Stormwater, Chevy Chase Lake and Coquelin Run
This article is a follow-up to one we posted previously on stormwater management. In this article, we asked an expert on the topic, George Washington University Associate Professor Ryan Engstrom, to take a look at what improved stormwater management might mean for Coquelin Run. Since we are in the very early stages of planning, there is no stormwater management concept plan yet for Chevy Chase Lake. However, the Chevy Chase Land Company has a strong track record with environmentally friendly design and is committed to incorporating modern stormwater management techniques in any new development. This is what Professor Engstrom had to say:
As mentioned in an earlier post, stormwater runoff is the water from precipitation (rainfall or snow melt) that flows across the landscape and enters into the local stream network without being treated. In the Chevy Chase Lake area, the local drainage is Coquelin Run which eventually flows into Rock Creek, the Potomac and on into Chesapeake Bay. When dealing with stormwater runoff there are two concerns 1) the amount water and 2) the quality of the water entering into the stream network. The amount of water that enters the system is strongly linked to the amount of impervious surfaces (roofs, pavement, and other areas where water cannot infiltrate into the soil). In a natural ecosystem, the trees, soil and other vegetation act like a filter and absorb, and slow down water moving over the land surface and into the stream network. When the natural ecosystem is built over by development, this filter affect is lost and the network becomes more like a funnel that directs water into the stream. Many older developments, such as in Chevy Chase Lake, were specifically designed like a funnel system to move water quickly away from these impervious surfaces by setting up drain pipes, downspouts and other methods of directing water towards the stream network. This was done to reduce flooding and standing water within the development. While this protects the development, it leads to increased stormwater runoff within the stream network and can, at times of heavy precipitation, cause very high water flows within the stream network. These high flows lead to erosion and the scouring of the stream channel which can lead to a poor stream ecosystem and possible flooding.
This funnel effect not only impacts the amount of water, but also the quality of the water that enters into the system. The quality is impacted because the water that flows over large surface parking lots and other impervious surfaces is moving quickly. The faster that water flows, the more pollutants it can carry. These pollutants include: sediment, oil, grease, and toxic chemicals from cars, heavy metals from roof shingles and autos, road salts, trash, and other material that it can pick up and transport into the stream network. Therefore, one of the major goals of stormwater management is to slow down the water that moves into the stream network. This not only reduces the total amount of water reaching the stream, but also the quality because if you slow the water down it cannot pick up as much material and it allows more water to infiltrate into the soil which can act as a filter to absorb material before it enters into the stream network.
In order to slow stormwater from moving to the stream there are currently many options available including, vegetated swales, permeable pavers, sidewalk storage such as curbside gardens, green roofs, buffers and many others. Some of these were described in the previous post. As mentioned previously, the Chevy Chase Lake area is an older development where the stormwater management design was to move water away from the development quickly, thus increasing the funnel affect. As the Environmental Protection Agency (EPA) notes on their web site http://www.epa.gov/owow_keep/NPS/urban_facts.html it is difficult and costly to retrofit older developments with stormwater management plans. What the EPA also notes is that in new developments, developers and city planners should aim to reduce runoff by maximizing surface roughness, infiltration opportunities and flow paths. Current regulations in Montgomery County require that any new developments need a stormwater management concept plan. http://permittingservices.montgomerycountymd.gov/DPS/waterresource/StormwaterManagementConcept.aspx. These stormwater management concept plans are designed to reduce impacts both during the construction phase and after. During construction, the primary concern is sediment entering the stream network. When preparing the landscape for construction, the soil is bare and allows for very little infiltration. Therefore, when precipitation occurs, there is water flowing across the soil which causes erosion and high sediment loads reaching the streams. This can be mitigated by creating buffers around the work site and/or creating sediment traps that pond the water and store the sediment prior to reaching the stream network.
In the case of Chevy Chase Lake, the current intent is to significantly improve the stormwater management capabilities with more trees, more pervious surfaces, and stormwater retention facilities. Due to the proximity to Coquelin Run a new, modern stormwater management system that uses currently recognized best practices will reduce the amount of stormwater and increase the quality of water reaching the stream network if the system is well designed and planned.
Ryan Engstrom is an Associate Professor at The George Washington University where he teaches courses on water resources, physical geography and geographic information systems. He holds a Ph.D in Geography from San Diego State University / University of California, Santa Barbara.
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