council of Peterborough are not as wide and deep as Flinders Ranges Council and the District Council of Orroroo and Carrieton. The terrain is relatively flat in comparison and the density of watercourses is less when compared to Flinders Ranges Council and the District Council of Orroroo and Carrieton. There was evidence that floodwaters had overtopped watercourses and eroded the floodplain, indicating that the land is susceptible to erosion and that significant flows had occurred in recent times. The District Council of Peterborough landscape is dominated by cropping and grazing.
Northern Areas Council landscape is dominated by cropping and grazing. The catchment runoff potential after harvest time, combined with the undulating terrain, is high. The creeks within Northern Areas Council exhibit the same characteristics as the other Councils in that they have a high erosion potential and have predominantly vertical side walls.
The Regional Council of Goyder landscape is dominated by cropping and grazing to the south of Burra and salt bush and grazing to the north of Burra. The landscape is undulating with more rugged areas around Mount Bryan. The landscape is less undulating to the east of Mount Bryan. There was evidence that many creeks had eroded down to bedrock. The creeks are similar to the four northern Council areas in that they are characterised by vertical side walls and have a high erosion potential.

3.3.1 Wildlife
There was evidence of wombat activity in the softer soils around Eudunda. In extreme cases the presence of wombat burrows could undermine or increase the risk of erosion around structures, resulting in premature failure. The presence of wildlife and there potential impact should be considered prior to the installation of any infrastructure on floodway sites within the study area. Care should be taken during construction works to minimise any impact to existing wildlife.

 

4. Review of Floodway Design Guides

4.1 Design Guides
The highways and major road networks across rural and metropolitan South Australia are controlled by the Department of Planning, Transport and Infrastructure (DPTI). The remaining road networks are under the jurisdiction and stewardship of Local Government Authorities. DPTI utilises the design procedures outlined in Austroads in conjunction with their own design standards and local knowledge of the area.
The publication “Waterway Design – A Guide to the Hydraulic Design of Bridges, Culverts and Floodways”, Austroads (1994) was developed as a national guide to provide design engineers a platform for progressing the design of waterway structures. The Guide covers the various aspects of waterway structures design, providing guidance on the selection of design floods, hydraulic design of waterway structures and measures to protect these structures during large flood events.
The Austroads guide is utilised nationally, however several states have created more detailed reference and design guides. Both Queensland and Western Australia Transport Departments have put resources into the generation of these documents to ensure that floodway design is understood and new cost effective floodways constructed to suit the site conditions.
The Department of Transport and Main Roads, Queensland has prepared the “Road Drainage Manual – A guide to Planning, Design, Operation and Maintenance of Road Drainage Infrastructure”, Department of Transport and Main Roads, (2010).
“The Road Drainage Manual provides guidance in relation to the planning, design, construction, maintenance and operation of road drainage structures in all urban and rural environments for main roads in Queensland. One of the key aspects of the manual is the integration of environmental considerations with the hydraulic aspects of road drainage. This manual is the department’s primary technical reference for people engaged in all aspects of road drainage and erosion and sediment control throughout Queensland. It provides sufficient information to undertake routine daily work without continual reference to other documents. Competent planners and designers should apply this manual in an intelligent way to tailor each design to the particular circumstances of a project.”
The Main Roads, Western Australia document titled “Floodway Design Guide”, Main Roads Western Australia, (2006) – provides further information specific to Western Australia’s hydrological conditions and preferences for hydraulic design and erosion control measures.
All these guides are useful resources when planning, designing, constructing and maintaining floodways.

4.2 Hydraulic Design
Where the floodway matches into the waterway invert level and does not impede the flow path the stage- discharge curve for the waterway can be simply estimated using the Slope Area Method. The Slope Area Method uses Manning’s Equation to calculate the flow rate for any number of sub-sections and water elevations across a creek cross-section.
The flow profile over a constructed floodway may be controlled by either the headwater or tailwater conditions.

 

During overtopping where a low tailwater condition exists, the outfall is under free conditions. These conditions occur initially where the road embankment acts like a weir. Discharge over the road embankment (floodway) is determined by the upstream velocity head and critical velocity will occur on the embankment. Under these conditions the tailwater level has no influence over the upstream water level or velocity.
Figure 2 shows plunging free flow with low tailwater, where a high velocity jet moves down the downstream batter, accelerating to maximum velocity at the base of the batter. Deceleration is caused by the sudden change in direction and bed friction until a hydraulic jump occurs downstream of the floodway.

Figure 2 Velocity Conditions for Plunging Free Flow over a Typical Floodway (Austroads Waterway Design Guide)

When the tailwater level is higher, weir flow over the embankment can produce either a submerged hydraulic jump on the downstream slope, as shown in Figure 3 or laminated flows downstream (flows over the surface of the downstream tailwater). The flow is still free plunging with the high velocity jet plunging into the turbulent tailwater.

Figure 3 Velocity Conditions for Submerged Hydraulic Jump over a Typical Floodway (Austroads Water Design Guide)

Subcritical flow conditions, as shown in Figure 4 develop when discharge over the floodway is controlled by the downstream channel capacity and the upstream velocity head. Under these conditions the floodway is considered submerged. The total head differential across the embankment is low resulting in the high velocity jet lifting from the boundary of the floodway and staying on the surface of the tailwater. The energy of the jet is dissipated in a harmless manner through the tailwater.

Figure 4 Velocity Conditions for Submerged Flow over a Typical Floodway (Austroads Waterway Design Guide)

 

 

Under these conditions, only nominal batter protection is required, however for the other two conditions, erosion protection will be required on the downstream batter and channel invert.
By limiting the differential between upstream and downstream water surface levels, minimal batter protection can be implemented. This can be achieved by either:
t Allowing the tailwater to rise to a high level before the floodway comes into operation by allowing sufficient flow to pass through culverts; or
t Constructing a low floodway so that the floodway ceases to act as a control at a relatively low upstream water surface level.

4.3 Erosion Protection
The type and extent of erosion protection is decided after consideration of the flow velocities (upstream and downstream), the orientation of the floodway with respect to the waterway, the natural downstream ground conditions and the performance of existing infrastructure in the area. Erosion can occur at the shoulders and batters of the floodway, within the channel (bank and invert) and on the pavement.

4.3.1 Upstream
The requirement for upstream erosion protection is determined by the approaching flow velocity, the period for which the floodway is submerged and the skew of the floodway to the direction of flow.
Generally, the extent of upstream erosion protection required for high flow velocities can be limited to the road shoulder and the top of the road batter.
For floodways submerged for extended periods, it is usual to provide a similar protection on the upstream batter to that provided on the downstream batter.

4.3.2 Pavement
The decision on pavement type is based on traffic volumes during wet and dry periods, the frequency and duration of overtopping, potential for erosion damage and the cost of construction and maintenance.
Most guides recommend the use of flexible (stabilised base course with two coat seal) or rigid (concrete) pavements. Within the five Councils the majority of floodways are unsealed.

4.3.3 Downstream Embankment
The type of erosion protection for downstream embankment batters can be either flexible or rigid. The treatment should form a continuous surface with the road pavement at the shoulder to avoid any sharp steps or grade changes which form high pressures.
Different types of flexible and rigid protection are shown below and have been sourced from “Floodway Design Guide”, Main Roads Western Australia, (2006).
Flexible Protection
t Dumped graded rock (rip-rap) defined as graded stone dumped upon a prepared slope. A typical example is shown in Figure 5. In most areas dumped rock is the least costly type of erosion protection. A suitable length toe (typically 1.0-1.5 times the embankment height) should be provided at the base of the rock to protect the embankment against high velocities at the change of grade.

 

The class and thickness of rock to be used can be determined from Table 2 and Table 3. Consideration should be given to the inclusion of a concrete cut-off wall at the interface between the pavement and rock rip-rap.

Figure 5 Rock Rip-Rap Protection (Dumped Rock) (Department of Transport and Main Roads, QLD)
t Hand placed graded rock which is inferior to dumped rock, is seldom used today.
t Rock mattresses (gabion mattresses) are rock placed in wire baskets or in wire covered mattresses. A typical example is shown in Figure 6. Wire enclosed rock is generally used in locations where suitable materials for dumped rock are not readily available or is not economic to import. The size of rock should be larger than the openings in the wire enclosure, and a suitable length toe is required as with rock rip-rap. The wire used should be PVC coated to reduce the risk of corrosion.

Figure 6 Rock Mattress Protection (Department of Transport and Main Roads, QLD)

Mattresses must be pinned or anchored and consideration should be given to constructing a concrete cut-off wall at the interface between the pavement and rock mattress and at the end of the toe of the rock mattress.

 

t Flexible mats comprise individual small high-density concrete blocks cast onto a geotextile loop matting. Each mat is generally about 5 m by 2.5 m and protection is provided by laying the mats side by side with an overlap. These are proprietary products and the designer should refer to the manufacturer’s technical literature for advice on their application and installation.
t Flexible pump-up revetment mattresses are concrete filled nylon mattresses where the concrete flows into discrete segments that are largely independent once the concrete has set, providing a degree of flexibility. These are proprietary products and the designer should refer to the manufacturer’s technical literature for advice on their application and installation.
t Vegetative cover can form an effective erosion protection system for floodways where the embankment is low and the approach velocity is low (less than 1.0 m/s). This type of protection is only appropriate in regions where adequate vegetation cover exists all year round (typically humid northern regions). The peak floodway velocities should also be kept low. This is not recommended for use as the primary erosion protection system, but is useful as an additional protective measure.
Rigid Protection
t Grouted rock is dumped or hand placed rock, and the voids filled with mass concrete. The concrete should be sufficiently fluid to fill all voids over the full depth of the rock layer. It is generally used in locations where stone of a size suitable for other forms of protection is not economically available. It is also useful where only a small depth is available for construction of rock protection (such as over culvert pipes) or where access to construct larger rock is difficult.
t Rigid pump-up revetment mattresses are nylon mattresses into which a small aggregate concrete is pumped. These are proprietary products and the designer should refer to the manufacturer’s technical literature for advice on their application and installation.
t Concrete slab protection is plain or reinforced concrete slabs poured or placed on the surface to be protected. Typical examples of concrete protection are shown in Figure 7. This type is not often used due to its high cost, but may be warranted at crossings subject to extended periods of inundation. It may also be warranted in exceptionally high velocity situations, where other types of protection are inadequate.

Figure 7 Concrete Protection (Department of Transport and Main Roads, QLD)

 

 

 

The cost of concrete protection systems will need to be justified, but in general performance is improved.
Rigid protection is susceptible to undermining by erosion, especially at the toe of batters, and should not be used unless the design engineer is confident that erosion will not compromise the structure.
Combinations of flexible and rigid systems may also be considered.
High velocities can cause erosion for a significant distance downstream of the floodways. Where high outlet velocities are expected, appropriate dissipation and/or protection measures will be required. Where possible, the design should ensure that velocities are appropriate to the level of treatment and the extent of protection (into the channel).
Generally, DPTI suggest the use of reinforced concrete, but defer to the Austroads Waterway Design Guide. Other treatments can be considered where availability of critical materials is low, or where significant cost savings can be realised. However, it is recommended that advice is sort from other Councils, the LGA, DPTI or other government agencies when alternative treatments are proposed to be used.

Table 2 Design of Rock Rip-Rap Slope Protection (Austroads Waterway Design Guide)

Velocity (m/s) Class of Rock Protection Wc (tonne) Section Thickness T (m)
<2 None –
2.0 – 2.6 Facing 0.5
2.6 – 2.9 Light 0.75
2.9 – 3.9 ¼ 1.00
3.9 – 4.5 ½ 1.25
4.5 – 5.1 1.0 1.60
5.1 – 5.7 2.0 2.00
5.7 – 6.4 4.0 2.50
>6.4 Special –

 

Table 3 Standard Classes of Rock Slope Protection (Austroads Waterway Design Guide)

Rock Class Rock Size (m) Rock Mass (kg) Minimum Percentage of Rock Larger Than
0.40 100 0
Facing 0.30 35 50
0.15 2.5 90
0.55 250 0
Light 0.40 100 50
0.20 10 90
0.75 500 0
¼ tonne 0.55 250 50
0.30 35 90
0.90 1000 0
• tonne 0.70 450 50
0.40 100 90
1.15 2000 0
1 tonne 0.90 1000 50
0.55 250 90
1.45 4000 0
2 tonne 1.15 2000 50
0.75 500 90
1.80 8000 0
4 tonne 1.45 4000 50
0.90 1000 90

Table 4 shows the size of rock required for various mattress thicknesses, where the Froude number is s;1. Critical velocity is the velocity at which the mattress will remain stable without movement of the rock fill. The limiting velocity is the upper limit although there is some movement of rocks resulting in deformation of the mattress.

Table 4 Indicative Thickness of Rock Mattresses

Thickness (m) Rock Fill Size (mm) D50
(mm) Critical Velocity (m/s) Limiting Velocity (m/s)

0.15 – 0.17 70 – 100
70 – 150 85
110 3.5
4.2 4.2
4.5

0.23 – 0.25 70 – 100
70 – 150 85
120 3.6
4.5 5.5
6.1

0.30 70 – 120
100 – 150 100
125 4.2
5.0 5.5
6.4

 

Concrete Cut-off Walls
Where high velocities are expected on the downstream shoulder it is recommended that concrete cut-off walls are used. The placing of rigid protection will reduce the risk of undermining the downstream embankment erosion protection and erosion of the road pavement.
Typically, these walls are constructed of low strength mass concrete, generally 0.5 – 1.0 m deep and
0.2 – 0.3 m wide.

Downturn Walls
Concrete downturn walls should be constructed at the end of a concrete apron and base slabs to reduce the risk of undermining the tow of embankment erosion protection. These walls are typically
0.50 – 1.00 m deep, however the depth of the wall will be dependent on site conditions and the presence of an erosion head within the waterway.

 

5. Current Practice

5.1 Variation in Approaches
There is a significant difference in the type and method of erosion protection adopted for floodways within the study area. Rather than conforming to the latest design guides, works are instead being influenced by Council’s need to find cost-effective solutions for their numerous floodways. The northern Councils prefer not to utilise culverts at floodways with no base flow and are in the process of removing many culvert structures. The southern Councils are more willing to adopt culverts in floodways, particularly where there is a base flow. It should be noted that neither solution is outside current practice, as there is literature discussing the advantages and disadvantages of both options. The main distinctions are:
t Higher cost of the associated infrastructure (culverts, headwalls and erosion protection), and the need to engage an experienced construction crew to ensure proper construction. Higher ongoing maintenance requirements should also be considered when using culverts.
t Drainage culverts induce a high erosion potential to the floodway. If localised erosion damage remains unchecked, it can progressively lead to failure of the floodway.
t Culverts increase the downstream tailwater level, prior to overtopping of the floodway. This reduces the risk of erosion and formation of a hydraulic jump immediately downstream of the floodway.
Erosion was observed at a small crossing utilising culverts installed in 2009/2010. Erosion damage was already occurring around the structure. Without regular inspection and maintenance this site could experience significant damage during the next flood event.
The majority of the floodways within the study area are located within the unsealed road networks and are therefore not sealed by either concrete or bituminous product. The key driver for this decision by the Councils was the cost of installation. All Councils are reluctant to use sealing product on floodways due to the incurred maintenance costs as the seal degrades.
The level of erosion protection provided at floodways across the study area varies from site to site. The damage caused to floodways as a result of large flood events over the last five years have highlighted the need for floodway designs that consider adequate erosion control for all elements of the floodway, including the peripheral areas surrounding the floodway.

5.2 Repairing Damaged Floodways
Presently, Councils are adopting a reactive approach to repair damaged floodways and open them to traffic as quickly as possible. These reactionary responses are driven by the limited funding resources readily available to Councils. The repairs tend to consist of simple replacement of lost material and limited compaction. Although this re-opens the routes quickly, it also leaves the site susceptible to further damage from future flood events.
It is easy to see how adopting a solution utilising large amounts of reinforced concrete and specifically sized and graded rock protection, as per conventional practice, would enable a viable solution to be built. However, the clear limitations on cost have been highlighted by both the CLGRSA and the Council representatives. This report aims to define floodway designs that will allow Councils to undertake floodway improvement works that will reduce the ongoing repair and maintenance costs.

 

5.3 Crossing Geometry
The floodway geometry can significantly reduce the requirement for physical infrastructure. Discussion regarding key crossing geometry elements is based on field observations only.

5.3.1 Crossing Location
The location of the creek/road intersection can have a significant impact on the scale of damage during flood events. Whilst it is accepted that it is not easy to realign the roadway, the funding and equipment/technology required can be considerably less than installing reinforced concrete erosion control infrastructure.
The majority of crossings experiencing significant bank erosion occur on the outside bends of a creek. Photo 3 shows an eroded creek batter on the outside bend in a creek and shallow vegetated batters on the inside bend in the creek. Note that there is vegetation establishing in the creek on the inside bend, indicating lower flow velocities and bed mobility.

Photo 3 – Photo taken looking upstream at F2 – Yednalue Road (Wirreanda Creek). The outside bend in the creek is on the right and the inside bend is on the left.

 

Roadways that do not cross at near perpendicular to the creek also increase the risk of flood damage. This is due to:
t Longer length of floodway.
t Increased risk of erosion and/or sediment deposition because low flows are more likely to flow along the roadway, which is often lower than the surrounding creek bed.

