Hydrologic, water quality and geomorphic indicators of catchment Effective Imperviousness

Abstract

Recent research has related waterway ecosystem health to a simple composite indicator of catchment Effective Imperviousness (EI). EI is defined as the impervious area that is directly connected, via a stormwater pipe or channel, to receiving waters. Empirical evidence has shown that catchments with EI greater than 5% have a significant impact on stream health. There are many hydrological and water quality indicators that could be used to define the EI of a catchment. Three primary indicators are proposed. Templates relating each of these indicators to catchment imperviousness were derived to enable (i) stormwater management strategies to be assessed in terms of catchment EI; and (ii) to examine the performance of current Victorian Best Practice Environmental Management (BPEM) objectives for stormwater. Meeting the current Victorian BPEM objectives has the effect of transforming the pollutant characteristics of a catchment of 60% imperviousness to an equivalent catchment with an EI of 10% or less. Rainwater tanks transform the hydrologic characteristics of a catchment to a lower equivalent EI, reducing a catchment with an EI of 60% to between 40% and 50%. Further reduction in catchment EI may be achieved by a combination of larger tank volume, higher stormwater use as a source and stormwater infiltration. Analyses also suggest that Victorian BPEM objectives for 1.5-year ARI peak flow reduction can significantly reduce the post-development Erosion Potential Index (EPI) of natural urban waterways by as much as 90%. This paper demonstrates that meeting the current Victorian BPEM objectives could contribute substantially to achieving low catchment EI.