Setting objectives for hydrologic restoration: from site-scale to catchment-scale

Abstract

The protection and eventual restoration of natural ecological functions and values in urban streams requires approaches to stormwater management that restore natural hydrologic processes at small scales, with the ultimate goal of returning catchment-scale flow-regimes towards their predevelopment behaviour. Adoption of such approaches is however, currently limited by a lack of stormwater management design objectives applicable at the site-scale. In this paper, we outline a conceptual framework for setting such objectives based on the role that an individual site plays in delivering catchment-scale hydrologic outcomes. Objectives are provided for the proportion of rain falling on impervious areas that should be lost (evapotranspired and/or harvested) and infiltrated. We also propose an objective for equivalent initial loss, which characterizes the probability of surface runoff from a given rain-event, with the aim of restoring natural catchment-scale retention of storm events. It is apparent that the management of flow-regimes at small scales will require both retention (and loss through use or evapotranspiration) of stormwater and restoration of baseflow processes. Stormwater harvesting and infiltration-based techniques are thus required to manage stormwater at small scales.

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