FLOOD MITIGATION FOR EXISTING URBAN ENVIRONMENTS

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1 FLOOD MITIGATION FOR EXISTING URBAN ENVIRONMENTS Justin Hinch Manager Infrastructure Services, Warrnambool City Council Abstract A significant proportion of the urban community of Warrnambool in Victoria s south west is centred on Russell Creek which is subject to flash flooding following significant storm events. The creek and the surrounding park lands provide an important social and environmental linkage which builds on the city s reputation as an open and green community, whilst also servicing the stormwater requirements from both urban and farmland areas. The creek corridor is narrow and bordered on both sides by residential properties. The confined nature of the floodplain is exacerbated by the proximity of the urban environment, aging infrastructure and the changing catchment characteristics which have been an ongoing concern for floodplain managers over a number of years. Further to this new development and intensification of existing residential areas continues to occur within the catchment, until recently without much consideration of future impacts. Investigations that span over a decade confirm the Council s own understanding of the flooding risk in and around the creek and have led to the identification of areas of significant flood hazard and depth in the Russell Creek catchments for flooding events up to and including the 1% Annual Exceedance Probability (AEP). These investigations identified hundreds of properties which were likely to be inundated during the 1% AEP flood event. A range of mitigation options were considered to minimise flood impacts and cost-benefit analysis related to property and building damage used as a means of comparing the available options. Delivered by the Warrnambool City Council in partnership with the Glenelg Hopkins Catchment Management Authority, the works which include the upgrade of a significant creek crossing and the construction of 1,200 m of flood walls (levee) have reduced the risk of above floor flooding for over 100 buildings and significantly reduced the flood hazard in and around the urban areas bordering the creek for a cost just under $4 million. Key Words: Stormwater, Flood Mitigation, Flooding, Floodplain, Urban Development, Partnership

2 Introduction A significant proportion of the urban community in Warrnambool, Victoria is centred on Russell Creek. Hydrology A RORB model was developed for the Russell Creek catchment, which was used to calculate flow hydrographs. The creek and the surrounding park lands provide an important social amenity to the local community, whilst also servicing the stormwater drainage flow from the surrounding area. The narrow creek corridor is considerably confined in a number of sections, with a number of dwellings and commercial buildings constructed close to the waterway flow path within this floodplain. The creek corridor is considered an important natural asset for the residents of Warrnambool and is used for walking, running, cycling and as open space with playgrounds. The creek corridor is readily accessible via a number of access ways which adjoin the creek. These unique characteristics along with the need for an aesthetically pleasing environment which does not compromise the privacy of adjoining properties formed many of the limitations in relation to flood mitigation planning. Floodplain Management The need for mitigation works including flood levees and improvements to the hydraulic capacity of Russell Creek has been established by a number of previous studies going back to Mapping produced in these studies ultimately indicated that during 1% Annual Exceedance Probability (AEP) flood events, a large number of properties and buildings are likely to be impacted by floodwaters. The nature of the Russell Creek floodplain is such that very little warning time is available to local residents in the event of a flood, being at risk of flash floods. This combined with the significant population at risk within this urban setting is the driver to establishing the need for structural works to reduce the flood risk. Figure 1 - Russell Creek catchment delineations (Sommerville, Design of North Warrnambool Flood Management Plan Phase 2 Mitigation Options, 2012) The 17 sub-catchments comprise of farming land, commercial and residential development areas and existing residential properties totals 32.7km 2. Russell Creek is an ungauged waterway without a calibrated model (from any historic events using anecdotal information). The Monte Carlo modelling method was adopted (a probabilistic approach), resulting in less uncertainty than a traditional deterministic approaches due to the range of input parameters considered. (Theilemann, Russell Creek Flood Mitigation - Revised Catchment Hydrology, 2017) The final parameters adopted for each design event were based on selecting a combination of parameters from the simulations which produces the peak flow along all reaches of Russell Creek. Design flows were validated against a range of other flow estimate methods including past studies, peak flow estimation equations and an existing RORB models. The hydraulic modelling included running four (4) critical storm durations including the 15 minute, 12 hour, 36 hour and 48 hour duration events for all AEP events including the 0.5%, 1%, 2%, 5%, 10% and 20% AEP.

