Draft – Study Report Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Golden Plains Shire
03 August 2018
Document Status
Version Doc type Reviewed by Approved by Date issued 01 Draft Julian Skipworth Julian Skipworth 28/06/2018 02 Draft Ben Tate Ben Tate 03/08/2018
Project Details
Project Name Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Client Golden Plains Shire Client Project Manager Angela Vary Water Technology Project Manager Lachlan Inglis Water Technology Project Director Julian Skipworth Authors Lachlan Inglis Document Number R07_V02c_Summary_Report_Inverleigh_FS
Front Cover: 1973 Floods, taken from Hotel looking north across High St (Source: Inverleigh Historical Society)
COPYRIGHT
Water Technology Pty Ltd has produced this document in accordance with instructions from Golden Plains Shire for their use only. The concepts and information contained in this document are the copyright of Water Technology Pty Ltd. Use or
copying of this document in whole or in part without written permission of Water Technology Pty Ltd constitutes an
infringement of copyright.
Water Technology Pty Ltd does not warrant this document is definitive nor free from error and does not accept liability for any loss caused, or arising from, reliance upon the information provided herein.
PO Box 436 Geelong VIC 3220 Telephone 0458 015 664 ACN 093 377 283 ABN 60 093 377 283
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EXECUTIVE SUMMARY
The Leigh and Barwon Rivers at Inverleigh Flood Risk Management Study investigated the flood behaviour of the study area and developed a detailed understanding of the flood risk profile through Inverleigh. A review of existing hydrology developed by the Corangamite CMA was undertaken to develop the design flows for the Leigh River and Warrambine Creek. An updated Flood Frequency Analysis were carried out on the Barwon River at Inverleigh streamflow gauge.
A hydraulic model was constructed using TUFLOW software with upstream boundaries at the Shelford gauge on the Leigh River, the Inverleigh gauge on the Barwon River, and the Warrambine Creek (5 km upstream of the Barwon River confluence). The hydraulic model was calibrated to three historic flood events; 1973 (considered the largest recorded flood event on the Leigh River), 1978 and 1995. Additional flood events from 2011 and 2016 were also used for validation. The calibration process utilised streamflow records, surveyed flood heights and community input including photos and anecdotal evidence. Upon completion of the calibration, design modelling was undertaken for a range of flood events from small events which mostly remain in channel to events larger than any flood previously observed. Modelling updates including flood depth, velocity and water surface elevation were used to develop proposed planning layers based on the 1% AEP flood events.
The modelling has also identified several structural mitigation options to reduce flood risk, with two of these being modelled to show significant benefits in terms of reducing the frequency and magnitude of flooding through the Inverleigh township. Flood related planning overlays were reviewed based on the updated flood modelling, with recommendations made for a planning scheme amendment. The structural mitigation options identified, along with the updated planning information aim to treat the existing flood risk through the township.
The study provides an improved understanding of flood behaviour through the study area. Flood intelligence information from the study was extracted and summarised to provide information to assist in raising awareness of flooding within the community and to improve the emergency response information available to VicSES, CFA and Council. A key component of the improved flood intelligence is using streamflow gauge predictions for upstream gauges (Leigh River at Inverleigh & Barwon River at Inverleigh) to provide predicted flood inundation at Inverleigh. The flood mapping data developed during this study has been formatted into the Victoria Flood Database format and it is recommended that this be uploaded to FloodZoom to allow emergency response agency staff access to the data.
Following the investigations undertaken for this study it is recommended that:
◼ Corangamite CMA
◼ Endorse the flood study and use the flood mapping data to inform floodplain risk management decisions.
◼ Upload the Victoria Flood Database mapping data and the excel spreadsheet of property inundation to FloodZoom.
◼ Make use of the flood level information produced to issue advice for any floodplain development.
◼ Golden Plains Shire
◼ Endorse the flood study before putting it out for public comment with aim of adopting the flood study and implementing a planning scheme amendment to update the flood related planning overlays to
introduce new LSIO and FO mapping into the planning scheme. R07_V02c_Summary_Report_Inverleigh_FS
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◼ Consider a detailed assessment of flood mitigation options, with a functional and detailed design project that includes a full flood damages assessment. It is strongly recommended that this option be pursued with all relevant stakeholders and considered within the Inverleigh Structure Plan.
◼ Make use of the flood level information produced to issue advice for any floodplain development.
◼ Request that the Bureau of Meteorology consider the following:
◼ Review the proposed Flood Class Levels of the Leigh River at Inverleigh from this study.
◼ Review the Flood Class Levels of the Leigh River at Shelford and bought in line with the proposed Flood Class Levels for Inverleigh from this study.
◼ Review the updated MFEP and discuss with VicSES the changes proposed by Water Technology prior to adopting the revised document.
◼ Actively promote the use of the VicEmergency website and App and the flood portal www.floodreport.com.au to the community to improve flood preparedness and awareness.
◼ Victoria State Emergency Service with assistance from Goulburn Broken CMA and Greater Shepparton City Council:
◼ Continue to engage the community through regular flood awareness programs such as the VICSES FloodSafe program.
◼ Review the updated MFEP and discuss with council the changes proposed by Water Technology prior to adopting the revised document.
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CONTENTS
GLOSSARY OF TERMS 6
1 INTRODUCTION 8 1.1 Study Area 8 1.2 Hydraulic Model and Mapping Extent 10
2 DATA REVIEW 11 2.1 Existing Flood Studies 11 2.2 Historic Flood Information 11 2.3 Topographic Data 12 2.3.1 Available Data Sets 12 2.3.2 LiDAR Verification 12 2.3.3 DEM Development 13 2.4 Hydraulic Structure Information 14 2.5 Hydrological Data 14 2.5.1 Daily Rainfall Data 14 2.5.2 Sub-Daily Rainfall Data 15 2.5.3 Streamflow Data 17
3 PROJECT CONSULATION 19 3.1 Steering Committee 19 3.2 Community Consultation 19
4 FLOOD BEHAVIOUR 20 4.1 Overview 20 4.2 Hydrology Summary 20 4.3 Hydraulics Summary 22 4.3.1 Hydraulic Modelling 22 4.3.2 Understanding Flood Behaviour 23
5 ASSESS AND TREAT RISK 27 5.1 Structural Mitigation 27
5.2 Planning Controls 29 5.3 Flood Warning 31
6 FLOOD BEHAVIOUR AND INTELLIGENCE OUTPUTS 32 6.1 Model Results 32 6.1.1 Data Sets 32
7 SUMMARY OF INVESTIGATION 34 7.1 Key Outcomes 34 7.2 Conclusions 36
R07_V02c_Summary_Report_Inverleigh_FS 7.3 Recommendations 36
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8 REFERENCES 38 Streamflow Gauge Quality Codes (BoM) 40
APPENDICES Appendix A Streamflow Gauge Data Records
LIST OF FIGURES Figure 1-1 Barwon River, Leigh River and Woady Yaloak Creek Catchments 9 Figure 1-2 Mapping Area and Additional Waterways modelled in 1D 10 Figure 2-1 Extent of LiDAR Datasets 12 Figure 2-2 Transect Survey on the Inverleigh-Winchelsea Road 13 Figure 2-3 Transect Survey on Napier St (in Inverleigh) 13 Figure 2-4 Daily and Sub-Daily Rainfall gauges in the broader catchment region 16 Figure 2-5 Key Streamflow Gauges for the Study 18 Figure 4-1 Flood Mapping Extent 23 Figure 5-1 1% AEP Depths for Mitigation Option 1 27 Figure 5-2 1% AEP Depths for Mitigation Option 2 28 Figure 5-3 Flood Level Difference Plot for Mitigation Option 1 28 Figure 5-4 Flood Level difference plot for mitigation option 2 29 Figure 5-5 Proposed Planning Overlays 30 Figure 6-2 Proposed Flood Class Level Extents for the Leigh River at Inverleigh 31 Figure 7-1 Properties Flooded Above Floor (Combined 1% AEP events) 35
LIST OF TABLES Table 2-1 Summary of Previous Flood Study reports 11 Table 2-2 Historic Flood Event Information 11 Table 2-3 LiDAR Survey Verification 13 Table 2-4 Active Daily Rainfall Gauges in the Barwon, Leigh, Warrambine and Woady Yaloak
Catchments 14 Table 2-5 Pluviograph Rainfall Stations in the broader study area 15 Table 2-6 Streamflow Gauge Record Summary 17 Table 4-1 Adopted Peak Flows for Design Events 21 Table 4-2 Design Event Combinations 21 Table 4-3 Summary of Flood Behaviour for Various Flood Events on the Leigh River 23 Table 4-4 Summary of Flood Behaviour for Various Flood Events on the Barwon River 25
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GLOSSARY OF TERMS Annual Exceedance Refers to the probability or risk of a flood of a given size occurring or being Probability (AEP) exceeded in any given year. A 90% AEP flood has a high probability of occurring or being exceeded; it would occur quite often and would be relatively small. A 1% AEP flood has a low probability of occurrence or being exceeded; it would be fairly rare but it would be of extreme magnitude.