5.3.2 Vertical Profile
The vertical profile relates to the levels at which the roadway crosses the creek. It has been found that the risk of damage to the floodway can be reduced when the roadway and creek bed are at the same level. The District Council of Orroroo and Carrieton have begun to remove previously installed infrastructure in the form of culverts that have failed and realigned the road to match the creek bed. This has been successful at several sites and has significantly reduced the cost of reinstating the crossing after subsequent flood events. It is important to note that the presence of an erosion head on the downstream side of the floodway makes it more difficult or is not cost effective to lower the road level to creek bed level.

5.4 Erosion Protection
The types of erosion protection solutions being utilised across the five Council districts are described below.

5.4.1 Use of Concrete Headwalls, Cut-off Walls and Aprons
Where there is a significant level difference between the road surface and the waterway invert (typically where an erosion head is present), concrete headwalls and aprons have been constructed.
Generally these end treatments are being undermined by an advancing erosion head. In many instances, inadequate supporting erosion protection has left the end treatments susceptible to an erosion head, resulting in catastrophic failure of the end treatments and floodway. The depth to which many of these structures have been placed below waterway invert level has not been sufficient to stop the progression of the erosion head.
The Councils have been adopting the use of concrete cut-off walls at the upstream and downstream ends of the roadway, with a significant amount of success, when the downstream batter is protected.

5.4.2 Use of Rock Mattress Protection
Assessment of Stream Crossings on State Roads, the Flinders Ranges Region, Earthtech, 2007 states that “Wire basket mattresses or revetment mattresses are generally unsuitable in these alluvial environments. In the cobble and gravel bed load streams the siltation and dragging of bed load is enough to tear open the wire baskets. As the rock fill is of similar size to the bed load it is then carried downstream. In sandier flood out reaches it is unlikely the baskets will be torn open but the sandy bed and suspended load is likely to abrade and remove the galvanising quickly allowing the baskets to corrode.”
This highlights the difficulties in using gabion baskets or similar technologies for erosion protection. The only solution recommended is to use suitably sized rock rip-rap. The cost of installation of rock mattress systems has also lessened Councils desire to adopt them.

 

6. Alternate Technology

Reinforced concrete is a solution that has been shown to work and is the default material in the design guides. Due to the remoteness of the floodway sites, the cost of the raw materials and labour costs makes it one of the more costly erosion control solutions. Developments in alternate technologies may represent a more cost effective sustainable solution. There are several products on the market that may have successful applications within floodway construction.

6.1 Concrete Canvas®
Concrete Canvas® is a recently developed technology and has been extensively used in both roadway drainage and military applications. It is both fully UV resistant and can achieve strength ratings of up to 40 MPa. The flexible nature of this product and the fact that it can be installed by hand means it is suited to both the remoteness of some of the sites considered as well as variety of sites considered. It is now being used extensively in drainage projects in Queensland and shows properties that merit its further investigation as a floodway erosion control measure.

6.2 Soil Stabilisation
The current practise of compacted road material to create the floodway road surface is not sufficient to withstand flood events. The following products are recommended for further investigation, for use as a means of reducing the risk of erosion on unsealed roadways.

6.2.1 Polymers
Stabilising polymers such as Polycom® and Soiltac® are widely used in unsealed roads for dust suppression and erosion control. There is a clear case for their use to minimise the risk of erosion of the road surface during flood events. The ease of transport and installation of polymers warrants further investigation for their use as a roadway erosion control measure. There is limited data available for these products and it is noted that some Councils in the study area have trialled polymer treatments with limited success.

6.2.2 Cement Treated Product
Existing technology using cement treated product to produce a more erosion resistance road surface is also recommended for further consideration and testing. It is avoided for large scale road construction in Council areas due to cost considerations, and has not been widely adopted as an erosion control measure on floodways in the study area. The aim is to provide a more erosion resistant surface to reduce the risk of failure and ongoing maintenance of floodways. The use of cement treated road material with a seal has been proven to work in the study area. Significant degradation can occur in areas of high velocity flow and therefore its placement on floodways must be carefully considered or recognise the fact that it will need to be replaced or patched on a more frequent basis than some other erosion control measures.

 

6.3 Non-engineered Solutions

6.3.1 Vegetation
There are many documents available that state the key to erosion control is vegetation. This provides a maintenance free, relatively low cost, self-regulating and non-technical solution.
The use of vegetation as an effective form of erosion control for floodways is limited to humid regions where adequate vegetation cover exists all year round. Vegetation is capable of withstanding low approach velocities, less than 1.0 m/s and as such is not a primary erosion control system.
The study area sites are in semi-arid regions and as such the establishment and survival of vegetation cannot be guaranteed. The study area is within a low rainfall and extreme temperature zone, and as such will impede the establishment of aquatic and riparian vegetation within the channel and the floodplain. In addition the soils within the channel and floodplains are highly erodible and in combination with fast flowing floodwaters, inhibit the effectiveness of vegetation to provide effective erosion control.
The issue presented by the regional Councils is that the erosion has reached critical points and impacted on road infrastructure in most of the sites identified for this project. The use of vegetation as a method of erosion control is best implemented in advance of the erosion reaching critical infrastructure.

6.3.2 Erosion Control and Turf Reinforcement Matting
Erosion control matting has been utilised in urban waterway as an in-channel protection option for many years. The use of matting is restricted to low velocity zones (approximately <2 m/s), stabilising bank batters, revegetation zones and in conjunction with primary protection treatments.
There are several types of matting which can be categorised as natural or man-made and are of varying thickness and weight.
A disadvantage of some erosion control matting is the necessity to establish and maintain vegetation or a protective layer as the matting itself will deteriorate over a short period when exposed to the elements.
It is not recommended that erosion control matting be used by the Councils within their floodways.

6.3.3 Other Products
Other products available for bank stabilisation include:
t Geocell
t Landlok
t Trinter
These erosion control measures are only recommended for use outside the critical areas of works.

 

7. Typical Failure Mechanisms

Although no two sites are ever the same, there are characteristic failure methods that account for most of the failures observed across the study area. These can be categorised according to their primary action and the floodway zone where they occur.

7.1 Failure Zones
For the purposes of this report, three zones have been defined within a floodway. The failure zones across the floodway include:
1. Upstream Zone – The upstream zone refers to the section of creek immediately upstream of the roadway shoulder. The upstream zone does not include any concrete cut-off walls on the upstream side of the roadway.
2. Roadway Zone – The roadway zone is the area enclosed by, and including the road shoulders.
3. Downstream – The downstream zone starts at the roadway shoulder and extends down the creek channel to a point where any creek erosion has a low risk of resulting in damage to the floodway.
The fourth zone of failure is the peripheral zone, which includes areas outside of the floodway zones.

7.2 Failure Modes
Floodways fail by three main causes: erosion, deposition and failure of infrastructure. The nature of these failures is discussed below with reference to the floodway zone it occurs in.

7.2.1 Erosion
Erosion is the primary cause of failure of many of the floodway sites in the study area. It can occur throughout the three floodway zones.
Upstream Channel Invert Erosion
This is a rare cause of failure for a floodway and can occur if an obstruction is placed in the creek bed or if the roadway is elevated above creek bed level without suitable erosion protection. No significant upstream erosion that contributed to floodway failure was identified at any of the sites visited in the study area. Many of the sites presented with upstream channel profiles showing little or no signs of erosion.
The most significant upstream erosion occurs at larger creek crossings, where the location of the low flow channel within the creek is not stable and changes direction and location after each flow event. Photo 4 shows the formation of a new low flow channel in a 50 m wide watercourse in the Flinders Ranges Council.

 

Photo 4 – New low flow channel (right-hand-side of picture) forming on the upstream side of a floodway in the Flinders Ranges Council.

Roadway Erosion
If the roadway sustains sufficient damage it becomes impassable to traffic and must be repaired. The floodway is deemed to have failed. Roadway erosion can occur as a result of:
t If drainage in the surrounding area and on the roadway are not managed correctly, then stormwater runoff may flow along the road alignment rather than in drainage swales or sheet flow across the roadway, which can lead to erosion and failure of the road surface.
t If a roadway is at creek invert level the change in material properties and the flow dynamics over it can cause turbulence which may lead to localised erosion of the road surface. This usually does not result in catastrophic failure of the crossing point. Erosion of the road surface at creek bed level can also occur as a result of shifting low flow channels in a creek.
t Increased flow velocities over the downstream end of a roadway raised above the bed level can result in erosion of the road surface. This is particularly evident during low flows before the floodway is submerged by rising downstream flood levels.
Downstream Channel Invert Erosion (Erosion Head)
This is the predominant and most destructive location for erosion to occur at a floodway site. It is typified by the presence of a large erosion head moving upstream at varying rates, dependent on the soil type.
This erosion head can range in size from half a metre up to a height of at least six metres.

Photo 5 shows an example of an erosion head that has formed in an open paddock approximately 300 m downstream of a floodway crossing. If not addressed this erosion head may ultimately result in failure of the upstream floodway.
Erosion heads develop at locations in the creek bed where the material is at a higher risk of erosion or the natural gradient of the creek bed changes causing an increase in flow velocity, resulting in erosion. Progressive clearing of vegetation within a catchment leads to increased runoff frequency and volume, which increases the risk of creek erosion.
As the erosion head progresses upstream it eventually encounters existing infrastructure. The erosion at the channel invert undermines the existing floodway infrastructure, causing it to fail.
If no infrastructure is present the erosion head simply continues upstream and removes either a portion of or the entire floodway.
Photo 5 – Example of an erosion head developing in a natural creek in the middle of an open paddock.

Erosion Around Infrastructure
Erosion around physical structures within a floodway, such as culvert headwalls, is also a common method of failure. Photo 6 shows erosion around the downstream end of an existing culvert headwall. The roadway has eroded behind the concrete headwall and pipe to a point where the pipe and headwall are completely exposed.

 

Photo 6 – Erosion around a culvert headwall.

Peripheral Zone Erosion
During flood conditions flow can occur outside the normal creek channel. If this occurs in areas of soft erodible soil it can create side channels which do not follow the normal creek alignment. If erosion of the banks is extreme it can lead to the failure of the roadway at an adjacent location to the existing floodway. Photo 7 shows the location of an erosion head forming perpendicular to the direction of flow in the creek. This is due to the spread of flow over the floodway being wider than the downstream creek width. If these side channels are not controlled they can result in damage to the road outside the area considered to be part of the “floodway”.

 

Photo 7 – Side channel cut by flood flows outside the creek channel. Main floodway site is shown by the position of the vehicle.

7.2.2 Deposition
Floodways that expand the creek cross-section at the floodway are more prone to sediment deposition. The expansion in creek cross-section results in a reduction in flow velocity, which enables rock, gravel and coarse sand to settle out on the creek bed. Floodways on the inside of a bend in the creek are also at a higher risk of sediment deposition due to reduced flow velocities. Although not strictly a “failure” of the infrastructure, large amounts of deposited material on a crossing can render it impassable to normal traffic. This is the “preferred” method of failure for the Councils and it is the most easily repaired.

7.2.3 Infrastructure Failure
Infrastructure failure at a floodway can quickly lead to failure of the floodway.

End of Design Life
All infrastructure has a design life. At some point the action of the creeks flooding, the environment and the stresses placed on a structure will cause it to fail.

 

Flood in Excess of the Original Design Intent
In the last five years there have been, at most sites in the study area, flood events that have exceeded any other flood event in the last 100 years. These flood events have caused damage never witnessed before by the Works Managers. The extent of inundation at the floodways has, at many sites, exceeded the extent of the original design floodway protection measures. The most common resultant failure is erosion around the extremities of the floodway. This method of failure can be difficult and costly to protect against, leaving the Council’s uncertain as to the extent of floodway protection to be provided.

 

8. Hydrology

A hydrological investigation is required at a floodway crossing to understand the flow rates that are to be mitigated to provide a particular level of service. With a given flow rate the:
t Extent of flooding across a floodway can be calculated.
t The velocity distribution across a floodway can be calculated.

8.1 Regional Precipitation
Recorded daily precipitation within the study area was collated from Bureau of Meteorology weather stations. Data sites were selected based on the length of recorded information and remaining open to at least 2010. Historical rainfall data at some stations commences in the late 1880s and has continued to present.
Daily rainfall data from 23 Bureau of Meteorology rainfall gauging station between Leigh Creek in the north and Tanunda in the south were analysed. A graphical representation of the 23 rainfall gauging stations across the study area is shown in Appendix B. This wide disbursement of sites was chosen to identify changes in rainfall patterns within the Flinders and Mid North districts. For the purpose of this study, only rainfall events with a daily precipitation in excess of 40 mm and during the dry season (October through to April) were included in the assessment. Winter storm events were excluded because they have historically not been a concern with respect to floodway damage. The lower rainfall intensities of winter storm events have historically only resulted in minor floodway repair works.
From this graphical representation of daily precipitation it can be summarised that more frequent large daily rainfall events, greater than 40 mm, occur approximately every 30 years from 1890 through to 2010. As the recorded data is daily rainfall, the duration of the rainfall events is not known, but generally large precipitation events in the dry months are associated with large storm events, which occur over a short period of time (six hours or less) rather than a steady sustained precipitation over the course of the day.
The largest daily rainfall event within central northern South Australia was recorded at Beltana Roadhouse weather recording station (BOM Station No. 017119). This site is approximately 120 km north of Hawker. The rainfall total recorded on 14 March 1989 was 235.6 mm and was the most significant rainfall event within that station’s recorded history.
A summary of the highest recorded daily rainfall events from a selection of seven representative sites within the study area are shown in Table 5. It is important to note that the daily totals are recorded from 9:00 am to 9:00 am on the following day. It is likely that some of the storm events may have commenced or finished outside of this time frame and some of the rainfall total for a particular storm event was recorded in the preceding or proceeding day.

 

Table 5 Highest Recorded Rainfall at Selected Sites Within the Study Area

Station No.
Name
Month
Year Daily Rainfall (mm)
019017 Hawker March 1989 173
019061 Yednalue January 2007 149
019032 Orroroo November 1977 119
019009 Carrieton March 1921 142.7
021055 Whyte Yarcowie February 2011 128.4
021047 Spalding January 1995 106.2
021100 Burra (Leighton) January 1995 172

Of interest are two large rainfall events in recent history (post 1970s) which are easily distinguished from the graphical representation of the daily rainfall data. The most significant rainfall event experienced in the study area occurred on the 14 March 1989. A summary of the daily rainfall totals for various gauging stations on the 14 March 1989 is shown in Table 6. The event can be seen to have extended as far south as Whyte Yarcowie.

Table 6 Rainfall Totals Recorded During March 1989 Storm Event

Station No. Name Daily Rainfall (mm)
017119 Beltana 235.6
019017 Hawker 173
019010 Cradock 100
019061 Yednalue 79.2
019009 Carrieton 82.2
019057 Yalpara 90
019032 Orroroo 90.2
021055 Whyte Yarcowie 48

It is noted that the 14 March 1989 event coincided with an intense geomagnetic storm which was experienced around the globe on 13 – 15 March 1989.
The most recent large storm event occurred on the 20 January 2007. The recorded daily rainfall totals at various stations in the study area are shown in Table 7. This storm event was more isolated than the 14 March 1989 storm event. The intensity of the storm focused on a smaller area, around the Hawker and Yednalue regions. By the time the storm front moved south some 70 km to Carrieton the recorded precipitation was only 40.4 mm and in the north rainfall totals were in the order of 20 – 30 mm and as low as 10.2 mm at Blinman.

 

In the 2007 storm event, there were reports that some areas received approximately 270 mm of rainfall in six hours. This is a significant difference to the recorded rainfall of 149 mm at Yednalue, which has one of the highest rainfall totals in the area on the day of the storm event. This demonstrates the localised nature of the storm events in the study area, and the difficulties in estimating peak flows within each catchment.
Table 7 Rainfall Recorded During January 2007 Storm

Station No. Name Precipitation (mm)
019017 Hawker 125.4
019010 Cradock 135
019061 Yednalue 149
019009 Carrieton 40.4

Analysis of the 2007 event was undertaken by Dr David Kemp of DPTI, due to the size of the event and the large scale flooding and substantial damage to infrastructure experienced in the Hawker area. The region experienced rainfall from the 18 to 20 January, with the majority of the rainfall occurring over a 12 hour period on the 20th January. It has been estimated for this location that this event had an Average Recurrence Interval (ARI) in the order of 1 in 1000 years.
8.2 Peak Runoff Analysis
The design of floodway infrastructure is based on the calculation of peak flood flow rates and velocities for various ARI storm events. Flows were derived using rainfall intensity estimates for storm events ranging from 10, 20, 50, 100, 200 and 500 year ARI. Rainfall intensities were derived from the Bureau of Meteorology web site at selected locations. Intensity Frequency Duration plots for various locations are shown in Appendix B. The 200 and 500 year ARI flows were extrapolated from the 10, 20, 50 and 100 year ARI flows.
Catchment hydrology is ideally based on reliable recorded stream flow data. Where gauging station information is not available, generally for remote or small rural catchments, determination of peak flows is based on regional relationships, rational method calculations or hydrological modelling using typical hydrological modelling parameters.
Based on discussion with the Department of Planning Transport and Infrastructure (DPTI), the Rational Method was applied to the Flinders Ranges Council, District Council of Orroroo and Carrieton and the District Council of Peterborough. Regional relationships were applied to Northern Areas Council and the Regional Council of Goyder catchments.
DPTI were able to provide an analysis of peak flood flows, for various catchments, for the January 2007 storm event. This information was used to compare flow rates.
It is important to note that the peak flows for various ARI documented in this report are estimates only. Estimating peak flows for various ARI within the study area is difficult due to a lack of rainfall data, including continuous rainfall monitoring (pluviograph), and flow gauging data. Flow gauging data is typically monitored on large catchments and the reliability of peak flow estimates (rating curve) is generally questionable. The localised nature of storm events in this area means that it is unlikely to be raining at the same intensity over a large catchment (800 km2) for the entire duration of the storm event. Any derived local relationships from flow gauging data in the study area are unlikely to be reliable when transferred to smaller catchments (less than 50 km2).

