Rippleside Catchment Main Drain Augmentation. Final Report

Transcription

1 Rippleside Catchment Main Drain Augmentation Final Report Prepared For: Prepared By: City of Greater Geelong WBM Oceanics Australia Offices Brisbane Denver Karratha Melbourne Morwell Newcastle Sydney Vancouver

2 DOCUMENT CONTROL SHEET WBM Oceanics Australia Melbourne Office: Level 5, 99 King Street MELBOURNE VIC 3000 Australia PO Box 604 Collins Street West VIC 8007 Telephone (03) Facsimile (03) ACN Document: Title: Project Manager: Author: Client: Client Contact: Client Reference: Synopsis: R.W FinalReport.doc Rippleside Catchment Main Drain Augmentation Final Report Allan Charteris Allan Charteris City of Greater Geelong Richard Wojnarowski Tender No. T00049 This report documents findings of flood investigations in the catchment of Rippleside, Geelong. REVISION/CHECKING HISTORY REVISION NUMBER DATE CHECKED BY ISSUED BY 1 28/02/2001 WJW ABC DISTRIBUTION DESTINATION REVISION City of Greater Geelong WBM File WBM Library

3 CONTENTS I CONTENTS Contents List of Appendices List of Figures List of Tables i iii iii iv 1 INTRODUCTION Background Catchment Description History of Flooding Lantana Ave Montgomery Ave The Fairway and Geelong Golf Club Calvert Ave and Lily Street Other Areas PROJECT METHODOLOGY Preliminary Tasks Project Initiation Data Collation and Review Barwon Water Digital Elevation Model Hydrological and Hydraulic Modelling, and Mapping of Existing Conditions Hydrologic Analysis Model Development Design Event Modelling Hydraulic Analysis Model Selection and Development Boundary Conditions Mitigation Option Assessment and Mapping Review of Existing Flooding Impacts Impact Identification Floor Level and Building Survey Flood Damage 2-5 I

4 CONTENTS II Mitigation Option Assessment DIGITAL ELEVATION MODEL Catchment DEM Study Area DEM EXISTING CONDITIONS Hydrology Model Description Global Parameters Loss Parameters Fraction Impervious Reach Storage Relationships Sub-Catchment Definition Model Verification Volume Checks Rational Method Checks Hydraulics Model Selection Model Development Model Geometry Model Resolution Model Boundary Conditions Flood Mapping Building Inundation Flood Hazard Mapping Flooding Issues MITIGATION ASSESSMENT Mitigation Option Elements Preferred Approach for Augmentation Flood Hazard Mapping Other Impacts ECONOMIC ASSESSMENT Flood Damages Assessment Stage Damage Curves 6-1 II

5 LIST OF APPENDICES III Outside Buildings Damages Benefit Cost Ratio Cost Benefit Benefit Cost Intangible Damages/Benefits Funding Mechanisms Special Rates and Charges Development Control in Flood Prone Areas Discussion STORMWATER MANAGEMENT PLAN CONCLUSION 8-1 LIST OF APPENDICES APPENDIX A: SURVEY REPORT A-1 APPENDIX B: MITIGATION OPTIONS ELEMENT IMPACTS B-1 APPENDIX C: SPECIAL RATES AND CHARGES LEGAL ADVICE C-1 APPENDIX D: DESIGN FLOW INFORMATION D-1 LIST OF FIGURES Figure 1.1 Study Area 1-2 Figure 1.2 Flooding at Ballarat Road December Figure 1.3 Flooding Cnr Marlo and Lantana December Figure 1.4 Flooding at Montgomery Ave December Figure 1.5 Flooding Downstream of Golf Club Car Park 1-4 Figure 1.6 Flooding Downstream of Golf Club Car Park 1-5 Figure 1.7 Flooding in The Fairway 1-5 Figure 1.8 Flooding in Baxter Road Retarding Basin December Figure 2.1 Underground Drainage Network Error! Bookmark not defined. Figure 3.1 5m DEM and Catchment Boundary 3-1 Figure 3.2 1m DEM of Flood Mapping Area 3-2 III

6 LIST OF TABLES IV Figure 3.3 1m DEM of Flood Mapping Area - Perspective View 3-3 Figure 4.1 RORB (Hydrology) Model Layout Error! Bookmark not defined. Figure 4.2 TUFLOW (Hydraulic) Model Layout Error! Bookmark not defined. Figure y Design Event Flood Inundation Error! Bookmark not defined. Figure 4.4 Inundation Above Floor Level Error! Bookmark not defined. Figure 4.5 Hazard Mapping Existing Case 100 Year Design EventError! Bookmark not defined. Figure 4.6 Flooding Issues in the Rippleside Catchment 4-1 Figure 5.1 Weddell Road Retarding Basin Storage Characteristics 5-4 Figure 5.2 Mitigation Option Elements Error! Bookmark not defined. Figure 5.3 Augmented System Elements Error! Bookmark not defined. Figure 5.4 Figure 5.5 Figure y Design Event Flood Extent for Augmented SystemError! Bookmark not defined. 100y Above Floor Level Inundation After AugmentationError! Bookmark not defined. Hazard Mapping Augmented Case 100 Year Design EventError! Bookmark not defined. Figure 6.1 Rippleside Flood Damage Curves 6-2 Figure 6.2 Areas Suitable for Alternative Flood Relief Strategies Error! Bookmark not defined. LIST OF TABLES Table 4-1 Impervious Fraction for Planning Scheme Zone 4-2 Table 4-2 Hydrological Model Verification 4-4 Table 4-3 TUFLOW Manning s n Coefficients 4-5 Table 4-4 Number of Properties Inundated Above Floor Level 4-7 Table 4-5 Hazard Categories 4-7 Table 5-1 Effect of Options on Existing Flooding 5-3 Table 6-1 Guideline Per metre Costs for Pipe Augmentation Works 6-3 Table 6-2 Estimated Cost of Augmentation Works 6-3 Table 6-3 Detailed Breakdown of Cost Estimate 6-4 IV