Australian Height Datum A common national surface level datum approximately corresponding to (AHD) mean sea level. Introduced in 1971 to eventually supersede all earlier datums.
Average Recurrence Refers to the average time interval between a given flood magnitude Interval occurring or being exceeded. A 10 year ARI flood is expected to be (ARI) exceeded on average once every 10 years. A 100 year ARI flood is expected to be exceeded on average once every 100 years. The AEP is the ARI expressed as a percentage.
Cadastre, cadastral base Information in map or digital form showing the extent and usage of land, including streets, lot boundaries, water courses etc.
Catchment The area draining to a site. It always relates to a particular location and may include the catchments of tributary streams as well as the main stream. Design flood A design flood is a probabilistic or statistical estimate, being generally based on some form of probability analysis of flood or rainfall data. An average recurrence interval or exceedance probability is attributed to the estimate.
Discharge The rate of flow of water measured in terms of volume over time. It is to be distinguished from the speed or velocity of flow, which is a measure of how fast the water is moving rather than how much is moving.
Flood Relatively high stream flow which overtops the natural or artificial banks in any part of a stream, river, estuary, lake or dam, and/or overland runoff before entering a watercourse and/or coastal inundation resulting from elevated sea levels and/or waves overtopping coastline defences.
Flood frequency analysis A statistical analysis of observed flood magnitudes to determine the probability of a given flood magnitude.
Flood hazard Potential risk to life and limb caused by flooding. Flood hazard combines the flood depth and velocity.
Floodplain Area of land which is subject to inundation by floods up to the probable
maximum flood event, i.e. flood prone land.
Flood storages Those parts of the floodplain that are important for the temporary storage, of floodwaters during the passage of a flood.
Geographical information A system of software and procedures designed to support the management, systems (GIS) manipulation, analysis and display of spatially referenced data. Hydraulics The term given to the study of water flow in a river, channel or pipe, and the evaluation of flow parameters such as stage and velocity.
Hydrograph A graph that shows how the discharge changes with time at any location.
Hydrology The term given to the study of the rainfall and runoff process as it relates to
the derivation of hydrographs for given floods. R07_V02c_Summary_Report_Inverleigh_FS
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Intensity frequency Statistical analysis of rainfall, describing the rainfall intensity (mm/hr), duration (IFD) analysis frequency (probability measured by the AEP), duration (hrs). This analysis is used to generate design rainfall estimates.
LiDAR Spot land surface heights collected via aerial light detection and ranging (LiDAR) survey. The spot heights are converted to a gridded digital elevation model dataset for use in modelling and mapping.
Peak flow The maximum discharge occurring during a flood event.
Probability A statistical measure of the expected frequency or occurrence of flooding. For a fuller explanation see Average Recurrence Interval.
Probable Maximum Flood The flood that may be expected from the most severe combination of critical (PMF) meteorological and hydrologic conditions that are reasonably possible in a particular drainage area.
RORB A hydrological modelling tool used in this study to calculate the runoff generated from historic and design rainfall events.
Runoff The amount of rainfall that actually ends up as stream or pipe flow, also known as rainfall excess.
Stage Equivalent to 'water level'. Both are measured with reference to a specified datum. Stage hydrograph A graph that shows how the water level changes with time. It must be referenced to a particular location and datum.
Topography A surface which defines the ground level of a chosen area.
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1 INTRODUCTION
This report is one of a series of reports documenting the outcomes of the Flood Risk Management Study on the Leigh and Barwon Rivers at Inverleigh. These analyses have been documented in a series of reports which are summarised below:
◼ R01 - Data Review & Project Inception Report (Water Technology 2017a)
◼ R02 – Hydraulic Calibration Report (Water Technology 2017b)
◼ R03 – Hydrology Summary (Water Technology 2018a)
◼ R04 – Flood Mitigation Report (Water Technology 2018b)
◼ R05 – Flood Intelligence, Warning and Planning Report (Water Technology 2018c)
◼ R06 – Study Report (Water Technology 2018d) This Report This report (R06) summarises the key findings of the study and the previous five reports including the hydrological and hydraulic analysis, mitigation modelling and draft planning layers. The reporting is supported by a number of standalone PDF flood maps and digital deliverables.
1.1 Study Area
This project has considered flood risk at the town of Inverleigh, a growing population located at the confluence of the Barwon and Leigh Rivers, shown in Figure 1-1. Inverleigh has experienced extensive flooding in its history which has hindered further development and presents a risk to a number of heritage values in the town. Complex hydrology exists within the study area as flood peaks from the Barwon and Leigh Rivers can occur at different times due to the large 2,700 km² combined catchment upstream of Inverleigh. The catchment stretches over 130 km from the Otway Ranges in the south to Ballarat in the north, storms can impact different parts of the catchment in isolation. The catchment area is shown in Figure 1-1.
Warrambine Creek flows into the Barwon River upstream of Inverleigh. This is considered the third waterway contributing to flows and complexity to Inverleigh flood risk. Warrambine Creek is a much smaller waterway compared with the Barwon River and Leigh River. It also receives diversion flows from the Woady Yaloak diversion channel. The Woady Yaloak Creek is diverted before it flows into Cundare Pool and channelled south of Lake Weering, across to Warrambine Creek approximately 13-14 km upstream of where the Warrambine Creek flows into the Barwon River on the outskirts of Inverleigh. Inflows from the diversion scheme need to be accounted for in any flood modelling at Inverleigh. Some recent work undertaken by the Corangamite CMA has estimated the local catchment flows from the Woady Yaloak catchment. More detail on flows derived from the catchment can be found in the previous Hydraulic Calibration report (R02) and the Hydrology Summary
Report (R03).
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FIGURE 1-1 BARWON RIVER, LEIGH RIVER AND WOADY YALOAK CREEK CATCHMENTS
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1.2 Hydraulic Model and Mapping Extent
Water Technology have extended the hydraulic model extent upstream and downstream to the location of relevant streamflow gauges. This has ensured the study outputs to be tied directly into the active streamflow gauges to improve flood intelligence and warning times to the gauge network. This was achieved by using a combination of 1D/2D hydraulic modelling reaches to route the flow from the upstream gauges to the study area and through to the downstream gauge on the Barwon River at Pollocksford. The additional model area and mapping extent is shown in Figure 1-2.
Previous flood modelling was completed by Water Technology in 2007 and has provided a basis for comparison of the results of this study along with the Barwon Regional Flood Mapping undertaken by GHD in 2016. It is recognised that new accurate mapping of Inverleigh is critical to the effectiveness of flood warning systems on the Barwon and Leigh Rivers as well as the implementation of updated planning controls for the area. Further information on the calibration of the hydraulic model can be found in the previous Calibration report (R02) and Hydrology Summary report (R03).
FIGURE 1-2 MAPPING AREA AND ADDITIONAL WATERWAYS MODELLED IN 1D
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2 DATA REVIEW
Shortly after the project inception meeting a detailed review was undertaken of all available flood related information as well as topographic data, structure information, and hydrological data. Details of this review are provided in the Data Review (R01), while a short overview is provided below.
2.1 Existing Flood Studies
Several flood studies and investigations into the Inverleigh township and waterway catchments have been undertaken in recent years. Table 2-1 summarises the available flooding and hydrology reports that were reviewed as part of this study.
TABLE 2-1 SUMMARY OF PREVIOUS FLOOD STUDY REPORTS
Year Study Type Notes CCMA Flood Mapping Hydraulic modelling of Inverleigh using Hydraulic Modelling & 2007 (Inverleigh & CCMA hydrology. Used for planning Mapping Winchelsea) scheme to date. (Water Technology) The modelling was completed at a Hydrology and Hydraulic regional scale and produced updated Barwon Regional 2016 Modelling of Leigh River mapping for several design events from Study and Barwon River the 20% AEP up to the 0.5% AEP. (GHD) Leigh River Flood Hydrology Investigation of Calibrated RORB model to 5 events and 2017 Study the Leigh River FFA assessment. (CCMA)
2.2 Historic Flood Information
Historic flood data recorded in the Victoria Flood Database (VFD) is summarised in Table 2-2 below. The flood extent for 1973 was derived from aerial photography, and the extent for 1995 was based on detailed flood mapping. Further historical information from the Corangamite CMA, Golden Plains Shire, steering committee members and the community (including photos and anecdotal evidence) was also collated. Using the information available, the hydraulic model was calibrated to the 1973, 1995 and 2011 flood events.