 

8.2.1 Northern Council Areas
The Rational Method (Equation 1) is a simple probabilistic method used to estimate a peak flow from a catchment area, for a given ARI event and average rainfall intensity. There are limits to this method of determining peak runoff within a catchment, and for rural areas the size of the catchment for which the Rational Method can be applied is typically less than 25 km2. In the absence of any other method of determining peak flow rates from larger catchments, the rational method has been used to calculate peak flow rates for two larger catchments (F2 and F4).

= 0.278 Equation 1
Q = Flow in m3/s, C = Runoff co-efficient, I = Rainfall intensity (mm/h), A = Catchment area (km2)
A key determinate of the Rational Method is the time of concentration. In line with Australian Rainfall and Runoff, 1986, the time of concentration for a rural catchment is estimated using the Bransby-Williams formula (Equation 2). In each case, an assessment of the flow path was undertaken to determine the catchment’s time of concentration and the Equal Area Slope was determined based on the selected flow path. The time of concentration is used to calculate the rainfall intensity, using Intensity Frequency Duration (IFD) charts, as shown in Appendix B.

A0.1*Se0.2

Equation 2

tc = Time of concentration (min), L = Mainstream length (km), A = catchment area (km2), Se = Equal area slope of mainstream
Flinders Ranges Council and the District Council of Orroroo and Carrieton have witnessed storm events that suggest the rainfall intensities calculated using the unmodified Bransby-Williams formula in Australian Rainfall and Runoff, 1986, for this area may not be representative of the actual rainfall intensities. A more representative estimate of the rainfall intensities could be achieved by halving the time of concentration calculated using the Bransby-Williams formula and therefore increasing the rainfall intensity. The time of concentration calculated using the Bransby-Williams formula was adopted to calculate the rainfall intensity for the District Council of Peterborough catchments.
The runoff co-efficient has been calculated using a method derived by DPTI, SA, Runoff Co-efficient Guide for Small Rural Catchments, Bill Lipp (date unknown). The Rational method and procedure used to calculate the runoff co-efficient for the northern Council areas of Flinders Ranges Council, District Council of Orroroo and Carrieton and the District Council of Peterborough are shown in Appendix C.

8.2.2 Southern Council Areas
An alternate method was adopted to calculate flows for the southern area Councils of the Regional Council of Goyder and Northern Areas Council. There are several Regional Flood Frequency analysis methods for South Australia. Regional Flood Frequency analysis for the Mount Lofty Ranges was used to develop peak flows for these catchments, “A Regional Flood Frequency Approach to the Mount Lofty Ranges” Unpublished thesis, University of Adelaide, June 1995, EUSUFF, T.H.
Previous studies undertaken by GHD in the Spalding and Gladstone regions concluded that Eusuff’s High Runoff Area approach provides the best estimation of flows for 10 to 100 year ARI events. The 200 and 500 year ARI flows were extrapolated using 10 to 100 year ARI flows.

 

The equations used to calculate the peak flow rates for the Regional Council of Goyder and Northern Areas Council are shown in Table 8.

Table 8 Equations to Calculate Peak Flow Rates for the Regional Council of Goyder and Northern Areas Council

Equation 10 Year ARI 20 Year ARI 50 Year ARI 100 Year ARI
Q = 1.64A0.81 Q = 2.18A0.79 Q = 3.02A0.75 Q = 3.8A0.73
A = Catchment area in km2

8.2.3 Catchment Areas
The catchment area for each of the identified floodways was estimated using 10 metre contour topographical data for South Australia. The catchment plans for floodways within each Council boundary are shown in Appendix D.
The catchment area for each floodway crossing is shown in Table 9 and Table 10. There are a range of catchment sizes from 1.5 km2 for the Angle Road floodway in Goyder to 1,200 km2 for Georgetown – Huddleston Road in the Northern Areas Council. Of the floodways selected for investigation in this study, half have catchment areas less than or equal to 6 km2, with only 6 catchments being greater than 170 km2.

8.3 Design Flows

8.3.1 Northern Council Areas
Table 9 summarises the calculated 10, 20, 50, 100 year ARI flows for 12 sites within the Council areas of Flinders Ranges Council, District Council of Orroroo and Carrieton and District Council of Peterborough.
The Rational Method and Regional Flood Frequency (high runoff area of the Mount Lofty Ranges) analysis were used to estimate peak flow rates at each floodway crossing. Two flow rates have been provided for the Rational method. The first flow rate (RM1) utilises the time of concentration calculated using the Bransby-Williams formula to obtain the rainfall intensity and the second flow rate (RM2) utilises half of the time of concentration calculated using the Bransby-Williams formula to calculate the rainfall intensity for each floodway catchment.
Previous flood investigations in Spalding and Gladstone indicate that the Eusuff High Runoff Area (EHRA) of the Mount Lofty Ranges Regional Flood Frequency method provides a reasonable estimate of the peak 100 year ARI flows in this area. The District Council of Peterborough is to the north of Gladstone and therefore could be expected to receive higher rainfall intensities and flow rates. The flow rates in Table 9 indicate that the Rational Method flow rates are higher than the Regional Flood Frequency flow rates for the 20, 50 and 100 year ARI events.
It was concluded that the Rational Method, utilising half of the time of concentration to calculate the rainfall intensity, provided a reasonable approximation of the flow rates at each floodway within the Flinders Ranges Council and the District Council of Orroroo and Carrieton. The Rational Method, utilising the full time of concentration to calculate the rainfall intensity, provided a reasonable approximation of the flow rates at each floodway within the District Council of Peterborough. The adopted flow rates in Table 9 are underlined.

 

Table 9 10, 20, 50 and 100 year ARI Flow Rates (m3/s) for the Flinders Ranges Council, District Council of Orroroo and Carrieton and District Council of Peterborough

Site Area 10 year ARI 20 year ARI 50 year ARI 100 year ARI
km2 RM(1) RM(2) EHRA(3) RM(1) RM(2) EHRA(3) RM(1) RM(2) EHRA(3) RM(1) RM(2) EHRA(3)
Flinders Ranges Council
F1 6.1 7 11 7 9 15 9 14 23 12 18 30 14
F2 178.9 60 97 110 82 133 131 122 199 148 161 260 168
F3 3.1 6 9 4 8 13 5 12 19 7 16 26 9
F4 262.6 69 112 150 95 153 178 140 229 197 183 298 222
District Council of Orroroo – Carrieton
O1 22.7 17 25 21 23 34 26 34 51 31 44 66 37
O2 2.1 4 6 3 5 8 4 8 12 5 10 16 7
O3 1.9 4 7 3 6 9 4 8 13 5 11 17 6
O3A 2.5 4 6 3 5 8 5 7 11 6 9 15 7
O4 5.1 6 10 6 8 13 8 13 20 10 16 26 13
District Council of Peterborough
P1 11.5 12 19 12 16 26 15 23 38 19 30 50 23
P2 27.1 21 27 24 28 37 30 41 53 36 53 69 42
P3 5.8 6 10 7 8 14 9 12 20 11 15 26 14
1. Rational Method – Full time of concentration
2. Rational Method – Half time of concentration
3. Regional Flood Frequency Analysis – Eusuff High Runoff Area

 

 

8.3.2 Southern Council Areas
Table 10 summarises the peak flows for the 10, 20, 50 and 100 year ARI storm events for Northern Areas Council and the Regional Council of Goyder.

Table 10 Regional Flood Frequency Analysis – Eusuff High Runoff Area Flow m3/s

Site Area km2 10 year ARI 20 year ARI 50 year ARI 100 year ARI
Northern Areas Council
N1 3.5 5 6 8 10
N2 586 286 335 360 399
N3 1158 497 574 600 655
N4 285 160 190 209 235
Regional Council of Goyder
G1 271 153 182 202 227
G2 1.6 2 3 4 5
G3 1.5 2 3 4 5
G4 24 21 26 32 38
G5 5.7 7 9 11 14

8.3.3 200 and 500 Year ARI Flows
The 200 and 500 year ARI flows have been extrapolated from the adopted 10, 20, 50 and 100 year ARI flows. The 200 and 500 year ARI flows are shown in Table 11.

 

Table 11 200 and 500 Year ARI Flows (m3/s)

Site Area (km2) 200 year ARI 500 year ARI
Flinders Ranges Council
F1 6.1 41 60
F2

F3 178.9

3.1 355 526
35 52
F4 262.6 405 598
District Council of Orroroo – Carrieton
O1 22.7 90 132
O2

O3 2.1

1.9 21 32
23 34
O3A

O4 2.5

5.1 20 29
35 51
District Council of Peterborough
P1 11.5 40 58
P2 27.1 70 102
P3 5.8 20 29
Northern Areas Council
N1

N2 3.5

586 10.9 12.8
429 471
N3 1158 698 755
N4 285 256 285
Regional Council of Goyder
G1

G2 271

1.6 247 275
7 10
G3 1.5 6 7
G4 24 43 49
G5 5.7 15 18

 

9. Hydraulics

A hydraulic investigation is required to calculate the velocity distribution across a floodway and to determine the extent of inundation across the floodway. Velocity calculations aid in determining appropriate erosion control and the extent of erosion control to meet the requirements of a particular level of service.

9.1 General
There are three types of floodways within the Central Local Government Region:
t At grade crossings – where the roadway is constructed to match the watercourse invert.
t Erosion Head – the road level is at or slightly raised above the waterway invert on the upstream side and the downstream invert level is significantly below the upstream waterway invert level. The level difference is typically the result of an erosion head that is migrating upstream.
t Culvert crossings – the floodway has a culvert within the road embankment to convey design flows without overtopping the roadway. Flows in excess of the culvert capacity will flow over the roadway.
A selection of representative sites was chosen for a basic hydraulic assessment. Each of these sites was chosen based on general site geometry being representative of a majority of the floodways within the study area. A full hydraulic analysis of each site is not the primary focus of this report and limited field information was gathered to undertake a hydraulic assessment.

9.1.1 At Grade Floodway Crossings
The following floodways classified within the at grade category are shown in Table 12.

Table 12 At Grade Floodway Crossings Within the Study Area

ID Road Waterway
F1 Carrieton – Quorn Road Unknown Creek
F2 Yednalue Road Wirreanda Creek
F4 Warcowie Road Wonoka Creek
N3 Georgetown – Huddleston Road Rocky River
G5 Ketchowla Road Unknown Creek

9.1.2 Erosion Head
These floodways have been severely eroded downstream of the floodway to produce a large ‘drop-off’ zone and destabilised downstream banks. The sites which were identified as falling into this category are shown in Table 13.
The flow profile over a constructed floodway may be controlled by either submerged or unsubmerged flow conditions.

 

For an unsubmerged floodway the flow over the floodway is assumed to behave as a broad-crested weir by assuming uniform critical flow is achieved over the floodway.
When the floodway becomes submerged the broad crested weir analysis is modified to incorporate a submergence factor in the early stages of submergence. Once the floodway becomes submerged and no longer constitutes a hydraulic control it can be modelled as an open channel.

Table 13 Floodways with Erosion Heads

ID Road Waterway
F3 Yednalue Road Yednalue Creek
O1 Belton Road Unknown Creek
O3 Johnburgh Road Unknown Creek
P1 Gumbowie Road Unknown Creek
P2 Bullyaninnie Road Black Fella Creek
P3 Bullyaninnie Road Little Black Fella Creek
N1 Hornsdale – Tarcowie Road Unknown Creek
N2* West Terrace Pine Creek
N4 Broughton Valley Rd Hutt River
G1 Bower Road Unknown Creek
G2 Caroona Road Unknown Creek
G3 Angle Road Unknown Creek
*The Pine Creek diversion was constructed with a drop structure.

9.1.3 Culvert Crossings
Incorporating culverts within the crossing eliminates ponding behind the road embankment during flow events and allows design flows to be conveyed through the culverts rather than over the roadway. Table 14 shows floodways with culvert crossings.

Table 14 Floodways with Culvert Crossings

ID Road Waterway
O2 Belton Road Unknown Creek
O3A Johnburgh Road Unknown Creek
O4 Orroroo Road Brambricks Creek
G4 Belalie Road Unknown Creek

 

The method for designing floodways is the same as described in Section 4 with consideration given to flow through the culvert(s) and weir flow over the roadway. A design flow is selected for the culverts, to convey floodwater without overtopping of the road surface. In events larger than the design flow, the road will be overtopped. Consideration needs to be given to applying a blockage factor for each culvert, as there is a high risk that the culvert will be partially or completely blocked during a flood event.
Typically, highway culverts are designed to convey a 100 year ARI flow without overtopping. Based on the culvert sizes observed during the site visit and calculated flow rates it is envisaged that the level of service provided by the culvert crossings is less than a 10 year ARI storm event. Damage to the floodway occurs when the culvert capacity is exceeded and floodwater overtops the roadway. In most cases scouring erosion around the culvert headwall results in failure of the floodway.
When the weir flow across the road embankment is sufficient to penetrate the tailwater surface a hydraulic jump will occur on the downstream side. Generally, the introduction of a hydraulic jump within the channel will produce greater erosive effects in the channel cross section. When the conditions are suitable the downstream flows appear to laminate or overlap with the tailwater. In this situation erosion velocities are still present but to a less extent.

9.2 Hydraulic Modelling
Manning’s Equation (Equation 3) and the hydraulic modelling program HEC-RAS were utilised t to analyse the hydraulic performance of three different types of floodways within the study area.
2 1

= A S2
n

Equation 3

Q = flow (m3/s), A = Cross-sectional area (m2), R = Hydraulic radius (m), S = Slope (%), n = Manning’s roughness
Average flow velocities within the channel at the at-grade floodways were calculated using Manning’s Equation. For both the erosion head and culvert crossings, channel and flow data was input into HEC- RAS and simulated for a range of flow scenarios.
As described previously, site inspections were undertaken to allow for classification of the different type of floodways. Given this is a high level study to develop appropriate remediation methods and preferred design solutions for floodways within the study region, detailed information to enable establishment of a hydraulic model was not considered essential at this stage as the focus is on developing a uniform approach for the design and maintenance of the numerous floodways across the study area.
Geometry data for the floodways simulated using HEC-RAS was obtained from photographs and site observations.

9.3 Results of Modelling

9.3.1 At Grade Floodway Crossings
Flow velocities for a 100 year ARI event at selected at grade floodway crossings were calculated using Manning’s Equation (Equation 3). The flow velocity is an average flow velocity for the cross section.
Table 15 shows that flow velocities for at grade floodway crossings are in the order of 2.0 to 3.2 m/s, which is sufficient to mobilise large gravel to small rocks (less than 500 mm in size) in the stream bed.

 

Table 15 Results of the Hydraulic Analysis using Manning’s Equation

Location
Waterway Flow Rate Q (m3/s) Flow Velocity V m/s)
F1 – Carrieton Unknown Creek 28 2.6
F2 – Wirreanda Wirreanda Creek 260 3.0
F4 – Warcowie Wonoka Creek 298 3.2
O2 – Belton Road Unknown Creek 16 2.0
P2 – Blackfella Creek Black Fella Creek 53 2.2
P3 – Little Blackfella Creek Little Black Fella Creek 15 2.2

9.3.2 Floodways with Erosion Heads and Culvert Structures
HEC-RAS modelling was undertaken for a selection of six erosion head sites and culvert crossings. A long-section extracted from HEC-RAS, through the O3 – Johnburgh Road floodway is shown in Figure 8. This floodway has an approximate 2 m drop on the downstream side of the culvert crossing.

Figure 8 Channel Long Section for O3 – Johnburgh Road Floodway

 

Table 16 shows a selection of floodways with erosion heads and culvert crossings. A 10 year ARI flow rate was entered into the HEC-RAS model and simulated to determine the velocity upstream of the floodway (VU), downstream end of the roadway (VR) and immediately downstream of the floodway (VD).

Table 16 Results of HEC RAS Modelling of Erosion Head and Culvert Crossing Sites

Location Waterway Q10 (m3/s) VU (m/s) VR (m/s) VD (m/s)
Erosion heads
F3 – Yednalue Road Yednalue Creek 9 2.3 2.9 4.6
O1 – Belton Road Unknown Creek 25 2.0 2.6 4.1
O3 – Johnburgh Road Unknown Creek 7 2.5 3.1 6.8
P1 – Gumbowie Road Unknown Creek 12 2.4 3.0 5.3
Culvert structures
O3A – Johnburgh Road Unknown Creek 6 0.6 2.4 2.7
O4 – Orroroo Road Bambricks Creek Q100 26 2.0 2.6 5.2

The results in Table 16 show that the flow velocity can be high, 4 to 7 m/s, depending on the height of the drop at the erosion head. The flow velocities upstream and at the downstream end of the roadway are also approximately 2.5 to 3 m/s, which is high enough to cause erosion and loss of pavement material. The increase in flow velocity at the downstream end of the roadway and at the toe of the downstream embankment increases the risk of erosion at these points. This was evident from the site visits, with most of the erosion occurring on the downstream side of the floodway.

