Flood Mitigation Benefits of Rainwater Tanks S. A. Gato-Trinidad1, D. Alexander, 1 J. Barker1 and T. Postlewaithe 1Centre for Sustainable Infrastructure, Swinburne University of Technology, John Street, Hawthorn, Victoria 3122, Australia 1. INTRODUCTION Flood mitigation can be achieved through on-site detention methods which also offers associated economic, environmental and social benefits. The purpose of this paper was to assess the flood mitigation benefits and the associated environmental, social and economic benefits of decentralised tanks within each household. The paper focused on how much reduction in peak flood volumes and other associated benefits can be achieved from rainwater tanks when calculated at a lot-scale in an existing catchment. The catchment in study is Sutton Lakes within Knox City Council, Victoria, Australia where there are several storm water related issues, including the traditional inadequate piped storm water network and various water quality concerns within the lake system. Based on flooding and water quality computer modelling it was revealed that during the 5 year ARI storm there was a 12.6% reduction in pipe flows from the outlet of the system and a reduction of 1.7% in a 100- year ARI event. A reduction of 17% and 11% in overland flows for 5-year and 100-year ARI events respectively and a decrease of 18% in wetland area required were also achieved. 1.1. Background Many of the developed parts of Australia have suffered severe drought conditions for more than a decade which makes water as a precious commodity. Yet some Australian cities also suffer from flooding. The growing population and booming economy means increasing water demand and competing uses of water. Greater Melbourne, Australia experienced low rainfall in the last 18 years which resulted to low inflows to storages (Figure 1). The reduction in inflows to storages forced water authorities to impose water restrictions and implementation of water conservation programs. Figure 1 Water flowing into Melbourne’s main water supply reservoirs – annual totals (GL/year), MW 2016 While other areas are experiencing drought, flooding especially flash flooding occurs in some areas due to increasing urbanisation which change the natural drainage regime. Increasing urbanisation leads to increasing runoff volume and increasing runoff speed as shown in Figure 2. -1- Figure 2 Typical hydrograph changes due to increase in impervious surfaces, Zevenbergen, et al (2010) 1.2. Measures in Place With drought and flooding affecting some areas of Australia, it has been recognised that an appropriate management of water cycle is the best way forward. This recognition has led to the development of the concept using runoff from city areas as a potential source of water supply. This enables a hybrid of decentralised and centralised water management solutions that ensure resilience to both water poor and water abundant areas at the same time delivers multiple benefits to people and environment. Rainwater tanks are essential resource that provides a renewable supply of water and can be used for a range of purposes including drinking, washing, bathing, laundry and gardening. In some parts of Australia it may be the main source of household water, while in others, it can supplement existing mains or town water supplies. Rainwater can assist self sufficiency and can provide a valuable alternative supply in times of drought or water restrictions. In March 2013, 34% of Australian households living in a dwelling suitable for a rainwater tank had a rainwater tank compared with 32% in 2010 and 24% in 2007 (ABS, 2013). The increase from 2007 to 2013 may be attributed to water restrictions, government rebate schemes, water regulations and water pricing (Figure 3). Rainwater tanks were a more prevalent feature for households residing outside capital cities (44%) compared with those living in capital cities (28%). Around 86% of South Australian households living outside of Adelaide had a rainwater tank installed at their dwelling, followed by 56% of Victorian households living outside of Melbourne. Of those households living in a state capital city, households in Brisbane and Adelaide were more likely to have a rainwater tank installed at their dwelling (47% and 44% respectively) followed by households in Melbourne (31%). In Victoria, rebates to install rainwater tanks were given since 2007 till June 2015. From then on, new housing construction are mandated to have rainwater tanks Figure 3. Percentage of Households with a rainwater tank installed by State Capital City, ABS 2013 -2- 1.3. Objectives of the Study While a number of previous studies were conducted on the cost benefit of rainwater tanks, the focus was more on water and cost savings for household owners and the government providing the rebates (Gato, 2014). Other studies focused on the efficient sizing of rainwater tanks and reliability. And although there are a number of studies in these areas, limitations on these studies still exist due to limitations of data and others