3 Maximum flows typically fell between 18 to 48 hour storm durations which were longer than expected given the catchment was known to respond very quickly and which has a time of concentration of about 3 hours. Figure 2 - Existing 1% AEP Flood Extent (Theilemann, Design of North Warrnambool Flood Mitigation Works, 2014) A climate change scenario was considered in the hydrologic model to understand the impact higher intensity storm events may have on flows in the catchment. An increased rainfall intensity of 32% was applied which is the value adopted by Melbourne Water and represents the upper range of CSIRO predictions regarding the impact of climate change on rainfall intensity by (Theilemann, Design of North Warrnambool Flood Mitigation Works, 2014) The 1% AEP flood extent under climate change conditions indicates flood levels that are slightly greater than those produced by the 0.5% AEP event under current existing conditions. During this scenario, the flood walls effectiveness is compromised as the levees are outflanked at a number of locations but not significantly changing the cost benefit analysis related to flood mitigation options. The hydraulic modelling has been through a number of iterations between the initial flood investigations in 2003 and subsequent reviews in 2007, 2010, 2014 and While ever effort was made by highly experienced consultants to provide accurate modelling outcomes, given the lack of calibration of the hydrology and hydraulic models for Russell Creek confidence in the modelling outcomes has been a gradual increase with each model run. Cost Benefit Analysis Flood damage assessment methodology adopted was based on that previously utilised for a number of studies in Victoria, combining aspects of the Rapid Appraisal Method, ANUFLOOD and other relevant flood damage literature. (Theilemann, Design of North Warrnambool Flood Mitigation Works, 2014) A range of mitigation options were considered for adoption along Russell Creek to mitigate during flood events. Ultimately each mitigation measure was assessed against properties inundated, building impacted and overall damages from each event. An ultimate scenario was created to attempt to mitigate the majority of the flood affected properties and would undoubtedly require a significant length of the creek to have a levee constructed and local transport routes (road and pathway) closed. Using the elements of mitigation available under the ultimate scenario, combinations were modelled comparing flood extents and impacts on properties and buildings, damage costs and any increases in flood depths. Existing Conditions Post Flood Mitigation Buildings flooded above floor Properties flooded below floor Total damage $7,514,598 $1,985,383 cost Average annual damage $390,948 $197,650 Table 1 Analysis of impacts and damage for existing conditions vs. adopted post mitigation condition (Theilemann, Design of North Warrnambool Flood Mitigation Works, 2014)

4 The implementation of the proposed mitigation works results in a reduced risk of above floor flooding for over 100 buildings. Of the 30 buildings identified as flooded above floor post mitigation works: - A small number did not experience above floor flooding prior to the proposed works (has occurred due to the mitigation). - A small number which already experience above floor flooding had an increase in flood depth. Figure 3 Culvert preparation works, prior to closure of Mortlake Road Flood Mitigation Works The mitigation works along Russell Creek include the construction of four (4) flood levees. These levees intercept overland flow paths and endeavour to confine the floodwaters within the creek corridor. Works to improve the hydraulic capacity including floodplain contouring and additional culverts at Mortlake Road achieve an increased flow area. Stage 1 Major Culverts Construction Installation of additional culverts under Mortlake Road used two cells of culverts m long by 2.40 m high and span of 3.6m, with one row of culverts used for pedestrian and cycle access under Mortlake Road. Figure 4 Pre-cast reinforced concrete crown units and bases being installed Use of cement stabilised sand backfill was critical in limiting the time of the road closure due to reduced layering of backfill and compaction process. One of the biggest community concerns in delivering Stage 1 works, was the traffic management on Mortlake Road (a state highway), and one of Warrnambool s busiest thoroughfares. The road was required to be closed for seven (7) days during the initial installation of precast reinforced concrete crown units and bases causing major disruption and was remained under traffic management for more than two months whilst construction of wingwalls, road batters, guard rail, shared paths and other associated works were completed. Figure 5 Additional culverts post mitigation Stage 2 Levee Construction The most common levees used in Victoria historically have been conventional earthen

5 banks. However, due to the already restricted floodplain capacity a number of options were evaluated. - Earthen levees; - Crib walls; - Concrete retaining walls; and - Temporary flood barriers. It was determined that a concrete retaining wall structure (flood walls) was the most appropriate as they enabled the floodplain to be maintained for public open space and reduce privacy issues that could arise out of public access to a raised embankment abutting private property. Figure 6 View along the La Bella Court flood wall The design heights applied have provided for a freeboard of 600 mm above the applicable 1% AEP flood level in line with Levee Management Guidelines. (The State of Victoria Department of Environment, Land, Water and Planning, 2015) The freeboard allows for uncertainty inherent in design and also allows for difficulties associated with flood estimation, wave action and levee settlement. Wall / Levee No. Wall Length Wall (range) Height Figure 7 View along the Garden Street north flood wall 1 184m m 2 319m m 3 233m m 4 484m m Total 1,220m 1.1m (average) Table 2 Levee basic details (Theilemann, Design of North Warrnambool Flood Mitigation Works, 2014) Maintaining pedestrian access to the creek corridor over and around these walls was an important design consideration. These access points can be rapidly closed off in the event of a major flood. The access design involved the installation of an earthen ramp on both sides of the creek in order to clear the constructed flood wall and achieve safe trafficable slope. Due to the challenges in achieving DDA compliant accesses and considering privacy of adjoining properties, the pathway level adopted at the flood wall was at the 1% AEP level without the freeboard allowance.