7 INTRODUCTION INTRODUCTION 1.1 Background The City of Greater Geelong has embarked on a process to investigate existing drainage capacity and to develop a plan for the upgrade of the system to mitigate existing flood problems. The Drainage Augmentation Study for the Rippleside Catchment is specifically concerned with: defining (simulating) and mapping existing flooding characteristics and drainage system deficiencies in the Catchment; assessing the nature and magnitude of impacts associated with existing flooding; identifying potential augmentation strategies to reduce damages associated with flooding impacts; and, evaluating and identifying opportunities of funding capital and maintenance works associated with the recommendations of the study. This report documents the findings of WBM s investigations of flooding in the Rippleside catchment and presents mitigation options to minimise flooding impacts. 1.2 Catchment Description The catchment of Rippleside is the largest urban catchment in Geelong (approximately 750 Ha) and is located north of the city ranging over the suburbs of Hamlyn Heights, Herne Hill, Manifold Heights, Geelong West and Geelong North (See Figure 1.1). The catchment is characterised by significant and almost complete urbanisation with few open space areas. These are limited to areas within the Geelong Golf Club, a dedicated drainage reserve between Weddell and Baxter Roads, and a small portion of undeveloped area at the top of the catchment. The existing drainage infrastructure is typically very old, much of it in place and unchanged since subdivision and development 30 to 60 years ago. Current drainage problems within the Rippleside Catchment are similar to those experienced by many older urban catchments which contain hydraulically constrained and aging stormwater systems. The characteristics of these catchments are usually typified by high levels of development, extensive areas of imperviousness and limited retention of dedicated drainage reserves or open channel systems. Additionally, the underground drainage systems are typically under capacity and there is a lack of conveyance and/or storage capacity above ground for major flows. 1-1

8 INTRODUCTION 1-2 Figure 1.1 Study Area The topography of the catchment is somewhat different from the typical floodplain catchment. Where one would usually find steeper upper catchment areas and a broad floodplain in lower areas, Rippleside exhibits quite the opposite relief. The upper parts of the catchment (above Vines Road) are very flat, almost as a plateau. The grade changes significantly below Vines Road and the lower parts of the catchment are relatively steep. The drainage paths become well defined and somewhat incised, particularly in the drain through the southern part of the Geelong Golf Club. The outlet of the catchment is to Rippleside Beach via a culvert that runs from the retarding basin at Baxter Road, below the Geelong-Melbourne Railway and under Rippleside Park. The railway virtually forms a dam wall preventing overland flows from progressing further down the catchment. 1.3 History of Flooding The flooding history in the catchment is varied with problems often attributed to limited underground capacity resulting in overland flows for which there exists no defined floodway. In a number of cases, dwellings have been built in the flowpath and occasionally as slab on ground (eg Lily and Graham St). 1-2

9 INTRODUCTION Lantana Ave Montgomery Ave Significant flooding was experienced in the Lantana Av Montgomery Ave area due to a local storm on 17 December At Ballarat Road, the outbound road was cut and flood depths of approximately m were observed due to ponding above the median strip (Figure 1.2). Ponding behind the railway lead to flooding at the Lantana Ave Marlo St intersection (Figure 1.3), again with flood depths of approximately m observed. Significant overland flows were also observed in Montgomery Ave (Figure 1.4). Figure 1.2 Flooding at Ballarat Road December 1978 Figure 1.3 Flooding Cnr Marlo and Lantana December

10 INTRODUCTION 1-4 Figure 1.4 Flooding at Montgomery Ave December 1978 Flooding in this event was observed to be brief and property inundation above floor level was not reported The Fairway and Geelong Golf Club Numerous flooding incidents have been reported by the Golf Club with particular problems in the carpark and practise fairway areas (See Figure 1.5 and Figure 1.6). Figure 1.5 Flooding Downstream of Golf Club Car Park 1-4

11 INTRODUCTION 1-5 Figure 1.6 Flooding Downstream of Golf Club Car Park Figure 1.7 Flooding in The Fairway Figure 1.7 illustrates the effect of this type of flooding on The Fairway, the road directly downstream from the effected Golf Club areas. The Golf Club report that this type of flooding is relatively frequent, in 1994 suggesting that the type of flooding presented in the above photos has occurred 2-3 times since

12 INTRODUCTION Calvert Ave and Lily Street Reports of flooding and inundation to almost floor level in these areas were recorded following flood events in Several properties were inundated and evidence of flooding of garages and garden sheds was observed. Reports indicated that flooding in these areas was a relatively frequent occurrence. Photographic documentation was not available Other Areas Significant flooding has been observed on many occasions in the Baxter Rd retarding basin. Figure 1.8 Flooding in Baxter Road Retarding Basin December 1992 Minor flooding in December 1992 resulted in the inundation illustrated in Figure 1.8. On other occasions, flood levels have been observed above the crest of Baxter Road (to the left of Figure 1.8). While the purpose of the retarding basin is to provide flood storage and is fulfilling that role, there is concern that peak flood levels in the area may impact upon adjacent housing during less frequent events. 1-6

