Cross Section–Wissahickon Sand Seepage Wetland Stormwater BMP (nts)
Wissahickon Valley Watershed Association Sand Seepage Wetland Stormwater BMP
sand/wood mix (80:20)
ephemeral stormwater
Wissahickon Creek
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his project consists of a woodland trail placed overtop of a gravity sewer paralleling the Wissahickon Creek. The trail was in poor condition due to stormwater discharge from an approximately 30-acre drainage area which was also eroding a channel between the receiving stream and the pipe outfall. The approach used to solve these problems and improve stormwater quality and quantity was an innovate ecological engineering technique called a ‘sand seepage wetland.’ This approach involved • covering the trail with a 3-foot deep layer of coarse sand amended with 20% (by volume) shredded hardwood, • building a boulder and cobble riffle grade control across the eroding channel that delivers the stormwater to the Wissahickon Creek, and • planting the side slopes of the sand seepage bed in native plants. The boulder and cobble riffle grade control restricts discharge to the Wissahickon during peak runoff periods, storing the water behind the sand seepage trail, which increases the water
surface area and enhances existing wetland water quality treatment and quantity attenuation. The mechanism for the water quality treatment is filtration through the carbon-rich sand filter media (sand trail with 20% shredded hardwood) as well as enhancement of natural wetland and floodplain treatment through runoff detention. As the peak discharge passes, water stored behind the sand berm and the boulder and cobble riffle grade control is able to find its way into the receiving stream over a period of 24 to 72 hours. The sand berm presents a firmer and cleaner substrate for hikers.
Typical Trail Seepage Berm Cross Section (nts)
Plan View–Wissahickon Sand Seepage Wetland Stormwater BMP
Typical Weir Profile (nts)
outfall
Legend sand seepage feature trail over existing sewer right of way bridge existing sewer alignment water surface for 72 hours after precipitation (light, medium and heavy rainfall) stream riffle grade control in outfall channel N
The hydrology of the wetland floodplain forest is improved, with supplemental benefits to the plant community, including suppression of non-native invasive species. The positive environmental stewardship effect of this project on the surrounding suburban community cannot be overstated. Similar projects that have been studied resulted in an order of magnitude reduction in peak discharge, and reductions of total suspended sediment, total nitrogen, and total phosphorus.
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Trail with bridge before restoration
Eroding outfall channel before restoration
Immediately after construction with one inch of rainfall
Path with bridge in distance one year after construction
Project Details • Drainage area to BMP is approximately 30 acres • Riffle grade control sized for conveyance of the 10-year storm discharge (approximately 65 CFS) • Sized to provide quality treatment for a 1.5-inch precipitation event • Maximum storage volume is the 25-year storm discharge • Designed and permitted in 2010, constructed in January and February 2011 • Design and construction cost: $79,000 (approximately three dollars per square foot)
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outfall
base map ©2010 Google
drainage area served by sand seepage wetland stormwater BMP
Bob Adams Director of Stewardship Wissahickon Valley Watershed Association 12 Morris Road Ambler, PA 19002 215.646.8866 info@wvwa.org
Joe Berg Biohabitats, Inc. 2081 Clipper Park Road Baltimore, MD 21211 410.554.0156 jberg@biohabitats.com
Sharon Yates A.D. Marble & Company 375 East Elm Street, Suite 101 Conshohocken, PA 19428 484.533.2500 syates@admarble.com
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