Water Journal : Water Journal September 2012-1
sustainable water management technical features 80 SEPTEMBER 2012 water • Australian Guidelines for Water Recycling: Stormwater Harvesting and Reuse, July 2009; • Model for Urban Stormwater Improvement and Conceptualisation (MUSIC); • CWW internally developed runoff and harvesting tools using historical climate data; and • Code of Practice: Irrigated Public Open Space South Australia (CoP). A concept design was developed for each project highlighting a number of underlying risks and assumptions. Investigation of the schemes' risks and assumptions was undertaken by the following method: 1. Demand and rainfall modelling was undertaken to assess whether sufficient runoff could be harvested and make the projects financially viable; 2. Harvesting impacts on the wetlands and downstream waterways; 3. Assessment of wetlands for suitability of stormwater harvesting and their ability to provide fit-for-purpose non-potable water; 4. Design and environmental site constraints: assessing flora and fauna and cultural heritage aspects. The initial concepts were to use the existing wetlands as storage and treatment by either using or raising the permanent pool depth. The permanent pool would be drawn upon directly from the last pond in the wetlands flow and would supply the end users on an as- needs basis. The last pond was chosen because the water was considered to be the cleanest, and because the pond was closest to the demand locations. The concept extraction points can be seeninFigures1and2bytheredXfor Schemes A and B, respectively. Discussion Using catchment characteristics, field data and the as-constructed designs, the wetlands' flow was recreated in MUSIC. Climatic data for the long-term average year of Western Melbourne was used to estimate the annual harvested volume. The wetlands' outfall data was then used in internally developed stormwater harvesting models. The internal models follow best practice using the (CoP) to determine turf irrigation demand. The outputs from the models indicate that, on average, a satisfactory and financially viable amount of water could be harvested from the wetlands provided relatively large storages for each scheme were provided. Separate modelling was undertaken to determine how harvesting from the wetlands' sub-catchments affected the flow into downstream water systems. The modelling indicated that, with harvesting, only 25% of flows had a reduction in greater than 10% instantaneous flow. This includes low flow conditions when the modelled flow was less than 1L/s. The modelling outputs were considered acceptable and it was decided to proceed further with the concept investigations. Options were explored to store the water. It was initially conceived to raise the permanent pool depth in both the wetlands; however, expert advice from the wetlands owner, Melbourne Water, indicated that this would hamper the performance of the wetlands in relation to flood mitigation and could affect the wetland ecology. This was an undesirable outcome. An extraction system that could operate within the extended detention depth was considered acceptable by Melbourne Water. Foxwood Dve Dunnings Rd Boardwalk Estate Wetlands Carranballac College, Boardwalk Campus City of Wyndham Reserve Emmanuel College X Figure 1. Scheme A.
Water Journal November 2012-1
Water Journal August 2012