Closure of “Watershed-Scale Evaluation of a System of Storm Water Detention Basins” by Clay H. Emerson, Claire Welty, and Robert G. Traver

Abstract

I consider this paper to be one of the most significant papers published in the past 10 years in the wateror hydrology-related journals of ASCE. That judgment is strongly influenced by the potential impact that its findings should have on correcting a widely applied but erroneous practice that has become—perhaps unwittingly—endemic certainly in Australia and presumably in the United States. I refer to the “sizing” of detention basins, used to mitigate mainstream flooding in developing urban catchments, by using tC “site time of concentration” instead of the catchmentwide “critical storm duration,” TC, used as the dominant storm duration in design. The two durations—tC and TC—in any given set of circumstances may differ by factors of two, four, eight or more; the derived rainfall depth differences and consequently the detention storage volume differences may be around two or three times. Widespread current practice therefore leads to undersized basins. The thoroughness of the authors in their quest to study the consequences of these differences, coupled with their professionalism in applying state-of-the-art techniques survey, measurement, model validation, etc. to a substantial field system in western Philadelphia, leaves no room for the skeptic to doubt the outcomes. Detention basin volumes fixed according to site-critical storm duration tC leads to minimal—the authors claim an average of 0.3%—mainstream peak flood reduction and, in some circumstances, peak flow increase, an even more daunting prospect. The authors conclude that “...on-site detention basins do not affect watershed-wide storm hydrographs resulting from frequent storm events.” This judgment I consider to be harsh as a generalization but certainly warranted for any system of undersized basins, as previously reviewed. A more favorable outcome—for detention adherents—may follow where basin volumes are fixed by using TC as the principal design parameter. The professional situation implied by the above considerations and the question of how it has developed is interesting to contemplate. In Australia—where anecdotal evidence suggests that 80% of basins are undersized—it stems from the municipal-agency approval process that accepts the tC-based design as satisfactory. The conscientious consulting engineer then faces a serious professional dilemma: should he or she design the basin correctly using TC , leading to greater cost for the developer, who is unlikely to be sympathetic and no support for a design based on “TC” from the Council officer for whom “what we’ve always done before” is the best guide. It is not surprising, given these