6 Figure 8 La Bella Court pedestrian access before flood wall construction Figure 10 Drainage located at the base of the flood wall with outfall provisions centred in each panel (as needed) Other features of the flood wall design included: - Modular UB column and pre-cast concrete panel configuration - Elcoseal X800 geo-membrane to limit saturation of the area around flood wall footings Figure 9 La Bella Court pedestrian access post flood wall construction Hence, temporary flood wall components were designed into the pedestrian crossing and located on-site in secured storage for emergency service agencies. It is intended that the creek could hold flows, until agencies could respond and install the temporary walls. Maintaining drainage outfalls were one of the more challenging design considerations due to the number of properties that sloped and discharged directly to the creek. Meaning flood walls needed to maintain drainage outfall without compromising the flood wall function. This was achieved through a number of major outfall backflow values, but also the installation of subsoil drains on the property side of the flood wall, allowing soakage from the surface and direct connection from buildings. All of which was directed to regularly space outfalls, also using backflow valve devices. - Corking / sealing of panels at columns with high density concrete wedges and Sikaflex Pro - Built-in provision for fencing to be attached later for added privacy / screening Key Considerations Funding The implementation of the flood mitigation works identified has joint funding from all three (3) levels of government, facilitated through the Natural Disaster Resilience Grants Scheme Victoria. Funding for the project commenced under this arrangement in 2010/2011 and will conclude in 2017/2018, over which time the project will be completed at the cost of just under $4 million. A major factor in justifying the project cost relates back to the cost benefit analysis being able to show the investment in flood

7 mitigation infrastructure will be offset by the savings in damages. Community Engagement The project was delivered by Warrnambool City Council in partnership with the Glenelg Hopkins Catchment Management Authority. Like the flood studies, community engagement was undertaken a number of times in 2003, 2005, 2006, 2007 and then again in 2013 more specifically on the flood mitigation works that were being proposed. It was necessary to develop formal communication engagement plans, which included: - A pre-alert for all local politicians with information on how to respond to potential enquiries from concerned residents; - A staff briefing to ensure all relevant staff, including customer service, are briefed on the issues and aware of their role in the project; - Property owners and residents mailout, based on distribution of three letters: for all properties in the Russell Creek catchment for properties that have their buildings impacted by above floor flooding and how the mitigation works will assist for properties where mitigation works will not assist - Joint media releases from Council and the Catchment Management Authority; - or SMS alert sign-up for project updates; and - All explanatory material (i.e. fact sheets) uploaded to websites. The most common enquiries received were the focus of fact sheets prepared addressing: - Why there were properties that would not benefit from the mitigation works - What impacts the flood modelling and subsequent planning controls might have on insurance - Whether land will be able to be developed following the flood mitigation works - Requests that private accesses from properties direct to the creek reserve (both pedestrian and vehicular) be retained Associated Activities Planning Scheme Amendment The Victorian Planning Provisions (VPPs) allow for zoning of land and the application of controls on the type of land use and permitted activities in areas prone to flooding to protect life, property and community infrastructure from flood hazard, and to preserve flood conveyance capacity, floodplain storage and natural areas of environmental significance. A planning scheme amendment has already been developed related to the 1% AEP existing conditions, however is scheduled to be updated with the 1% AEP post mitigation condition. Much discussion has occurred on whether it is appropriate to adopt the mitigated conditions with respect to failure risks. Through the proposed amendment hearing process, it is expected this issue to be clarified. Flood Warning System A Total Flood Warning System (TFWS) action plan was developed for Russell Creek. The favoured approach is based around Event-Reporting Radio Telemetry System (ERTS) equipment, and the Bureau of Meteorology (BoM) provided Enviromon software with SMS alerting to opted-in community members and stakeholder entities via a commercial service provider.

