Flood Report Cape Otway Road Modewarre
COESR Pty Ltd
September 2019
Document Status
Version Doc type Reviewed by Approved by Date issued V01 Draft Ben Tate Ben Tate 04/10/2017 V02 Draft Ben Tate Ben Tate 17/10/2017 V03 Draft Ben Tate Ben Tate 14/11/2017 V04 Draft Ben Tate Ben Tate 19/01/2018 V05 Final Ben Tate Ben Tate 31/01/2018 V06b Final Ben Tate Ben Tate 07/03/2018 V07b Final Ben Tate Ben Tate 11/05/2018 V07c Final Ben Tate Ben Tate 07/08/2018 V08 Final Ben Tate Ben Tate 30/08/2019 V09 Final Ben Tate Ben Tate 09/09/2019 V10 Final Ben Tate Ben Tate 16/09/2019
Project Details
Project Name Cape Otway Road Modewarre Client COESR Pty Ltd Client Project Manager Tract Consultants Water Technology Project Manager Ben Tate Water Technology Project Director Warwick Bishop Authors Alex Simmons, Lachlan Inglis, Craig Flavel, Scott, Evans, Ben Tate Document Number R01v10
COPYRIGHT
Water Technology Pty Ltd has produced this document in accordance with instructions from COESR Pty Ltd 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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16 September 2019
Simon Loader Associate Town Planner Tract Consultants 4/65 Brougham Street GEELONG 3220
Dear Simon Cape Otway Road Modewarre – Flood Report
This report documents the existing and proposed developed flood risk at Cape Otway Road and Connies Lane, Modewarre. The report has investigated flooding from both backwater flooding from Lake Modewarre and flooding directly from the waterway that passes across the site flowing into Lake Modewarre. The investigation has shown that the peak flood level is likely to be caused by backwater flooding from Lake Modewarre which could be caused by a rare storm event creating catchment runoff filling the lake, or more likely a series of wet years that fill the lake to high levels.
Modelling has suggested that since year 1911 the largest water level observed in Lake Modewarre may have been reached in 1977, at a level of approximately 115.37 m AHD. Water Technology recommends adopting a design level of 115.37 m AHD for development of the site and applying an appropriate freeboard (suggest 0.6 m) for setting floor levels for the development. This means that floor levels of the development should be set no lower than 116 m AHD.
If you have any queries regarding this report, please contact me directly.
Yours sincerely
Ben Tate Senior Principal Engineer (03) 8526 0800 [email protected] WATER TECHNOLOGY PTY LTD
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CONTENTS
1 INTRODUCTION 7 1.1 Catchment 7 1.2 Proposed Development 7
2 FLOODING FROM LAKE MODEWARRE BACKWATER 10 2.1.1 Lake Outlets 11 2.2 Rainfall – Runoff Modelling (Source) 15 2.2.1 Model Setup 15 2.2.2 Climatic Conditions 17 2.2.3 Validation of Model 17 2.3 Long Term Results 21 2.3.1 Wet Spells 21 2.3.2 Dry Spells 21 2.4 Flood Risk from Lake Modewarre Backwater 22 2.4.1 Impact of Loss of Floodplain Storage on Lake Modewarre 22
3 FLOODING FROM WATERWAY 23 3.1 Overview 23 3.2 Hydrology 23 3.2.1 RORB Model Construction 23 3.2.2 Design Modelling 34 3.3 Hydraulics 36 3.3.1 Overview 36 3.3.2 TUFLOW Model Construction 36 3.4 Existing Conditions Flood Modelling Results 42 3.4.1 Tailwater Sensitivity 46 3.5 Proposed Developed Conditions 47 3.5.1 Development Impacts 51 3.5.2 Access and Egress 56
4 FLOOD RISK MANAGEMENT 57
LIST OF FIGURES Figure 1-1 Modewarre catchments and site location 8 Figure 1-2 Site Plan (provided 2/9/2019) 9 Figure 2-1 Lake Modewarre Crown Land Reserve 10 Figure 2-2 Lake Modewarre Natural Topography Levels 12 Figure 2-3 Disused Piped Outlets 13 Figure 2-4 Former Lake Modewarre Reserve Committee Operating Level of 114.2 m AHD 14 Figure 2-5 Schematisation of GR4J Rainfall-runoff Catchment Model (e-Water Source) 16
Figure 2-6 Lake Modewarre Long Term Modelled Water Levels 19
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Figure 2-7 1977 Flood extent – (SRWSC) & Modlled Peak in 1977 (Red) 20 Figure 2-8 Level-Duration Curve for modelled levels within Lake Modewarre 21 Figure 2-9 Flood Extent at 115.37 m AHD with the proposed layout 22 Figure 3-1 RORB Sub-areas and reaches 24 Figure 3-2 Catchment land use from lga zoning overlays 26 Figure 3-3 Fraction impervious Sub-area Weighted averages 27 Figure 3-4 Regions adopted for Loss Prediction Equations 28
Figure 3-5 ARR recommended median ILs values 29
Figure 3-6 ARR recommended median CL values 30 Figure 3-7 Design Modelling Process Diagram 34 Figure 3-8 Outflow hydrographs for the 10%, 5% and 1% aep 36 Figure 3-9 TUFLOW Model and Boundary Conditions 37 Figure 3-10 Model Boundary and Topography 38 Figure 3-11 Detailed Hydraulic roughness mapping 39 Figure 3-12 Left: Cape Otway Road Bridge Looking South, Right: Batsons Road culvert Looking North to Lake Modewarre 40 Figure 3-13 sITE plan layout 41 Figure 3-14 1% AEP Flood – Maximum Depth for Existing Conditions 43 Figure 3-15 1% AEP Flood – Maximum Velocity for Existing conditions 44 Figure 3-16 1% AEP Flood – Maximum Surface Water elevation for Existing Conditions 45 Figure 3-17 Long Section along waterway from development site to Lake Moddewarre 46 Figure 3-18 Existing Conditions Tailwater Sensitivity Difference Plot (Low Lake LEvel Minus Full Lake Level) 46 Figure 3-19 1% AEP Flood - Maximum Depth for Developed Conditions 48 Figure 3-20 1% AEP Flood - Maximum Velocity for Developed Conditions 49 Figure 3-21 1% AEP Flood - Maximum Water Surface Elevation for Developed Conditions 50 Figure 3-22 Maximum Water Level Difference Plot (Developed Minus Existing 52 Figure 3-23 Maximum Velocity Difference Plot (Developed Minus Existing) 53 Figure 3-24 Change in Floodplain Storage (1% AEP) 55
LIST OF TABLES Table 2-1 Lake Modewarre Level/Storage/Surface Area Relationship 11 Table 2-2 e-Water Source GR4J Model Inputs 15 Table 2-3 Annual Rainfall for Extended "Wet or Dry Spells" in Lake Modewarre 17 Table 3-1 Adopted Fraction Impervious values 25 Table 3-2 Storm losses comparison table 30 Table 3-3 Adopted losses 31 Table 3-4 Equation based kc estimates 31 Table 3-5 1% AEP Peak flow and critical durations with varying kc 31 Table 3-6 ARR Regional Flood Frequency Estimation model results 32 Table 3-7 VicROADS Rational Method results 32 Table 3-8 1% AEP Peak flow Summary of Approaches 33
Table 3-9 RORB critical durations for catchment 35
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Table 3-10 Temporal pattern (ARR2016) that closest matched monte carlo analysis 35 Table 3-11 Manning’s value for the tuflow model area 38 Table 3-12 Hydraulic Structures 40 Table 3-13 Floodplain Storage Volumes for Individual Parcels in a 1% AEP 12 hour Event 56
Table 3-14 Lake Modewarre surface area and relative increase in flood levels 56
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1 INTRODUCTION
The Cape Otway Road Australia (CORA) Comprehensive Development Plan (Tract, August 2019) outlines a long term plan for the staged development of currently low yielding agricultural land to the south-east of Lake Modewarre, into retail, sports, accommodation and tourism precincts. The Comprehensive Development Plan describes the future layout of these precincts, and provides a set of objectives, requirements and guidelines that will guide the development of the project.
