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.
