An alternative method for on-site stormwater detention design

Abstract

A widespread approach for the protection of hydrologic conditions in regional catchments is the attenuation of peak discharges through on-site detention. It is common practice and policy for detention to be dimensioned via analysis of the catchment area including or immediately surrounding a development site, with the objective being maintenance of pre-development peak flow conditions at the development site’s outlet. The Regional Effect is a term given to the phenomenon of adverse hydrologic conditions that result from the inappropriate location of stormwater detention systems in a regional catchment. In this paper, an analysis is presented that highlights the inadequacy of site-focussed stormwater detention design with the definition of a Regional Effect Point, at which detention located in the downstream catchment is not beneficial. It is also reasoned that unnecessarily large detention volumes can exacerbate the Regional Effect and should be avoided. In Queensland, Australia, current guidelines recommend the use of runoff-routing models for dimensioning of on-site detention. As an alternative, some Queensland local councils provide deemed-to-comply solutions that involve basic inputs of site area and land use to calculate on-site detention. Via observations, the volumes produced by deemed-to-comply solutions greatly exceed those calculated by runoff-routing methods. To improve material and construction efficiency, limit unnecessary land dedication, and seek to reduce the potential for the Regional Effect whilst complying with current mandates for on-site detention, an alternative detention dimensioning method is presented that does not require packaged computer software. The process can be summarised by three interrelated modules: (i) graphical extrapolation of the Rational Method for hydrograph approximation; (ii) depth-storage-discharge programming; and (iii) numerical runoff routing using an alternative solution to the continuity equation given by the Queensland Urban Drainage Manual (2013).