9.4 Treatment Options
There are several options for reducing flow velocities within the waterways on the downstream end of the floodways. These include:
t Stilling pools
t Rock wall

Both these solutions have been utilised effectively in many urban waterways to protect banks and inverts from fast flowing stormwater.
Each of these options was simulated in HEC-RAS to assess the suitability of reducing flow velocities and therefore erosion at the downstream end of the floodway. The two options suggested aim to contain the high velocities within a shorter length (2 m compared to 5 to 10 m), where heavily engineered erosion control is located. Initial modelling of a stilling pool with a depth of approximately 0.5 m over a length of two metres immediately downstream of the bottom of the downstream batter was sufficient to reduce flow velocities back to approximately 2 m/s at the downstream end of the stilling pond.
The rock / gabion wall option involves constructing a small obstruction (wall of approximately 0.5 m above channel invert level) in the flow path downstream of the floodway. This option essentially creates a stilling basin with an overflow weir to reduce flow velocities to approximately 2 m/s.
In each case a full engineering design will be necessary to determine the level of erosion protection for each site.

 

10. Climate Change

This report has reviewed historical rainfall data to assess whether the probability of occurrence for more intense rainfall events has altered. This is driven by recent high intensity storm events in the study area over the last five years. There is increasing concern that these larger events are increasing in frequency.
The recent heavy rainfall and flooding across regions of Australia is largely explained by two consecutive La Niña events. The very strong La Niña event in 2010 followed by another in 2011 brought the highest two-year Australian average rainfall total on record “State of the Climate 2012”, CSIRO and Australian Bureau of Meteorology paper.
The recent drought in south-eastern Australia (prior to the rainfall events of 2010 and 2011) was the worst on record; CSIRO 2010 has indicated the event was due to large scale climatic changes to the region as a result of global warming:
t There were no wet years over the entire thirteen year period.
t It was confined to the southern Australia and did not extend across the continent.
t The main decrease in rainfall was in Autumn rather than winter or spring.
t Temperatures have been steadily rising across the region.
The challenge when trying to predict the effects of climate change is the uncertainty. The Australian Academy of Sciences addresses the issue of uncertainty, particularly in rainfall by stating:
How climate change will affect individual regions is very hard to project in detail, particularly future changes in rainfall patterns…Uncertainty about future climate change works in both directions: there is a chance that climate change will be less severe than current best estimates, but there is also a roughly equal chance that it will be worse.

10.1 Published Information
The following has been sourced from information published by the Australian Bureau of Meteorology (BOM) and the Commonwealth Scientific and Industrial Resource Organisation (CSIRO).
State of the Climate 2012 http://www.csiro.au/Outcomes/Climate/Understanding/State-of-the-Climate-2012.html Fewer rain storms across southern Australia, Reference 11/67
http://www.csiro.au/en/Organisation-Structure/Divisions/Marine–Atmospheric-Research/Fewer-rain- storms-across-southern-Australia.html
Annual Climate Summary for South Australia http://reg.bom.gov.au/climate/current/annual/sa/archive/2010.summary.shtml http://reg.bom.gov.au/climate/current/annual/sa/summary.shtml

 

10.1.1 Rainfall
Australia’s rainfall is highly variable. During recent decades, there has been a general trend towards increased spring and summer monsoonal rainfall across Australia’s north, higher than normal rainfall across central parts of the continent, and decreased late autumn and winter rainfall across the south.
Climate models suggest long-term drying over southern areas during winter and over southern and eastern areas during spring. This will be superimposed on large natural variability, so wet years are likely to become less frequent and dry years more frequent. Droughts are expected to become more frequent in southern Australia; however, periods of heavy rainfall are still likely to occur.
A very strong La Niña event in 2010, followed by another La Niña event in 2011, brought the highest
two-year Australian-average rainfall total on record. Many rainfall records were broken during this period.
The record rainfall of 2010 and 2011 fell in spring and summer in the southeast of the continent. Rainfall was below average for the period April to July 2011, continuing the longer-term drying trend observed over the winter half of the year for this region. Recent drying trends across southern Australia in autumn and winter have been linked to circulation changes. The causes of these changes are an area of active research.
April to September rainfall deciles from 1997 to 2011 for Australia are shown in Figure 9 (a decile rainfall map shows whether the rainfall is above average, average or below average for the most recent 15-year period, in comparison with the entire rainfall record from 1900). Areas of highest on record and lowest on record are also shown.

 

Figure 9 Autumn Winter Rainfall Deciles from 1997 – 2011

Rainfall for South Australia has followed a decreasing trend since 1900, more so in the second half of the century. Annual rainfall since 1970 has decreased by 10 – 40 mm /decade although the rainfall in the 1970s was relatively high in comparison to the other decades.
The trend in annual total rainfall from 1970 to 2011 is shown in Figure 10 and the trend in annual total rainfall from 1900 to 2011 is shown in Figure 11. It would be pre-mature to act on data from only the last 30 years (which shows a significant downward trend in rainfall) without considering the longer term trends.

 

Figure 10 Trend in Annual Total Rainfall 1970 – 2011

Figure 11 Trend in Annual Total Rainfall 1900 – 2011

 

10.1.2 Future Australian Temperature, Rainfall and Extreme Weather Events
Australian average temperatures are projected to rise by 0.6 to 1.5 °C by 2030 when compared with the climate of 1980 to 1999. The warming is projected to be in the range of 1.0 to 5.0 °C by 2070 if global greenhouse gas emissions are within the range of projected future emission scenarios considered by the Intergovernmental Panel on Climate Change. These changes will be felt through an increase in the number of hot days and warm nights, and a decline in cool days and cold nights.
Climate models suggest long-term drying over southern areas during winter and over southern and eastern areas during spring. This will be superimposed on large natural variability, so wet years are likely to become less frequent and dry years more frequent. The future predictions for rainfall in southern Australia indicate that there will be an increase in the number of dry days, but it is also likely that there will be an increase in intense rainfall events.
Models generally indicate an increase in rainfall near the equator globally, but the direction of projected changes to average rainfall over northern Australia is unclear as there is a lack of consensus among the models.
For Australia as a whole, an increase in the number of dry days is expected, but it is also likely that rainfall will be heavier during wet periods.
Projections suggest that it is likely (with more than 66 per cent probability) that there will be fewer tropical cyclones in the Australian region, on average, but the proportion of intense cyclones is expected to increase.

10.2 Impact on Mid North & Northern Districts
The Central Local Government Region of SA commissioned a report titled “Central Local Government Region Integrated Climate Change Vulnerability Assessment – 2030”, November 2011. This report discusses the impact of climate change on rainfall intensity. For South Australia the suggested increase in rainfall intensity is likely to be only minor (perhaps only about 2% above current levels, Darren Ray, BOM, Head Climatologist, South Australia, pers comms.)
The predicted reduction in rainfall and hotter days could potentially reduce vegetation coverage within these regions. These predicted deviations could lead to a decrease in a catchments time of concentration and increased stormwater runoff for flood events due to less ground infiltration and interception by vegetation. Increasing the significance of rain events may result in greater occurrence of erosion to floodways and possibly failure of infrastructure. This is supported by data from BOM showing that although 2010/2011 had a higher annual average rainfall there was a relatively low number of consecutive wet days compared to previous years.
The localisation of rainfall and flood events describe in Section 8 complicates the issue of trying to explain or fit climate change information to the observed data.
The daily rainfall plots shown in Appendix B indicate that the most significant daily rainfall totals have generally occurred in the last 20 to 25 years of record. Of particular interest is whether the more intense and greater daily rainfall totals observed in the Flinders Ranges are now being observed as far south as Eudunda. More data collection will be required to confirm if this is the case.

 

11. Classification of Sites

For Councils to be able to allocate funds for works, the crossings must be graded or ranked in order of importance to determine the most benefit for capital expenditure to upgrade or reinstate existing floodways. The criteria against which rankings could be based are discussed below.

11.1 Council Road Classification
The Councils already rate the roads and access tracks under their control. This classification provides detail as to the level of construction standard as well as maintenance of the road. This report utilises this classification to propose a base ranking system for crossing sites. A summary of the road categories, road description and level of importance is shown in Table 17. There are two road category ranking systems used by the Councils. The rankings have been grouped together for ease of assigning a level of importance.

Table 17 Road Category versus Importance Rating

Council Road Category

Road Description Level of Importance
for Floodway Repair/Route Reinstatement
Category 1 Category A Major council road – local arterial route. Higher traffic volumes. Would be sealed if funds allowed.
High
Category 2 Category B Major route for local traffic. Used by high priority local traffic i.e. school buses etc.

 

Medium

 

 

Low
Category 3A Category C1 All year access track to residences and farms.
Minimum 100 mm sheeting thickness.

All year access track to residences and farms.
Minimum 75 mm sheeting thickness.

All year access track to residences and farms. No minimum sheeting thickness.
Category 3B Category C2

Category 3C
Category 4 Not constructed

11.2 Definitions
The levels of importance have been defined by GHD. The Councils may wish to redefine the levels of importance to suit their individual Council requirements.

11.2.1 High Importance
A high importance classification can be defined as:
t A high priority route for Council. The floodway would be inspected for damage immediately following a significant storm event.

 

t Route should only be closed for the duration of flow during storm events.
t Required maintenance of infrastructure should be minimal.

11.2.2 Medium Importance
A medium importance classification can be defined as:
t Priority route for Council. Council would inspect the floodway for damage within three days of a significant storm event.
t Crossing can accept limited closures following storm events.
t Maintenance and repair of infrastructure can be expected following a significant storm event.

11.2.3 Low Importance
A low importance classification can be defined as:
t Low priority route for Council. Local residents accessing the floodway will report any damage to Council within three days of a significant storm event.
t Closure of crossing does not significantly impact users.
t Alternate routes available.
t Council would undertake repairs to the floodway within 10 days of a significant storm event. The road classifications for the sites considered in the report are shown in Table 18.
Table 18 Floodway Sites Road Classifications

Location Road Classification
The Flinders Ranges Council
F1 Carrieton Quorn Road Cat 2
F2 Yednalue Road Cat 2
F3 Yednalue Road Cat 3C
F4 Warcowie Road Cat 3C
The District Council of Orroroo – Carrieton
O1 Belton Road C2
O2 Belton Road C2
O3 Johnburgh Road B
O3A Johnburgh Road B
O4 Orroroo Road A

 

Location Road Classification
The District Council of Peterborough
P1 Gumbowie Road C2
P2 Bullyaninnie Road C2
P3 Bullyaninnie Road C2
Northern Areas Council
N1 Hornsdale – Tarcowie Road C
N2 West Terrace A
N3 Georgetown – Huddleston A
N4 Broughton Valley Rd C2
The Regional Council of Goyder
G1 Bower Road Cat 3C
G2 Caroona Road Cat 3C
G3 Angle Road Cat 3C
G4 Dares Hill Cat 2
G 5 Terowie site Cat 2

11.3 Adjustment Factors
There are many factors that can affect how a route is classified. It may be that the original route classification needs to be adjusted for changing circumstances of the route. Some additional factors that may be considered are discussed below. Ultimately, it will depend on the experience of the assessor as to whether the importance is upgraded or downgraded.

11.3.1 Traffic Volume and Designated Tourist Routes
For higher classified routes the volume of traffic would be a good gauge as to how important the route is to the local area/Council and would in theory serve as the preferred ranking criteria. However, it cannot be used as the sole measure of importance of a route, as many of the routes show traffic volumes in the order or one or two vehicles a week and it is impossible to differentiate between these routes.
There is an expectation that the Councils provide a higher level of service for tourist routes. The level of importance of a floodway crossing can be assessed against the volume of tourist traffic.

11.3.2 Population Serviced
This relates to the occupants of the properties serviced by a floodway. For example it may be that a route serves only a single property, which may result in a low importance. Council may consider upgrading the importance of a floodway crossing if the occupants of a property, serviced by a particular floodway, are elderly or in poor health. The challenge is that populations can change quickly so justification for a route upgrade on this basis alone would be difficult.

 

11.3.3 Local Economy
A particular route may be critical to transport livestock or grain during harvest time. Consideration needs to be given to the time of year that a particular route may be used to transport livestock or grain and the risk of storm events resulting in damage to a floodway.

11.3.4 Alternate Route Availability
If a route is of high importance but does not provide the sole access to an area or residence the level of importance may be lowered accordingly.

11.4 Serviceability Level
The various floodway design guides discussed in Section 4 of this report define different serviceability criteria for floodways. The WA Floodway design guide for example uses the critical depth of flow over the floodway as the defining criteria. For minor rural Council roads these flood levels are expected to be exceeded once every 20-50 years.
The ephemeral nature of the flows and the lack of accurate flow data from sites within the study area make it difficult to adopt a serviceability system such as this. The serviceability criteria for designs in this report are based solely around the robustness and predicted design life of a proposed design with maintenance. It must be accepted that the crossings will be closed during large storm events.
Three serviceability levels have been adopted for this project. They can be categorised as shown below. The definitions can be used to apply solutions to sites not considered in the scope of this study.
Reference is made to the floodway design sections shown in Section 4 and the alternate floodway design sketches shown in Appendix E.

11.4.1 Level 3
Level 3 is the highest level of protection that can be provided at a floodway crossing. Level 3 protection focuses on the use of reinforced concrete infrastructure throughout the design. It provides the least risk that infrastructure will be damaged during a significant storm event. The Councils must consider the whole of life costs for floodway sites, although concrete is the most expensive of the installed infrastructure options, it may still be less expensive than simply repairing the floodway with standard road materials, especially if an erosion head is present. Table 19 shows possible floodway protection measures considered for level 3 protection. The option of doing nothing is present for level 3 protection where a site has experienced no damage or sediment deposition is the primary cause of failure.
Site factors such as the geometry of the crossing, as discussed in Section 5 should also be implemented as part of these solutions.

 

Table 19 Floodway Protection Measures for Level 3 Serviceability

Construction Zone
Serial Upstream Roadway Downstream Peripheral
1 Reinforced concrete batter protection and cut- off wall. Reinforced concrete deck and cut-off walls. Reinforced concrete batter and cut-off wall. Bank reshaping works with rock rip-rap protection.
2 Cement treated material to 4 MPa in low flow velocity areas Cement treated material to 4 MPa with concrete cut-off walls in low flow velocity areas Cement treated material to 4 MPa in low flow velocity areas Bank reshaping works with rock rip-rap protection.
3 Do Nothing Do Nothing Do Nothing Do Nothing

11.4.2 Level 2
Level two protection provides a suite of mitigation options that are expected to return a design life of 25 years, however the risk that the infrastructure will be damaged during a major storm event is higher than level 3. Level 2 mitigation methods are likely to require maintenance during the design life of 25 years. Table 20 shows possible floodway protection measures considered for level 2 protection. It is expected that the Councils will focus on these solutions in order to maximise the level of service with limited available funds. The results of the trials will provide a better understanding of the achievable design life of some of the solutions. The option of doing nothing is present for level 2 protection where a site has experienced no damage in a particular area or sediment deposition is the primary cause of failure.
Site factors such as the geometry of the crossing, as discussed in Section 5.3 should also be implemented as part of these solutions.

Table 20 Floodway Protection Measures for Level 2 Serviceability

Construction Zone
Serial Upstream Roadway Downstream Peripheral
1 Concrete Canvas® Cement treated material
to 4 MPa with concrete cut-off walls. Concrete Canvas® Bank reshaping works and
Concrete Canvas® and/or rock rip-rap.
2 Rock riprap Stabilising Polymer with concrete cut-off walls Rock rip-rap Bank reshaping works and
Concrete Canvas® and/or rock rip-rap.
3 Do Nothing Use of good quality road material Precast concrete blocks Bank reshaping works and
Concrete Canvas® and/or rock rip-rap.

 

Construction Zone
Serial Upstream Roadway Downstream Peripheral
4 Cement treated material to 4 MPa in low flow velocity areas Cement treated material to 4 MPa with concrete cut-off walls Rock rip-rap Bank reshaping works and Concrete Canvas® and/or rock rip-rap.
5 Do Nothing Do Nothing Do Nothing Do Nothing

There are a number of combinations of options that could be adopted for the roadway and downstream batter.

11.4.3 Level 1
Level 1 presents mitigation measures that can be implemented with a minimum of cost. It is based on solutions that provide the Councils with quick fix options that can be implemented until more funds and construction effort are available. This level includes the options of ‘do nothing’ and maintaining current techniques of post storm event repair. Table 21 shows possible floodway protection measures considered for level 1 protection.
Site factors such as the geometry of the crossing, as discussed in section 5 may also be implemented as part of these solutions.
Although engineering design may be utilised for these solutions it is expected that the works manager will have sufficient knowledge and experience to conduct the works necessary with limited external input.