13 PROJECT METHODOLOGY PROJECT METHODOLOGY The methodology to undertake the Rippleside Drainage Augmentation Study was detailed in WBM s proposal and forms part of the contract documentation. This section provides a brief overview of the methodology adopted during the project with particular reference to any alterations required to overcome difficulties encountered during the course of the project. The methodology was developed in accordance with the requirements of the study brief with a particular emphasis on ensuring that recommended augmentation options are realistically implemented within local hydraulic, economic and political constraints. 2.1 Preliminary Tasks Project Initiation Project initiation occurred in May 2000 with a meeting of the City of Greater Geelong s Project Officer (Mr Richard Wojnarowski) and WBM project staff (Allan Charteris, Wesley Walden, Lloyd Heinrich). Following the meeting a ½ day site inspection was undertaken where Council s Project Officer identified key elements of the system and related flooding issues to WBM. During the course of the project additional site inspections were undertaken by WBM staff to clarify flood and drainage issues Data Collation and Review Upon commissioning, all relevant data and information for the Catchment and its drainage systems was obtained from Council and Barwon Water. Review of this information was undertaken to identify any significant data gaps and to gain a complete understanding of issues in the Rippleside catchment. Council provided detailed information regarding systems that are currently under their responsibility as well as some information suitable for providing infill to the data gaps identified in the City of Greater Geelong data set. Additional data was retrieved from Barwon Water, including cadastral information, ground level contours over the entire catchment, and planning scheme information. The underground drainage information has been comprehensively reviewed. Figure 2.1 shows the major underground drainage network within the floodmapping area. The key features of the drainage system, typically pipes with diameter greater than 600mm, are illustrated. In some areas, limited or very old information has been provided for the drainage network. Additional information regarding these areas was provided by the City of Greater Geelong and the data set was deemed complete. 2-1

14 PROJECT METHODOLOGY Barwon Water Barwon Water have supplied the following data: Cadastre over the Rippleside catchment Planning Scheme Zones over the catchment 1m contours over the catchment The initially supplied contour data set covered only part of the catchment of Rippleside. Additional time was required to complete the data set by digitising the remainder of the catchment from the Corio 2500 series maps. 2.2 Digital Elevation Model Preparation of the Digital Elevation Models used in the study is discussed in Section 3. Briefly, the process for the preparation of the DEMs is described below. The DEM for the mapping area was developed using the points and break lines derived from photogrammetry. Establishment of the DTM involved the development of Triangulated Irregular Network (TIN) using these points and breaklines to establish a continuous triangular 3D shape of the area. For mapping purposes the continuous TIN is sampled at the requisite model resolution to create a grid of data points forming the DEM. Vertical Mapper, which operates within the MapInfo GIS package, was used to create the DEM. Within the flood mapping area the sampling resolution for the DEM is 1m. Based on our past experience, we have found that this level of detail is well suited to simulating the topography of heavily urbanised environments. The DEM was reviewed to identify any anomalies. Site inspections were undertaken to verify terrain shape. 2.3 Hydrological and Hydraulic Modelling, and Mapping of Existing Conditions This Section discusses our approach to developing hydrological and hydraulic models, simulating existing flooding characteristics and developing flood maps for existing conditions Hydrologic Analysis Model Development A RORB (Version 4.2) hydrological model was developed for the entire catchment to simulate rainfall runoff characteristics and generate inflows for the hydraulic model. The RORB hydrological model simulates catchment storage and routing characteristics, including the incorporation of retarding basins or areas that provide natural retardation of flows. The development of hydrological model involves the following key steps: subcatchment definition was undertaken using available topographical information for the study area. Existing information is considered sufficient in this regard. Subcatchment delineation took 2-2

15 PROJECT METHODOLOGY 2-3 into consideration compatibility with the required extent of hydraulic analysis as well as land use characteristics. landuse characteristics were identified from the current planning scheme zonings. This information along with available aerial photography was used to derive proportions of perviousness and imperviousness within each subcatchment. model loss parameters were applied in accordance with the catchment characteristics and based on similar hydrological investigations in the region. channel routing methods within RORB was selected in consideration of reach type and incorporating physical data relating to length and slope. storage and routing parameters (k and m) were determined from theoretical calculations for the derivation of default values. Historical stream flow information for hydrological model calibration does not exist in the study area. Verification of peak flows generated by the RORB models was undertaken in consideration of values derived using the Rational Method. As part of this process RORB model parameters were adjusted to achieve consistency between both approaches Design Event Modelling RORB was used to derive hydrographs as input to the hydrodynamic model of the drainage system. Temporal patterns and design rainfall intensities were derived from Australian Rainfall and Runoff IFD curves and maps. This process involved the derivation of design rainfall events covering a range of return periods and durations for each drainage sub area. The 5, 10, 20, 50, and 100 year ARI events were simulated Hydraulic Analysis Model Selection and Development Originally, the study was to be undertaken using MIKE11, a 1D hydraulic modelling package from DHI. Initial testing of the model shows good correlation with anecdotal evidence of flooding within the catchment, particularly in the lower portions. However, the complex distribution of flows in the upper sections of the model has difficult to model with an appropriate degree of certainty. This is due to a number of factors, including: The requirement in MIKE11 to predefine flow paths as model branches. These are typically defined in consideration of relatively low flows. However the alignment of these paths can change in higher flow situations, either by short circuiting, breakouts with looped anabranch flows or transverse flow between parallel channels. The ability of MIKE11 to handle sheet flow situations through, for example, residential areas. Predefinition of these types of flow paths as 1 dimension branches does not take into consideration the effect of momentum in the 2D horizontal plane. 2-3

