Scenarios for redesigning an urban drainage system to reduce runoff frequency and restore stream ecological condition

Monitoring the hydrochemical efficiency of urban stormwater treatment devices is not straightforward as the traditional, automated, In urban areas, efficient drainage of impervious surfaces means that polluted stormwater is frequently delivered to streams. Commonly, catchment urbanization can increase runoff frequency by a factor of 10 or more, as the effective imperviousness - the proportion of the catchment that consists of impervious surfaces drained to streams - is increased. This causes a decline in stream health. To decrease runoff frequency, effective imperviousness must be reduced. This requires urban drainage systems to be redesigned, using techniques such as infiltration and rainwater harvesting, so that stormwater from small rain events is not piped directly to streams but instead is infiltrated, reused or retained. We have developed scenarios that explore alternative urban drainage systems appropriate for a small partly urbanised catchment in Melbourne’s east. These scenarios incorporate, biofiltration basins, swales and dual purpose rainwater tanks that supply water for householders. Our results suggested that sufficient reductions in effective imperviousness and runoff frequency are possible to achieve improvements in stream health.