The Comprehensive Development Plan addresses flooding specifically, stating that the development should maintain the free passage and temporary storage of floodwaters in order to not cause any significant rise in flood level or flow velocity, and to ensure that development maintains or improves river and wetland health, waterway protection and floodplain health.
To achieve the flooding objectives the Comprehensive Development Plan includes the following requirements:
◼ Buildings must be sited so as to sit above the Q100 flood event as determined by the Corangamite Catchment Management Authority.
◼ Any proposed flood mitigation works must be designed and undertaken to the satisfaction of Corangamite Catchment Management Authority and the responsible authority.
These guiding principles and requirements have been developed as a result of stakeholder engagement with relevant referral bodies such as the Corangamite CMA, Surf Coast Shire Council, Environment Protection Authority Victoria, and technical investigations completed by specialist consultants.
This report investigates the flood risk at the subject site situated at Cape Otway Road and Connies Lane, Modewarre for backwater flooding from Lake Modewarre and from flooding direct from the waterway that flows adjacent to the development site. 1.1 Catchment
The subject site is situated to the south-east of Lake Modewarre. The catchment is predominately farmland and low density rural living. The lake is a terminal lake, with four drainage catchments contributing inflows to Lake Modewarre as described below, and as shown in Figure 1-1.
◼ Unnamed waterway that flows through the subject site, draining from Wensleydale and the Otway National Park.
◼ North-east catchment draining from Gnarwarre.
◼ North-west catchment draining the Lake Dubban area.
◼ Local catchment fringing the lake.
An existing Land Subject to Inundation Overlay covers some of the proposed development site indicating that it may be flood prone, confirming the need for the investigation outlined in this report. 1.2 Proposed Development
The proposed CORA Concept Masterplan features a multi-faceted array of sports facilities, retail precinct and a range of accommodation including a hotel and eco-lodges. The site is situated on the fringes of the Lake Modewarre floodplain and proposes to adopt best practice approaches to floodplain risk management, stormwater treatment and reuse. A large portion of the site is located on high land and is not subject to inundation from the 1% AEP flooding event. The proposed plan will treat and reuse wastewater and stormwater
runoff on-site.
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A map of the Lake Modewarre catchment is shown in Figure 1-1 while the proposed development site is shown in Figure 1-2.
Lake Modewarre
Proposed
development site
FIGURE 1-1 MODEWARRE CATCHMENTS AND SITE LOCATION
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FIGURE 1-2 SITE PLAN (PROVIDED 2/9/2019)
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2 FLOODING FROM LAKE MODEWARRE BACKWATER
Lake Modewarre is a shallow crater lake located near the town of Moriac in South West Victoria and is located to the north-west of the subject site. The lake sits on a 533 hectare crown reserve as shown in Figure 2-1, however the surface area of the lake varies considerably depending on water levels. An EPA1 report indicates the lake averages a surface area of 414 hectares, while an analysis of storages and levels undertaken by CCMA suggest the lake surface area may be up to 920 ha (at its natural spilling capacity of 118.3 m AHD). A level/storage/surface area relationship was provided by the CCMA, minor changes to provide more detailed information at low lake levels were undertaken for the catchment modelling. This relationship is shown in Table 2-1.
FIGURE 2-1 LAKE MODEWARRE CROWN LAND RESERVE
1 A review of Historic Western Victorian Lake Conditions in relation to fish deaths (2007), accessed from 01_R01v10_CapeOtwayRoad_FloodInvestigation
- http://www.epa.vic.gov.au/~/media/Publications/1108.pdf 4525
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TABLE 2-1 LAKE MODEWARRE LEVEL/STORAGE/SURFACE AREA RELATIONSHIP
Level (m AHD) Volume (ML) Surface Area (ha) 109 0 0 109.1 1 100 109.2 2 175 109.5 200 300 111 3,000 400 112 4,000 460 113 5,199 520 114 5,490 549 115 6,991 699 116 7,887 789 117 8,500 850 118 9,241 924
2.1.1 Lake Outlets
The Lake is considered a terminal lake (closed system), with no natural outlets until levels reach 118.3 m AHD and water can then spill to the north east of the Lake into the Thompson Creek catchment as shown in Figure 2-2.
Information provided by the CCMA included several reports undertaken by the former State Rivers and Water Supply Commission (SRWSC)2, the CCMA3 and the Victorian EPA3, which document two man made outlets to the south east of the Lake in the area known as the backwash. The first of these was constructed in the 1850’s and drained excess water in Lake Modeware into Browns swamp and into Thompson Creek shown in Figure 2-3. The tunnel became inoperable and the Barrabool Shire Council replaced it with a 600 mm diameter culvert sometime in 1976 to drain private property following extended periods of flooding. The water flowed from the culvert beneath a lunette ridge to the south where it discharged to an open drain north of Cape Otway Rd. The drain then runs to Cape Otway Road and into Browns Swamp.
A memo from 2006 shows the CCMA did not object to the easement (over old tunnel outlet) to be
decommissioned on the subject Land. The memo (to the Surf Coast Shire) explains the drainage scheme was managed by the Shire of Barrabool. Under the previous drainage scheme arrangement, it appears the Barrabool Shire Council would seek permission from the State Rivers and Water Supply Commission based on rules for operation set by the Lake Modewarre Reserve Committee. The outlet invert has been surveyed at a level of 113.1 m AHD (CCMA pers. Comm), with the operating rules aiming for a minimum water level of 0.50 m above the obvert of the outlet (114.2 m AHD). At this level, the surface area of the lake is 552.5 ha (not including the backwater) as shown in Figure 2-4.