16 PROJECT METHODOLOGY 2-4 In consideration of these, and in consultation with the City of Greater Geelong, it was decided to migrate the modelling to a 2D platform, TUFLOW, which has the capability to overcome the problems above while providing the necessary level of detail. Section 4.2 describes the selection and development of the 2D hydraulic model Boundary Conditions The hydraulic model requires downstream boundary conditions and lateral inflow boundary conditions representing the runoff from the catchment. The downstream boundary condition has been specified as a constant tailwater level reflecting the mean tidal level in Corio Bay. Catchment runoff was modelled using the hydrological (RORB) model, as previously discussed, and the output will subsequently provide inflows to the hydraulic model. The hydrological model provides flow outputs at nodes at designated locations throughout the catchment. The conventional approach is to then simulate these inflows in the hydraulic model as a series of individual point discharges. This approach is suitable when simulating the discharge from a single pipe outlet, however, in reality runoff will also enter the drainage system as dispersed inflows (overland flows) along the length of the drainage system. Based on our previous experience with modelling urban drainage systems similar to the Rippleside Catchment, we have found that the performance of the hydraulic model in simulating discharges and water levels is sensitive to the proposed method of simulating inflows. In response to the above issue, WBM Oceanics Australia has developed in-house software that interfaces hydrological model output and TUFLOW. This software provides the flexibility of generating a number of smaller lateral inflow boundary conditions along flow paths in the model to simulate dispersed overland flow more accurately. 2.4 Mitigation Option Assessment and Mapping This section outlines our approach when assessing the flooding characteristics of the existing drainage system and identifying possible mitigation options to alleviate flooding problems Review of Existing Flooding Impacts Impact Identification Results from the existing flood simulations would be reviewed to identify major constriction or conveyance deficiencies that are contributing to flooding impacts. In this regard, the impact identification involved: Design event simulation for a range a design flood events to consider the capacity of various components of the existing system, and when and where flood problems are likely to occur; Identification and documentation of flood prone areas where properties and infrastructure are subject to flooding under varying ranges of event frequency; 2-4

17 PROJECT METHODOLOGY 2-5 Identification of under sized components of the system that may be causing flooding and backwater effects; Assessment of the influence of hydrograph peak timings from various subcatchments within the study area and the influence that this has on flooding impacts at particular locations; and GIS database analysis to provide a comprehensive list of all properties and infrastructure that is affected by flooding in the catchment Floor Level and Building Survey A detailed survey was conducted by City of Greater Geelong focussing on properties where model results indicate a significant threat from flooding. Floor level, property size and condition and other information was collected to be used in subsequent damages assessments Flood Damage A damages assessment for properties and infrastructure was carried out based on the ANUFLOOD method. The method contains appropriate relationships between depth of flooding and damages for a range of building types (eg. low level house of low value, high set house of medium value, high value commercial property) etc. These relationships account for such factors as the time of flood warning and the relative degree of flood preparedness of the community. Annual Average Damage predictions for urban areas are calculated from the damage curves and are used as a starting point for establishing cost/benefit ratios of the augmentation options considered Mitigation Option Assessment From the review of existing flooding a series of mitigation options were developed. Structural options will be considered to alleviate drainage bottle necks and storage problem areas. Typical structural measures considered included: Modification and resizing of major culverts and waterway openings which may be causing flooding impacts; Development of structural measures such as levee banks to protect properties from flooding; and Construction of major flood retarding structures to protect downstream areas. 2-5

18 PROJECT METHODOLOGY

19 DIGITAL ELEVATION MODEL DIGITAL ELEVATION MODEL 3.1 Catchment DEM A broad scale Digital Elevation Model (DEM) was developed for the entire catchment for use in hydrology components of the study. Barwon Water provided contours over the entire catchment at approximately 1.5m intervals (data derived from 6 contour information). These data were used to create a DEM with a 5m horizontal resolution. Figure 3.1 shows the entire catchment DEM. Figure 3.1 5m DEM and Catchment Boundary 3.2 Study Area DEM The detailed Digital Elevation Model (DEM) of the study area was developed from a range of data sources. It involved a combination of existing photogrammetry, new photogrammetry from existing low level aerial photography and ground controlled terrestrial survey. Information collected as part of the survey program was as follows: 3-1

20 DIGITAL ELEVATION MODEL 3-2 Existing Photogrammetry existing photogrammetric data from within the study area gathered as part of the 1992 survey. Low Level Aerial Photography (Barwon Water 1992) low level aerial photography collected by Barwon Water was obtained and reviewed for use in photogrammetric analysis. The photography was used to prepare digital terrain information of the remainder of the study area. Aerial Survey Photo Control photo control survey collected as part of the Barwon Water survey in 1992 was used to validate photogrammetry. Appendix A presents the survey report regarding the photogrammetry data extracted from the existing photography. Vertical accuracy of the photogrammetry was 0.1m. The DEM for the mapping area was developed using the points and break lines derived from photogrammetry. Figure 3.2 and Figure 3.3 illustrate the detailed DEM of the study area. Figure 3.2 1m DEM of Flood Mapping Area 3-2