Table 21 Floodway Protection Measures for Level 1 Serviceability

Construction Zone
Serial Upstream Roadway Downstream Peripheral
1 Cement treated material to 4 MPa with cut-off wall. Cement treated material to 4 MPa with cut-off walls. Cement treated material to 4 MPa with cut-off wall. Bank reshaping works
2 Stabilising Polymer Stabilising Polymer with concrete cut-off walls. Scattered rock rip- rap protection. Bank reshaping works
3 Do Nothing Use of good quality road material with concrete cut-off walls. Do Nothing Do Nothing

 

12. Cost Matrix
12.1 Construction Zones
Due to the absence of any significant damage on the upstream side of any of the floodways visited in the study, only the roadway and downstream zones are considered in the cost analysis.

12.2 Costs of Various Floodway Protection Measures
The cost per metre section of floodway for various floodway protection measures on the roadway and downstream zones is shown in Table 22. It is important to note that the costs will vary from Council to Council and each Council should establish their own spreadsheet to calculate the costs for a particular treatment measure. The assumptions associated with each of the floodway protection measures are shown in Table 23. The peripheral erosion protection measures have not been included in Table 23. No allowance has been made for future maintenance in the per metre erosion protection measure costs.
Future maintenance costs are considered in the cost matrix shown in Table 24.
Table 22 Floodway Protection Measures Assumptions

Treatment Measure Assumptions
Roadway Protection Measures

Reinforced concrete deck Costs based on 8 m wide roadway. Concrete deck 150 mm thick. Approximate cost for reinforced concrete $450/m3

Cement treated base Costs based on 8 m wide roadway. Treated to 300 mm depth. Approximate costs for Cement Treated (CT) material $50/m3

Polymer stabilising compound

Continual repair Costs based on 8 m wide roadway. Treated to 200 mm depth. Approximate costs for polymer $300/kg which will treat 25 m3 fill.
Costs based on 8 m wide roadway. Assumed loss of road material
is 200 mm depth per flood event. Approximate cost of replacement material $15/m3
Downstream Protection Measures
Reinforced Concrete Embankment Protection Based on standard DPTI floodway design drawing shown in Appendix E.

Concrete Canvas® Option

Rock Rip-Rap Based on section shown in sketch 33-16230-SK001, Appendix E. Costs for fill not included as these can be locally won.
Based on section shown in sketch 33-16230-SK001, Appendix E. Costs for fill not included as these can be locally won.
Precast Concrete Block option Based on section shown in 33-16230-SK002, Appendix E.

Continual Repair Costs based on 8 m wide roadway. Assumed loss of material =
50% of crossing to full depth of downstream erosion head. Cost of replacement material is $15/m3

 

12.3 Costs Matrix
A cost matrix was developed using the per metre costs shown in Table 23. The cost matrix shown in Table 24 includes both the capital costs and a present value cost including all estimated maintenance and future capital costs. The costs have been calculated over a 25 year period, as this represents a realistic expectation for the serviceability level of the alternate technology designs proposed.
An important assumption with respect to maintenance is that there will be 15 storm events requiring maintenance of the floodway over the next 25 years.
It is important to note that the assumed maintenance costs in Table 23 will vary from Council to Council. Each Council is encouraged to develop their own spreadsheet with typical year one construction costs and ongoing maintenance costs.

12.3.1 Roadway Assessment
As expected the use of reinforced concrete is the most expensive option over 25 years, however a reinforced concrete deck may be expected to have a design life of 50 plus years. It is important to note that failure of reinforced concrete decks can occur well below an expected 50 year design life if they are not adequately protected or erosion surrounding the structure is greater than expected. Other options such as Concrete Canvas® are likely to require replacement every 25 years. The design life of concrete blocks is difficult to estimate, as failure of the blocks themselves is unlikely to be the ultimate failure mechanism. The Councils are encouraged to undertake a more detailed life cycle cost analysis if a greater design life than 25 years is expected for a particular erosion control measure.
It should be highlighted that the use of a stabilisation product is only of use where the roadway experiences limited erosion. If the mode of failure for a site is deposition on the roadway the use of any treatment will be ineffective. The most cost effective treatment for those sites is the continuation of post flood event maintenance.

12.3.2 Downstream Assessment
The progression of an erosion head through the roadway requires significant expenditure to mitigate this problem. Simply repairing the roadway as per current methods is an expensive unsustainable option and should be avoided wherever possible.
Concrete Canvas® is stated as having a design life of 25 years. If this can be realised it represents a cost saving of almost 33% over reinforced concrete, even with minor repair work and maintenance being conducted throughout the life of the infrastructure. As discussed above, if a design life comparison with reinforced concrete is required, considering a design life in excess of 25 years than a more detailed life cycle cost analysis is required.
Precast concrete blocks show a saving of nearly 15% for smaller sites (those with approximately 1 m erosion head. But these savings are quickly lost as the depth of the erosion head increases to 2 m.

12.4 Limitations
The following assumptions and limitations are provided to guide any future cost assessment of erosion control measures:

 

t The cost of concrete is taken as a standard $300/m3. This cost varies greatly throughout the study area. For remote sites, either specific design mixes including retarders or on site batching will be required. The standard cost matrix does not include either of these additions.
t Detailed site investigations (survey or geotechnical) and detailed design fees have been considered in the costs.
t There is no allowance for site set up costs or transport of machinery.
t There is no contingency built into the costs.
t All costs are approximate and subject to market conditions.
t Operating and maintenance costs for continual repairs are based on 15 storm events occurring over a 25 year period.
t Maintenance costs for non-traditional techniques are calculated at 2% of capital costs.

 

Table 23 Costs (per metre) of Various Floodway Protection Measures

Serial
Construction Zone Option
Scenario Material Plant – with Operator Labour Total Cost
$/m
Remarks
Type Quantity Cost $ Quantity Cost $ Quantity Cost $

1a

 

 

 

 

Roadway
Reinforced concrete deck
1.2 m3
540 Excavator 140
2 men
4 hours
520
1,255
50% of hourly rate used for plant except excavator
Roller 55
1b

 

1c
Cement treated base

Polymer stabilising compound
2.4 m3

 

0.032 kg
120

 

20 Excavator 28
1 man
1 hour

 

1 man
1 hour
55

 

55
214

 

280
10% of hourly rate used

 

50% of hourly rate used
Roller 11
Excavator 70
Grader 80
Roller 55

1d
Continual repair – roadway only

1.6 m3

25 Grader 16

1 man
1 hour

55

117

10% of hourly rate used
Roller 11
Tipper 10

 

Serial
Construction Zone Option
Scenario Material Plant – with Operator Labour Total Cost
$/m
Remarks
Type Quantity Cost $ Quantity Cost $ Quantity Cost $

 

2a

 

 

 

 

Downstream

Continual Repair

1 m drop

4 m3

60 Excavator 140

1 man
1 hour

55

293
Assume loss of 50% of crossing
10% of hourly rate used for plant except excavator
Grader 16
Roller 22
Tipper 11

 

2b
Reinforced Concrete Embankment Protection

1 m drop

1.0 m3

450

Excavator

280

4 men
4 hours

1040

1,770

2c Concrete
Canvas® Option
1 m drop
7.7 m2
620
Excavator
280
2 men
1 hour
110
1,010

2d Precast Concrete
Block option
1 m drop
4 blocks
1,200
Excavator
280
2 men
1 hour
110
1,590

 

Serial
Construction Zone Option
Scenario Material Plant – with Operator Labour Total Cost
$/m
Remarks
Type Quantity Cost $ Quantity Cost $ Quantity Cost $

 

3a

 

 

 

 

Downstream

Continual Repair

2 m drop

8

120 Excavator 280

1 man
1 hour

55

553

10% of hourly rate used for plant except excavator
Grader 32
Roller 44
Tipper 22

3b Reinforced Concrete Embankment Protection

2 m drop

2.01 m3

905

Excavator

420
4 men
6 hours

1560

2,885

3c Concrete Canvas® Option
2 m drop
10 m2
800
Excavator
420
2 men
2 hours
220
1,440

3d Precast Concrete Block option
2 m drop
7 blocks
2,100
Excavator
420
2 men
2 hours
220
2,740

Table 24 Cost Matrix

 

Option

Failure Type

Treatment Option
Year 1 Capital Cost
$/m Reduced Future Capital Costs $/m
O&M Costs
$/m
8%
Discount Factor
– 25 years
Present Cost
$/m
Treatment for Roadway
1a Roadway Erosion Reinforced concrete deck 1,255 10.5 1,255

1b
Roadway Erosion Stabilise roadway with 10 yr design life
214
150
10.5
364

1c
Roadway Erosion Stabilise roadway with 5 yr design life
280
470
10.5
750

1d
Roadway Erosion Continual repair – traditional techniques
117
80
10.5
957
Treatment Options for 1 m Drop at Erosion Head

2a Erosion head developed. Crossing damaged to 1 m depth Continual repair – traditional techniques
293
180
10.5
2,183

2b Erosion head developed. Crossing damaged to 1 m depth Reinforced concrete embankment protection
1,770
10.5
1,770

2c Erosion head developed. Crossing damaged to 1 m depth
Concrete Canvas®
1,010
30
10.5
1,325

2d Erosion head developed. Crossing damaged to 1 m depth
Precast concrete blocks
1,590
40
10.5
2,010

 

 

Option

Failure Type

Treatment Option
Year 1 Capital Cost
$/m Reduced Future Capital Costs $/m
O&M Costs
$/m
8%
Discount Factor
– 25 years
Present Cost
$/m
Treatment Options for 2 m Drop at Erosion Head

3a Erosion head developed.
Crossing damaged to 2 m depth Continual repair – traditional techniques
553
340
10.5
4,123

3b Erosion head developed.
Crossing damaged to 2 m depth Reinforced Concrete Embankment Protection
2,885
10.5
2,885

3c Erosion head developed.
Crossing damaged to 2 m depth
Concrete Canvas®

Precast concrete blocks
1,440

2,740
40

60
10.5

10.5
1,860

3,370

3d Erosion head developed.
Crossing damaged to 2 m depth

 

13. Option Selection
As discussed in Section 12 of this report all erosion control measures have an assumed maximum design life of 25 years. The selection of an appropriate erosion control design at a particular site can depend upon a number of issues, including the availability of materials, funds and machinery and resources to undertake the works. Worked examples are provided in this section to assist Councils in future assessments.
13.1 Floodway Options Selection Flow Chart
A floodways option selection flow chart shown on page 64 has been developed to assist Councils with the process. The following step by step flow chart procedure is recommended to assist Councils in recommending an erosion control design or repair of a particular floodway:
1. Use Councils road classification to rate the importance of the floodway for repair or upgrade. A High, Medium or Low importance is assigned.
2. Adjust the level of importance based on secondary factors as described in Section 11.3 of this report, if necessary.
3. Define the primary failure mechanism for the site. Refer to Section 7 of this report for descriptions of typical failure mechanisms. It may be that the site experiences more than one failure mechanism. The dominant failure mechanism should be addressed in the first instance. See worked example Flinders F4 in this section.
4. Consider the consequence of the failure mechanism:
a) Catastrophic. The crossing has been closed by the failure of the site and requires significant construction effort to reopen it. Significant delays can be expected before the road is reopened by either patching the failure or providing an alternate roadway around the failed floodway.
b) Limited. The crossing has been closed by the failure but can be opened with available plant and manpower. Delays can be expected before the road is reopened by reconstructing the road surface. The floodway may be cut to one way traffic for some time.
c) Maintenance only. The crossing is passable in certain vehicles but not all traffic. Site to be repaired as soon as possible.
d) Deposition. The crossing experiences no physical damage, but becomes impassable due to deposition of gravel and rock. Site to be repaired as soon as possible.

5.
a. Site infrastructure
Is there infrastructure present at the crossing?
Yes
Go to 5) b
No Go to 6)
b. Has this infrastructure failed?
Yes
Go to 5) c
No Go to 6)
c. Can the structure be patch repaired? Yes Consider repair. Go to Step 7)
No Structure is beyond repair Go to 6)
6. Use the level of serviceability matrix shown in Table 25 to select the level of serviceability.

 

Table 25 Level of Serviceability Matrix

Consequence of Failure
Route importance Catastrophic Limited Maintenance Deposition

High
Level 3
Level 2
Level 1 No works recommended

Medium
Level 2
Level 2
Level 1 No works recommended

Low
Level 2
Level 1 No works recommended No works recommended

7. Identify scope of works for replacement / repair options. Refer to Section 11.4 of this report for floodway protection measures for each level of serviceability.
8. Conduct cost analysis to determine if sufficient funds are available for preferred option.
a. Funds available: Conduct site specific design and implement.
b. Funds not available: Go to Step 6) and consider next level down and redo cost analysis.
9. Monitor solution to determine if maintenance will be required or if secondary mode of failure will become more apparent.

 

 

13.2 Worked Examples

13.2.1 Goyder Site G1: Bower Road (Unknown Creek)
This site has experienced infrastructure failure, erosion head progressing through road. The flow chart process is summarised in Table 26.

Table 26 Site G1 Flow Chart Summary

Step Response Outcome
1 Council road category Cat 3C Road (from Table 18) Low

2 Consider adjustment factors to
determine if floodway level of importance warrants upgrading.
Alternate access routes are available
No change

3
Failure method Erosion head progressing through road No secondary mode of failure observed

4

 

5a

Failure type

Is there existing floodway infrastructure? A large storm event would likely result in erosion head progressing significantly through road to over 1 m in depth

Headwall

Catastrophic

 

Yes
5b Has this infrastructure failed? Failed due to undermining Yes

5c Can the structure be patched repaired? Structure has suffered complete failure
No
6 Determine the level of repair Level 2

 

 

 

 

Costs to be determined upon undertaking additional site analysis.

 

7

 

Identify scope of works Floodway protection (downstream) – Concrete Canvas® / precast block / rock rip rap.
Stabilise road surface
Peripheral works required around floodway

8

Cost analysis Projected costs for 25 year life
Stabilising road and Concrete Canvas® solution
Stabilising road and precast blocks

9 Monitor site for erosion from different flow paths in flood Monitor peripheral erosion and downstream floodway erosion and protect as necessary

 

13.2.2 Flinders Site F4: Warcowie Road (Wonoka Creek)
This is a large creek crossing. The roadway has experienced limited erosion and deposition. The flow chart process is summarised in Table 27.

Table 27 Site F4 Flow Chart Summary

Step Response Outcome
1 Council road category Cat 3C Road (from Table 18) Low

2 Consider adjustment factors to see if road warrants upgrading Assume traffic analysis shows low traffic levels No change

3
Failure method Deposition deemed critical failure mode.

Erosion deemed secondary

4
Failure type The floodway can be reopened by simple grading of crossing Deposition

5a

5b Is there existing floodway infrastructure? No Go to 6
Has this infrastructure failed?

5c Can the structure be patched repaired?

6
Determine the level of repair Primary failure is sediment deposition. No Works Recommended

7
Identify scope of works No Works Recommended

8
Cost analysis Manage budget to allow current practise to continue No Works Recommended

9
Monitor site for erosion from different flow paths in flood If erosion of the western approaches becomes an issue after future storm events consider localised use of stabilisation material.

 

13.2.3 Orroroo Site O3 – Belton Road (Unknown Creek)
Small creek crossing with an erosion head and existing infrastructure. The flow chart process is summarised in Table 28.

Table 28 Site 03 Flow Chart Summary

Step Response Outcome
1 Council road category Cat B Road (from Table 18) Medium

2 Consider adjustment factors to see if road warrants upgrading Low traffic volumes

No alternate route No downgrade of route rating
3 Failure method Erosion head and significant erosion around structure

4

5a
Failure type

Is there existing floodway infrastructure? In the event of failure the road would require significant works to reopen it Catastrophic

Yes
Headwall
5b Has this infrastructure failed? Failure caused damage to road Yes

5c Can the structure be patched repaired? Patch repairs are possible due to the localised failure Yes
6 Determine the level of repair Level 2

7

Identify scope of works Floodway protection (downstream) – Concrete Canvas® / precast block / rock rip rap.
Stabilise road surface

 

8

Cost analysis Projected costs for 25 year life

Stabilising road and Concrete Canvas® solution Costs to be determined upon undertaking additional site analysis.
Precast blocks

9 Monitor site for erosion from different flow paths in flood Monitor secondary erosion on site to ensure no further works are required.

 

13.2.4 Peterborough Site P1 – Gumbowie Road (Unknown Creek)
Small creek crossing with a small erosion head and significant road erosion. The flow chart process is summarised in Table 29.

Table 29 Site P1 Flow Chart Summary

Step Response Outcome
1 Council road category Cat C2 Road (from Table 18) Medium

2 Consider adjustment factors to see if road warrants upgrading Low traffic volumes No alternate route No downgrade of route rating
3 Failure method Roadway erosion and erosion around infrastructure

4

Failure type Roadway can be opened with available plant and manpower.
Erosion around existing headwall is deemed secondary (at present) Limited

5a

5b Is there existing floodway infrastructure?

Has this infrastructure failed? Headwall

Structure intact but at risk of being undermined at the toe. Yes

No

5c Can the structure be patched repaired?
6 Determine the level of repair Level 2
Use of stabilisation material in roadway.
7 Identify scope of works Use of riprap material to protect infrastructure from further erosion.
Extend concrete headwall.

 

8

 

Cost analysis Projected costs for 25 year life. Stabilising road.
Rock rip-rap protection on downstream side of concrete headwall.
Extend concrete headwall. Costs to be determined upon undertaking additional site analysis.

9
Monitor site for erosion from different flow paths in flood Monitor erosion on site to ensure no further works are needed to
protect infrastructure. If infrastructure suffers damage, repeat process from Step 5.