2 State Rivers and Water Supply Commission- Drainage of Lake Modewarre into Thompsons Creek, 1977
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- CCMA - Memo SCS 2006: Easement of over tunnel outlet, Batsons Road Modewarre 4525
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Drain Channel Outlet 118.3 m AHD (to Barwon River catchment)
Open Channel Outlet 118.3 m AHD
Outlet 119.23 m AHD
FIGURE 2-2 LAKE MODEWARRE NATURAL TOPOGRAPHY LEVELS
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Tunnel
600 mm pipe
Open Drain to Browns Swamp
FIGURE 2-3 DISUSED PIPED OUTLETS
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FIGURE 2-4 FORMER LAKE MODEWARRE RESERVE COMMITTEE OPERATING LEVEL OF 114.2 M AHD
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2.2 Rainfall – Runoff Modelling (Source)
A rainfall-runoff model was developed using e-Water Source (Carr, R, Podger, G.)4. Source allows the user to input historical rainfall and evapotranspiration data to determine long term flows, or in this example water levels within a storage (Lake Modewarre).
2.2.1 Model Setup
The model was developed using the GR4J hydrological model within Source. The model contains two hydrological stores and has four parameters. A schematisation of the model is shown in Figure 2-5.
Key inputs into the model are summarised in Table 2-2. The model was simulated at a daily time step from 1911 to 2015.
TABLE 2-2 E-WATER SOURCE GR4J MODEL INPUTS
Parameter Definition Adopted Parameter Catchment Contributing catchment size (ha) 7,200 ha Catchment delineated using ArcHydro Storage Level/Storage/Surface Area of Lake Relationship shown in Table 2-1 Calculated by CCMA from LIDAR Rainfall Daily rainfall (up to 9am from BoM data for Winchelsea (gauge 090167 previous day) 1911-2015) Evapotranspiration Evapotranspiration rate (up to 9am BoM data for (gauge 087126 1969-2015, from previous day) average monthly values were then calculated for 1911-1969 Seepage Seepage rate of Lake (mm/hr) 0.36-0.40 mm/hr (Based on heavy clay seepage rate) x1 Capacity of the production soil 350 (SMA) store (mm) x2 Water exchange coefficient (mm) -1 x3 Capacity of the routing store (mm) 40 x4 Time parameter (days) for unit 0.5
hydrographs
4 Carr, R, Podger, G. eWater Source — Australia's Next Generation IWRM Modelling Platform 34th 01_R01v10_CapeOtwayRoad_FloodInvestigation
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FIGURE 2-5 SCHEMATISATION OF GR4J RAINFALL-RUNOFF CATCHMENT MODEL (E-WATER SOURCE)
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2.2.2 Climatic Conditions
Daily rainfall and evapotranspiration data is required for the GR4J rainfall-runoff catchment model. There are a number of daily rainfall gauges within the catchment, however very few have an extended period of rainfall suitable to undertake a long-term assessment. The Winchelsea Post Office daily rainfall gauge (#090167) has over 100 years of daily rainfall data dating back to 1898. The rainfall gauge is located 10 km to the north west of the site at a similar elevation. It is expected that the rainfall gauge location and the site would have similar long term rainfall totals given their proximity to each other.
Daily evapotranspiration data was available from nearby Wurdiboluc reservoir from 2015 back to 1955. To obtain daily rates further back than 1955, the monthly average was calculated for each month and then applied to the daily data back to 1911.
2.2.3 Validation of Model
No surveyed levels were available for the lake water levels, therefore the validation of the model relied on anecdotal periods of wet and dry years as well as information from the SRWC report (1977), EPA report (2007) and the CCMA.
Based on known information the lake was dry during 1945, 1948, 1967-69, 1997-2000, 2007-10. Periods of wet years included 1952 and 1973-78. The rainfall for the wet and dry periods mentioned above is summarised in Table 2-3. The average annual rainfall at the Winchelsea Post Office is 546.6 mm.
TABLE 2-3 ANNUAL RAINFALL FOR EXTENDED "WET OR DRY SPELLS" IN LAKE MODEWARRE
Year Annual Rainfall for Period Summary 1945 405 25% Below Average 1948 492 10% Below Average 1952 840 53% Above Average (Highest annual rainfall on record) 1967-69 418 24% Below Average (including 288 mm in 1967) 1973-1978 674 20% Above Average (3 of the top 6 highest annual rainfall totals) 1997-2000 471 14% Below Average 2007-2010 443 19% Below Average (including 299 mm in 2006)
To simulate the impact of the tunnel and 600 mm pipe outlet from the lake a separate scenario was modelled which included an outlet based on the surveyed invert levels of the 600 mm diameter outlet pipe. The spillway rate was based on a manning’s pipe flow calculation at several depths, and design of the outflow pipe. The pipe was considered operational at all times which differs from the operating conditions discussed in Section 2.1.1. While this is a limitation in the modelling of the lake levels, it helps provide validation of the levels for the wet years and the impact of the outlet when compared with “natural conditions”. Additionally, no reliable information is available on the volume of additional flow provided to the lake from releases of Wurdiboluc into Lake Modewarre. This may result in the modelled levels being lower than what was experienced during periods of additional flow releases.
Despite the limitations, the catchment modelling allows for long term assessment of expected levels in the lake
to allow for the flood risk at the proposed site from high lake levels to be determined.
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During the validation of the model, the seepage rate was found to have largest impact on the fluctuation of lake levels. Median values as per the e-Water manual were adopted for the remaining parameters with the exception of X2 which controls the interaction between surface water and groundwater. A search of the Bureau of Meteorology Groundwater Dependent Ecosystem Atlas5 showed the likelihood of an aquatic Groundwater Dependent Ecosystem (GDE) at the site to be of “High Potential GDE from a regional study”. Given the likelihood, the value was changed from the median value of 0 to a -1 to represent the interaction between surface water and groundwater.
Following the validation of the model using the “piped conditions”, the model was then run with the outflow pipe removed to provide an estimation of what the lake levels may have got to had the outlet not been operated.
A timeline for the two scenarios are shown in Figure 2-6. An extent of flooding was produced in the SRWSC 1977 report. This was digitised and compared against the region contour for the modelled peak for 1977 (115.37 m AHD). Figure 2-7 shows the comparison in the backwash area is extremely close and provides
further confidence in the validation of the model.
5 Bureau of Meteorology – Groundwater Dependent Ecosystem Atlas, accessed from 01_R01v10_CapeOtwayRoad_FloodInvestigation
- http://www.bom.gov.au/water/groundwater/gde/map.shtml 4525
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Flooding Piped Outlets 1973-74 & 1977 “Natural Conditions”
Dry Period Dry Period 1967-69 Dry Period 1945-48 1998-2001 2004-2010
FIGURE 2-6 LAKE MODEWARRE LONG TERM MODELLED WATER LEVELS
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FIGURE 2-7 1977 FLOOD EXTENT – (SRWSC) & MODLLED PEAK IN 1977 (RED)
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2.3 Long Term Results
Based on the modelled lake levels for the “no-piped” option, a level exceedance curve was developed to assess the periods when certain level thresholds were exceeded. Figure 2-8 shows the level exceedance curve for the two modelled scenarios in Lake Modewarre. The results show the difference between the piped outlet and the natural scenario varies at the higher lake levels but makes no difference to the majority of the period.
