Single lot on site detention requirements in New South Wales Australia and its relation to holistic storm water management

Storm water run-off from urban developments need to be controlled to minimize its impacts on the quality and hydrodynamics of receiving waters. Holistic management strategies and treatment at the source have been introduced in Australia to mitigate the effects of run-off from urban developments on waterways. The adoption of conventional storm water quality and quantity controls and Water Sensitive Urban Design (WSUD) principles by councils, developers, industry and catchment managers assist in mitigating this impact. Pending on local council requirements, lot scale (re)development may include a combination of a rainwater tank, an On Site Detention (OSD) system and possibly an infiltration or bio-retention system, with overflows discharging to the existing drainage systems. It is argued in this paper that designing these systems independently may result in over-design, increasing development costs. This paper considers the effect of rainwater tanks on water quantity discharges on a lot scale, and how this can affect the design of OSD systems. This major research study, conducted in Western Sydney, indicated that where planning development controls require that storm water management be implemented to ensure run-off flow rates do not increase as a result of the development, a reduction can be applied to the OSD design, if a rainwater tank is installed on the same site as the OSD and is connected to multiple end-uses. This combination of systems can contain up to the 1-year Average Recurrence Interval (ARI) runoff in the rainwater tanks, thereby significantly reducing the OSD volume and orifice size.