8 Urban Drainage Strategy As it was critical that no additional flows enter the catchment through intensification of developable land, an Urban Drainage Strategy was prepared. This strategy ensures that sufficient supporting policy and area of land have been allocated to achieve the following outcomes: - Waterway reserves which provide consistency between developments - Consideration of flooding and ensuring no new properties are flood prone Figure 11 Decorative feature of the formal pedestrian underpass - Ensuring downstream properties and current mitigation works are not negatively impacted as a result of the new development - Ensuring best practice management of stormwater is undertaken The strategy identified the requirements to prevent future urban flooding and improve the urban design in new growth areas. Water quality was also identified as negatively impacted, with the increase in impervious area within each catchment. Hence, more detailed Water Sensitive Urban Design principles were prepared. (Law, 2013) Public Art Initiative Detailed design of the culverts, particularly at road level is considered an unremarkable bridge by most, however a splash of colour in the tunnel underneath has transformed the Russell Creek underpass which was redecorated in an eye-popping display by a former Warrnambool artist with a range of birds, fish and other native animals depicted in a mural completed over eight days. Noting that the underpass had significant users, from dog walking and school student, bike riders and the like, a project was instigated in collaboration with the Warrnambool Art Gallery to make it a place in which people can pause and reflect on life in the community. Figure 12 Decorative feature of the second culvert cell The artwork was financially supported by the Department of Justice Anti-Graffiti Fund, Brophy Family and Youth Services, and the Warrnambool Art Gallery s outreach program which aims to help local artists develop their skills while also engaging the wider community in the arts. Conclusion Mitigation of flood risk in the Russell Creek area through the design of flood wall levees for the highly urbanised floodplain provides great benefit in terms of reduction in flood impacts and flood damages to the community relative to the cost of implementation. The increase in hydraulic capacity of the creek through the additional Mortlake Road underpass adds to the overall improves and mitigated outcome for the catchment.

9 Due to the flash flooding nature of the Russell Creek catchment, flood warning, emergency management and planning controls for reducing flood risk are also appropriate mitigation measures to further reduce the flood risk in the catchment. Key Learnings Establishment of the hydrology and hydraulic parameters for modelling one of the most importance aspects of the planning process. Obtain extensive terrain / surface data such as building floor levels, detailed feature survey, waterway gauge readings and confirming land use. All of which improve the accuracy of the flood modelling. Community engagement at the early design phases was critical to inform property owners and residents of the proposed change to the environment. Keep the message consistent between stakeholders and avoid long periods without information going out. Stormwater and Floodplain Management policy will assist in making sure the outcomes of mitigation works are maintained into the future. Acknowledgements Such a significant project has many years of effort from a number of people along the way. My involvement and this report would not have been possible with the support of the following: - Sheree Cahill Environmental Engineer Glenelg Hopkins Catchment Management Authority - Johanna Theilemann Project Engineer Water Technology Pty Ltd - Rohan McKinnon Senior Project Engineer Warrnambool City Council - Rhyce Milward Construction Engineer Warrnambool City Council References Law, S. (2013). Warrnambool Urban Drainage Strategy. Notting Hill: Water Technology Pty Ltd. Sommerville, H. (2010). Design of North Warrnambool Floodplain Management Works. Cardno Victoria Pty Ltd. Sommerville, H. (2012). Design of North Warrnambool Flood Management Plan Phase 2 Mitigation Options. Cardno Victoria Pty Ltd. The State of Victoria Department of Environment, Land, Water and Planning. (2015). Levee Management Guidelines. The State of Victoria Department of Environment, Land, Water and Planning. Theilemann, J. (2014). Design of North Warrnambool Flood Mitigation Works. Notting Hill: Water Technology Pty Ltd. Theilemann, J. (2017). Russell Creek Flood Mitigation - Revised Catchment Hydrology. Notting Hill: Water Technology Pty Ltd. Author Biography Mr. Justin Hinch is currently Manager Infrastructure Services with the Warrnambool City Council in South West Victoria, a role where over the past five (5) years he has achieved a number of professional successes recognised by Institute of Public Works Engineers Australasia (IPWEA) including two Best Capital Works awards and Young Engineer / Practitioner of the Year.

10 Making up the remainder of his 17 years in Local Government, Justin progressed from Cadet Engineer to Senior Civil Engineer in the rural municipalities of Horsham Rural City Council and Mansfield Shire Council, and he has completed a Bachelor of Engineering Science and Diploma in Business Management. Justin maintains a strong focus on infrastructure planning and delivery, with a particular interest in stormwater management along with building young engineering professionals through leadership roles in the Local Government Infrastructure Design Association (LGIDA) and IPWEA Vic. Contact Details: Justin Hinch Manager Infrastructure Services Warrnambool City Council 25 Liebig Street Warrnambool 3280 P.O Box 198 Warrnambool 3280 T: E: jhinch@warrnambool.vic.gov.au