TABLE 2-2 HISTORIC FLOOD EVENT INFORMATION
Event Description Data Available
Largest recorded flood on Barwon Photos, single flood mark at shearing shed 1952 River (not surveyed) 1973 Largest recorded flood on Leigh River Surveyed flood marks, aerial photos 1976 Leigh River flood Surveyed flood marks 1978 - Leigh River flood Surveyed flood marks 1995 - November Barwon River flood Aerial photos 2011 - January Leigh River flood Aerial photos, survey marks
2016 - September Photography of flood debris marks R07_V02c_Summary_Report_Inverleigh_FS
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2.3 Topographic Data
2.3.1 Available Data Sets
Three main LiDAR data sets were available to use for the generation of the Digital Terrain Model (DTM) for the hydraulic modelling. The three datasets included a 5 m resolution dataset (NAP LiDAR) from 2003, 1 m resolution dataset (ISC Rivers) flown in 2010 and a 1 m resolution dataset (Golden Plains LiDAR) flown in 2014, Figure 2-1. Bathymetry of the river confluence was captured to provide an estimate of channel capacity (not well represented in the LiDAR). Culvert and bridge structure information was also collected in the field to include important hydraulic structures within the hydraulic modelling.
FIGURE 2-1 EXTENT OF LIDAR DATASETS
2.3.2 LiDAR Verification
Additional field survey (2x 100 m long transects at 5 m spot intervals) previously captured along two roads at Inverleigh) including several transects was available to verify the Lidar data and is shown in Figure 2-2 and
Figure 2-3 and summarised in Table 2-3. The Golden Plains LiDAR (flown in 2014) matched extremely well to the transect survey, with a mean difference of -0.038 m and a standard deviation of 0.038 m. The ISC Rivers LiDAR (flown in 2010) was consistently higher with a mean difference of 0.203 m compared to the transect survey. As a result, the ISC Rivers dataset was lowered by 200 mm. The NAP LiDAR (flown in 2003) dataset was also lowered by 200 mm to provide a closer match to the other datasets outside of the township. The NAP
LiDAR was only required for a small area of the model domain located on the edge of the floodplain. R07_V02c_Summary_Report_Inverleigh_FS
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FIGURE 2-2 TRANSECT SURVEY ON THE INVERLEIGH-WINCHELSEA ROAD
FIGURE 2-3 TRANSECT SURVEY ON NAPIER ST (IN INVERLEIGH)
TABLE 2-3 LIDAR SURVEY VERIFICATION
NAP (2003) ISC Rivers (2010 ) Golden Plains (2014) Min -0.173 0.127 -0.102
Max 0.245 0.268 0.016 Mean 0.017 0.203 -0.038 St. Dev 0.098 0.040 0.038
2.3.3 DEM Development
The DEM used for the hydraulic model was developed with the Golden Plains LiDAR dataset as the preferential dataset followed by the adjusted ISC Rivers LiDAR dataset (lowered by 200 mm), with a small remaining area
on the edge of the floodplain using the NAP LiDAR dataset. R07_V02c_Summary_Report_Inverleigh_FS
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2.4 Hydraulic Structure Information
Survey of the Bells Bridge and Hamilton Highway bridge was collected during the 2007 flood study and was available for use in the hydraulic model. The survey was verified with a site visit where a number of other culverts in the township were measured and included in the hydraulic model.
2.5 Hydrological Data 2.5.1 Daily Rainfall Data
A total of 64 gauge records within the four study catchments were identified from the BoM online Weather Station Directory (BoM, 2017) dating back to 1856 (Scotsburn, Mt. Buninyong). There are currently 17 active daily rainfall gauges across the four catchment regions which are listed in Table 2-4. In the broader region, there are close to 50 daily rainfall gauges which can be used to generate spatial distribution patterns for historic rainfall events, the locations of these are shown in Figure 2-4.
TABLE 2-4 ACTIVE DAILY RAINFALL GAUGES IN THE BARWON, LEIGH, WARRAMBINE AND WOADY YALOAK CATCHMENTS
Gauge Location Rainfall Station Number Start Date Years of Record
Ballarat Aerodrome 89002 1908 109
Ballarat Hopetoun Rd 89111 2004 13
Barwon Downs (Gerangamete) 90189 2001 16
Benwerrin 90188 2000 17
Birregurra (Post Office) 90008 1903 114
Boonah 90187 2001 16
Buckley (Balliwindi) 87124 1968 49
Bungaree (Kirks Reservoir) 87014 1881 136
Colac (Mount Gellibrand) 90035 2000 17
Cressy 89010 1901 116
Forrest State Forest 90040 1898 119
Glen Park (White Swan Reservoir) 89048 1953 64
Leigh River @ Mount Mercer 89104 2000 17
Pennyroyal Creek 90061 1885 132
Rokewood 89068 1908 109
Scotsburn (Mount Buninyong) 87046 1856 161
Warrambine Ck At Warrambine 89084 1972 45 R07_V02c_Summary_Report_Inverleigh_FS
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2.5.2 Sub-Daily Rainfall Data
The Bureau of Meteorology (BoM) have six current rainfall stations that record rainfall in 6 minute increments which are relevant to the study area. Three other pluviograph stations in the broader catchment area have been closed since 1995, however provide important information for historic flood events between 1954 and 1995 A summary of the pluviograph stations in the broader study area are listed in Table 2-5 while a number of sub-daily gauges in the broader region including those listed in Table 2-5 are shown in Figure 2-4. The data collected from these pluviograph stations allows for temporal patterns associated with historic rainfall events to be developed across the catchment.
TABLE 2-5 PLUVIOGRAPH RAINFALL STATIONS IN THE BROADER STUDY AREA
Pluviograph Station Station Number Period of Record Years of Record
Ballarat Aerodrome 89002 1954-1999 45
Blackwood 87017 1974-2017 43
Bullengarook East 87075 1966-2017 51
Colac (Mt Gellibrand) 90035 2000-2017 17
Geelong North 87133 1968-1995 27
Little River 87033 1965-2017 52
Melton 87039 1984-2017 33
Weeaproinah 90083 1974-2017 43
Winchelsea Shire Depot 90166 1974-1997 23
Wyelangta 90087 1952-2017 65
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R07_V02c_Summary_Report_Inverleigh_FS FIGURE 2-4 DAILY AND SUB-DAILY RAINFALL GAUGES IN THE BROADER CATCHMENT REGION
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2.5.3 Streamflow Data
An extensive streamflow data network exists throughout the Barwon, Leigh and Warrambine Creek systems with key gauge locations shown in Figure 2-5. The record of data available and quality of the data ranges across the network, however provides a good representation of streamflow conditions and key flood events at Inverleigh. The period of record for all streamflow data within the catchments is shown in Table 2-6, BoM quality codes are included in Appendix A.
A full summary of data quality for the three systems is shown in Appendix A and uses available data up to April 2017.
TABLE 2-6 STREAMFLOW GAUGE RECORD SUMMARY
Streamflow Gauge Record Quality A Data Missing Data
Barwon River @ Ricketts Marsh 1971-2017 94.00% 1.03%
Barwon River @ Winchelsea 233201 2000 – 2017 97.58% 0.10%
Barwon River @ Inverleigh 233218B 1966 – 2017 97.29% 0.14%
Barwon River @ Pollocksford 233200B 1969-2017 94.42% 0.39%
Leigh River @ Mt Mercer 233215A 1956-2017 96.74% 0.05%
Leigh River @ Shelford 233213A 1954-2017 7.20% 77.94%1
Leigh River @ Shelford (Golf Hill) 233248A 1994-2012 97.87% 0.03%
Warrambine Creek @ Warrambine 233223 1970-2017 44.77% 0.12%
Birregurra Creek @ Ricketts Marsh 1953-2017 88.85% 0.74%
1The Leigh River @ Shelford 233213A was inactive between 1963-2011. If this data is excluded Quality A = 28.89% and Missing Data = 5.74%
A review of the streamflow gauges within the study area was carried out during the project as well as survey of gauge zero levels for the Leigh River at Shelford (#233213) and Leigh River at Inverleigh streamflow gauges. A review of the Leigh River at Shelford gauge identified several inconsistencies with the current rating table at high flows. Additional work on the Shelford streamflow gauge rating curve was carried out for the Corangamite CMA at the same time as this investigation and helped inform the flood intelligence information of this study.
