Selection of appropriate water-sensitive systems for stormwater quantity control in South Australia

Conventional Australian practice in managing stormwater generated in new residential sub-divisions uses traditional pit-and-pipe systems linked to detention basins to reduce peak discharges to pre-development levels. The question arises: can similar flow management be achieved cost-effectively in sub-divisions using infiltration systems and rainwater tanks? The layout of a contemporary residential sub-division in Strathalbyn, South Australia, was selected for a case study to answer this question. The sub-division consists of 56 developed allotments varying in size from 491 m2 to 2787 m2. Flood management performances of the sub-division is compared on the basis of 10 year and 20 year ARIs. Three soil types - sandy clay and two grades of medium clay are employed in the modelling. Clearance distance criteria to footings and boundaries are recognised for the three soils. Three infiltration systems - leaky wells, infiltration trenches and soakaways are considered. The emptying time criteria adopted are 2 days for the 10 year ARI and 2.5 days for the 20 year ARI. The rainwater tank option was employed only where all three infiltration options could not be used because of clearance distance or emptying time criteria being violated. Stage 1 of the project involved fitting three infiltration systems - leaky wells, infiltration trenches and soakaways into the allotments, taking account of required clearance distances and emptying times. It was found that these systems gave satisfactory performance in all 56 allotments where the soil was sandy clay. Infiltration systems could only be used in 49 allotments in the high medium clay case; they all failed to meet the required criteria in the low medium clay soil. In this case the rainwater tank option was the only water-sensitive measure that could meet the required criteria.