Downstream Effects of Stormwater Management Basins

Urbanization decreases the natural storage of a watershed, which changes the timing characteristics of the runoff. Stormwater management (SWM) basins are an attempt to put the storage lost through development back into the runoff process. While SWM basins provide the proper volume of storage, they fail to return the timing characteristics to those that existed prior to development. The changes in storage and timing characteristics may have adverse effects on flow rates and bedload transport in downstream channel reaches. A study of a 2.12-sq mile watershed in Montgomery County, Md., showed that a SWM basin increased both peak flows and bedload transport rates in the channel downstream from the facility. This results from both the change in timing characteristics and the increased duration of bankfull flow. These results indicate that SWM policies must require evaluation of SWM plans on a regional basis and not just using on-site control criteria. Also, methods that can evaluate storage and timing changes must be used in the design of SWM facilities.