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R07_V02c_Summary_Report_Inverleigh_FS FIGURE 2-5 KEY STREAMFLOW GAUGES FOR THE STUDY
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3 PROJECT CONSULATION
An important element of the flood mapping study was the active engagement of residents in the study area. This engagement was developed over the course of the study through community consultation sessions and meetings with a Steering Committee. The aims of the community consultation were as follows:
◼ To raise awareness of the study and to identify key community concerns; and
◼ To provide information to the community and seek their feedback/input regarding the study outcomes including the existing flood behaviour and proposed flood mapping extents.
3.1 Steering Committee
The Inverleigh Flood Study incorporated a Project Steering Committee group that consisted of representatives from the Golden Plains Shire, Corangamite CMA, VicSES, DELWP as well as representatives from the local community. Water Technology attended three Project Steering Committee meetings to present on the status of the project and the findings of the study.
3.2 Community Consultation
Further community consultation was carried out with representatives from the community attending an afternoon and evening session in August 2017 at which Water Technology and Corangamite CMA staff gained historical flood information and discussed the project with community members. This data was used to help validate the hydraulic model results for the historical flood events modelled. An online mapping portal was also provided on the Golden Plains Shire “Have Your Say” website.
Additionally, letters were sent to residents who were likely to be directly impacted by the results of the flood study. This included land holders whose properties were likely to be included the proposed Land Subject to Inundation Overlay and Floodway Overlays.
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4 FLOOD BEHAVIOUR
4.1 Overview
Inverleigh can be directly impacted by riverine flooding from the Leigh River and Barwon River. The majority of the township is affected by flooding from the Leigh River. Flooding from the Leigh River usually occurs due to prolonged heavy rainfall in the upper catchment around Mt Mercer and Ballarat. Flooding from the Barwon River is usually a result of prolonged heavy rainfall in the upper catchment in the Otway Ranges. Localised rainfall around the township is likely to cause flash flooding issues but will generally cause only a minor rise in Leigh River and Barwon River levels. Flows from the Warrambine Creek catchment are relatively minor compared to flows in the Barwon River. Additional flow from the Woady Yaloak Diversion Channel are minor and are insignificant compared with flows from the Barwon River at the Warrambine Creek outfall.
The flood behaviour associated with catchment flooding mechanisms has been assessed using a range of industry standard approaches and tools:
◼ Hydrological analysis – this involved the analysis of the magnitude of previous flood events in the catchment, the use of an existing rainfall-runoff model for the Leigh River catchment, flood frequency analysis of a streamflow gauge on the Barwon River and the prediction of the likelihood of future flood events of a given magnitude.
◼ Hydraulic analysis – showed how a given flood event may behave as the Leigh River or Barwon River flows break out of bank. The impacts of varying flow regimes where a dominant water way flood occurs and the joint probability of a Leigh and Barwon River flood occurring at the same time was modelled. This was based on existing joint probability work carried out by the Corangamite CMA in the Leigh River Flood Study (2017). A hydraulic model was used to predict the extent of flooding, flood depths and flow velocities for a range of possible future flood events. The different flood mechanisms and the results of the hydrologic and hydraulic analysis for the study area are discussed in detail in the following sections.
4.2 Hydrology Summary
The current flood study has built on and utilised existing information that has been developed through a range of previous studies. The following section summarises the hydrological analysis undertaken in this study as well as previous investigations within the Barwon, Leigh and Warrambine Catchments. Detailed Information on the Hydrology can be found in the Hydrology Summary Report (R03).
The Leigh River hydrology investigation undertaken by Corangamite CMA (2017) resulted in an increase in the 1% AEP design flow estimate for the Leigh River at Shelford compared to the previous design flow estimate
used for the flood modelling project carried out in 2007 (Water Technology, 2007). There was an increase in
the 1% AEP peak flow for the Barwon River at Inverleigh and an increase in peak flow rates in the Warrambine Creek at the Barwon River outfall. The Barwon River increase was a result of additional streamflow estimates for the 1951 and 1952 flood events on the Barwon River and the extension of the streamflow record back to 1922. The increase in design flows for the Warrambine Creek were based on hydrological modelling (RORB) developed by the Corangamite CMA.
A summary of the design flow rates for the three waterways is shown in Table 4-1. The adopted design scenarios for all design events have been adopted from the Leigh River Flood Study (Corangamite CMA, 2017). The previous work found the Barwon River floods have a moderate correlation with Leigh River flows, but Leigh River floods have a low correlation with Barwon River flows. A stronger correlation of flows exists between the Leigh River and Warrambine Creek flood events. This is likely a result of the Leigh River and Warrambine catchment proximities to each other in comparison to the Barwon River catchment which is much
R07_V02c_Summary_Report_Inverleigh_FS further south. A summary of the concurrent events is shown in Table 4-2.
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 20
Warrambine Creek can also receive additional flows form the Woady Yaloak Diversion scheme. The Woady Yaloak Diversion channel has a capacity of around 490 ML/d and the Lough Calvert drain is even smaller. These flows are minor compared to the flows from the Barwon River at Winchelsea during major flood events. In the 1995 event for example the peak flow at Winchelsea was around 50,000 ML/d, while the Leigh River flows at Mt Mercer was around 12,000 ML/d for the same event. In such events the Woady Yaloak Diversion, even if flowing at capacity, would contribute less than 1% of total flow at Inverleigh.
The Probable Maximum Flood (PMF) is the flow generated from the theoretical maximum precipitation for a given duration under current climate conditions. Table 4-1 includes an estimate of the PMF for the three waterways. This was calculated using the “Quick Method” of Nathan as recommended in ARR2016 as the design flows are based on a flood frequency analysis method. This method is suitable as the RORB model for all upper catchments was not available, which is required for a GSAM assessment.
TABLE 4-1 ADOPTED PEAK FLOWS FOR DESIGN EVENTS
Barwon River at Leigh River at Shelford Warrambine Ck (US of Design Event Inverleigh (ML/d) (ML/d) Barwon River) (ML/d) 50% AEP 6,700 2,300 2,000 20% AEP 13,900 6,200 4,100 10% AEP 21,400 10,600 6,100 5% AEP 31,500 16,600 9,100 2% AEP 50,200 27,800 13,600 1% AEP 69,900 39,000 18,200 0.5% AEP 95,900 54,250 23,000 0.2% AEP 143,200 80,400 30,100 0.1% AEP 192,100 106,300 39,700 PMF 915,000 740,000 686,000
TABLE 4-2 DESIGN EVENT COMBINATIONS
Design Event on Leigh River Dominant Flood Barwon River Dominant Flood Dominant River Probability of concurrent flow for minor Probability of concurrent flow for minor tributaries tributaries Barwon River Warrambine Ck Leigh River Warrambine Ck
0.1% AEP 2% 0.2% 1% 1% 0.2% AEP 5% 0.5% 2% 2% 0.5% AEP 10% 0.77% 3.28% 3.28% 1% AEP 20% 1.31% 5% 5% 2% AEP 28.35% 2.15% 6.89% 6.89% 5% AEP 50% 5% 13.31% 13.31% 10% AEP 50% 10% 22.12% 22.12% 20% AEP 50% 20% 22.12% 22.12%
50% AEP 50% 50% 50% 50% R07_V02c_Summary_Report_Inverleigh_FS
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4.3 Hydraulics Summary
This section discusses the application of the hydraulic model to simulate flood behaviour (extents, depth, velocities) for a range of flood magnitudes. The hydrologic analysis previously discussed, provided flood inflow hydrographs for the hydraulic model. These inflow hydrographs were routed through the calibrated hydraulic model. This enabled the modelling of flood depths, extents and velocities over a range of flood magnitudes. It also provided a tool for understanding the flood behaviour across the study area. A detailed description of the hydraulic model setup, calibration, validation, sensitivity tests and design event simulation is provided in the Hydraulic Calibration Report (R02) and Hydrology Summary Report (R03). This section summaries the general model development and key outcomes from the hydraulic modelling investigation.
4.3.1 Hydraulic Modelling
A dynamic 1D/2D hydraulic model was developed using the TUFLOW modelling package. The main river channels of the Barwon and Leigh Rivers and Warrambine Creek within the mapping extent were modelled as 1D components. The remainder of the floodplain and township was modelled using a 2D grid at a 5 m resolution. The Leigh River upstream of the mapping extent (Inverleigh-Teesedale Road) to Shelford and the Barwon River downstream of the weir structure to the Pollocksford streamflow gauge were modelled entirely as 1D components. These sections were included in the hydraulic model in order to achieve better timings in relation to existing streamflow gauges. The extent for which all flood mapping deliverables were prepared, is shown in Figure 4-1. A number of culvert structures within the Inverleigh township and to the west of the township along the Hamilton Highway were modelled as 1D structures.
