Streams in Auckland Region have been found to exhibit loss of natural functioning of freshwater systems and the degradation of their values as a consequence of intensive urban development. Conventional development, unless mitigated, significantly increases runoff volumes and the duration and frequency of elevated peak flows which consequently degrade the morphological and ecological functions of freshwater systems. In areas where stream values and functioning have not been significantly compromised by past development, and there is potential future development, there is an opportunity to manage streams to support multiple values including healthy in-stream ecosystems and community and Mana Whenua values.
The TP108 design storm approach has limitations in modelling small storm events and may predict the incorrect amount of storage needed to control sequential storms properly. Continuous simulation models are better at predicting more realistic performance of bioretention devices, rather than the performance for a single, design storm condition. Long-term continuous simulation allows a quantifiable assessment of changes in hydrological regime due to urban development over the full range of high to low flow conditions.
A continuous simulation modelling investigation has been carried out to undertake a quantitative performance assessment of a bioretention device on the changes in hydrologic regime of watershed runoff in terms of reduction in frequency, magnitude, duration and volume of runoff flows. The model results provide relationships between the runoff volume reductions, bioretention device sizes, catchment imperviousness and subsoils infiltration rates.