FIGURE 2-8 LEVEL-DURATION CURVE FOR MODELLED LEVELS WITHIN LAKE MODEWARRE
2.3.1 Wet Spells
The daily level data was fed into the River Analysis Package developed by e-Water. This allows for detailed analysis of wet and dry periods to be calculated at varying thresholds. To assess periods of high lake levels,
a threshold of 114 m AHD was used to calculate the period above this level for the data between 1911-2015 under natural conditions (i.e. no additional inflows form Wurdiboluc reservoir and no releases via the piped outlets).
The results showed 5 periods where this level was exceeded, with an average period above this threshold of 101 days. The longest period above the 114.0 m AHD threshold was 164 days.
2.3.2 Dry Spells
Similar to the “wet spell” analysis, a dry threshold was used to assess the expected time when the lake is likely to dry under natural conditions. A threshold of 109.1 m AHD (200 mm depth in the lake) was used with a minimum spell length of 60 days (to register) as a dry spell. The results showed 22 dry spells with the longest
dry spell 141 days and an average duration of the dry spell of 88 days.
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2.4 Flood Risk from Lake Modewarre Backwater
The catchment modelling undertaken has allowed for an understanding of the potential flood risk from Lake Modewarre backwater for the proposed development to be quantified. The Lake Modewarre backwater flood risk is a result of extended periods of above average rainfall as discussed in Section 2.2.3 that occurs over a number of months and years to produce high lake levels. It should be noted that for the period for which the recorded high lake levels occurred, there was a likelihood of flows from Wurdiboluc Reservoir entering Lake Modewarre. Since the upgrade of the Wurdiboluc Reservoir in the 1990’s, additional height on the dam wall is likely to limit potential flows into the Lake Modewarre system. Figure 2-9 shows the Lake Modewarre backwater flood extent for the highest modelled level of 115.37 m AHD in 1977. This impacts on the wellness centre, hotel area, and several eco-lodges in the south west (now removed from the plan). This flood risk can be reduced by setting minimum floor levels for these buildings based on this flood level plus appropriate freeboard.
FIGURE 2-9 FLOOD EXTENT AT 115.37 M AHD WITH THE PROPOSED LAYOUT
2.4.1 Impact of Loss of Floodplain Storage on Lake Modewarre
Raising the retail area above the flood level of 115.37 m AHD removes around 23,800 m3 of floodplain storage. A simple water balance calculation based on estimated volume and flood extent area was used to estimate the increase in water levels. This results in an increase of around 3.32 mm across the site, neighbouring properties and the lake and floodplain. This is based on an estimated surface area of 7,147,800 m2). This slight loss of floodplain volume and associated water level rise does not produce a noticeable increase in the flood extent.
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3 FLOODING FROM WATERWAY 3.1 Overview
As opposed to flooding from the lake backwatering, this section investigates potential for flooding of the site from a large storm event and associated runoff along the waterway passing through the development site and onto Lake Modewarre. The methodology for the flood modelling is described below in two main components; hydrology and hydraulic modelling. The hydrology has used RORB, which is an industry standard modelling software commonly used in Victoria. The hydraulic modelling has used TUFLOW (a commonly used hydraulic model in Australia) to determine flood levels, depths and velocities across the site. 3.2 Hydrology 3.2.1 RORB Model Construction
The RORB model is comprised of multiple inputs and model parameters that determine how much runoff is generated from certain storm events, these include:
◼ Sub-catchments and reach delineation
◼ Fraction impervious of the catchment
◼ Rainfall depths and spatial and temporal patterns
◼ Kc and m – model parameters, discussed in sensitivity testing
◼ Rainfall-runoff losses
Each of these inputs are discussed in the following sections.
3.2.1.1 Catchment Delineation
Catchment delineation was undertaken by assessing the available topography datasets, this included detailed field survey of the subject site as well as several LiDAR datasets and the Statewide 10 m topography DEM. The Statewide 10 m topography DEM was used to determine catchment areas external to the subject site using ESRI’s ArcHydro for ArcGIS and delineated into 12 sub-catchments with associated drainage reaches. The RORB manual suggests a minimum of 5 and maximum of 20 sub-areas should be used with Boyd (1985) suggesting a minimum number of 8 sub-areas. The RORB catchment delineation is provided in Figure 3-1.
EQUATION 3-1 BOYD (1985) SUB-CACTHMENT DELINEATION CALCULATION: 0.1 푆푚푖푛 = 5.20(퐴) 0.1 푆푚푖푛 = 5.20(37.76)
푆푚푖푛 = 7.5 ~ 8.0
3.2.1.2 Reaches and Nodes
The reaches and nodes were constructed using ArcRORB. Nodes were placed throughout the sub-areas at junctions between any two reaches and at centroids to the sub-areas. These were connected via reaches, each with an ArcGIS calculated length, slope and type.
All reaches within the catchment were represented as “natural” reaches within RORB this is due to the open
grassed areas and natural waterways present.
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FIGURE 3-1 RORB SUB-AREAS AND REACHES
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3.2.1.3 Fraction Impervious
The estimated percentage of impervious surface within each sub-catchment was devised from the land use planning (zoning) within the catchment. Specific values are allocated to each zone and are represented in Table 3-1, with an associated map shown in Figure 3-2. An area weighted fraction impervious for each sub- area was then calculated, as shown in Figure 3-3.
TABLE 3-1 ADOPTED FRACTION IMPERVIOUS VALUES
Land use Zone Classification Adopted FI6 Road Zone – Category 2 RDZ2 0.6 Farming Zone FZ 0.0 Rural Living Zone RLZ 0.2 Public Use Zone – Service and Utility PUZ1 0.05 Rural Conservation Zone RCZ 0.1 Public Conservation and Resource Zone PCRZ 0.1
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- Melbourne Water – Table 1: Effective Impervious values for source nodes: MUSIC Guidelines (2016) 4525
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01_R01v10_CapeOtwayRoad_FloodInvestigation FIGURE 3-2 CATCHMENT LAND USE FROM LGA ZONING OVERLAYS
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FIGURE 3-3 FRACTION IMPERVIOUS SUB-AREA WEIGHTED AVERAGES
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3.2.1.4 Rainfall Depths
Rainfall depths for the catchment area were determined using the Australian Rainfall and Runoff (2016) rainfall Intensity-Frequency-Duration (IFD) data from the Bureau of Meteorology. Areal Reduction Factors and temporal patterns were sourced from the ARR Data Hub.
Due to the small size of the catchment, the design event critical durations are shorter than 24 hours.
3.2.1.5 Losses
An Initial Loss (IL) and Continuing Loss (CL) model was used in RORB. These losses may vary depending on a wet/dry catchment. Losses for the catchment were determined using methods described in ARR 2016 Book 5, Chapter 3, this included initial estimates recorded via Data Hub, regional estimates and equation based estimates.