The modelling process involved the following stages:
◼ Model setup and calibration to three historic, calibration events with surveyed flood marks (1973, 1978 & 2011).
◼ Validation of model results to photos and community consultation of two historic events (1995 & 2016).
◼ Sensitivity tests (blockage of Hamilton Highway bridge, joint probability of non-dominant waterway).
◼ Design flood simulations (events from 50% AEP through to 0.1 AEP and the PMF event).
The calibration, validation, and sensitivity assessments are an iterative investigative process and all outcomes
from these stages inform the final design flood simulations.
R07_V02c_Summary_Report_Inverleigh_FS
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FIGURE 4-1 FLOOD MAPPING EXTENT
4.3.2 Understanding Flood Behaviour
Table 4-3 provides a summary of key flood behaviour and roads within the study area that are inundated in flood events up to a 0.2% AEP flood event. They are shown in likely order of inundation (i.e. roads that are likely to flood first are shown first). The table was developed to be read from top to bottom, with each subsequent larger magnitude event reporting on the incremental changes in consequences. For example, if the reader is wishing to understand the consequences of a 2% AEP event, then the flood characteristics should be read for the 20%, 10%, 5% and 2% AEP events in succession. It is also recommended that the reader refer to the standard PDF maps provided with this study. There is a separate map for each modelled design event providing peak flood depths, extents and water surface elevations. After completion of the study, it is recommended that the mapping be incorporated into the Victoria Flood Database (VFD) and uploaded to FloodZoom. The flood levels within the table refer the flood class levels currently provided for the Leigh River at Shelford. As discussed in the Flood Warning, Intelligence and Planning Report (R05) it is recommended that the current flood class levels be reviewed and adopted to be in line with flood consequences at Inverleigh.
TABLE 4-3 SUMMARY OF FLOOD BEHAVIOUR FOR VARIOUS FLOOD EVENTS ON THE LEIGH RIVER
Design Gauge Leigh Event for Height for River at Consequences Leigh River Leigh River Inverleigh at Shelford at Shelford 50% AEP 2.00 m 1.60 m ~2,600 ML/d
20% AEP 5.53 m 3.00 m Teesdale – Inverleigh Road overtopped (depth below 0.3 m) ~6,000 ML/d
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Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 23
Design Gauge Leigh Event for Height for River at Consequences Leigh River Leigh River Inverleigh at Shelford at Shelford
7,800 ML/d 6.00 3.30 Minor Flood Level at Shelford
10% AEP Hamilton Hwy culverts west of town begin to flow (potential erosion at old Hwy alignment). ~11,000 ML/d 6.64 m 3.92 m Teesdale – Inverleigh Road inundated (depth above 0.3 m).
Park and River reserve between township and confluence begin to inundate.
12,500 ML/d 7.00 m 4.30 m Moderate Flood Level at Shelford
Federation Bridge inaccessible. 5% AEP 7.63 m 4.62 m Dawber Road overtopped (depth below 0.3 m). ~17,000 ML/d Burkes Road overtopped (depth below 0.3 m).
18,500 ML/d 8.00 m 4.76 m Major Flood Level at Shelford
Private property north of Cambridge Street inundated. Northern football ground inundated. Levee located beside the tennis courts and Inverleigh Primary School overtopped. Private property in town and to the west of town inundated. Railway culverts at East Street begin to flow. 2% AEP Hamilton Highway west of town overtopped (depth below 0.3 m). ~28,000 ML/d 8.31 m 5.53 m Cambridge St overtopped (depth below 0.3 m).
Hamilton Hwy west of town inundated (depth below 0.5 m). Cambridge St inundated (depth below 0.5 m). High St inundated (depth below 0.5 m). Newman St inundated (depth below 0.5 m).
East St inundated (depth below 0.5 m). Terrier Road inundated (depth below 0.5 m). 5 houses flooded above floor (in and west of town). 1% AEP Inverleigh-Winchelsea Road overtopped (depth below 0.3 m). ~39,000 ML/d 8.44 m 6.00 m High St inundated (depth below 0.5 m).
Significant external property damage. 50 houses experience external property damage as well as a 1973 flood number of shops. event 8.47 m 6.20 m Inverleigh-Winchelsea Road inundated (depth below 0.5 m). ~48,000 ML/d
Inverleigh-Shelford Road inundated (depth below 0.5 m). R07_V02c_Summary_Report_Inverleigh_FS
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 24
Design Gauge Leigh Event for Height for River at Consequences Leigh River Leigh River Inverleigh at Shelford at Shelford 33 houses flooded above floor (in town and west of town) & another 59 within 150 mm of above floor flooding. Recreation Reserve building flooded above floor. 0.5% AEP 8.50 m 6.40 m CFA shed inundated. ~54,000 ML/d Inverleigh-Winchelsea Road cut (depth above 0.5 m) Hamilton Hwy west of town cut (depth above 0.5 m). High St cut (depth above 0.5 m) 82 houses flooded above floor (in and west of town) 0.2% AEP Above Police Station Inundated extrapolated 6.97 m ~80,000 ML/d rating curve. CFA Shed flooded above floor Inverleigh-Shelford Road cut (depth above 0.5 m) Leigh River at Shelford Gauge Zero = 82.77 m AHD Leigh River at Inverleigh Gauge Zero = 54.00 m AHD
Table 4-4 provides a similar summary of flood behaviour as provided previously for the Leigh River dominated flooding, but for a Barwon dominated flood.
TABLE 4-4 SUMMARY OF FLOOD BEHAVIOUR FOR VARIOUS FLOOD EVENTS ON THE BARWON RIVER
Barwon River at Consequences Inverleigh 50% AEP Park and River reserve between Winchelsea-Inverleigh Road and confluence 2.55 m begin to inundate. ~6,700 ML/d 20% AEP Anabranches along Barwon floodplain (upstream of confluence) begin to flow. 3.42 m River Road overtopped (depth below 0.3 m). ~13,900 ML/d 10% AEP Water backing up against the Inverleigh – Winchelsea Road River Road 3.99 m Inundated (depth below 0.5 m). ~21,400 ML/d
A number of rural properties to the south of Inverleigh begin to flood.
5% AEP Inverleigh-Winchelsea Road overtopped (depth below 0.3 m). 4.57 m Costains Road overtopped (depth below 0.3 m). ~31,500 ML/d Rawson Road overtopped (depth below 0.3 m).
River Road cut (depth above 0.5 m). R07_V02c_Summary_Report_Inverleigh_FS
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Barwon River at Consequences Inverleigh Railway culverts at East Street begin to flow (north towards town). Properties between McCallum Road and River Road inundated. 1 property flooded above floor level. 2% AEP Inverleigh-Winchelsea Road cut (depth above 0.5 m). 5.38 m Fuller Road overtopped (depth below 0.3 m). ~50,200 ML/d Weatherboard Road overtopped (depth below 0.3 m). Jubilee Street cut (depth above 0.5 m). Railway Street cut (depth above 0.5 m). Saleyard Road overtopped (depth below 0.3 m). Rawson Road cut (depth above 0.5 m) Inverleigh township cut (Hamilton Highway & Inverleigh – Winchelsea Road). 1995 flood event Bells Bridge cut (depth above 0.5 m). 5.59 m Houghton Lane cut (depth above 0.5m). ~55,600 ML/d Saleyard Road cut (depth above 0.5 m). 2 properties flooded above floor level. 1% AEP Properties within Inverleigh township start to experience external damage. 6.04 m Fuller Road cut (cut above 0.5 m). ~69,900 ML/d Mahers Road overtopped (depth below 0.3 m).
Weatherboard Road cut (depth above 0.5 m). Flooding between Houghton Lane and Park Street 0.5% AEP 9 properties flooded above floor level. Above extrapolated Extensive flood damage through the Inverleigh township. rating curve. High St overtopped (depth less than 0.3 m). ~95,900 ML/d Mahers Road cut (depth above 0.5 m).
Days Road cut (depth above 0.5 m). 0.2% AEP Extensive flooding along Park Street. Above extrapolated 133 properties flooded above floor level. rating curve. Railway line overtopped.
~143,200 ML/d
R07_V02c_Summary_Report_Inverleigh_FS
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5 ASSESS AND TREAT RISK
5.1 Structural Mitigation
A report detailing the existing 1% AEP flood risk and options to treat the flood risk within Inverleigh (R04) was produced following the hydraulic calibration and hydrology summary. Two structural mitigation options assessments were presented to the steering committee and community meeting held in November 2017. The two structural mitigation options are listed below and covered in more detail later in this section. A third option (a levee on the southern edge of Inverleigh) to protect flooding from the Barwon River was also assessed.