 

13.2.5 Northern Areas Site N1 – Hornsdale – Tarcowie Road (Unknown Creek)
Creek crossing with a significant reinforced concrete headwall structure. The flow chart process is summarised in Table 30.

Table 30 Site N1 Flow Chart Summary

Step Response Outcome
1

2 Council road category

Consider adjustment factors to see if road warrants upgrading Cat C Road (refer to Table 18) Medium
Low traffic volumes

No alternate route No downgrade of route rating

3
Failure method Failure of reinforced concrete headwall. Erosion head will develop through road if failure is not addressed.

4
Failure type Damage to the road will be severe requiring significant effort to repair. Catastrophic

5a

5b Is there existing floodway infrastructure? Headwall

Multiple failure locations Yes

Yes
Has this infrastructure failed?

5c
Can the structure be patched repaired? Patch repairs not a viable option. Headwall has reached the end of useful design life and degradation of structure is substantial No
6 Determine the level of repair Level 2

 

7

 

Identify scope of works Infrastructure is severely decayed. Cost of complete removal of infrastructure will be significant. Treat as a special case with deliberate engineering input and design.
Consider adding new headwall behind existing infrastructure, leave existing infrastructure in place and add Level 2 protection measures, most likely rock rip-rap, on downstream side.
Stabilisation of the road should also be considered.

8
Cost analysis To be undertaken once options considered. Costs to be determined upon undertaking additional site analysis.

9 Monitor site for erosion from different flow paths in flood Monitor site to ensure mode of failure does not change as a result of works

 

14. Trials

There is a need to investigate the use of alternate technology as a means of providing erosion protection at floodway sites, in the study area. The cost of utilising reinforced concrete or rock rip-rap for erosion control on floodways is in many instances not economically viable. Alternate technologies like Concrete Canvas® may be able to provide cost savings without a significant reduction in design life.
Product trials present an opportunity for the Councils to benefit from shared knowledge. It is not anticipated that each of the Councils conduct a trial with each of the products highlighted, but a selection of Councils trial the various products and report back to the other Councils on the performance of the tested products.

14.1 Manufacturers
Concrete Canvas® has only been on the market since 2009 and is still a developing product. The manufacturer is willing to be involved in any trial conducted with their product and have stated that they will provide on-site consultation as well as design and installation assistance.
PolyCom® suppliers have also stated they are keen to be involved in a trial. This is primarily to ensure the correct placement and use of the product to maximise the chances of success.

14.2 Economy of scale
Each of the products recommended can be used in different configurations and in different quantities. The Councils must therefore engage with the manufacturers in order to achieve the most cost effective solution. By sharing the results and combining resources the Councils can maximise both their buying power and their savings potential on these products. Combined with the long shelf-life of the products, the Councils can increase their order quantity, therefore maximising the use of the long shelf-life of these items.

14.3 Whole of Life Costs
The key to these trials is that they are not being implemented to represent either a “one-size fits all” solution or a replacement product to match the performance of reinforced concrete. What each of the products represents is an opportunity to present real cost savings over its own design life. Consider PolyCom®, this product will almost certainly not last 50 years once placed, however using it costs approximately $12/m3 of treated fill. This is less than 1/25 of the cost of a cubic metre of reinforced concrete. If PolyCom® therefore lasts 2 to 5 years, it represents a real-time cost saving to the Councils.

14.4 Site Selection
Each Council has several hundred sites, however, the selection of site should best test the properties of the trial material to provide a sound understanding of its application. However, the challenge is that flood events cannot be predicted and it may be that a trial site remains untested for some time. Each Council must determine sites that are both suitable and likely to experience significant flow in the foreseeable future, in order to maximise the benefit of each trial.

 

14.4.1 Trial Sites
The District Council of Peterborough has expressed an interest in undertaking a trial using PolyCom® at the P1 – Gumbowie Road site.
The Regional Council of Goyder has expressed an interest in undertaking a trial using Concrete Canvas® at the G2 – Caroona Road site.
Flinders Ranges Council has expressed an interest in undertaking a trial using concrete blocks at the F3
– Yednalue Road site.
The District Council of Orroroo and Carrieton has expressed an interest in undertaking a roadway cement stabilisation trial at one of its crossings.
Northern Areas Council has expressed an interest in undertaking a rock rip-rap trial at the N1 – Hornsdale – Tarcowie Road site.

 

15. Site Specific Recommendations

This study has not sought to identify a single solution for each site considered during the investigation. The variability of many of the input factors such as the capital and maintenance costs (which change with both time and location) mean that identifying the “best” option must rest with Council representatives.
The site specific recommendations are shown in Table G.1, Appendix G. Site specific recommendations are provided for three floodway zones and three different levels of service to provide the Councils with options, dependent on the serviceability level required. The peripheral zone has also been included to identify any specific works surrounding the floodway. Councils should work through the options selection process described in Section 13 of this report and the floodway protection measures described in Section 11 of this report. Reference is also made to the floodway design sections in Section 4 of this report and the alternate design sketches shown in Appendix E.
The solutions are interchangeable for a specific site, which allows the Councils to select a different level of protection for each of the construction zones on a single site in order to reduce capital costs. For example, Council may wish to provide a reinforced concrete roadway with rock rip-rap on the downstream side to overcome an ongoing issue at a particular site where the primary failure mechanism is erosion of the road surface.
If a variety of serviceability level options are chosen then the overall level of serviceability for a single site is reduced to the lowest level of service selected.

 

16. Management of Floodway Assets

16.1 Asset Tracking
The report titled “Assessment of Stream Crossings on State Roads, the Flinders Ranges Region”, Earthtech, 2007, following the flood events in The Flinders Ranges Council stated that:
“The cutting of a channel from Wonoka Creek back to Wirreanda Creek will require the construction of an additional road crossing or substantial works to attempt to stop the headward erosion. Damage from the avulsion may be a long way off but if erosion has occurred during the recent floods it may be timely to record this to better understand the process.”
It highlights the need to undertake regular monitoring and assessment of floodways. It is recommended that the Councils initiate a system to assess each of the floodways in their Council area to allow for a more economical integration of floodway maintenance into their Road Maintenance Program and/or Asset Management Register. The use of tablets to record GPS co-ordinates, take photos and note observations for each site should be investigated. This would enable works managers to more easily document claims due to flood damage and track the condition of floodway assets.
One of the key challenges of this project has been identifying the exact means of failure of some of the floodway sites. In some cases there were multiple causes of failure.
At site G2-Bower Road it was not possible to confirm whether the erosion had undermined the structure from the downstream side causing it to fail or whether the erosion had initially started on the road side and developed behind the headwall. It is possible that regular monitoring would have identified the beginning of these erosion points, particularly if erosion at the toe of the headwall had been getting deeper over time. This knowledge would allow the design development for new infrastructure to include measures which reduce the risk of the same type of failure(s) occurring again.
The major threat to any floodway is erosion undermining or eroding around the structure. This erosion can generally be observed to develop over a number of flood events. By taking a proactive approach and monitoring the erosion at each site, damage can be tracked and repaired before it becomes critical and the floodway fails catastrophically, therefore requiring expensive repair or replacement works.

16.2 Monitoring Program
It is acknowledged that the establishment of a monitoring program presents a significant challenge for Councils, given the number of floodway sites within each Council district and shortage of resources. However, it will provide an invaluable tool for tracking damage at each site. In addition, it can provide Council will a continual record of information on construction methods, costs and maintenance programs. The key aim is to establish what works best for the least capital expenditure and to provide a means of passing this information onto future works managers.
The initial stages of asset management could consist of a photographic journal of sites to allow for tracking of erosion at each site. This will allow Councils to build up a continual photographic record of each of the sites through regular inspections after flow events or at scheduled road maintenance.

 

16.3 Shared Knowledge
As highlighted in Section 5 Current Practice, information relating to either floodway damage or repair is not actively shared between the Councils. By adopting common practises and sharing knowledge the Councils can improve their understanding and ability to respond to floodway failures, as well as avoiding costly solutions that have been unsuccessful in other areas.

 

17. Conclusions and Recommendations

17.1 General
This floodway research project has highlighted a number of issues surrounding the ongoing maintenance and repair of the thousands of floodways within the five Council boundaries. The issues surrounding the ongoing maintenance and repair of the floodways include:
t More frequent and intense storm events over the last five years have resulted in significant damage to floodway infrastructure.
t The Councils do not have the capital and resources to provide the level of service at floodways that is consistent with the road category.
t A lack of resources results in a reactionary response to floodway maintenance and repair, which ultimately impacts on the level of service provided.
t Uncertainty with respect to identifying the extent of floodway erosion control works to mitigate large flood events.
t Insurance claims as a result of flood damage to floodways are only sufficient to reinstate damaged infrastructure and not replace the damaged floodway with an erosion control measure that is less at risk of failure.
The Works Managers within each Council have a wealth of experience in maintaining and protecting floodways. Their efforts to maintain and repair floodways with insufficient capital are to be commended. The key to the successes that they have had is due to their ability to recognise the issue at a site and set about rectifying the issue to reduce the future maintenance and repair costs. This is no more evident in the northern Councils decision to remove culverts where possible and drop the floodway down to creek bed level. The infrequent nature and short duration of storm events in the north means that there is no significant impact on the level of service provided by the floodway and the primary failure mechanism of the floodway is minor erosion or sediment deposition, which is much more cost effective to manage.
This floodway research project has built upon the works managers knowledge and ideas to develop cost effective floodway erosion control measures. A process has been developed to assist in recommending the most appropriate floodway erosion control measures that considers the:
t Primary and any secondary floodway failure mechanisms.
t Existing floodway infrastructure.
t Level of service to be provided by a floodway.
t Construction and maintenance costs.
t Availability of materials.

17.2 Floodway Sites and Failure Mechanisms
Twenty one floodway sites were investigated during the course of this project. The sites were chosen because they represent examples of ongoing issues with other floodways or the Councils were looking for cost effective erosion control measures to replace or enhance an existing floodway. Each floodway is different, however there are consistent failure mechanisms, including:

 

t An erosion head that advances upstream with each successive flood event until eventually it undermines the downstream floodway infrastructure, often resulting in failure of the floodway.
t Erosion around infrastructure such as culvert headwalls.
t Erosion of road surfaces.
t Peripheral erosion adjacent to a floodway that results in damage to a roadway some distance from the floodway.
t Sediment deposition on the roadway.
The least expensive and most easily dealt with failure mechanism is sediment deposition. The Councils can grade the roadway after a flood event and restore traffic far more quickly than a roadway that has suffered erosion and loss of pavement material.

17.3 Hydrology and Hydraulics
A hydrological and hydraulic assessment was undertaken for the floodway sites. A simplified approach to estimate flow rates for a range of average recurrence intervals has been adopted. This will enable the Works Managers to calculate flow rates for any floodway catchment. It is noted that there is a lack of flow gauging and pluviograph rainfall data collection within the five Council areas. The localised nature of storm events in the study area also makes it difficult to accurately predict flow rates.
Flow velocities and the extent of flooding for a given flow rate for at grade floodway crossings can be estimated using Manning’s Equation. HEC-RAS modelling was undertaken to estimate flow velocities at floodways with a drop on the downstream side (erosion head). The flow velocity for at grade floodway crossing is typically 2 to 3 m/s. The flow velocity variation across a floodway with a drop on the downstream side varies considerably with flow velocities at the toe of the drop in the order of 4 to 7 m/s.

17.4 Floodway Designs and Cost Matrix
The first step in redesigning or enhancing an existing floodway is to establish the importance of the floodway. The level of importance is firstly based on the road classification then adjusted, if required, considering a number of factors such as traffic volume, population serviced, local economy and alternate route availability.
The next step is to define the serviceability level. The serviceability level in this report is calculated by assessing both the level of importance of a floodway and the consequences of failure. Once the serviceability level is defined an appropriate level of erosion protection can be defined. Floodway erosion protection works for each serviceability level have been defined.
The large variation in raw material availability and cost across the region reduces the possibility of a “one size fits all” solution. The floodway design options allow the Councils to choose the best technology and configuration for each floodway. This project has also identified a range of both materials and techniques for floodway construction to be used in areas other than those identified in the study area.
The level of serviceability can be applied to four zones within and surrounding a floodway, including:
t Upstream zone – upstream of the roadway.
t Roadway zone – roadway area including cut-off walls at the roadway.
t Downstream zone – downstream of the roadway to the base of any embankment.

 

t Peripheral zone – areas surrounding the floodway including the downstream watercourse and areas adjacent to the floodway.
A cost matrix was developed so that each Council can input their own construction and maintenance costs, for a 25 year period and 8% discount rate, for each floodway erosion control measure and floodway zone. This will enable the Councils to identify the most cost effective floodway protection measures based on localised issues at each site.

17.5 Trials
The challenge for this study has been the need to find alternate products to reinforced concrete. Reinforced concrete presents a robust and durable solution, but one that comes at a cost. There are no products marketed as “floodway solutions”. Alternate products such as Concrete Canvas® and polymers such as PolyCom® will be trialled before any widespread implementation. The manufacturers are keen to be involved in trials of their product and the process of implementing trials has commenced.

17.6 Site Specific Recommendations
Floodway erosion control measures or enhancement works at each of the 21 sites have been recommended for each floodway zone and for three levels of serviceability. This information provides the Councils with options depending on their funding constraints and the localised issues at each site.

17.7 Shared Knowledge and Asset Management
One of the key issues identified during this study is the need to share knowledge between Councils. A lack of resources at each of the Councils means that a reactive approach to asset management has been adopted. Significant cost savings can be realised by the Councils with a pro-active monitoring and maintenance regime of floodway assets. It is recognised that the Councils have limited resources and funds available in order to maintain such a programme but these challenges must be overcome to realise long term cost savings.

17.8 Recommendations
It is recommended that:
t Councils undertake a review of the vertical profile and horizontal alignment of floodways prior to adopting erosion control measures. It may be more economical in the long term to remove a culvert crossing than implement erosion control measures to retain the culvert crossing.
t A factor of safety in the form of a minimum 300 mm additional flow depth is added to the calculated flow depth for a particular ARI flood event. The additional flow depth is included in the extent of floodway erosion control measures to account for the uncertainties surrounding the hydrology for this area and the hydraulic analysis.
t The Councils record storm event in excess of 40 mm between the months of October and April to establish any trends in increasing storm activity and intensity. The location of these storm events should also be recorded to establish any patterns.
t The use of small rock rip-rap with concrete infill is used with caution and that suitably sized rock rip- rap be used where possible.

 

t Subject to the outcomes of the trials, the five Councils consider purchasing products in bulk to recognise any cost saving benefits this may have.
t Each of the Councils review their current methods of detailing flood damage for insurance claims and tracking general erosion at each site. Consideration needs to be given to purchasing tablets for each of the Works Managers so that they can document flood damage and general erosion at problem sites.
t The Councils undertake trials using the alternate floodway erosion control methods and communicate the effectiveness of each product to each of the other Councils.
t The Council’s undertake a cost analysis of the floodway sites utilising their own available cost data to assess the most cost effective floodway treatment options for each site.
t The Councils trial the options selection process documented in this report and adjusts the process accordingly to meet their individual requirements.
t The Councils initiate a system to assess each of the floodways in their Council area to allow for a more economical integration of floodway maintenance into their Road Maintenance Program and/or Asset Management Register.

