Stormwater Runoff Control Using Full Spectrum Detention

The goal of controlling peak flow rates at individual sites to pre-developed levels can be met using detention basins for design storms from 2- to 100-year return periods. However control of peak storm runoff flows along receiving streams by multiple detention basins operating simultaneously within larger urban catchments are much more difficult to achieve. The latter topic was studied in the past by several investigators, but the findings were very limited in scope and were focused primarily on larger runoff events such as the 10-year to 100-year flows. At the same time, there is evidence that stream geometry, water quality and aquatic habitat are impacted significantly whether this type of detention is used, or not, as areas urbanize. The profound hydrologic and geomorphic changes caused by urbanization require more robust control of the frequently occurring, smaller, runoff events. In response to this, different approaches toward designing stormwater detention were investigated and modeled by the authors. This testing was first done using design storm protocols employed by the Urban Drainage & Flood Control District for the Denver area of Colorado and then followed up using the EPA SWMM 5.0 model calibrated to 15-years of recorded 5-minute rainfall (5 gages) and runoff (2 gages) data for a 3.1 square mile watershed. This paper presents the findings most applicable for the Denver region and other locations having similar precipitation patterns; however, the underlying principles used to develop this concept can be used to develop design protocols for other hydrologic regions of the USA and other countries. This approach for the sizing of detention basins has been labeled by the authors as full-spectrum detention and was based on the following: 1) The first stage of a two-stage full-spectrum detention (FSD) basin captures approximately the volume equal to 1.1 times the excess urban runoff volume (EURV). EUVR is the difference in volumes between urbanized and predevelopment land use conditions, 2) This EURV is then released over an extended period of time (72 hours in the final version), and 3) The upper stage is sized to capture the larger events to control the peak flow rates to predevelopment for the 10- 25-, 50- or 100-year or storm (100-year event in the case tested). This paper describes how this concept was developed and evaluated and that it shows promise in providing simple on-site detention design protocols that reasonably controls stormwater peak flow rates not only at individual sites but also along receiving waters when multiples of similarly designed on-site detention basins are used in larger watersheds. The controls provided are quite robust for a very broad spectrum of runoff events; from very small to very large.