Based on the 1% AEP mapping, the implementation of a levee on the south of the township did not appear to provide significant reduction in flood risk and will also likely adversely impact properties outside of the levee. Based on this and a low ranking in the prefeasibility assessment this option did not undergo detailed modelling.
◼ Option 1 - involved increasing the height of the existing levee near the school to above the 1% AEP flood level. This incorporates a minimum 300 mm freeboard above the 1% AEP flood level (Figure 5-1).
◼ Option 2 - involved increasing the existing levee to above the 1% AEP flood level (with a 300 mm freeboard) and extending the levee east following Cambridge Street to the Hamilton Highway Bridge
(Figure 5-2).
FIGURE 5-1 1% AEP DEPTHS FOR MITIGATION OPTION 1 R07_V02c_Summary_Report_Inverleigh_FS
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 27
FIGURE 5-2 1% AEP DEPTHS FOR MITIGATION OPTION 2
Option 1 reduces above floor flooding to 25 buildings while removing 19 parcels of land within Inverleigh from the 1% AEP flood extent. This option raises the existing informal levee/embankment at the tennis courts to above the 1% AEP flood level and is estimated to cost approximately $19,000. A flood level difference plot for Option 1 (Figure 5-3) shows the change in flood levels for the 1% AEP flood event. This results in a reduction in flood extent (pink), the reduction in flood levels (green) and the increased flood levels (yellow-orange) as a
result of the Option 1 earthworks.
FIGURE 5-3 FLOOD LEVEL DIFFERENCE PLOT FOR MITIGATION OPTION 1 R07_V02c_Summary_Report_Inverleigh_FS
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 28
Option 2 reduces above floor flooding to 25 buildings while removing 45 parcels of land within Inverleigh from the 1% AEP flood extent. This option raises the existing informal levee/embankment at the Tennis courts and constructs a new levee from the Inverleigh Primary School along Cambridge Street to the Hamilton Highway Bridge to above the 1% AEP flood level. This option is estimated to cost approximately $327,000.
A flood level difference plot for Option 2 (Figure 5-4) shows the change in flood levels for the 1% AEP flood event. This results in a reduction in flood extent (pink), the reduction in flood levels (green) and the increased flood levels (yellow-orange) as a result of the Option 2 earthworks.
Buildings with minor increased flood levels
FIGURE 5-4 FLOOD LEVEL DIFFERENCE PLOT FOR MITIGATION OPTION 2
Both these options reduce the flood risk to a number of properties within Inverleigh with minimal increase to flood risk at existing buildings. It is recommended that if the Golden Plains Shire wish to pursue further investigation of mitigation options, a detailed mitigation study involving full benefit-costing analysis, detailed modelling, considerations for long term maintenance of the structure, an assessment of cultural heritage and environmental impacts of such works, and extensive consultation with impacted landholders be completed.
5.2 Planning Controls
An assessment of the existing planning controls for Inverleigh was undertaken by Planning and Environmental Design resulting in a document outlining a number of recommended planning scheme amendments which could be implemented to further treat flood risk within Inverleigh. Further planning outputs for the project include revised draft Floodway Overlay (FO) and Land Subject to Inundation Overlay (LSIO) extents shown in Figure 5-5. The LSIO is based on the flood extent of the 1% AEP design flood modelling and mapping produced during this study. The FO delineation is in line with the Australian Rainfall and Runoff 2016 Revision – Project 10 Safety Criteria for vehicles. These criteria assess depth and velocity of water over the area in question during a 1% AEP flood event as follows:
◼ Depth no greater than or equal to 0.3 metres; and
◼ Velocity no greater than or equal to 3.0 metres per second
R07_V02c_Summary_Report_Inverleigh_FS ◼ The product of depth multiplied by velocity no greater than or equal to 0.3 m2 per second
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 29
FIGURE 5-5 PROPOSED PLANNING OVERLAYS
R07_V02c_Summary_Report_Inverleigh_FS
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5.3 Flood Warning
The effective warning time for flooding at the Inverleigh township catchment varies between flood events. Travel times from the upstream streamflow gauges in the Leigh River catchment include between 10-16 hours from the Leigh River at Mt Mercer streamflow gauge and 4-6 hours from the Leigh River at Shelford streamflow gauge. Travel times from the upstream streamflow gauges in the Barwon River catchment range from 16-20 hours from the Barwon River at Ricketts Marsh streamflow gauge, 8-12 hours from the Barwon River at Winchelsea streamflow gauge and 3-5 hours from the Barwon River at Inverleigh streamflow gauge.
Proposed Flood Class Levels for Inverleigh
The establishment of Minor, Moderate and Major Flood Class Levels at Inverleigh will help provide information for Inverleigh residents as well as providing additional streamflow information between Shelford and Pollocksford. Based on the modelling undertaken, the following flood class levels for the Leigh River at Inverleigh are recommended:
◼ Minor Flood Level = 4.70 m
◼ Moderate Flood Level = 5.50 m
◼ Major Flood Level = 6.00 m
Figure 5-6 shows the flood extent for the three proposed flood class levels. The minor flood class level has minimal extent outside of the river channel through the township, with some private farmland and river frontage inundated. The Moderate Flood Class level shows private property within the township flooded along Cambridge Street, the levee located beside the Inverleigh tennis courts is overtopped, the Inverleigh Recreation Reserve is flooded and the Hamilton Highway overtopped west of the township. Above floor
flooding within the township begins shortly after the Major Flood Class level has been exceeded.
FIGURE 5-6 PROPOSED FLOOD CLASS LEVEL EXTENTS FOR THE LEIGH RIVER AT INVERLEIGH
R07_V02c_Summary_Report_Inverleigh_FS
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6 FLOOD BEHAVIOUR AND INTELLIGENCE OUTPUTS
6.1 Model Results
The model result data, including grids and extents, have been provided in specified Victoria Flood Database (VFD2) format for each flood event. The following result components were generated:
◼ Flood level, flood depth, flood velocity and flood hazard grids
◼ Flood elevation contours
◼ Flood extent data
◼ Hydrographs at key locations
◼ Long-section of river water levels
Grids and shapefiles (ESRI/VFD format), and Data tables (Excel csv/xlsx format) were provided on a Study USB upon completion of the study.
6.1.1 Data Sets
The following datasets were provided as final deliverables to the Golden Plains Shire and Corangamite CMA.
Grids
Gridded datasets of model results were provided for the following:
◼ Design events (50%, 20% 10%, 5%, 2%, 1%, 0.5%, 0.2% and 0.1%) – maximum depth, velocity, velocity x depth, Flood Hazard using AR&R Safety criteria for vehicles, Flood Hazard using AR&R Safety criteria for people and water surface elevation.
◼ Calibration events (1973, 1978, 1995, 2011 and 2016 events) – maximum depth velocity, velocity x depth, and water surface elevation.
◼ Model Topography
The hydraulic analysis provides regular grids of flood elevations across the hydraulic model study area. The flood extent was defined by converting the 2.5 m grid flood elevations to an extent polygon. The extent was smoothed to remove the sharp edges of the grid cells for cartographic / presentation purposes.
Flood depths were classified for mapping using the following classifications:
◼ 0 m to 0.25 m
◼ 0.25 m to 0.50 m
◼ 0.50 m to 1.00 m
◼ 1.00 m to 2.00 m
◼ Greater than 2.00 m
R07_V02c_Summary_Report_Inverleigh_FS
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Vector Data
ERSI shapefiles in VFD2 format were provided for the following:
◼ Peak flood extents
◼ Peak flood elevation contours
◼ Mapping limits
◼ Recommended Flood Overlay & Land Subject to Inundation Overlay
◼ Flood Affected Properties
◼ Surveyed Floor Levels
◼ Study Area Extent
Maps
The flood response inundation maps have been produced for the following design flood events for both Barwon River and Leigh River dominant scenarios:
◼ 0.2% AEP event
◼ 0.5% AEP event
◼ 1% AEP event
◼ 2% AEP event
◼ 5% AEP event
◼ 10% AEP event
◼ 20% AEP event
Each map includes:
◼ Flood extent,
◼ Flood level contour at 1m intervals,
◼ Depth of inundation,
◼ Identification of essential services,
◼ Major Road/street names
◼ Cadastral base
◼ Gauge height indication for the Leigh River at Shelford, Leigh River at Inverleigh and Barwon River at Inverleigh streamflow gauge.