 

18. Bibliography

Publications
Quigley, M., Sandiford, M., Fifield, K. and Alimanovic, A., in press. Bedrock erosion and relief production in the northern Flinders Ranges, Australia. Earth Surface Processes and Landforms, DOI:10.1002/esp.1459.
EUSUFF, T.H. (1995) “A Regional Flood Frequency Approach to the Mount Lofty Ranges” Unpublished thesis, University of Adelaide, June 1995
Government of South Australia – ASSESSMENT OF STREAM
CROSSINGS ON STATE ROADS, THE FLINDERS RANGES REGION 2007, PP34. Earth Tech.
ASCE, 1990, Retaining Forest Roads, Civil Engineering (Dec 1990), ASCE (D, E, T)
Berger L, Greenstein J, Arrieta J, 1987, Guidelines for the Design of Low Cost Water Crossings, TRR 1106, Transportation Research Board, Washington
(A, B, D, M)
DTEI, 2007. January 2007 flood images. Typical road damage caused by flooding between 18 January and 20 January 2007. Department of Transport, Energy and Infrastructure, South Australia, Adelaide, photographs.
Flavell D (ed), 1994, Waterway Design, Austroads, Sydney (B, C, D, H)
Kemp, D., 2007. Preliminary report, 9/02/2007. Department for Transport, Energy and Infrastructure, Adelaide, pp. 1.
Walsh, A & Kemp D eds (2002) Protecting Waterways Transport SA, Walkerville SA.
Central Local Government Region of SA, 2011. Central Local Government Region Integrated Climate Change Vulnerability Assessment – 2030.
Websites
http://www.news.com.au/breaking-news/floodrelief/south-australia-braces-for-serious- storms/story-fn7ik2te-1225985877595 South Australia Braces for Serious Storm. Viewed February 2012.
http://www.lowecol.com.au/lfw/gfwmeminfo/RoadDrainage.pdf Land Notes, Natural Resource Management – Road Drainage. Viewed February 2012.
http://www.codepublishing.com/wa/Ferndale/Ferndale18/Ferndale1820.html FERNDALE MUNICIPAL CODE – Chapter 18 Floodway Zone. Viewed January 2012.
http://ingenierosdeminas.org/biblioteca_digital/libros/00010-hyd- hydraulic%20design%20manual.pdf – Texas Department of Transportation – Hydraulic Design Manual. Viewed February 2012.
http://www.arrb.com.au/admin/file/content13/c6/ARR368%20Stormwater.pdf Allan Alderson – the Collection and discharge of stormwater from the road infrastructure. Viewed February 2012

 

http://www.samdbnrm.sa.gov.au/Portals/9/PDF’s/Biodiversity/Improving%20the%20success%20of
%20revegetation.pdf Sarah Lance
Revegetation research in the Murray Mallee – Improving the success of revegetation for biodiversity and habitat restoration. Viewed March 2012
http://www.ecy.wa.gov/programs/sea/pubs/93-30/index.html – Slope stabilisation and erosion control. Viewed February 2012.
http://www.ncwe.org.au/arr/Website_links/ARR_Project_5_Stage1_report_Final.pdf Australian Rainfall and Runoff Stage 1 final Report. Viewed March 2012.
Causes of erosion http://images.wool.com/pub/lww_Rivers_Preventing-creek-erosion.pdf
State of the Climate 2012
http://www.csiro.au/Outcomes/Climate/Understanding/State-of-the-Climate-2012.html
Fewer rain storms across southern Australia, Reference 11/67
http://www.csiro.au/en/Organisation-Structure/Divisions/Marine–Atmospheric-Research/Fewer- rain-storms-across-southern-Australia.html
Annual Climate Summary for South Australia http://reg.bom.gov.au/climate/current/annual/sa/archive/2010.summary.shtml http://reg.bom.gov.au/climate/current/annual/sa/summary.shtml

 

 

 

 

 

 

Appendix A
Site Information

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

33/16230/50302 Floodway Research Project
Final Report

 

Flinders Ranges Council Sites
Site information for the four sites in the Flinders Ranges Council, obtained during a site visit on the 9 February 2012, and provided by Council is shown in Table A.1. The locations of the Flinders Ranges Council sites are shown in Figure 2, Appendix D.
Table A.1 Site Information for the Four Sites in the Flinders Ranges Council

Site and Photos General Description Current and Future Failure
F1 Carrieton – Quorn Road (Unknown Creek)

Photo taken looking downstream

Photo taken looking east Council have made significant changes to this site. The creek was realigned to remove an ongoing erosion issue at a floodway crossing to the east of the current floodway. The roadway has been constructed at the bed level of the creek and is aligned at approximately 30 degrees to the creek. The outside bend of the creek immediately downstream of the roadway has vertical side walls. The realigned creek on the upstream end of the roadway has vertical side walls. The creek bed material ranges from sand to rock, typically less than 200 mm is size. Current: Realigning the creek has had a significant impact in reducing the risk of erosion. During recent flood events, sediment deposition has been the primary failure mechanism, which is largely due to the expansion of the watercourse and reduced flow velocity at the floodway.
Future: It is expected that the site will continue to experience deposition from future floods. At some point the banks of the creek may become degraded. As the flow path alters the mode of failure may change.

 

Site and Photos General Description Current and Future Failure
F2 Yednalue Road (Wirreanda Creek)

Photo taken looking east

Photo taken looking west This is a large creek crossing (approx. 80 m wide) with the road at creek bed level. The road crosses the creek at an oblique angle upstream of a bend in the creek. There is evidence of sediment deposition in the creek bed and evidence of erosion at the western end of the crossing as the road cuts through western creek bank. During recent storm events the entire creek bed is inundated, however, during smaller flows the creek does not take a defined path and can cause minor erosion or sediment deposition at any point across the floodway. Current: During low flows the roadway experiences minor erosion 100 to 200 mm deep. The western approach into the creek bed has recently been eroded during significant storm events, requiring repair to open the roadway to traffic. Sediment deposition has also occurred on the roadway during recent significant storm events, requiring regrading to open the roadway to traffic.
Future: It is likely that the two modes of failure described above will continue to occur. In addition the creek banks adjacent to the road are showing signs of erosion. It is likely that this erosion will continue and may lead to further erosion of the roadway.

 

Site and Photos General Description Current and Future Failure
F3 – Yednalue Road (Yednalue Creek)

Photo taken looking upstream

Photo taken looking downstream This is a small tributary crossing with a modest upstream creek profile. There is a downstream erosion head that has progressed up the creek channel and has now reached the roadway. A previously installed headwall has failed and turned inwards towards the centreline of the creek, as shown in the photo looking downstream. There is little embedment depth of the wall and it was limited to the width of the creek channel. Recent storm flows would appear to have been large enough so that the flow was beyond the extent of the concrete cut-off wall/headwall, resulting in erosion around the end of the wall. Current: The primary failure mechanism on this site is the progression of the erosion head up the creek channel that has now reached the roadway. The road surface also requires replacement following significant storm events.
Future: With no intervention the erosion head will progress through the roadway during subsequent flood events.

 

Site and Photos General Description Current and Future Failure
F4 – Warcowie Road (Wonoka Creek)

Aerial photo of site

Photo midway along floodway looking east This is a large creek crossing, approximately 70 m across, with the road cutting at an oblique angle and traversing along the creek bed for approximately
120 m. There are several large gum trees in the creek bed. The creek bed is mobile and during low flows the channel bed tends to form a different flow path with each subsequent storm event. During major flood events the entire crossing fills to a depth of approximately 2 m. There is visual scarring on the trees from debris carried down the creek during previous flood events. Current: During a large flood event there is significant deposition along the roadway. This is normally well graded material that is graded to the upstream side of the roadway. During minor flood events the low flow channel does not follow a predictable flow path and results in erosion of the road surface.
Future: Damage to the roadway due to bed mobility issues are likely to continue. Sediment deposition is also likely to be an ongoing issue.

District Council of Orroroo and Carrieton Sites
Site information for the five sites in the District Council of Orroroo and Carrieton, obtained during a site visit on the 10 February 2012, and provided by Council is shown in Table A.2. The locations of the District Council of Orroroo and Carrieton sites are shown in Figure 3, Appendix D.
Table A.2 Site Information for the Five Sites in the District Council of Orroroo and Carrieton

Site and Photos General Description Current and Future Failure
O1 – Belton Road (Unknown Creek)

Photo taken on the downstream end looking north

Photo taken looking upstream This floodway previously had a culvert crossing. Due to ongoing repair works following flood events, Council decided to remove the culvert crossing. The upstream channel shows little sign of erosion or vegetation removal. On the downstream side there is significant existing erosion protection infrastructure in the form of rock rip rap and left over erosion protection from the culvert crossing. Current: The primary failure mechanism has been stopped by removing the culverts, however erosion of the downstream side of the roadway has commenced.
Future: If left un treated the erosion on the downstream side of the roadway will continue upstream in the form of an erosion head, resulting in an ongoing maintenance issue for Council.

 

Site and Photos General Description Current and Future Failure
O2 – Belton Road (Unknown Creek)

Photo taken looking downstream

Photo taken looking upstream This floodway has a twin pipe culvert crossing. There is a significant amount of erosion behind the culvert headwall and wing walls. The southern wing wall has been moved by floodwaters. The upstream creek is stable with gentle vegetated batter slopes. The downstream channel is stable with signs of deposition on the channel bed and minor erosion at the bottom of the batters. The batters on the downstream side are vegetated and have a gentle batter slope. Current: On the downstream side of the crossing significant erosion has occurred around the existing culvert headwall and wing wall infrastructure. This has caused the headwall to fail. Erosion behind the culvert headwall is encroaching onto the roadway.
Future: The site will continue to experience erosion of the roadway and the next significant storm event will result in significant damage to the road. Further damage is likely to occur to the culvert infrastructure.

 

Site and Photos General Description Current and Future Failure
O3 – Johnburgh Road (Unknown Creek)

Photo taken looking at the downstream headwall and floodway

Photo taken looking at the downstream headwall This site has an approximate 50 year old concrete headwall constructed to halt the progress of an erosion head that threatened the roadway. The upstream channel is relatively shallow (bed level at road level) with sparse vegetation. There is an existing concrete headwall that has a large hole in it. Recent storm events have resulted in erosion around the edge of the headwall. Edge protection in the form of large rocks concreted into place has been placed at the edges of the structure. There is no evidence of erosion at the base of the headwall. The downstream creek is badly eroded with overhanging batters. Current: Erosion has occurred on the roadway immediately upstream of the concrete headwall. This resulted in failure of the concrete headwall. The road surface is also eroded after each storm event, requiring replacement. It is unclear whether the failure of the concrete headwall led to the erosion behind it, or whether the erosion in the road surface caused the headwalls failure. There is also significant erosion around the ends of the headwall.
Future: In the short term, the erosion of the road behind the headwall will continue with each subsequent flood event. There is also evidence of erosion at both ends of the headwall, which may encroach onto the roadway and possibly result in failure of the headwall. Erosion of the downstream channel is also a high risk.

 

General Description Current and Future Failure
This is a very similar site to the O3 site in that the upstream channel is relatively small, and is not showing signs of significant erosion. The banks are well vegetated. There is a large drop off on the downstream side of the road that is supported by a well-constructed headwall. This one differs however, in that there is a 1200 mm diameter culvert under the road. Unlike other examples of this construction in the Orroroo area, the culvert appears to be working well. There is no evidence of significant erosion at the upstream headwall and there is localised erosion surrounding the downstream headwall. Both headwalls have a significant embedment depth into the banks of the creek.
The downstream creek banks are highly eroded with vertical side walls. There is no significant erosion at the culvert outlet, even though there is no significant erosion protection.
The site has been highlighted in this report for the main reason that there is an erosion head on the northern side of the downstream end of the creek. This erosion head has developed perpendicular to the creek flow direction. Current: This site does not require any significant maintenance following a flood event. The erosion head perpendicular to the flow direction on the downstream end is of concern. There is evidence of erosion around the southern headwall on the downstream end, which is threatening to expose the end of the headwall.
Future: The erosion head forming on the downstream side of the road is progressing at approximately 1 to 2 m per flood event. This erosion head is likely to cut the road to the north unless works are undertaken to halt its progress.
The erosion in the downstream creek will most likely expose the ends of the headwall and form an erosion head at the ends of the headwall, ultimately resulting in failure of the roadway.
O3A – Johnburgh Road (Unknown Creek)

Photo taken looking south on the downstream end Photo taken looking at the downstream headwall Photo taken looking at the upstream headwall

 

Site and Photos General Description Current and Future Failure
O4 – Orroroo Road (Bambrick Creek)

Photo taken looking at the downstream culvert headwall

Photo taken looking upstream The catchment area for this floodway is 5.1 km2. The floodway crossing is approximately 100 m wide, which is disproportionate to the size of the catchment. This floodway crossing is the only sealed floodway in the 21 sites investigated.
There is an upstream headwall with a box culvert under the roadway. The box culvert is completely filled with sediment and no longer operates as designed. Any flow in the creek will overtop the roadway. There is a headwall on the downstream side and an approximate 3 m drop from the roadway to the creek bed level.
There is an existing concrete spillway downstream, however this is aligned with the old road alignment and does not provide complete erosion protection. Significant erosion protection has been placed on the downstream side of the crossing although this has not been designed, graded or specifically placed. The roadway has a concrete cut-off wall on the upstream and downstream sides of the roadway. Current: Council have dumped rock rip-rap on the downstream embankment to repair erosion of the embankment that occurred during the last flood event.
Future: The downstream embankment is a high risk of being eroded, which could also result in erosion of the road verge and possibly the roadway. The extent of rock rip-rap on the downstream embankment may not be sufficient to protect the full extent of the embankment.

District Council of Peterborough Sites
Site information for the three sites in the District Council of Peterborough, obtained during a site visit on the 8 February 2012, and provided by Council is shown in Table A.3. The locations of the District Council of Peterborough sites are shown in Figure 4, Appendix D.
Table A.3 Site Information for the Three Sites in the District Council of Peterborough

General Description Current and Future Failure
This site has a very shallow upstream profile with the road at creek bed level. On the downstream side of the road there is a drop-off of approximately 1 m, which is supported by a mass concrete headwall. This headwall is acting as a retaining wall and is restricting the progress of the erosion head up the creek. The concrete headwall does not appear to be embedded into the creek bed. This is not a recent construction; however it has with stood the test of time reasonably well. It is an isolated piece of infrastructure and is not tied in to either wing walls or base erosion protection. Recent floods have resulted in erosion around the northern end of the headwall.
The road approaches to the floodway channel a significant amount of stormwater runoff into the floodway. This has resulted in erosion of the roadway and is contributing to erosion around the ends of the headwall. Current: Stormwater runoff along the roadway is resulting in erosion of the road pavement and is contributing to erosion around the concrete headwall. The toe of the concrete headwall is also being undermined and is at risk of failure.
Future: If this site is left untreated the road will continue to erode as before. The erosion around the headwall will undermine the structure and begin to erosion around the ends leading to a complete failure of the head wall structure which will allow the erosion head to progress
through the roadway.
P1 – Gumbowie Road (Unknown Creek)

 

 

 

 

Photo taken looking at the downstream headwall Photo taken looking at the northern end

Photo taken looking south, showing flood damage

 

Site and Photos General Description Current and Future Failure
P2 – Bullyaninnie Road (Black Fella Creek)

Photo looking upstream at existing floodway This site is only 1 km downstream from the little blackfella creek floodway. The progression of an erosion head up the creek channel is evident, with bed material being stripped down to bedrock on the downstream side of the existing floodway.
The location of the existing floodway crossing is temporary due to the previous floodway being washed away. The previous floodway was located approximately 20 to 30 m downstream of the existing floodway, where the existing rock is exposed.
The upstream side of the existing floodway crossing is a shallow well defined channel with minor deposition of well graded material. Current: The primary failure of this site is due to the erosion head moving upstream. The roadway has been moved upstream of the advancing erosion head to maintain access.
Future: The erosion head will continue to move upstream and the existing roadway will continue to fail.

Photo taken looking upstream at flood damage Photo looking downstream

 

Site and Photos General Description Current and Future Failure
P3 – Bullyaninnie Road (Little Black Fella Creek)

Photo taken looking south after the Feb 2011 flood event This is a small creek crossing with the upstream channel at road level. It has a narrow upstream cross-section with significant vegetation and minimal signs of erosion. The floodway crosses the creek at the upstream end of the start of a bend in the creek. The downstream end of the crossing shows significant erosion of the banks and sediment deposition in the channel. The deposited material is well graded from sand to boulders less than 300 mm.
There was also evidence on site to suggest that the creek overtopped upstream of the floodway and flowed around the floodway to the south, before rejoining the creek downstream of the floodway. Current: The primary failure of this crossing is due to an erosion head progressing upstream. The focal point of the erosion appears to be on the northern side of the creek, at the start of the outside bend in the creek. Fast flowing stormwater exiting the narrow upstream creek profile further exacerbates the erosion problem.
Future: Council have repaired the erosion caused by the February 2011 flood event by filling the eroded section of the floodway and dumping rock rip-rap on the downstream side. There is a high risk that erosion will occur at the interface of the rock rip-rap and the roadway during the next flood event.

Photo taken looking north after Feb 2011 flood event

Photo taken looking upstream

Northern Areas Council Sites
Site information for the four sites in Northern Areas Council, obtained during a site visit on the 2 March 2012, and provided by Council is shown in Table A.4. The locations of Northern Areas Council sites are shown in Figure 5, Appendix D.
Table A.4 Site Information for the Four Sites in Northern Areas Council

Site and Photos General Description Current and Future Failure
N1 – Hornsdale – Tarcowie Road (Unknown Creek)

Photo taken looking at the downstream headwall structure The watercourse is relatively wide (approximately 50 m) in relation to the upstream catchment size of 3.5 km2.
The upstream creek bed level is at road level and has a narrow upstream cross-section with significant vegetation and minimal signs of erosion. The downstream side of the roadway is supported by a reinforced concrete headwall of varying height as the creek bed is at two levels. It is unclear whether this headwall was constructed as a single structure of whether it has been expanded to suit the changing creek conditions.
The concrete headwall is showing signs of severe decay and there is significant undermining on the downstream side. Current: The existing concrete headwall structure has failed in several locations and the downstream concrete apron is being undermined.
Future: The headwall structure has reached the end of its design life. The decay of the concrete and the erosion under the structure will continue. There is a high risk that the headwall will suffer complete failure leading to the erosion head progressing through the roadway.

 

Site and Photos General Description Current and Future Failure
N2 – West Terrace (Pine Creek)

Photo taken looking at downstream end of floodway

Photo taken looking upstream This is an important floodway site forming part of the flood defences for the township of Laura. It forms part of a man made series of structures including stormwater channel and flood protection levee.
The upstream channel has a relatively small cross- section and is well vegetated, showing no significant signs of erosion. The channel has a straight alignment and the crossing is perpendicular to the road.
The floodway consists of a stable roadway and a large intact concrete spillway which extends to the top of the adjoining levee.
There is erosion occurring both at the downstream toe of the spillway and the ends of the concrete spillway. Current: The site has not experienced any failure at this time.
Future: If left unprotected the erosion will continue to undermine the floodway structure on the downstream side. This erosion will eventually lead to the failure of the concrete spillway. The levee is showing signs of erosion at the interface with the concrete spillway, which will continue to threaten its integrity if not addressed.