The ARR Data Hub determined initial loss and continuing loss parameters to be 25 mm and 4.0 mm/hr
respectively. The study area is located within Region 3 of the loss prediction equations, shown in Figure 3-4.
Lake Modewarre catchment
FIGURE 3-4 REGIONS ADOPTED FOR LOSS PREDICTION EQUATIONS
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EQUATION 3-2 MEDIAN INPUT CALCULATIONS:
ILs (Storm Initial Loss) and CL (Continuing Loss) equations are outlined below.
퐼퐿푠 = −1.57 ∗ 푠0푤푟푡 + 0.14 ∗ 퐷퐸푆푅퐴퐼푁24퐻푅 + 18.8
퐶퐿 = 0.03 ∗ 퐷퐸푆푅퐴퐼푁24퐻푅 + 0.06 ∗ 푆푂푚푎푥 + 5.1
Where:
ILs is the storm Initial Loss (mm)
CL is the Continuing Loss (mm/h)
s0_wtr is the soil moisture in the surface store in winter season (mm)
DES_RAIN_24HR is the design Rain Intensity (I24,50) (mm)
SOmax is the maximum storage of the surface soil layer (mm)
Based on median input values these equations determined an ILs value of 27.5 mm and a CL of 3.1 mm/hr
ARR2016, Book 5, Chapter 3, Figure 5.3.18 and Figure 5.3.19 also outline median ILs and CL values of 30 mm
and 6 mm/hr respectively for the catchment, as shown in Figure 3-5 and Figure 3-6.
Lake Modewarre catchment
FIGURE 3-5 ARR RECOMMENDED MEDIAN ILS VALUES
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Lake Modewarre catchment
FIGURE 3-6 ARR RECOMMENDED MEDIAN CL VALUES
The loss values above are for a complete storm. If they are to be used with the IFD burst rainfall from the Bureau of Meteorology, either preburst rainfall needs to be added to the rainfall burst, or the losses can be reduced to be suitable to use with burst rainfall not a complete storm. The ARR data hub gives a median pre- burst rainfall depth of between 0.4 to 2.4 mm for events from 50% AEP to 1% AEP for a 12 hour duration, giving some indication of how much ILs may be reduced by if using burst rainfall.
The CL values must also be factored up to account for models running at a timestep of less than an hour, the factor applied to this model is around 1.2 to 1.3.
The initial losses and continuing losses represented in Table 3-2 give a relatively close match between values.
The differential between losses can be attributed to the regionalisation of the various approaches.
TABLE 3-2 STORM LOSSES COMPARISON TABLE
Loss Type ARR Data Hub ARR Region Equation Median from ARR Map Initial Loss 25 mm 27.5 mm 30 mm Continuing Loss 4.0 mm/hr 3.1 mm/hr 6 mm/hr
By adopting the losses from the ARR Region 3 equation and factoring the continuing loss up to account for the smaller timestep, and reducing the initial loss to convert the complete storm loss to a burst loss, we have adopted the losses shown in Table 3-3.
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TABLE 3-3 ADOPTED LOSSES
Loss Type Loss Initial Loss 25 mm Continuing Loss 4.0 mm/hr
3.2.1.6 RORB Kc and m
Kc is a RORB model routing parameter estimated using empirical equations that generally represent a wide range of fitted data for Australian catchments and dictates the attenuation along reach models. In gauged catchments, the kc value is one of the major parameters used to calibrate the RORB model, varying peak flow and timing. In ungauged catchments (such as Cape Otway Road) as mentioned, the kc can be estimated using empirical equations.
With RORB there are multiple equation-based estimates available for Victoria, these are outlined in Table 3-4. The equations vary in dependence on the catchment area (A) and the average reach distance (Dav). Generally, those associated with the Dav are preferred.
TABLE 3-4 EQUATION BASED KC ESTIMATES
Description Equation kc estimate Victoria (Mean Annual Rainfall 푘푐 = 0.49 ∗ 퐴0.65 5.19 <800mm)
Victorian based data (Pearse et 푘푐 = 1.25 ∗ Dav 9.06 al, 2002)
Australian based data (Dyer, 푘푐 = 1.14 ∗ Dav 8.26 1994)
Australian based data (Yu, 1989) 푘푐 = 0.96 ∗ Dav 6.96
Sensitivity testing of the kc values was completed using the RORB Monte Carlo analysis, comparing RORB model peak flows at the model outlet. The modelled peak flows and critical durations are shown in Table 3-5.
TABLE 3-5 1% AEP PEAK FLOW AND CRITICAL DURATIONS WITH VARYING KC
kc calculation method Duration Peak Flow (m3/s) MAR 12hr 50.10
Pearce et. al. 12hr 33.21 Dyer et. al. 12hr 37.07 Yu et. al. 12hr 41.93
As a method of verification for the most appropriate kc, the ARR Regional Flood Frequency Estimation Model7, the VicRoads Modified Rational, the Rational (Adams Method), and Hydrological Recipes approximation methods were used to calculate an estimated peak flow for the catchment, the model produced the peak flow estimates shown in Table 3-6, Table 3-7 and Equations 3-3 and 3-4.
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TABLE 3-6 ARR REGIONAL FLOOD FREQUENCY ESTIMATION MODEL RESULTS
AEP (%) Discharge Lower Confidence Limit Upper Confidence Limit (m3/s) (5%) (m3/s) (95%) (m3/s) 50 10.0 3.80 26.7 20 18.4 7.38 46.8 10 25.6 10.1 66.1 5 33.7 12.8 90.3 2 46.3 16.5 130.0 1 57.3 19.4 170.0
TABLE 3-7 VICROADS RATIONAL METHOD RESULTS
AEP (%) Iy (mm/h) Py Discharge (m3/s) 50 8.59 0.11 9.60 20 10.97 0.13 14.71 10 12.54 0.15 18.68 5 14.69 0.16 24.07 2 17.72 0.17 31.69 1 20.20 0.18 39.11
EQUATION 3-3 RATIONAL (ADAMS METHOD)
푄100 = (퐶 × 퐼 × 퐴)/360
3 푄100 = 33.07 푚 /푠
EQUATION 3-4 HYDROLOGICAL RECIPES URBAN AND RURAL ESTIMATES 푹풖풓풂풍 푪풂풕풄풉풎풆풏풕:
0.763 푄100 = 4.67 × 푎푟푒푎
0.763
푄100 = 4.67 × 37.76
3 푄100 = 74.57 푚 /푠
푼풓풃풂풏 푪풂풕풄풉풎풆풏풕:
0.71 푄100 = 10.29 × 푎푟푒푎
0.71 푄100 = 10.29 × 푎푟푒푎
3 푄100 = 135.55 푚 /푠
Table 3-8 below summarises the various approaches to estimating the 1% AEP peak flow.
