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Uncontrolled or inadequately controlled stormwater runoff results in significantly increased peak flows to streams during storm events, as surface water which would otherwise infiltrate into the ground is conveyed through channels, stormdrains, and other impervious surfaces directly to the stream system. These increased peak flows enter streams at very high velocities, resulting in stream erosion as well as increased stream temperatures and pollutant loads. Increased stormwater flow and reduced groundwater infiltration alters natural stream hydrology and increases stream erosion and sedimentation with detrimental effects on aquatic communities and stream habitat.
As an urbanizing community, Montgomery County continues to increase the amount of land covered by impervious surfaces, altering the natural hydrology in many of our stream systems. As new development occurs, County agencies require new stormwater controls and reserve stream buffer areas to help mitigate the impacts of the altered land surface on increased surface runoff, runoff quality, and groundwater replenishment. The goal of the stormwater controls and related site planning, stream buffer, and forest conservation and reforestation requirements is to minimize the extent of necessary impervious surfaces associated with approved land uses, capture and slow down runoff peak flows to mimic predevelopment flows to the extent feasible, and reduce pollutants in runoff. Older developed areas of the County (e.g. Silver Spring, Bethesda) were originally established long before stormwater controls were required. The goal in these areas is to retrofit modern controls as opportunities arise and to restore riffle, pool and wetland habitat in stream environments damaged by the lack of adequate stormwater management. County agencies have major cooperative watershed restoration initiatives now underway in priority subwatersheds identified through the Countywide Stream Protection Strategy.
The County’s efforts to effectively control stormwater runoff impacts must always be balanced against other important, but often competing needs and realities. In newly developing areas, there is a great deal of pressure to balance environmental and stream protection needs with the economic development, housing, and transportation needs of the future. In already urban areas, there is a general lack of land for adding strategically located stormwater controls. Use of underground or innovative rooftop controls is often complicated by conflicts with underground utilities, lack of gravity drainage solutions, or extraordinary costs that limit practicality.
To assess stormwater control conditions for impervious areas, the percentage of developed land, by subwatershed, with functional stormwater quantity and quality controls will be measured through GIS mapping yielding a delineation of drainage area to existing stormwater ponds. A final map will reflect controlled, uncontrolled and SWM exempt areas.