 

Site and Photos General Description Current and Future Failure
N3 – Georgetown – Huddleston Road (Rocky River)

Photo taken looking along the floodway

Photo showing cracks in the concrete road deck This creek has a shallow profile with a creek width of approximately 40 m. The creek has a constant cross section both upstream and downstream of the floodway. The creek is in constant flow.
The existing infrastructure consists of a concrete deck secured by concrete edge strips with two small, centrally placed culverts of approximately 300 mm in size. The concrete deck extends beyond the limits of the creek. Current: The concrete deck has significant cracking. These are likely to be a result of movement of the road foundation. As the cracks grow, sections of the roadway separate and the crossing become impassable.
Future: Without repair, the roadway will continue to fail. If this is not address there is a risk of catastrophic failure of the crossing, although this is unlikely in the short term.

 

Site and Photos General Description Current and Future Failure
N4 – Broughton Valley Road (Hutt River)

Photo taken looking east along roadway

Photo taken looking downstream This site is a small crossing that occurs just before the Hutt river joins the Broughton river. It is a perpendicular crossing site in a straight section of river. It has a natural erosion head that has been prevent from developing by the presence of bedrock. The local quarry has placed rock and concrete in small amounts as the erosion threatens the road. This local protection stops at the extents of the crossing. The route only services a local quarry, which is important but not vital. Current: This crossing does not fail in the typical sense. The floodway has some erosion on the downstream side of the road. All surface material has been removed leaving only the bedrock exposed.
Future: The concrete placed by the quarry continues to wash away in small sections. In a large flood event erosion will occur beyond the zone of protection installed and could lead to erosion of the roadway. If the site is not maintained, the downstream erosion protection will fail and the erosion will progress through the roadway.

Regional Council of Goyder Sites
Site information for the five sites in the Regional Council of Goyder, obtained during site visits on the 08 February and 07 March 2012, and provided by Council is shown in Table A.5. The locations of Goyder District council sites are shown in Figure 6, Appendix D.
Table A.5 Site Information for the Five Sites in the Regional Council of Goyder

Site and Photos General Description Current and Future Failure
G1 – Bower Road (Unknown Creek)

Photo taken looking downstream

Photo taken looking upstream The upstream profile is shallow with well vegetated banks. There is an erosion head of approximately 1.5 m deep that is progressing up the creek channel. The creek crosses the road at approximately a 30 degree angle.
There are two concrete headwalls on the site. The first is approximately 20 m downstream of the crossing and has completely failed. Little is known about the time of construction of this headwall or the creek configuration at its time of failure. The creek is approximately twice as wide after this headwall and suggests that erosion around the edge of the headwall may have led to its failure.
There is a second concrete headwall immediately downstream of the roadway, which is of very similar construction to the first. The headwall is approximately 1 m in depth, 200 mm wide and it extends 1 m beyond the creek banks. This headwall has also failed. Current: The primary failure of this site is due to the progression of the erosion head up the creek.
Previously installed infrastructure has not been sufficiently tied into the surrounding environment nor been provided with sufficient protection at the tow of the concrete headwalls to prevent undermining. It is likely that failure of the second headwall was caused by erosion at the toe of the wall, resulting in undermining of the concrete headwall.
Future: Without remedial works, the erosion head will encroach into the road corridor. Any erosion control works at this site will need to consider flow over the floodway on the eastern side of the creek and how stormwater is directed back into the creek.

 

Site and Photos General Description Current and Future Failure
G2 – Caroona Road (Unknown Creek)

Photo taken looking downstream This approximate 100 year old floodway has a small upstream channel that is well vegetated and showing no significant sign of erosion. There is alluvial material deposited at the base of the upstream channel. There is a deep (5 to 6 m) erosion head on the downstream side of the floodway. The downstream zone of the floodway is protected by a stacked rock wall which is stapled together at the top course of rock. The downstream channel is approximately 20 m. Current: The downstream rock erosion control infrastructure has failed in the central region allowing the erosion head to progress into the roadway. It is unclear whether erosion developed behind the structure and was able to undermine it or whether the erosion began at the base of the structure causing it to collapse.
Future: Without immediate enhancement the erosion head will progress through the road rendering the crossing completely impassable. The remainder of the rock erosion protection infrastructure is unlikely to survive another large storm event, if left in its current state.

 

Site and Photos General Description Current and Future Failure
G3 – Angle Road (Unknown Creek)

Photo looking at downstream end. This is a relatively small crossing site and the road services a single residential property. The upstream channel is approximately 2 m in depth and is well vegetated. It has a narrow base showing signs of deposition of well graded material 40 mm.
The road and the creek intersect at right angles and the creek has a relatively straight alignment at the crossing site.
An erosion head has formed in the downstream channel of approximately 2.5-3 m depth, with near vertical banks. The soil in the area is extremely soft and there is evidence of wombat activity and burrows around the site. Current: The primary cause of failure at this site is the progression of the erosion head up the creek channel and erosion at the downstream end of the road.
Future: The erosion head will continue upstream and continue to erode the roadway if it is not protected.

 

Site and Photos General Description Current and Future Failure
G4 – Belalie Road (Unknown Creek)

Photo taken looking east

Photo taken looking downstream The creek runs parallel with the roadway upstream of the floodway. There is a small spring that produces a base flow in the creek, which is serviced by existing culverts under the road. At the time of inspection the creek was dry. The creek crosses the road at an acute angle of approximately 30 degrees. The creek is approximately 0.5 m below road level on the upstream side and 1 to 1.5 m below on the downstream side. Current: Once the culvert reaches capacity the creek flows over the road and erodes the road surface. The attached photo shows erosion following a significant storm event. There is no significant damage to surrounding infrastructure although there is evidence of failure at the culvert joints, with water flowing between culvert sections during a storm event.
Future: It is likely that a significant storm event will result in further damage to the roadway and culvert.

 

Site and Photos General Description Current and Future Failure
G5 – Ketchowla Road (Unknown Creek)

Photo taken looking south This site occurs at the end of sequence of five crossings. The roadway is at creek bed level. The creek is approximately perpendicular to the roadway. There are signs of deposition in the downstream creek. There is exposed bedrock under the roadway on the eastern side. The creek banks, both upstream and downstream are showing signs of erosion.
Approximately 50% of the crossing is covered by a fibre reinforced concrete deck that was placed in 2008. The other half of the crossing is exposed bedrock, where the concrete deck has failed.
The remaining fibre reinforced deck is on the inside of a bend and the failed section is on the outside of a bend in the creek. Current: The concrete deck has been lifted off its position and washed down the creek. This has likely been caused by a lack of integration of the deck with other infrastructure and the failure to adequately tie the deck to the creek bed.
Future: Without enhancement the remaining deck is at risk of failing. The creek bed is otherwise stable due to the presence of bedrock. There is a risk of erosion of the creek banks causing erosion/failure of the roadway but this is not likely in the immediate future.

 

 

 

 

 

 

 

Appendix B
Hydrological Data

Historical Flood Data Daily Rainfall Data IFD Data

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

33/16230/50302 Floodway Research Project
Final Report

 

 

Historical Flood Events
Table B.1 below shows comments from local newspapers over the period 1907 – 1995.
Table B1 Newspaper Reports of Flood Events (1907 – 1995)

Date Selected
Comments from Newspaper Articles
16/01/1995 Burra Police Station and a number of private properties flooded. Creek to west of
Burra caused a lot of damage to the commercial centre. Shops from Elders Ltd to the
Burra Motor Company were flooded. Many roads were closed.
26/01/1993 Major crop damage throughout district. Some flooding in Farrell Flat due to Wakefield
River and tributaries. Floods in Burra Booborowie and Mt Bryan described as worst
for 50 years. Students were billeted in Burra as they could not return home. Some
homes inundated in North Burra and Mt Bryan. Roads closed and fences damaged.
Road damage of approx. $ 100 000 reported.
17/12/1992 Burra Creek in flood. Roads and crops damaged
25/09/1992 Roads closed in and around Burra. Major flooding in Clare
14/03/1989 Extensive damage to roads (estimated at 80 km) especially in and around Hallett and
Jamestown. Fences damaged and erosion.
4/10/1976 Water inundated a number of houses and shops.
13/05/1974 Wettest Jan to May on record. Continual flooding of watercourses and creeks.
Damage to fencing and paddocks.
31/01/1974 Adelaide Road to slaughterhouse inundated. Water up to the bellies of cattle in the
yards. Large flows from Mount Bryan and Hallett catchments. Part of Copperhouse
Bridge gave way. Severe damage to Burra Creek crossing at Worlds End. Damage to
fencing, roads and culverts. Parts of Adelaide Road, Mount Bryan to Hallett and
Booborowie Roads closed. Peak occurred at noon on 30 Jan and was within 1 to 2
feet of the top of Gully’s Wharf Bridge (Young St). Water was still crossing the road
over the Pig and Whistle on 1 Feb.
12/02/1955 Widespread damage to fences and roadways. In some instances the intake of water
into dams was so great that holes were washed in the banks allowing water to run
straight through. Flooding between Worlds End hills and Gordon’s Lagoon.
Causeway near Studholme damaged. Gordon’s Lagoon 1 ft over road. Burra Creek at
Burra to Robertstown Road rose to a depth of 8 to 9 feet over the crossing.
26/01/1941 Previous drought was broken by torrential rains. Greatest and most general
downpour ever known. Whole country resembled an inland sea. Roads and railways
were impassable. Race course and homes in the vicinity were one large lake.
Significant road, fencing and property damage and stock losses. Water was 4 to 5
feet over the bridge which is ten feet from the creek bed.

 

Date Selected
Comments from Newspaper Articles
1/12/1937 In Hallett and parts of Booborowie all creeks were flowing bank full. Some damage to
property, poultry, fences and crops.
2/09/1937 Heaviest downpours experienced for a few years. Burra creek was flooded. All creeks
overflowed banks. Flood in Burra creek the biggest seen since 1915.
20/08/1937 Dust Hole Creek carried a very large flood. Trees and heavy timber carried along like
corks.
10/01/1936 Severe rain storm. Flow of water became too much for the stormwater drains and
streets were flooded. Burra Creek ran the biggest flood for years – one of the biggest
floods known. Roads and fences damaged.
12/08/1932 Farrell Flat roads were impassable – flood waters over the top of fences. Burra Creek
flooded – 3 ft over the ford at the Pig and Whistle. Fences damaged at Black Springs.
First time since 1915 that Burra Creek flooded.
21/02/1928 Cars became bogged, roads were impassable and the train line to Broken Hill was
cut off. Fences damaged. A large lake (1.5 x 1.5 miles and up to 8 ft deep) formed at
Quandong Station and similarly at Oakbank.
31/05/1921 Market Square was flooded. Shops and homes damaged. Road damage including 16
crossings washed away. Flood levels were the biggest seen on Adelaide Road.
Repetition of the 1915 flood was feared and realised.
19/02/1919 Flood report from Hallett Flooding to the east did a lot of damage to roads and
fences. Roads were impassable. Creek crossings washed away. Several fords were
washed away at Mt Bryan. Washaways between Terowie and Peterborough. Train
delayed.
9/04/1915 Great drought was broken. Market Square was a rushing torrent and a number of
business premises were invaded by big streams of water. One of the biggest floods
that ever occurred in Burra. Footbridge and bridge swallowed by water. Property and
contents damage.
5/09/1910 Roads were impassable with heavy damage. Mt Bryan Council was forced to cancel
existing contracts to pay for repairs. Over 50 chains metal was washed from Hallett to
Burra. Flooding was the worst in 20 years. Burra Creek carried a record flood.
Damage to roads, bridges and fences. Stock losses.
2/09/1908 Large rains in the north. Biggest flood in 20 years. Flooding in Burra Creek. Mail run
was delayed. Footbridge in front of Post Office was covered. Water was several feet
up the walls of Henderson’s black bridge.
13/06/1907 Rain overflowed all waterways and brought rubbish from near and far. The dayman of
the Corporation was kept busy clearing culverts and footbridges in the rain and mist
for several days.

 

Recent Flood Events
The issue of floodway damage was highlighted in 2006/2007 when several major route crossings were destroyed, and DPTI invested heavily in replacement infrastructure to reduce the risk of future damage. More recent flood events of December 2010 and February 2011 occurred so close together that the Councils had not fully repaired the damage from the first event, when the second struck.
The following photos were taken of a DPTI floodway crossing that was reconstructed in 2008. The reinforced concrete elements of the floodway appeared to be functioning as per the design intent. The culvert was 100% blocked with gravel and the downstream rock rip rap (encased in concrete) was showing signs of deterioration and undermining.

 

 

 

240

Daily Precipitation for Blinman- Beltana (017014) Daily Precipitation > 40mm from 1874 to 2011

Daily Precipitation for Hawker (019017) Daily Precipitation > 40mm from 1882 to 2011

Daily Precipitation for Cradock (019010) Summertime Daily Precipitation > 40mm from 1888 to 2010

Daily Precipitation for Yednalue (019061) Summertime Daily Precipitation > 40mm from 1917 to 2011

Daily Precipitation for Carrieton (019009) Daily Precipitation > 40mm from 1883 to 2011

Daily Precipitation for Orroroo (019032) Summertime Daily Precipitation > 40mm from 1884 to 2011

Daily Precipitation for Peterborough and Surrounds (019034) Summertime Daily

Daily Precipitation for Jamestown (021027) Daily Precipitation > 40mm from 1877 to 2011

Daily Precipitation for Whyte Yarcowie (021055) Summertime Daily Precipitation > 40mm from 1877 to 2011

Daily Precipitation for Spalding (021047) Summertime Daily Precipitation > 40mm from 1902 to 2011

Daily Precipitation for Burra and Surrounds (021034) Daily Precipitation > 40mm from 1897 to 2011

Daily Precipitation for Clare (021025) Summertime Daily Precipitation > 40mm from 1882 to 2011

Daily Precipitation for Eudunda (024511) Daily Precipitation > 40mm from 1880 to 2011

Daily Precipitation for Kapunda (023307) Summertime Daily Precipitation > 40mm from 1861 to 2011

Daily Precipitation for Tanunda (023318) Summertime Daily Precipitation > 40mm from 1868 to 2011

LOCATION 32.350 S 138.725 E * NEAR.. Bendleby

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 32.075 S 138.550 E * NEAR.. Craddock

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 32.050 S 138.550 E * NEAR.. Craddock 2

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 32.350 S 138.575 E * NEAR.. Carrieton j

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

 

LOCATION 32.325 S 138.350 E * NEAR.. Boolcunda East

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

 

LOCATION 32.775 S 138.575 E * NEAR.. Orroroo ,

UST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 33.000 S 138.525 E * NEAR.. Tarcowie

.!,.IST OF COEFFICIENTS TO EQUATIONS OF THE FORM

 

LOCATION 32.850 S 139.275 E * NEAR.. P2 Blackfella

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 32.850 S 139.250 E * NEAR.. P3 Little Blackfella

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

 

LOCATION 33.025 S 138.900 E * NEAR.. P1 Gumbowie

UST OF COEFFICIENTS TQ EQUATIONS OF THE FORM

 

LOCATION 33.175 S 138.300 E * NEAR.. Laura

L!ST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 33.275 S 139.200 E * NEAR.. ‘ Ketchowla .

b!. L PF J:;OEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 33.325 S 138.325 E * NEAR.. Georgetown

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 33.525 S 138.600 E * NEAR.. Splading

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 33.675 S 139.075 E * NEAR.. . Caroona road

LIST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 34.075 S 139.200 E * NEAR.. Bower Road

UST OF COEFFICIENTS TO EQUATIONS OF THE FORM

LOCATION 34.250 S 139.275 E * NEAR.. Angle road

UST OF COEFFICIENTS TO EQUATIONS OF THE FORM

 

 

 

 

 

 

 

Appendix C
Rational Method Calculation

 

 

Appendix D
Catchment Plans

Appendix E
Alternate Floodway Design Sketches

Appendix F
Alternate Technology Brochures

Concrete Canvas® PolyCom® Soiltac®

Appendix G
Site Specific Recommendations

33/ 16230/50302 Floodw ay Research Project
Final Report

 

This Floodway Research Project Report (“Report”):
1. has been prepared by GHD Pty Ltd (“GHD”) for Central Local Government Region of South Australia;
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GHD and its servants, employees and officers otherwise expressly disclaim responsibility to any person other than Central Local Government Region of South Australia arising from or in connection with this Report.
To the maximum extent permitted by law, all implied warranties and conditions in relation to the services provided by GHD and the Report are excluded unless they are expressly stated to apply in this Report.
The services undertaken by GHD in connection with preparing this Report:
• were limited to those specifically detailed in Section 1.2 of this Report;
• did not include detailed investigations or design and should be read in conjunction with the limitations shown in Section 2.3 of this report.
The opinions, conclusions and any recommendations in this Report are based on assumptions made by GHD when undertaking services and preparing the Report (“Assumptions”), as documented throughout the report.
GHD expressly disclaims responsibility for any error in, or omission from, this Report arising from or in connection with any of the Assumptions being incorrect.
Subject to the paragraphs in this section of the Report, the opinions, conclusions and any recommendations in this Report are based on conditions encountered and information reviewed at the time of preparation and may be relied on until 12 months, after which time, GHD expressly disclaims responsibility for any error in, or omission from, this Report arising from or in connection with those opinions, conclusions and any recommendations.

 

Acknowledgement
“This project has been assisted by the Local Government Research & Development Scheme”