Copies of the maps were provided in PDF format. R07_V02c_Summary_Report_Inverleigh_FS
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7 SUMMARY OF INVESTIGATION
The hydraulic modelling undertaken for the Inverleigh Flood Risk study assessed flood risk for the town of Inverleigh and identified locations along the Leigh River and Barwon River through Inverleigh that pose a high flood risk. The modelling has also identified several potential mitigation options to reduce flood risk, with two of these being modelled to show significant benefits in terms of reducing the frequency and magnitude of flooding through the Inverleigh township. The mitigation options identified along with the updated planning information aim to treat the existing flood risk through the township. Flood intelligence information from the study aims to provide emergency response information to VicSES, CFA, Council and for use within FloodZoom. A key component of the improved flood intelligence is using streamflow gauge predictions for upstream gauges (Leigh River at Inverleigh & Barwon River at Inverleigh) to provide predicted flood inundation at Inverleigh.
7.1 Key Outcomes
Key outcomes from this study in relation to flood risk are summarised below.
Hydrology
A thorough review of existing hydrology investigations into the Leigh River, Barwon River and Warrambine Creek was undertaken to provide an estimate of design flows and hydrographs for a range of AEP events. A Flood Frequency Analysis (FFA) was also undertaken for streamflow gauges on each of the contributing waterways. The design flows from the FFA were generally consistent with previous investigations. The adopted peak 1% AEP design flow for the Leigh River has increased around 23% from the previous flood mapping investigation carried out in 2007 (370 m3/s to 455m3/s). The updated hydrology analysis on the Barwon River resulted in the adopted 1% AEP flow decreasing from 815 m3/s to 674 m3/s, a decrease of 17%. An increase of 44% in 1% AEP peak flow for the Warrambine Creek (145 m3/s to 210 m3/s) was also adopted. Additional flow contribution to Warrambine Creek from Woady Yaloak Diversion Channel is relatively minor (due to channel capacity) compared with flows from the Barwon River, and are not considered significant in relation to large flood events on the Barwon River.
The likelihood of a Barwon River and Leigh River flood event peaking at the Inverleigh township at the same time is low. There is little correlation between flood events between the two major catchments (Leigh River and Barwon River) and the timing of flood peaks at Inverleigh from the beginning of a rainfall event has not historically occurred.
Hydraulic Characteristics
Overbank flows of the Leigh River are likely to inundate parts of the Inverleigh township from the north. Initially
flows will enter the township via the Leigh River near the tennis courts and over the Hamilton Highway. Prior to this, the Hamilton Highway west of the township will become inundated. Floor level survey was gathered during the flood study and results show that in a 1% AEP flood event 10 properties experience above floor flooding. Flooding from the Barwon River up to a 1% AEP event is not likely to result in above floor flooding within the central Inverleigh township, two properties to the south of Inverleigh are flooded above floor. Eleven properties considered to be flooded above floor (i.e. within a 50 mm tolerance of surveyed floor level) and 30 properties within 300 mm of being flooded above floor for Leigh River or Barwon River 1% AEP flood events
are shown in Figure 7-1. R07_V02c_Summary_Report_Inverleigh_FS
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 34
Sheds
FIGURE 7-1 PROPERTIES FLOODED ABOVE FLOOR (COMBINED 1% AEP EVENTS)
Assess and Treat Risk
Using the hydraulic modelling results, two mitigation options were investigated, costed and modelled to assess the impact on flooding. The mitigation options modelled showed a significant reduction in flood risk for many flood prone properties within Inverleigh. It is recommended that a detailed flood mitigation study be considered to further develop a functional and detailed design of flood mitigation at Inverleigh.
A review of the existing planning scheme was undertaken and suggested LSIO and FO planning maps were produced.
Flood Warning
The effective warning time for flooding at the Inverleigh township varies between flood events. Travel times from the upstream streamflow gauges in the Leigh River catchment include between 10-16 hours from the Leigh River at Mt Mercer streamflow gauge and 4-6 hours from the Leigh River at Shelford streamflow gauge. Travel times from the upstream streamflow gauges in the Barwon River catchment range from 16-20 hours from the Barwon River at Ricketts Marsh streamflow gauge, 8-12 hours from the Barwon River at Winchelsea
streamflow gauge, and 3-5 hours from the Barwon River at Inverleigh streamflow gauge.
Additional work using the information developed from this study was completed to link gauged levels/flows at the Leigh River at Shelford gauge to the Leigh River at Inverleigh gauge board. This will assist in raising flood awareness in the community and will assist emergency response during future floods.
R07_V02c_Summary_Report_Inverleigh_FS
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7.2 Conclusions
The following key conclusions are made based on the findings of the investigation:
◼ Parts of the Inverleigh Township are susceptible to out of bank flooding from the Leigh River. In a 1% AEP flood event on the Leigh River, 10 properties are inundated above floor. For the 1% AEP event on the Barwon River, 2 properties are flooded above floor.
◼ Through the series of steering committee and community meetings, many community members understand that the largest risk of flooding to Inverleigh is from the Leigh River, and there is good understanding regarding the areas where flooding occurs more frequently. Most understand that flooding is a natural occurrence however the potential flood risk to lives and private assets is of concern.
◼ Mitigation of flooding within the township may be possible through the use of levee embankments along the north of the township to restrict out of bank flows from the Leigh River entering the township. For the purposes of this study, Water Technology assessed two mitigation options which have not previously been assessed. It is recommended that a detailed mitigation study of these options be undertaken should they wish to be pursued. The mitigation options assessed within this study have a positive impact on reducing flood risk within the centre of the Inverleigh townships significantly reducing the number of properties flooded above floor. As a result of the embankment levees, there is an increase in flood levels across a number of private properties, with the impacts predominantly confined to agricultural land.
◼ A review of the existing planning scheme was undertaken and draft Land Subject to Inundation (LSIO) and Floodway Overlay (FO) planning maps were produced.
◼ The Municipal Flood Emergency Plan (MFEP) was updated with flood intelligence from this study. This should be utilised during future floods.
7.3 Recommendations
With regard to the study outcomes, the following recommendations for this study have been listed based on three key stakeholders from the project:
◼ Corangamite CMA
◼ Endorse the flood study and use the flood mapping data to inform floodplain risk management decisions.
◼ Upload the Victoria Flood Database mapping data and the excel spreadsheet of property inundation to FloodZoom.
◼ Make use of the flood level information produced to issue advice for any floodplain development.
◼ Review the proposed Flood Class Levels of the Leigh River at Inverleigh from this study.
◼ Golden Plains Shire
◼ Endorse the flood study before putting it out for public comment with aim of adopting the flood study and implementing a planning scheme amendment to update the flood related planning overlays to introduce new LSIO and FO mapping into the planning scheme.
◼ Consider a detailed assessment of flood mitigation options, with a functional and detailed design project that includes a full flood damages assessment. It is strongly recommended that this option be pursued with all relevant stakeholders and considered within the Inverleigh Structure Plan.
◼ Make use of the flood level information produced to issue advice for any floodplain development.
◼ Request that the Bureau of Meteorology consider the following: R07_V02c_Summary_Report_Inverleigh_FS
Golden Plains Shire | 03 August 2018 Flood Risk Management Study – Leigh and Barwon Rivers at Inverleigh Page 36
◼ Review the proposed Flood Class Levels of the Leigh River at Inverleigh from this study.
◼ Review the Flood Class Levels of the Leigh River at Shelford and bought in line with the proposed Flood Class Levels for Inverleigh from this study.
◼ Review the updated MFEP and discuss with VicSES the changes proposed by Water Technology prior to adopting the revised document.
◼ Actively promote the use of the VicEmergency website and App and the flood portal www.floodreport.com.au to the community to improve flood preparedness and awareness.
◼ Victoria State Emergency Service with assistance from Goulburn Broken CMA and Greater Shepparton City Council:
◼ Continue to engage the community through regular flood awareness programs such as the VICSES FloodSafe program.
◼ Review the updated MFEP and discuss with council the changes proposed by Water Technology
prior to adopting the revised document.
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8 REFERENCES
References used throughout all reports have been included below. Individual reports produced for this study have not been included in this reference list.
