There is increasing awareness in the stormwater industry of the more subtle long term effects of urban development on urban stream morphology and ecology. Historically we have moved from a flood management approach, through stormwater quality improvement to water sensitive design (WSD) and sustainable management of urban streams. The recent Auckland Unitary Plan included measures targeted at protecting higher value streams.
Despite increased awareness, the practicality of implementing effective measures through statutory and design processes remains a challenge, with outcomes still uncertain. In part this is due to two factors: the difficulty of fully addressing the principle of hydrological neutrality; and the complexity of assessing the effectiveness of those measures on diverse stream environments in terms of base flow, erosion and ecology.
The underlying mechanisms typically used within WSD are retention, extended detention and peak flow attenuation, with treatment design contributing to the mix. While there are guidelines in place for such devices, rules vary across the country, and are typically borrowed and reinterpreted from elsewhere, rather than being validated for a region or site. There appears to be little hydrological simulation to support the design guides.
This paper has drawn on the earlier work investigating urban hydrology and stream erosion that lies behind the current TP10 extended detention requirements. It explores the relative effects of retention and extended detention on stream response to frequent rainfall events, and uses simplified continuous simulation to explore the relative effects of sizing parameters and configuration on the frequency of runoff events and the long term flow duration curve. It is not a comprehensive study, but rather is intended to inform regulators on potential outcomes of their decisions, and spark further analytical investigation into this important subject.