21 DIGITAL ELEVATION MODEL 3-3 Figure 3.3 1m DEM of Flood Mapping Area - Perspective View The detailed 1m DEM was used as the basis for the preparation of the hydraulic model for the study area. It also provides the foundation for the derivation of flood extent and presentation of other flood mapping. 3-3

22 EXISTING CONDITIONS EXISTING CONDITIONS Assessment of the existing flood conditions throughout the catchment is presented in this section. This information provides the base for the assessment of the effectiveness of mitigation measures tested in Section Hydrology Model Description Hydrological modelling was performed in this study using the runoff and stream routing program RORB (Laurenson and Mein, 1997). RORB simulates the hydrological behaviour of a catchment by discretising the area into a series of sub-catchments joined by a series of reaches. Rainfall-runoff is simulated for each catchment with the hydrographs routed through the stream network using a nonlinear storage routing procedure. RORB is widely used in Victoria and is the package recommended by Melbourne Water for the assessment of hydrological characteristics of ungauged catchment in metropolitan areas Global Parameters Loss Parameters RORB generates runoff by subtracting losses at each timestep from the rainfall occurring in that time period, losses were assumed to comprise an initial loss followed by a continuing loss. An initial loss of 12.5 mm was adopted, and continuing losses were considered as a constant proportion of the rainfall in each timestep. Continuing loss in RORB for impervious areas is a predefined fraction of the rainfall where 10% of the rainfall per timestep is lost. The following volumetric runoff coefficients were adopted: 100y event y event y event y event y event Fraction Impervious The fraction of the catchment that is impervious is a key input to the hydrologic modelling. Impervious fractions for various planning scheme codes were identified based on previous experience in similar studies within metropolitan Melbourne (undertaken on behalf of Melbourne Water) and adopted in consultation with City of Greater Geelong. Key impervious fractions are presented in Table 4-1 below: 4-1

23 EXISTING CONDITIONS 4-2 Table 4-1 Impervious Fraction for Planning Scheme Zone Zone Impervious Fraction General Residential 0.4 General Commercial 0.8 General Industrial 0.8 Public Open Space 0.1 Roads 0.9 General Farming 0.1 Special Public Purpose (e.g. school) 0.6 The planning scheme data were used to establish a spatially weighted average of the impervious fractions for each hydrologic modelling sub-area Reach Storage Relationships RORB simulates the linkages between sub-catchments as reach storages with the storage discharge relationship defined by the following equation; S = 3600kQ m Where S represents the storage (m 3 ), Q is the discharge (m 3 /s), m is a dimensionless exponent and k is non-dimensional empirical coefficient. k is defined by the product of the catchment value k c and the individual reach k i. Both m and k c are defined as calibration parameters. These were defined based on previous experience in RORB modelling in metropolitan Melbourne. For all catchments m was assumed to be equivalent to 0.8 and k c was calculated using the following equation; k c = 1.53A 0.53 where A is equivalent to the catchment area. This equation was sourced from Melbourne Water and is recommended by Melbourne Water for use in urban catchments throughout metropolitan Melbourne. Melbourne Water has undertaken considerable research to establish this relationship and is considered the most appropriate for use in metropolitan Melbourne Sub-Catchment Definition Sub-catchments used in the hydrologic modelling process were defined using a combination of City of Greater Geelong drainage scheme plans and topographic data. Where drainage scheme data was not available sub-catchments were defined using topographic data. Assessment of the hydrological characteristics of the catchment was undertaken using RORB. Sub-catchment breakdown is presented in Figure 4.1, also showing the location (in red) of checkpoints for hydrological model verification (Section 4.1.4). 4-2

24 EXISTING CONDITIONS Model Verification No data were available for model calibration. To determine the overall validity of the hydrologic modelling, checks of peak flow against recognised calculation methods were undertaken and are presented below Volume Checks As a preliminary check to ensure the RORB model was correctly configured, the runoff volume generated by the numeric model was compared against the volume derived by distributing the rainfall depth over the catchment area. The primary purpose of this check is to ensure there were no model leaks or short circuits. The runoff volume for each catchment was estimated by distributing a 2 hour 100yr ARI rainfall event over the catchment and recording the volume of runoff generated at the outlet of the catchment. To ensure no loss of rainfall occurred the catchment parameters were set as follows, initial loss = 0 mm and runoff coefficient = 1. The volume of runoff generated by the model was compared to the volume calculated by multiplying the catchment area by the rainfall depth. In all cases, the difference in runoff volume estimated by both methods was less than 1% of the total runoff volume and thus the volume checks were satisfied Rational Method Checks In addition to the volume checks a further check of each RORB model was performed to verify the peak flows generated. The RORB model was configured and run assuming that the catchment contained no storages, these results were compared to the peak flow estimated using the Rational Method, defined by the following equation; Q100 = C100. I, 100 A t c Where Q 100 Peak discharge Runoff coefficient for a 1% probability event C 100 I tc,100 t c A 100 year ARI rainfall intensity for a storm of duration t c Time of concentration of the catchment Catchment Area A runoff coefficient of 0.6 was adopted in line with the hydrologic modelling assumptions. The rainfall intensity was estimated using the Algebraic Method outlined in Book 2 of Australian Rainfall and Runoff (Institution of Engineers, 1998). The critical storm duration was checked by comparing the duration of the critical storm from the RORB results with the time of concentration estimated using the Bransbury Williams method. Bransbury Williams The Bransbury Williams formula is recommended as a reasonable approach to estimate the time of concentration of a catchment as it includes provision to allow for slope of the catchment when calculating response times; 4-3

