Low Impact Stormwater Management Approaches for College Gardens

The City of Rockville Department of Public Works (City), working with the College Gardens community, Biohabitats, Inc. and the Center for Watershed Protection (CWP), embarked on an effort to demonstrate the potential for low impact development (LID) stormwater management approaches and identify implementation opportunities in the 160-acre College Gardens subwatershed of Watts Branch in Rockville, Maryland. Specific project objectives included: 1) providing a technical basis for innovative strategies and stormwater demonstration projects that can be applied to this existing urban neighborhood; 2) placing priorities on techniques that reduce nutrient loads, slow runoff rates, improve water quality, and utilize drainage area capacity to absorb runoff and recharge groundwater; and 3) providing the basis for technology transfer of similar approaches to other similar neighborhoods. Field reconnaissance and data collection utilized the Unified Subwatershed and Site Reconnaissance (USSR) protocol developed by CWP and a residential characterization and awareness survey. The subwatershed was divided into 9 catchments based on drainage patterns and land use to identify priority locations and types of LID practices to be implemented throughout the study area. A modeling effort using the Watershed Treatment Model (WTM) (CWP, 2001) was conducted to predict the nutrient load reduction potential of recommended LID practices. The priority LID practices identified and sited included bioretention, rain barrels, raingardens, and filters. The suite of priority practices were estimated to be able to provide partial or full water quality treatment for a total combined drainage area of about 30 acres. This includes a rain barrel and raingarden pilot program on residential lots, which has resulted in the installation of 110 rain barrels and 9 raingardens as of the end of 2005. This study suggests that implementing low impact stormwater management measures is feasible and beneficial with regard to the project goals. However, because this subwatershed is highly developed (50% impervious cover) and space for siting practices is limited, opportunities to provide more substantial management of runoff volume (i.e., channel protection storage) are limited. In fact, a more conventional retrofit of an ornamental pond located near the lower end of the drainage area is viewed as a necessary supplement to the LID practices to provide sufficient storage capacity to slow runoff rates and reduce the erosive velocities of larger storm events in this urbanized subwatershed.