Estimating the Treatment Performance and OSD Characteristics of Both Proprietary and Non-Proprietary WSUD Systems at Castle Hill in Sydney

Abstract

In Water Sensitive Urban Design (WSUO) there is often a focus on water quality control. In practice however, there is normally a need to provide extensive flood control, often through some form of onsite stormwater detention (OSO). Techniques such as stormwater infiltration and stormwater reuse allow opportunities for on-site retention, with little or no downstream stormwater discharge. However in heavy clay soils this is not always achievable and a hybrid WSUD/OSD system is often desirable. The first part of this paper examines the conceptual development of such a hybrid system that was able to achieve good WSUD objectives while still satisfying the local government authority's OSO requirements, which is one of the most stringent in Sydney. The issues arising from the realisation of that concept design through the detailed design and construction phases are also described. The second part of the paper explores various techniques for modelling such hybrid systems using the MUSIC program. There are several Water Sensitive Urban Design (WSUD) models available in Australia, including MUSIC, ERWIN and Switch2. However, while such software can readily model standard WSUD components such as bioretention systems, it can be difficult to use the models for non-standard hybrid systems and proprietary products such as the Rocla ecoRAIN and the HydroCon Stormwater System. The examples used to illustrate the techniques are all taken from an actual development at Castle Hill, involving a 64-lot residential subdivision in north-western Sydney that incorporates an integrated WSUD system. The modelling of both water quality treatment and the provision of on-site stormwater detention (OSO) are examined and detailed guidelines for adapting off the- shelf software are provided. Comparisons with simpler analysis procedures, such as regression equations and the NSW EPA Level One Type Assessment are also made, and the limitations of such approaches are explained.