Improving stream health in urban areas by reducing runoff frequency from impervious surfaces

In urbanised catchments, the frequent direct delivery of water and pollutants from impervious surfaces to streams has a detrimental effect on stream health. Recent studies of a range of ecological indicators have shown that where there is opportunity for attenuation of these inputs, that is, where the link between impervious surfaces and streams is less direct, the damage to stream health may be mitigated. This suggests that improving stream health, in areas subject to urbanisation, involves finding ways to decrease the efficiency of water delivery from impervious surfaces. We have undertaken a feasibility study to examine options to reduce the area of impervious surfaces that are directly connected to waterways by efficient drainage systems. These options include: rainwater tanks where the captured water is used to replace a portion of the mains water supply; permeable pavements that reduce runoff from roads; and swale drains and bioretention systems along roads, rather than piped drainage direct to streams. Models of the relationship between effective imperviousness and a range of ecological indicators, from previous studies, are used to assess the likely effect of these options on stream health. A large-scale experiment is now proposed to test this new approach to urban stream restoration.