Assessing the ability of infiltration-based WSUD systems to manage channel-forming flow regimes in greenfield catchment developments: a catchment scale investigation

Abstract

Catchment urbanisation is an inevitable and growing form of land-use change. These land-use changes have profound impact on catchment hydrology by altering the quantity and quality domains of flow characteristics in stream channels resulting in increased flood hazards, degraded aquatic ecosystem health and changed channel geometry. The concept of stormwater source control emerged from the need for managing stormwater in urbanised catchments with the aim of alleviating the detrimental impacts of catchment urbanisation on flow regimes, associated stream geometry and ecosystems health. These source control techniques are called Water Sensitive Urban Design (WSUD) in Australia and are increasingly being endorsed and adopted for managing stormwater in urbanising catchments. Consequently, the hydrologic response in urban and peri-urban catchments will be decided or influenced by the WSUD systems in those catchments in the future. This paper focuses on assessing the ability of infiltration-based WSUD systems in maintaining channel- forming flow regimes in greenfield catchment developments. The performance evaluation methodology is demonstrated by constructing hydrologic models for natural, urban, and managed (with WSUD) conditions for a selected case study catchment. The US Environmental Protection Agency Storm Water Management Model (SWMM) is used as the modelling tool. WSUD systems were designed based on the use of the flow duration control approach based on continuous simulation of flows. Two urbanisation levels were investigated: 30% and 70% of directly connected impervious area (DCIA) of the catchment in the study. The system performance was assessed using selected flow indices. The results of the study depicts that urbanisation dramatically change the natural flow regime. Under urbanised conditions of the catchment, magnitude of peak flow was notably increased while low flow was reduced. The increased frequency and duration of channel-forming flow was also evident under catchment urbanisation. The implementation of infiltration-based systems helps to reduce the impact of urbanisation by bringing flow values close to their natural conditions value. The infiltration-based WSUD systems adopted in this study can manage the channel-forming flow magnitude, frequency and duration close to their predevelopment levels. The adopted WSUD systems effectively reduce increased runoff volumes under urban conditions close to natural volumes. The percentage of time that flows exceeded Q1.67 increased from the natural value of 0.71% to 1.9% at 30% DCIA scenario and employing bioretention systems in the developed catchment effectively reduced flow frequencies of Q1.67 back to 0.63%. The magnitude of Q1.67 increased by 586% from the natural catchment value at 30% DCIA and stormwater management using bioretention systems reduced the statistic down to 13% of natural value. The duration of Q1.67 flow was increased by 170% from its natural value at 30% DCIA scenario and with the implementation of WSUD systems, DQ1.67 reduced up to 11% below its natural value. Similar results were obtained for 70% DCIA scenario. These improvements in streamflow regime would reduce the impact of urbanisation on channel geometry and ecological health.