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TABLE 3-8 1% AEP PEAK FLOW SUMMARY OF APPROACHES
Approximation Method 1% AEP Peak Flow (m3/s) RORB (MAR Kc) 50.1 RORB (Pearce et. al. Kc) 33.2 RORB (Dyer et. al. Kc) 37.1 RORB (Yu et. al. Kc) 41.9 ARR Regional FFA 57.3 VICROADS Rational Method 39.1 Rational Method (Adams) 33.1 Hydrological Recipes (Rural) 74.6
Water Technology has found the Pearce et. al. (2002) kc prediction equation to work well in many Victorian catchments. Given the equation determined by Pearce et. al. (2002) is based on Victorian data and gives a 1% AEP peak flow well within the range of possible values, it was adopted for this study.
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3.2.2 Design Modelling
3.2.2.1 Overview
ARR 2016 recommends the use of Monte Carlo analysis within RORB to determine the design peak flow. ARR 2016 also recommends an Ensemble approach using various temporal patterns. Both approaches were used to compare flows. The Monte Carlo analysis was used to adopt peak design flows, and the events from the Ensemble analysis which matched the peak flows from Monte Carlo were used to produce the hydrographs for input into the hydraulic model. A flow chart showing the modelling process is shown in Figure 3-7.
This process resulted in a single temporal pattern chosen for each design run, leaving three durations and three AEPs to be modelled, totalling nine design model runs.
RORB input parameters are determined using ARR 2016 methods
RORB Monte Carlo modelling was used to determine peak flows and event critical durations at 4 locations within the catchment
RORB Ensemble modelling was used to determine which temporal patterns best matched the Monte Carlo peak flows
The most appropriate temporal pattern for
each AEP is chosen. This results in a range of AEPs and event durations to be modelled and the event critical durations determined Expert Reviewer Ben Tate Warwick Bishop
FIGURE 3-7 DESIGN MODELLING PROCESS DIAGRAM
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3.2.2.2 Monte Carlo Analysis
The RORB Monte Carlo Analysis was undertaken adopting a median initial loss of 25 mm and using the initial loss distribution from ARR 2016, a median continuing loss of 4.0 mm/hr, and the Pearce Kc of 9.08. Monte Carlo Analysis samples an extensive range of temporal patterns and rainfall losses, in combination with the other set of model parameters of rainfall intensities, spatial pattern, continuing loss, aerial reduction factors, kc and m. The model then takes the hydrographs from all model runs and produces a statistical design peak flow at each RORB output location.
Hydrographs were exported at the outflow of the RORB model and the corresponding hydraulic model inflow boundaries. The Monte Carlo Analysis showed that the critical durations varied from two hours to twelve hours across the design events, as represented in Table 3-9.
TABLE 3-9 RORB CRITICAL DURATIONS FOR CATCHMENT
Location Critical Durations 50% AEP 20% AEP 10% AEP 5% AEP 2% AEP 1% AEP Lower Catchment (Outflow) 2 hr 6 hr 12 hr 12 hr 12 hr 12 hr
3.2.2.3 Ensemble Analysis
The RORB Ensemble Event Analysis was run utilising all the 10 ARR2016 recommended temporal patterns for each duration. The loss parameters and kc were adopted from the Monte Carlo simulation. For this case, three design events were modelled, resulting in 30 design temporal patterns for each of the ten durations, 300 model simulations. The peak flows determined in the Monte Carlo analysis were used to find a temporal pattern from the Ensemble Analysis producing a hydrograph with a similar peak flow. This comparison of peak flows between the Monte Carlo and Ensemble Analysis was completed at the outlet location at Lake Modewarre, this is summarised in Table 3-10.
TABLE 3-10 TEMPORAL PATTERN (ARR2016) THAT CLOSEST MATCHED MONTE CARLO ANALYSIS
Design Event Temporal Patterns AEP (%) Lower Catchment (Outlet) 10 13 5 13
1 30
The selected single temporal patterns were utilised to reduce the number of hydraulic model runs for each of the outlined AEPs.
3.2.2.4 RORB Modelling Outputs
The RORB model was used to produce flows at the hydraulic model boundaries and at Lake Modewarre. The
RORB model outflows to Lake Modewarre for the twelve hour duration for each AEP are shown in Figure 3-8.
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40
35 10% AEP
5% AEP 30 1% AEP 25
20
Flow Flow Q(m3/s) 15
10
5
0
1 2 3 4 5 6 7 8 9
13 10 11 12 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Time Time (hrs) FIGURE 3-8 OUTFLOW HYDROGRAPHS FOR THE 10%, 5% AND 1% AEP 3.3 Hydraulics 3.3.1 Overview
The flood modelling approach for the study area used a 1D/2D TUFLOW model. The waterway and floodplain were represented in 2D, with culverts represented in 1D. There are three primary inputs for a hydraulic model; the topography, hydraulic roughness and boundary conditions. The hydraulic model development was staged throughout the investigation with initial modelling providing an indication of ‘existing’ or base case conditions and an initial development plan assumed that the development area contained a large amount of fill and no cut on site. Over the course of the investigation the site layout has changed several times, with the layout at the time of the modelling previously shown in Figure 1-2. It is noted that the site layout has since been revised, removing the ecolodges to the south-west of the site and the rural residential lots to the north-east.
3.3.2 TUFLOW Model Construction
The model was built with TUFLOW software at a 5 m grid resolution, with key hydraulic controls (e.g. bridges and culverts) represented in 1D. The model was not calibrated to any historic events.
3.3.2.1 Boundary Conditions
3.3.2.1.1 INFLOW BOUNDARIES
Two inflows were included in the model as shown in Figure 3-9. The hydrographs for the Cape Otway Road Modewarre catchment were determined through the hydrologic design event RORB modelling described in
Section 3.2.
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3.3.2.1.2 OUTFLOW BOUNDARIES
The downstream boundary, at the end of the model was located at Lake Modewarre. The boundary was a Height/Time (HT) type boundary at the inlet channel of the lake, located north of the Batsons Road crossing. This was set at a constant 114 metres AHD. Long term water balance modelling of the Lake Modewarre catchment confirmed that this was an appropriate tailwater level, with that level exceeded rarely (only 1.4% of the time) over the modelled timeseries. A sensitivity analysis of the tailwater was undertaken and is detailed in Section 3.4.1.
FIGURE 3-9 TUFLOW MODEL AND BOUNDARY CONDITIONS
3.3.2.2 Grid Extent and Resolution
High resolution Light Detection and Ranging (LiDAR) data was available for the study area, Figure 3-10. This data was used to construct a model DEM of the floodplain to enable 2D hydraulic modelling.
The model extends well upstream of Cape Otway Road through to the Lake Modewarre outlet at the train line to the north-east of the lake.
A key consideration is establishing an appropriate grid resolution for the 2D hydraulic model. A grid resolution of 5x5 m was adopted, which provides a reasonable level of accuracy, and fast model run times. Given the
wide and flat nature of the floodplain, a grid size of 5x5 m represents the flood behaviour well in a large flood.