◼ ARR (1987) Institution of Engineers Australia, Australian Rainfall and Runoff: A Guide to Flood Estimation, Vol. 1, Editor-in-chief D.H. Pilgrim, Revised Edition 1987 (Reprinted edition 1998), Barton, ACT
◼ ARR (2016) Ball J, Babister M, Nathan R, Weeks W, Weinmann E, Retallick M, Testoni I, (Editors) Australian Rainfall and Runoff: A Guide to Flood Estimation, © Commonwealth of Australia (Geoscience Australia), 2016
◼ BoM (2017), Water Data Online Portal & Climate and past weather portal, Bureau of Meteorology, http://www.bom.gov.au/waterdata/ & http://www.bom.gov.au/climate/
◼ Corangamite CMA (2017) Leigh Flood Study Report prepared by the Corangamite CMA
◼ Connel Wagner (2005) Inverleigh Structure Plan Review, produced for the Golden Plains Shire, 2005
◼ DELWP (2016), Victorian Floodplain Management Guidelines, Department of Environment, Land, Water and Planning (DELWP), 2016
◼ DELWP (2017), Victorian Water Monitoring web portal, Department of Environment, Land, Water and Planning (DELWP), accessed various dates from April 7, 2017 – June 2017 at accessed from https://data.water.vic.gov.au/monitoring.htm
◼ GHD (2016) Regional Flood Mapping – Barwon River, Thompson Creek and Woady Yaloak Creek Hydrology and Hydraulic Modelling, prepared by GHD for the Department of Environment and Primary Industries, August 2016
◼ Inverleigh Community (2013) Inverleigh Community Plan, 2013
◼ Rawlinsons (2016), Rawlinsons Australian Construction Handbook, 2016
◼ Water Technology (2007), Corangamite CMA Flood Mapping Inverleigh and Winchelsea. Produced for the Corangamite Catchment Management Authority, 2007.
◼ Water Technology (2018e) Inverleigh Flood Intelligence – Shelford Rating Curve Review, prepared by Water Technology for the Corangamite Catchment Management Authority, 2018
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APPENDIX A
STREAMFLOW GAUGE DATA RECORDS
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Streamflow Gauge Quality Codes (BoM)
Description to be used by the Data Quality Description Provider for transferring data to the Description for Ratings Bureau of Meteorology
The record set is the best The data provider releases the The data provider releases the available given the recorded measurements for use, measurements for use, declaring technologies, techniques declaring that it is the best available that it is the best available given Quality-A and monitoring objectives given the technologies, techniques and the technologies, techniques and at the time of monitoring objectives at the time of monitoring objectives at the time classification. classification. of classification.
The data provider releases the The data provider releases the The record set is measurement for use with the recorded measurement for use with the compromised in its ability provision that the data is Quality-B provision that the data is compromised to truly represent the compromised in its ability to in its ability to represent the monitored parameter. represent the monitored parameter. parameter.
The data provider releases the record The data provider releases the The record set is an Quality-C set for use, declaring that the data is an record set for use, declaring that estimate. estimate the data is an estimate
The data provider releases the The record set's ability to The data provider releases the record record set declaring that the truly represent the set declaring that the data's ability to Quality-E data's ability to represent the monitored parameter is represent the monitored parameter is monitored parameter is not not known. not known. known.
The data provider releases the The record set is not of The data provider releases the record record set declaring that the data Quality-F release quality or set declaring that the data is missing, is missing, void or known to be contains missing data. void or known to be incorrect. incorrect.
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Barwon River Barwon River @ Ricketts Marsh Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality A 41.01 91.49% Quality B 0.71 1.58% Watercourse Daily Quality C 0.45 1.00% m3/s 26/07/1971 09/05/2016 discharge mean Quality E 2.22 4.96% Quality F 0 0.00% Missing 0.44 0.97% Quality A 42.13 94.00% Quality B 0.79 1.75% Daily Quality C 0.47 1.06% Watercourse level m 26/07/1971 09/05/2016 mean Quality E 0.97 2.16% Quality F 0 0.00% Missing 0.46 1.03%
Barwon River @ Winchelsea 233201 Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality A 13.95 81.56% Quality B 0.3 1.76%
Watercourse c Daily Quality C 2.09 12.24% 11/01/2000 12/02/2017 discharge m3/s mean Quality E 0.75 4.36% Quality F 0 0.00% Missing 0.01 0.08% Quality A 16.69 97.58% Quality B 0.3 1.78%
Daily Quality C 0.09 0.54% Watercourse level m 11/01/2000 12/02/2017 mean Quality E 0 0.00% Quality F 0 0.00% Missing 0.02 0.10%
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Barwon River @ Inverleigh 233218B Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality 47.8 95.37% A Quality B 0.04 0.09% Watercourse Daily m3/s 04/04/1966 04/05/2016 Quality C 0.42 0.85% discharge mean Quality E 1.85 3.68% Quality F 0 0.00% Missing 0.01 0.01% Quality 48.76 97.29% A Quality B 0.06 0.12% Daily Watercourse level m 04/04/1966 04/05/2016 Quality C 0.25 0.51% mean Quality E 0.97 1.94% Quality F 0 0.00% Missing 0.07 0.14%
Barwon River @ Pollocksford 233200B Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality A 43.14 91.58% Quality B 0.27 0.57%
Watercourse Daily Quality C 0.26 0.56% m3/s 13/05/1969 07/06/2016 discharge mean Quality E 3.25 6.91% Quality F 0 0.00% Missing 0.18 0.38% Quality A 44.47 94.42% Quality B 0.27 0.57%
Daily Quality C 0.26 0.55% Watercourse level m 13/05/1969 07/06/2016 mean Quality E 1.92 4.08% Quality F 0 0.00% Missing 0.18 0.39%
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Birregurra Creek @ Ricketts Marsh 233211 Time Quality Total Parameter name Unit Start date End date Percentage series type code years Quality A 52.46 83.13% Quality B 0.18 0.28%
Watercourse Daily Quality C 4.96 7.85% cumec 18/06/1953 15/03/2017 discharge mean Quality E 5.21 8.25% Quality F 0 0.00% Missing 0.31 0.49% Quality A 56.67 88.85% Quality B 0.17 0.27%
Daily Quality C 0.38 0.60% Watercourse level m 18/06/1953 15/03/2017 mean Quality E 6.09 9.55% Quality F 0 0.00% Missing 0.47 0.74%
Leigh River Leigh River @ Mt Mercer 233215A Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality A 56.97 95.50% Quality B 0.29 0.48% Daily Quality C 0.4 0.68% Watercourse discharge m3/s 10/10/1956 22/05/2016 mean Quality E 1.99 3.33% Quality F 0 0.00% Missing 0.01 0.01% Quality A 57.72 96.74% Quality B 0.28 0.48% Daily Quality C 0.39 0.66% Watercourse level m 10/10/1956 22/05/2016 mean Quality E 1.23 2.07% Quality F 0 0.00%
Missing 0.03 0.05%
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Leigh River @ Shelford 233213A Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality A 4.38 6.95% Quality B 6.86 10.88% Daily Quality C 0.02 0.03% Watercourse discharge cumec 11/05/1954 02/05/2017 mean Quality E 2.9 4.60% Quality F 0 0.00% Missing 48.87 77.55% Quality A 4.54 7.20% Quality B 8.19 12.99% Daily Quality C 0.02 0.03% Watercourse level m 11/05/1954 02/05/2017 mean Quality E 1.15 1.83% Quality F 0 0.00% Missing 49.12 77.94%
Leigh River @ Shelford (Golf Hill) 233248A Time Quality Total Parameter name Unit series Start date End date Percentage code years type Quality A 16.03 91.64% Quality B 0.07 0.41% Watercourse Daily Quality C 0.12 0.66% m3/s 25/07/1994 15/01/2012 discharge mean Quality E 1.27 7.27% Quality F 0 0.00% Missing 0.01 0.03% Quality A 17.12 97.87% Quality B 0.1 0.58% Daily Quality C 0.12 0.66% Watercourse level m 25/07/1994 15/01/2012 mean Quality E 0.15 0.86% Quality F 0 0.00% Missing 0.01 0.03%
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Warrambine Creek Warrambine Creek @ Warrambine 233223 Time Total Parameter name Unit series Start date End date Quality code Percentage years type Quality A 44.54 95.40% Quality B 0.52 1.12% Watercourse Daily Quality C 1.54 3.30% m3/s 01/06/1970 23/01/2017 discharge mean Quality E 0.04 0.08% Quality F 0 0.00% Missing 0.05 0.10% Quality A 44.77 95.90% Quality B 0.33 0.70% Daily Quality C 1.52 3.26% Watercourse level m 01/06/1970 23/01/2017 mean Quality E 0.01 0.02% Quality F 0 0.00% Missing 0.06 0.12%
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Melbourne Brisbane 15 Business Park Drive Level 3, 43 Peel Street Notting Hill VIC 3168 South Brisbane QLD 4101 Telephone (03) 8526 0800 Telephone (07) 3105 1460 Fax (03) 9558 9365 Fax (07) 3846 5144
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Wangaratta Wimmera First Floor, 40 Rowan Street PO Box 584 Wangaratta VIC 3677 Stawell VIC 3380 Telephone (03) 5721 2650 Telephone 0438 510 240
www.watertech.com.au
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