25 EXISTING CONDITIONS 4-4 t c = 58L A S e Where t c time of concentration L mainstream length measured from catchment divide A catchment area S e equal area slope of the main stream projected to the catchment divide The Bransbury Williams method is used principally for the estimation of flows in ungauged rural catchments. Limitations in the use of this method in urban areas chiefly relate to the lumping of parameters at a single point in the catchment such that there is no distribution of catchment characteristics. Nevertheless, past experience has shown these methods to be very useful in providing first pass estimates of time of concentration and peak flow. Table 4-2 below presents the results of the model verification. Table 4-2 Hydrological Model Verification Location Rational Method Flow (m 3 /s) RORB Peak Flow (m 3 /s) Node 13 Node 35 Node 52 North Node 52 South Node 52 Total The estimates of flow obtained from the Bransbury Williams method agree with the values obtained from RORB. This comparison yields a high degree of confidence in the performance of the RORB model and is considered suitable for use in the preparation of design event hydrology information for subsequent hydraulic modelling. 4.2 Hydraulics Model Selection Hydraulic assessment of the flooding characteristics within the Rippleside catchment was undertaken using a fully 2D modelling approach that included the provision of key elements of the underground drainage system. The model provided full dynamic interaction of the overland and underground systems. WBM s modelling system TUFLOW was favoured over other similar systems (eg. Mike21) for the following reasons: Fully geo-referenced flood modelling capability allowing direct integration of the DEM information within the model. 4-4

26 EXISTING CONDITIONS 4-5 Dynamic simulation of 1D elements means that culverts, bridges, weirs and other 1 dimensional elements are modelled at the same time as the overland system and allow direct interaction between the two conveyance mechanisms. Fully integrated with GIS for floodmapping of the resulting flood extent and inundation depths allowing easy interrogation of flood information against other GIS related data (eg. floor levels, property boundaries). Suitability for time varying simulations with robust wetting and drying algorithms promoting model stability and accurate representation of floodplain hydraulics Model Development Drainage design information was used to provide detailed information in the development of the TUFLOW hydraulic model. Overland flood flow paths were established from on-site inspections and review of the detailed Digital Elevation Model Model Geometry The geometry of the 2D model was established by constructing a uniform grid of rectangular cells using the following information: The detailed Digital Elevation Model developed from photogrammetry of the study area; Major waterway openings associated with road crossings (eg. Weddell Rd); and Key control points based on specific ground survey. As discussed previously, 1D components of the model included key elements of the underground drainage system, hydraulic structures and extensions upstream and downstream of the 2D modelling domain. These components of the model were established using the detailed drainage information provided by City of Greater Geelong. The model was developed using typical roughness parameters for the urban areas. Table 4-3 below presents the key Manning s n coefficients used in the model. Table 4-3 TUFLOW Manning s n Coefficients Model Resolution Commercial/Industrial Residential Roads Grassed Floodways Railway One of the key considerations in establishing a 2D hydraulic model relates to the selection of an appropriate grid size. Grid size or model resolution must be balanced in consideration of the goals of 4-5

27 EXISTING CONDITIONS 4-6 the study and computation efficiency. Accordingly, the grid resolution must be selected to provide a suitable compromise of the following: The grid resolution must be fine enough to provide sufficient representation of the modelling domain to accurately simulate the physical characteristics the study area; and The grid resolution must result in a model with number of elements that will not result in unrealistically long run times. Model run times of greater than 10 hours are generally not considered to be practical. The computation cell size for the TUFLOW model was set at 5m. In adopting the grid size for the Rippleside 2D model the above mentioned issues were considered in accordance with the final objectives of the study and production of detailed flood maps. A 5m grid size over the study area provides a good definition of land shape, key controls and waterways. In TUFLOW, the computation nodes are located at cell centres, midsides and corners, such that the spatial resolution of the underlying terrain is twice that of the model (ie. 2.5m). Water levels are calculated at cell corners and velocities at cell midsides. The model domain and underground network are shown in Figure 4.2. All hydraulic parameters describing the underground pipe network were provided in the City of Greater Geelong data set. It has been assumed that there is no restriction to the entry of stormwaters into the underground system and that the pipes are free from debris, etc such that they flow at design capacity Model Boundary Conditions Boundary conditions for flood water inflows are provided by the hydrologic (RORB) modelling. Point inflows, from an adjoining sub-catchment, are input at the edge of the model. Local inflows, derived from a catchment within the 2D model domain, are distributed as inflows at a number of locations along the reach, depending on the length of reach and size of catchment. The downstream boundary condition for the mapping area is a constant tail water level set at mean sea level in Corio Bay. Sensitivity testing indicated that setting this level at high or low water did not influence flooding in the study area of the Rippleside catchment. 4.3 Flood Mapping For each event, simulations of the range of selected durations were undertaken. The maxima of these simulations provides the flood level envelope. Flood extents are derived from comparison of these data with the DEM and are shown in Figure 4.3. Figure 4.3 displays the inundation extent for the design 100y event. Complete mapping for the existing conditions for the 5y, 10y, 20y, 50y and 100y design flood events is provided on separate plans. The number of properties inundated above floor level is shown in Table