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FIGURE 3-10 MODEL BOUNDARY AND TOPOGRAPHY
3.3.2.3 Hydraulic Roughness
The hydraulic roughness was mapped using planning layers and verified from aerial imagery. The catchment comprises many different land use types including road reserves, water bodies (dams), and agricultural land. Table 3-11 gives the Manning’s roughness values adopted for this model and Figure 3-11 shows the spatial representation of the roughness values applied. These values are based on standard industry values (VicRoads Road Design Guidelines).
TABLE 3-11 MANNING’S VALUE FOR THE TUFLOW MODEL AREA
Land Type Manning’s Value (n)
Pasture, open grassed 0.04
Shrubs and trees, medium vegetation 0.075 medium density Residential 0.25 Sealed Road 0.025 Unsealed road 0.035 Scattered vegetated waterway 0.06 Water Body 0.03 Low Density Residential/Rural Residential 0.1 Dense Bushland 0.1
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FIGURE 3-11 DETAILED HYDRAULIC ROUGHNESS MAPPING
3.3.2.4 Timestep
A timestep of 2.5 seconds was adopted for this study. This means that during the model simulation the model will run the full suite of hydrodynamic calculations on every active wet cell, every 2.5 seconds of model time. In general, the smaller the grid cell size the smaller the timestep required for the model to run stable. Normal recommendations aim for a 2D time step of between ½ to ¼ of the grid resolution.
3.3.2.5 Key Hydraulic Structures
Key hydraulic structures were simulated using 1D layered structures which enabled key road deck, span, and topography features to be accounted for accurately. 1D structures are linked to the 2D flow paths and allow
for the model to simulate water flowing through a hydraulic structure.
There are two hydraulic structures incorporated in the catchment:
◼ The Cape Otway Road bridge traversing the floodway, Figure 3-12 (left).
◼ The culvert and road deck located at Batsons Road upstream of Lake Modewarre, Figure 3-12 (right). A site visit conducted in October 2017 measured the dimensions of these structures. These were collated and added to the hydraulic model to accurately represent the structures as they were present on site. The structure dimensions are summarised in Table 3-12.
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TABLE 3-12 HYDRAULIC STRUCTURES
Flood Study ID Type Span Dimensions FC01.2_B Bridge (B) Single Pier (450 mm wide) L 28.8 m x W 11 m x H 2.0 m (surface obtained from LIDAR) FC01.2_C Culvert (C) Single Ø 750 mm diameter pipe
FIGURE 3-12 LEFT: CAPE OTWAY ROAD BRIDGE LOOKING SOUTH, RIGHT: BATSONS ROAD CULVERT LOOKING NORTH TO LAKE MODEWARRE
3.3.2.6 Proposed Developed Conditions
Developed conditions were modelled based on a site plan that was provided and then georeferenced as shown in Figure 3-13. This included changes to the model topography to include raising the infrastructure above the 1% AEP flood level.
The 1% AEP event was modelled for existing and developed conditions for the 12-hour duration event which was calculated as the critical duration in the hydrology model. The results including maximum depth, velocity, water surface elevation and flood hazard (the velocity and depth product) were generated as TUFLOW output
grids and used to carry out the flood risk assessment.
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FIGURE 3-13 SITE PLAN LAYOUT
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3.4 Existing Conditions Flood Modelling Results
The maximum depth for the 1% AEP flood under existing conditions can be seen in Figure 3-14. A large area of the proposed development in the south-west of the site is inundated, with a maximum depth of 1.1 metres.
The modelled velocities show a significant reduction in speed as the floodwaters enter the flatter wide storage areas of the floodplain, as seen in Figure 3-15. The average velocity over this area of the site is 0.1 to 0.25 m/s. Velocities upstream of the site are higher, in the order of 0.5 to 2 m/s.
Water surface elevations across the site range from 122.00 m AHD at the western property boundary to 114.43 m AHD at the northern property boundary. Due to the ponded backwater effect of the storage area, 114.43 m AHD is the 1% AEP flood level from the waterway across much of the site, as seen in Figure 3-16. As water levels increase during the flood event, the area downstream of the development site slowly fills up resulting in a backwatering effect of the development site. The current farm dam located on site is overtopped as a result of the backwater from downstream. A long section from the current farm dam along the waterway, across Batsons Road to the Lake Modewarre outfall illustrates the hydraulic control created around Batsons
Road (Figure 3-17).
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FIGURE 3-14 1% AEP FLOOD – MAXIMUM DEPTH FOR EXISTING CONDITIONS
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FIGURE 3-15 1% AEP FLOOD – MAXIMUM VELOCITY FOR EXISTING CONDITIONS
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FIGURE 3-16 1% AEP FLOOD – MAXIMUM SURFACE WATER ELEVATION FOR EXISTING CONDITIONS
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FIGURE 3-17 LONG SECTION ALONG WATERWAY FROM DEVELOPMENT SITE TO LAKE MODDEWARRE
3.4.1 Tailwater Sensitivity
The downstream tailwater representing Lake Modewarre was adjusted to 113 m AHD (one metre lower than the full level of 114 m AHD previously adopted). Existing conditions were again simulated with the difference in water levels shown in Figure 3-18. These results show that lowering the tailwater by 1 m only reduces the flood levels across the site by around 2 cm. As a conservative approach, the higher tailwater level of
114 m AHD was adopted.
wayRoad_FloodInvestigation
FIGURE 3-18 EXISTING CONDITIONS TAILWATER SENSITIVITY DIFFERENCE PLOT (LOW LAKE LEVEL
MINUS FULL LAKE LEVEL)
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3.5 Proposed Developed Conditions
As discussed previously, the proposed developed conditions were represented by raising the roads and building footprint areas associated with the Retail Village and Hotel above the 1% AEP flood level.
In addition to this, an area immediately between the Retail Village and Hotel will become a wetland, used to treat stormwater runoff from the proposed development. This will incorporate vegetation and landscaping to become one of the many treatment points for stormwater runoff across the site. The waterbody will be created by realigning an existing earth berm to create the northern edge of the waterbody. All other buildings have been sited outside of the 1% AEP flood extent. The resulting flood depth, velocity and water surface elevation
for the 1% AEP flood event can be seen in Figure 3-19, Figure 3-20, and Figure 3-21 respectively.
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FIGURE 3-19 1% AEP FLOOD - MAXIMUM DEPTH FOR DEVELOPED CONDITIONS
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FIGURE 3-20 1% AEP FLOOD - MAXIMUM VELOCITY FOR DEVELOPED CONDITIONS
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FIGURE 3-21 1% AEP FLOOD - MAXIMUM WATER SURFACE ELEVATION FOR DEVELOPED CONDITIONS
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3.5.1 Development Impacts
Existing and developed model results for the 1% AEP flood event were analysed and compared to determine the impact the development has on the water levels and velocity for the site and surrounding properties. This comparison is calculated by subtracting the existing conditions water levels from the developed conditions water levels as follows: ‘Developed Conditions – Existing Conditions’
3.5.1.1 Developed Flood Level Impacts
The result shows positive values where the development has caused water level increases, and negative values where there are water level decreases. This calculated comparison also provides information on the previously wet and now dry, and previously dry and now wet areas as seen in Figure 3-22. Due to the filling of locations within the flood extent to accommodate the proposed development, a flood level increase up to 3 cm compared with existing levels was observed throughout the majority of the site. The flood extent increased only slightly at the southern end of the subject site. The impacts are generally confined to the site and have only minor impacts to the parcels of land to the immediate north of the site. This increase in flood levels is between 2-3 cm and extends to Batsons Road, with only a minor increase to the flood extent observed. The affected land holder’s to the north of the site have confirmed their acceptance of the minor increases in flood extent and depth, see acceptance letters in Appendix A.