were conducted based on hypothetical studies (Eroksuz and Rahman, 2010). Previous studies recognised the water savings from rainwater tanks, however, the full benefits of rainwater tanks is not limited to water savings and the corresponding cost benefits from the water saved. In order to determine the full extent of the benefits of rainwater tanks, there is a need to study the benefits of these tanks from the flood mitigation point of view. It cannot be denied that these tanks collect water from the roofs during rainfall events thus decreasing the amount of runoff and reducing peak flows and the time to peak. In this regard, the objective of this paper is to preliminary assess the effectiveness of household rainwater tanks in terms of flood mitigation. Specifically, to determine the reduction in peak flood volumes and other associated environmental, social and economic benefits. 1.4. Project Area Sutton Lakes is a residential subdivision located in Rowville approximately 26km South East of Melbourne CBD and is currently experiencing flooding (Figure 4). The total catchment size is 52.6 ha consisting of open space areas, approximately 400 households with an average block size of 740m2 and an average roof size of 280m2, and a trio of ornamental lakes that aid in flood mitigation. These lakes are serviced by both Melbourne Water and Knox City Council and suffer from water quality issues such as algal blooms that can be harmful to residents in the area. The catchment consists of traditional drainage pipe network connecting into the lakes/retarding basins which then flows into downstream Corhanwarrabul Creek. Figure 4 The Sutton Lakes Catchment 2. MODELLING Prior to the modelling of the pipe network and any additions of decentralised stormwater systems certain aspects of the project site had to be manipulated for the purpose of research. Given the current retarding basins service significant upstream catchments, a new retarding basin design was developed to focus only on the project site and best gauge the effects of any stormwater system additions to the catchment. The current retarding basin -3- system limits post development flows for 100 year ARI to those of predevelopment conditions. Therefore the new retarding basin system was designed to mimic these parameters in order to service the project site only. Decentralised stormwater systems were then added to this model in order to gauge any flood mitigation benefits through the analysis of reduction in pipe sizes required to convey flows and any overland flows. Environmental and economic benefits were determined through a pollutant and flow generation model which calculated the reduction in pollutant levels through various stormwater systems as well as various flow reductions through stormwater detention and re-use. 2.1. RORB and Retarding Basin Design In order to match predevelopment flows from the site a retarding basin was resized to allow a discharge from the site equivalent to predevelopment flows and a runoff model was created using an industry standard commercial program, RORB. A sub catchment analysis was required to be performed prior to model construction in order to determine appropriate catchments and their properties using the corresponding Rainfall Intensity-Frequency- Duration data of the project site (Bureau of Meteorology 2015). Critical flows for 100 year ARI are 2.35m3/s and occur at 9 hours duration. Therefore post development flows for the 100 year ARI were restricted to these flows through an appropriate retarding basin design. The retarding basin design is governed by the inflows to the system and the restrictions of the outlet structure. The depth of detention zone of the basin has a maximum value due to downstream boundary conditions, therefore the conditions that required designing included length, width and equivalent storage and the size of the outlet. Through the use of the orifice equation the outlet structure was determined to have a diameter of 930mm. RORB required the insertion of an elevation vs storage vs discharge relationship which was determined through use of the equivalent height above the orifice and the basin volume, in accordance with open water safety guidelines for slopes of the basin (Melbourne Water 2005). Once outflows were limited to 2.35m3/s the basin physical characteristics were determined to include a length of 80m, width of 60m and a total detention volume of 8,800m3. 2.2. Drains The following steps present the process taken in setting up the stages 2 and 3 models that have been configured in DRAINS. Before utilising the DRAINS program, extensive manipulation and data collection of the subcatchment was performed via a desktop study using GIS software, AutoCAD and Google maps Street view. The following data was created in order to fulfill the DRAINS stormwater management model: Contour smoothing (acts as a guide to develop sub-catchments) Pit identification including elevation sourced from LIDAR data (Pit types: Closed, sag or on-grade pit and AHD values) Pipe network simplification.