28 EXISTING CONDITIONS 4-7 Table 4-4 Number of Properties Inundated Above Floor Level Design Event 5y 10y 20y 50y 100y Number of Properties Inundated above Floor Level Building Inundation The modelling indicates flood inundation to a number of residential and commercial properties in the drainage system. Floor level survey was gathered by City of Greater Geelong and compared with the inundation depths generated by the model. In total, approximately 130 floor levels were gathered and of these it was found that 21 properties were inundated above floor level. Figure 4.4 below shows the 100y design flood extent and surveyed property locations. Properties shown in red indicated a flood level above the floor level at that location, while points in green indicate a floor level above 100y design flood level. The floor level survey revealed many properties with floor levels near the 100y design flood level. Of the 130 properties surveyed, 56 were within 25mm of the 100y design flood level at that location. Due to sub-scale features (ie topographic features on a less than 5m scale) flooding of individual properties may or may not occur. 4.5 Flood Hazard Mapping Public Safety is a significant issue for Council. In terms of flooding and drainage, public safety relates to direct risks from flooding (eg. injury or death resulting from drowning) and indirect (eg. injury from car accidents resulting from poorly drained roadways). A measure of Public Safety or Risk is often defined relating to flood depth and/or flood velocity. In this investigation, safety criteria have been adopted using velocity and depth criteria as defined by Melbourne Water (as instructed by Council). The criteria are defined in Table 4-5 below. The quantity RISK is defined as: RISK = maximum ( Velocity x Depth, Depth ) Table 4-5 Hazard Categories Risk Category Criteria Low RISK 0.4 Moderate 0.4 < RISK 0.8 High RISK >

29 EXISTING CONDITIONS 4-8 Within the catchment of Rippleside RISK is presented for the existing case (related to the 100 year design event) in Figure 4.5. The figure indicates that areas of high and unacceptable RISK are limited to flooded areas of significant depth inundation. Areas of particular concern are: Lantana Ave Grace McKellar Golf Course areas Lily Street Retarding basins upstream of Weddell Road Baxter Road Retarding Basin 4.6 Flooding Issues The design flood modelling has illustrated a number of flood related issues for investigation. Residents within the area have previously identified many of these issues. Figure 4.6 below identifies the key flooding issues for which subsequent mitigation options will seek to minimise. 1 Flooding in Colville Ct 2 Overland flow through Bayview Parade properties 3 Overland flow through Lily St properties 4 Flooding Behind Railway in Lantana Ave and Marlo St 5 Flooding through Montgomery Street properties 6 Flooding along Hepner Place 7 Overland flow along The Fairway 8 No increase in Flooding at Baxter Road The identification of these issues leads to the preparation of mitigation options. These are presented in the next section. It should be noted that: Floods greater than the one percent flood can occur. During such floods an area greater than that shown would be inundated. Conversely properties within the area shown can be affected by floods of lesser magnitude. Local flooding of other areas, or in excess of levels shown, may occur. The extent of flooding shown relates to flooding from mapped reaches of Council's drainage systems only, and does not adjacent catchments or private drainage systems. Local increases in flood levels, depths and/or velocities shown may result from local factors such as drain blockages, and local obstructions to overland flows such as fences, buildings and cars. 4-8

30 EXISTING CONDITIONS

31 EXISTING CONDITIONS

32 EXISTING CONDITIONS

33 EXISTING CONDITIONS

34 EXISTING CONDITIONS

35 EXISTING CONDITIONS

36 MITIGATION ASSESSMENT MITIGATION ASSESSMENT Review of the existing flood situation in Rippleside has identified a number of significant flooding issues. In this section drainage augmentation options to address these issues are evaluated. 5.1 Mitigation Option Elements Figure 5.2 below presents the various mitigation option elements tested at Rippleside. Following consultation with the City of Greater Geelong a range of option elements were developed. The following describes 12 of the elements tested (selected as practical options for addressing individual flooding issues). A brief rationale behind their development is included. 0 Do Nothing The Do Nothing or maintain the status quo option needs to be considered as a base from which comparisons can be made. 1 Lily Street Augment pipe system along the length of Lily Street This option is aimed at addressing flooding issues in Lily Street and potentially in the Bayview Parade area. Pipes located under Lily St as locating the pipes under the drainage line would require placement through properties with residences (some as slab on ground) 1a Lily Street Augment pipe system in lower Lily Street only The most sever flooding in Lily Street is at the lower end. This option to address the most severe flooding directly with possible follow on improvements in the upper system 2 Retarding Basin upstream of Lantana Ave This option is designed to reduce/control the volume of floodwater entering the Lantana Ave area from the upper catchment. Some redirection of upper cathcment flows in included to minimise the depth of flooding in Lanatana Ave with possible follow on effects downstream in Montgomery Ave and Ballarat Rd 3 Upgrade pipe capacity under Ballarat Rd This option is to directly address the lack of capacity under Ballarat Rd with follow on effects at Montgomery Ave 4 Upgrade pipe capacity under Hepner Pl This option is to address flooding in Hepner Place due to exceeded pipe capacity. 4a Upgrade pipe capacity under Hepner Pl and add Retarding Basin below Hepner Pl Increasing flow conveyance in Hepner Pl may increase flooding in downstream areas. This option is aimed at reducing any such adverse impacts 5 Retarding Basin upstream of Hepner Place An alternative to Option 4/4a. Construction of a retarding basin upstream of Hepner Place in the Golf Course to reduce/control flood flows along Hepner Place 6 Pipe upgrade through Grace McKellar An attempt to reduce flooding in the Grace McKellar property by increasing the capacity of the underground system. Includes pipe upgrades under Ballarat Rd. 7 Retarding Basin in Hurst Reserve This option designed to reduce/control overland flood flows through the Bayview Parade area. Some excavation of the park is included to maximise storage 8 Pipe upgrade along The Fairway Flood flows along The Fairway restrict access to and from properties in the street and pose a 5-1