3.5.1.2 Developed Velocity Impacts
The maximum velocities over the site have fluctuated due to the developed areas raised above flood levels, Figure 3-23. The areas where flow paths have been narrowed may see an increase in maximum velocity of 0.2 m/s higher than existing conditions. Some locations within the proposed development show decreased flood velocities due to newly created backwater areas. This is a worst case scenario, as some of the development may not be raised on fill pads but rather piers (for example) which would allow the free passage of flood waters under the buildings.
The maximum velocities are experienced for a short period of time as the floodplain fills, then velocities slow dramatically as the majority of the site becomes a backwater from the slightly higher elevation of the bed of
the inlet to Lake Modewarre, to the north of the site.
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FIGURE 3-22 MAXIMUM WATER LEVEL DIFFERENCE PLOT (DEVELOPED MINUS EXISTING
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FIGURE 3-23 MAXIMUM VELOCITY DIFFERENCE PLOT (DEVELOPED MINUS EXISTING)
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3.5.1.3 Developed Flood Storage Impacts
The increase in flood level is in large part due to the loss of storage within the floodplain generated through the filling of building footprints to above the 1% AEP flood level and the creation of a wetland adjacent to the Retail Village and Hotel. The approach to filling the development footprint areas is a conservative approach but provides the ‘worst case’ scenario for filling the areas identified within the concept development layout. The results show that using the conservative approach there is only a minimal increase to flood levels to the immediate neighbouring properties to the north. The land holder has negotiated with the neighbouring land holder’s to the north to gain permission to increase the flood levels in a 1% AEP through the properties.
To assess the changes to floodplain storage as a result of the proposed development, the maximum flood depth on each parcel within the floodplain was calculated for existing and proposed developed conditions. The results shown in Table 3-13 with the Parcel ID’s and change in volume shown in Figure 3-24. While there is a reduction in floodplain storage of 18,700 m3 across the thirteen parcels, this equates to just over 2.5% of the total volume of floodplain storage on the thirteen properties (not including the volume of Lake Modewarre at this water level). The additional volume will also be spread across into Lake Modewarre. The developed conditions modelling results show an additional 31,500 m3 of volume entering the lake during the 1% AEP flood event. Lake Modewarre covers an area of between 474-550 hectares. Based on a water level of 114.0 m AHD, the area of the lake attributed to crown land is 549 hectares. The additional volume entering the lake may result in an increase of between 5 to 6 mm based on the additional volume spread evenly over the lake surface. These estimations are conservative and assume the lake to be at full water level. No bathymetry data is available for Lake Modewarre below a level of around 112.5 m AHD, the surface area of Lake Modewarre for elevations of 113.0 and 114. 0 m AHD have been calculated and are shown in Table 3-14. This provides an indication that the additional flow volume entering the lake will have no noticeable impacts on flood extent. The slight increase in volume stored within the floodplain is not likely to have impacts on the hydrological regime of the floodplain and surrounding area. The area has undergone significant changes to the hydrological regime through earth works and land use practices since European settlement, the works proposed as part of this development aim to have minimal impact on environment and aim to improve the quality of water leaving the site through treatment of stormwater runoff and plans to establish native vegetation
along the floodplain.
ation
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FIGURE 3-24 CHANGE IN FLOODPLAIN STORAGE (1% AEP)
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TABLE 3-13 FLOODPLAIN STORAGE VOLUMES FOR INDIVIDUAL PARCELS IN A 1% AEP 12 HOUR EVENT
Parcel ID Existing Conditions Developed Conditions Change in Volume (m3) Volume (m3) Volume (m3) 1 32,105 35,296 3,191 2 170,153 174,982 4,830 3 91,039 50,727 -40,311 4 34,581 35,633 1,052 5 33,600 35,127 1,527 6 2,528 2,667 139 7 265,033 275,718 10,685 8 8,403 8,823 420 9 8,525 9,453 928 10 11,617 11,617 0 11 6,888 6,888 0 12 70,881 69,702 -1,178 13 7 7 0 Total 735,357 716,639 -18,718
TABLE 3-14 LAKE MODEWARRE SURFACE AREA AND RELATIVE INCREASE IN FLOOD LEVELS
Elevation (m AHD) Surface Area (ha) Increased Flood Levels from development (m) 2017 Aerial Photography (~ 111 m AHD) 346 0.0060 VicMAP Crown Land Tenure Extent 474 0.0044 113.0 520 0.0040 114.0 550 0.0038
3.5.2 Access and Egress
During preliminary modelling and design of the site, the inclusion of ecolodges in the south-west of the site raised concerns regarding safe access and egress during a flood. The proposed ecolodges in the south-west have since been removed. The development consequently now has no access and egress issues during a flood. There is clear road access from the development to Cape Otway Road.
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4 FLOOD RISK MANAGEMENT
As described within this report, the site is subject to potential flooding from backwatering of Lake Modewarre and from flooding direct from the waterway running through the site. The flood levels modelled indicate that the Lake Modewarre backwater flooding mechanism may produce the peak flood levels at the site.
Water Technology recommends that a flood level of 115.37 m AHD be used for design purposes. This flood level was the maximum modelled flood level from Lake Modewarre during 1977 over the 104 year simulation period, assuming no piped outfalls from the lake. It is recommended that a freeboard of 0.6 m be applied on top of this design flood level, meaning that floor levels should be set at 116 m AHD.
Given the conservative flood modelling of the proposed development, filling the development to above the flood level has shown a slight increase in water level across the neighbouring properties to the north. The flood modelling has demonstrated that this increase in water level is approximately 2 cm for a 1% AEP flood on the waterway, and only 3 mm for the larger Lake Modewarre backwatering flood mechanism. COESR Pty Ltd has already held discussions with the landholders and reached an in-principle agreement, see acceptance letters from the landholders in Appendix A.
Water Technology is satisfied that the CORA site can satisfactorily manage acceptable flood risks and meet the requirements of the Corangamite CMA, who were consulted regarding the development proposal during the course of this investigation.
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APPENDIX A
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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
Adelaide Perth 1/198 Greenhill Road Ground Floor Eastwood SA 5063 430 Roberts Road Telephone (08) 8378 8000 Subiaco WA 6008 Fax (08) 8357 8988 Telephone 0438 347 968
Geelong Gippsland PO Box 436 154 Macleod Street Geelong VIC 3220 Bairnsdale VIC 3875 Telephone 0458 015 664 Telephone (03) 5152 5833
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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