Application of HEC-HMS to Model the Additive Effects of Multiple Detention Basins over a Range of Measured Storm Volumes

Southeastern Pennsylvania has experienced dramatic population growth during the past 30 years. Implementation of stormwater management plans during this period has left the landscape dotted with stormwater detention basins. The design objective of the majority of these stormwater facilities has been to limit on-site peak outflow rate to near predevelopment levels for a large design storm of a low return frequency. Problems with this design objective include the following issues. (1) In urban watersheds, flooding and in-stream erosion often result from smaller, more frequent storm events. (2) Low return-frequency storm events account for only a small percentage of the total annual precipitation and therefore the design of these detention basins has not focused on the bulk of the yearly precipitation. (3) Historically, designs have not provided for any attenuation of total storm runoff volume, whereas in nearly all cases the on-site volume has been greatly increased compared to predevelopment conditions. (4) Design of these detention basins has solely been focused on reducing on-site flowrates and has not taken into consideration the effect of neighboring detention basins on total streamflow. In this paper we quantify the composite effect of multiple detention basins operating in an uncoordinated fashion on the total streamflow in a small urbanizing watershed. The case study to be presented is for Valley Creek watershed, located in Chester County Pennsylvania. The 62-square kilometer watershed is undergoing rapid urbanization and is now covered by approximately 17% impervious surfaces, and contains more than. 100 detention basins. In order to assess the additive effect of stormwater detention facilities, all detention basins were surveyed and incorporated into a watershed-scale rainfall-runoff model. A range of storms was measured using a network of five recording rain gages. The model was created using HEC-HMS in conjunction with the HEC-GeoHMS extension in ArcView 3.3. Model results show that the detention basins have essentially no attenuating effect on peak streamflow for a typical storm event. This analysis points to the need for a fundamental re-evaluation of the basic premise for stormwater detention basin design.