Retention Capacity: A Metric to Link Stream Ecology and Storm-Water Management

Abstract

A major impact on stream ecosystems in urban catchments is the increased frequency of runoff of poor quality storm water. This problem has not been commonly addressed in storm-water management, which usually focuses on pollutant load reduction or abatement of peak flows, or in some cases on minimization of impervious area. If stream ecosystems are to be restored or protected, a greater focus is required on the minimization of hydraulic connection between impervious surfaces and streams for small, frequent rain events. We propose new objectives for storm-water management. We suggest a primary objective of zero directly connected imperviousness for catchments where the ecological objective is to protect stream ecosystems, because the direct connection between impervious surfaces and streams is a severe stressor to stream ecosystems. We then propose an index to quantify the degree of connection of impervious surfaces, scaled to runoff frequency (retention capacity, RC), which can be used as a design objective for treatment measures. RC can be combined with total imperviousness to calculate attenuated imperviousness, analogous to effective imperviousness, but with an objective quantification of the degree of disconnection of impervious surfaces. RC emphasizes the importance of mimicking natural runoff behavior, with the greatest emphasis on small, frequent storms, to reduce the frequency of flow disturbance experienced by streams closer to the preurban frequency. It provides a practical design objective for storm-water treatment systems, which is better targeted toward the protection of stream ecosystems than the pollutant load or runoff volume reduction targets which are commonly used.