37 MITIGATION ASSESSMENT 5-2 public safety risk. This option is designed to minimise that risk by providing additional underground capacity. 9 Upgrade pipe capacity from Lantana Ave through to Ballarat Rd This option designed to address flooding issues in the Lantana Ave/Montgomery Ave area and at Ballarat Rd. Each of these elements has various impacts on the existing flood characteristics of Rippleside. Appendix B illustrates the flooding impact graphically as an increase or decrease in flood level resulting from the hydraulic influence of each element. The floor level survey data has been integrated with the option assessment. Of the 130 properties that were surveyed, comparisons of flood and floor levels indicate that 21 buildings are inundated above floor level in the 100y design event. Only 9 floors are inundated by greater than 0.1m. Table 5-1 below indicates the relief provided to relevant flooded floors by each of the elements tested. In the table, Floors Affected means the number of floors flooded in the existing case for which the element is attempting to provide relief. Floors Relieved is the number of those floors for which relief is provided. Elsewhere indicates secondary flood effects (positive or negative) of the option. Within the catchment there are two areas where flooding is of particular concern due to the frequency of flooding (from resident complaints) and the severity of the depth of flooding leading t a significant public safety risk. These areas are: Drainage line parallel to Lily Street Lantana Ave through to Ballarat Rd In both cases only a limited number of properties are inundated above floor level for the 100y design event. Lily Street Although only 2 floors are inundated in the 100y event, 27 properties in this area are affected by flooding. Pipe upgrades in Lily Street (Option 1 and 1a) appear to adequately address the flooding of floors at units at the corner of Lily and Graham Streets and reduce flood levels in the area. However, the gully line runs through many properties and complete flood relief through the provision of 100y capacity drainage is unlikely to be practical. Lantana Ave to Ballarat Rd Flooding in this area is due primarily to the ponding of flood waters behind barriers. In Lantana Ave the Railway line constrains flood conveyance while on Ballarat Rd, the road and the median strip prevent adequate flood conveyance. In total, some 30 properties are affected with 5 properties inundated above floor level. Options 2, 3, 6 and 9 all provide differing mechanism for flood relief. However, in each case, flood levels reduce below floor level for only 2 of the properties flood affected above floor level. 5-2

38 MITIGATION ASSESSMENT 5-3 Table 5-1 Effect of Options on Existing Flooding Case Floors Affected Floors Relieved Elsewhere 1. Lily St Pipe Upgrade 2 2 Increase in flood levels in Baxter Rd RB 1a. Lower Lily St Pipe Upgrade 2 2 Increase in flood levels in Baxter Rd RB 2. Lantana Ave Retarding Basin additional floor relieved 3. Ballarat Rd Pipe Upgrade Hepner Pl Pipe Upgrade 2 2 Increase in flood levels in Baxter Rd RB 4a. Hepner Pl Pipe Upgrade and Weddell Road RB Hepner Pl Retarding Basin Grace McKellar Pipe Upgrade additional floor relieved 7. Hurst Reserve Retarding Basin additional floors flooded 8. The Fairway Pipe Upgrade Lantana Ave Ballarat Rd Pipe Upgrade

39 MITIGATION ASSESSMENT Preferred Approach for Augmentation From review of the affect of each of the flood options a combination of elements was identified that best addresses flooding issues in the Rippleside catchment. Figure 5.3 below illustrates the combined elements, as listed below: 1a Lily Street Augment pipe system in lower Lily Street (2 x 1350 RCP x 150m) 4a Upgrade pipe capacity under Hepner Place (1 x 1500 RCP x 25m, 2 x 1200 RCP x 235m, 2 x 1350 RCP x 35m) and add Retarding Basin below Hepner Place 6 Pipe upgrade through Grace McKellar (1 x 1050 RCP x 70m, 1 x 1200 RCP x 70m, 2 x 1050 RCP x 130) and under Ballarat Rd (4 x 1050 RCP x 20m, 4 x 1200 RCP x 20m) 8 Pipe upgrade along The Fairway (2 x 900 RCP x 380m) The retarding basin proposed below Hepner place is designed with a bund wall set to a level of 14.5m AHD. The design capacity at full level is 15,850m 3. The storage curve for the retarding basin is shown in Figure 5.1 below Water Level (m AHD) Storage (m 3 ) Figure 5.1 Weddell Road Retarding Basin Storage Characteristics Existing Case design flow information is provided in Appendix D. Figure 5.4 displays the inundation extent for the design 100y event for the preferred combination of options. Complete mapping for the preferred case for the 5y, 10y, 20y, 50y and 100y design flood events is provided on separate plans. Of the 21 properties that experience flooding above floor level in the 100y design event the augmentation relieves the threat to only 7. Properties that remain likely to be inundated in the 100y design event are shown in Figure