On-site detention (OSD) of storm runoff decreases catchment peak flows through the routing effect of temporary storage; on-site retention (OSR) achieves the same objective by abstracting part of the urban flood wave and passing the retained water to disposal on site. The investigation explored both strategies applied to a set of hypothetical present/re-developed urban catchments ranging in size from 14 ha to 210 ha. Comparisons were made on the basis of site storage required (SSR) to achieve the same global peak flow reductions, environmental aspects and cost. OSR practice was shown to out-perform OSD generally in medium-large catchments with respect to SSR and, hence, cost. The retention option also has clear environmental benefits that fall beyond the scope normally ascribed to OSD practice. The paper cautions against use of OSR in unsuitable circumstances.

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