Improving stream low flow regimes in urbanised catchments using water sensitive urban design techniques

Abstract

As a catchment becomes urbanised, the natural hydrological processes are significantly altered and this leads to a deterioration of the stream’s ecological condition. Low flow characteristics of a stream are useful hydrological statistical indicators for assessing the health of an ecosystem. An investigation was carried out to assess the capacity for water sensitive urban design (WSUD) measures to maintain “natural” low flow characteristics when a catchment is urbanised. A calibrated Storm Water Management Model (SWMM) of a natural catchment in South Australia was developed using historical rainfall and stream flow data. Prior to the model calibration, the sensitivity of the SWMM parameters was assessed against hydrological responses on the low flow region of the flow duration curves (FDCs) and the deficit volumes at three selected threshold flows over the calibration period. The calibrated model was then modified to generate flow data for three selected urbanised scenarios by altering the percentage of effective impervious areas. Model resolution effects on low flow regimes were assessed to ensure spatially distributed effects did not influence the results. Two selected WSUD measures were applied to one of the urbanised scenarios and low flow characteristics were examined. Flow duration and spell duration-frequency statistics of the urbanised cases were compared against those of the natural catchment. The study revealed that urbanisation causes more frequent low flow spells during both the wet and the dry seasons. As expected, the frequencies of the low flow spells with duration up to 20 days increased dramatically due to urbanisation. Both WSUD measures were capable of achieving similar frequencies for the same “natural” spell durations. The frequency of low flow spell duration events was reduced as WSUD measures were applied to the urbanised catchment case, indicating that the low flows with WSUD measures will generally be higher than those without WSUD. For the case study catchment, WSUD infiltration measures that are capable of diverting up to half of the natural groundwater input will enable low flow characteristics to be maintained.