A Potential Shortcoming of Event-based Design for Stormwater Detention Systems

M. Pennington (New Zealand)


Stormwater, hydrological model, detention, design event.


The design of a typical detention basin, used for mitigation of adverse effects resulting from development of rural land, is investigated for compliance with the relevant design standard. The design is shown to meet required performance standards in response to a design rainfall event of 50-year ARI. Following this the response of the detention basin is tested using actual rainfall that was recorded near to the site. A one-year time series (1975) is simulated, in which there was a 50-year rainfall event that occurred. Given the precedent rainfall and the slow response of the designed detention basin, its performance under an actual 50-year rainfall event fell short of the performance standard specified for the design event. The point is made that while a design 50-year rainfall event is used to set performance criteria, this does not necessarily mean that the facility will meet these criteria in response to real rainfall. The 50-year design event is seen as a level-of-service, and may not guarantee performance under real conditions.

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