Sink or SWMM: Simulating the Hydrological Effects of Retention Tanks in a Small Urban Catchment

Abstract

A 12 ha urban catchment in Adelaide, South Australia is experiencing infill development. The impacts of urban developments on the catchment’s hydrological regime were examined. Rather than undertaking a costly upgrade of the stormwater system to mitigate these impacts, the City of West Torrens Council proposed the catchment-wide provision of rainwater retention tanks. A stormwater management model (SWMM) was used to simulate the runoff from the catchment at various stages of urbanization. Aerial imagery was used to develop a model of the catchment in 1993. Subsequent models were produced based on observed infill and impervious area increases in 2007 and 2015, with a projected scenario of 2040. The impacts of three different retention tank capacities (2, 5 & 10 kL) were investigated, along with three rates of household adoption of rainwater tanks (50, 75 & 100% of properties). Three usage levels for retained rainwater were also investigated: (1) All purposes except kitchen use and bathing; (2) solely outside the house, largely for irrigation; (3) solely for flushing toilets and laundry. Increases in the uptake of tanks in the catchment, tank sizes and water use from the tanks reduced total catchment runoff and peak flows, and augmented water savings. For the 2040 development scenario, runoff volume was reduced by 7.8–17.5%, five-year peak flow was reduced by 0.4–6.7% and each house saved around 24 kL of water a year with a 5 kL tank. The most influential rainwater tank variable was the percentage of properties equipped with tanks. It is recommended that as many houses as possible have 5 kL tanks installed, with the tanks plumbed to enable daily water use. This would lower the peak flow and total runoff below 1993 levels, cost the council less than $550 000 and provide homeowners with an alternative water source.