Evaluation of Stormwater Wetland and Wet Pond Forebay Design and Stormwater Wetland Pollutant Removal Efficiency
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2007-01-08
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Abstract
This thesis is the combination of two research studies involving stormwater wetlands and wet ponds. In the first study, forebays, designed to trap total suspended solids and dissipate inflow energy to stormwater wetlands and innovative wet ponds, were examined. Current forebay design standards in North Carolina arbitrarily size the surface area of wetland forebays to be 10% of the total surface area and the volume of wet pond forebays to be 30% of the total volume. Using this method to size forebays does not allow for variability in watershed characteristics, particularly projected sediment load. Between October 2004 and March 2006, 15 wetland and wet pond forebays in the North Carolina piedmont were surveyed. All sites are located in urban or suburban watersheds in either stable or unstable watersheds. The changes in the topography of the forebay bottom were digitized and analyzed for scouring and aggredation.
The surveys revealed that most forebays built in the last 10 years have not been sized according to the NCDENR standard; six of ten stormwater wetland forebays were oversized. The forebays did not adequately remove influent solids, as 11 forebays had a negative overall fill rate, indicating a net soil removal. Statistical analysis determined that the relative percent area of the forebay and stability of the watershed were the only factors affecting forebay fill rate. Watershed area, curve number and the number of runoff events between surveys did not impact the volume change of the forebays.
Findings from the surveys suggest that forebay design should include two zones for energy dissipation and sedimentation. The energy dissipation zone should consist of a preformed scour hole excavated next to the inlet; the hole should be between 50 and 400 mm deep, depending on culvert diameter, and have a diameter at least as large as the culvert diameter. The sedimentation zone should include a long and shallow flow path to increase treatment time and reduce particle-settling distance. Engineers can use these new forebay-sizing guidelines to more accurately size forebays. These standards can also be used in scheduling maintenance for stormwater wetlands and wet ponds.
The second chapter of this thesis discusses two stormwater wetlands in North Carolina that were monitored for water quality. A wetland installed at the Bruns Avenue Elementary School in Charlotte, NC, was monitored from September 2004 — December 2005. The 6.4-ha watershed consists of a residential development and school property. Samples from 15 storms were analyzed for a variety of pollutants. Event mean concentrations (EMCs) and event mass loadings (EMLs) were determined for each storm.
The concentrations of all nutrient pollutants, including Total Nitrogen (TN), Total Kjeldahl Nitrogen (TKN) and total phosphorus (TP), fecal coliform (FC), total suspended solids (TSS), copper and zinc were significantly reduced during the monitoring period (p< 0.05). Growing season did not significantly (p<0.05) affect influent and effluent EMCs.
The Smithfield-Selma High School (SSHS) stormwater wetland captured runoff from the school's 10.5-ha watershed. Monitoring of the wetland's water quality treatment during base flow and storm flow events took place from December 2003, until February 2005. The SSHS wetland significantly (p<0.05) reduced the influent concentrations of NO3--NO2-, NH4+, TN, TKN, TP, PO43-, TSS, and Cu, but did not significantly (p<0.05) reduce Zn. Only the removal of TN varied significantly (p<0.05) between base flow and storm flow; TN concentrations were reduced significantly (p<0.05) more during base flow events. Unlike at the BES wetland, there was a significant (p<0.05) increase in the reduction of PO43- and TN during the dormant season. There was not a significant (p>0.05) seasonal difference for the other monitored parameters.
Stormwater wetlands have been assigned a nitrogen removal efficiency of 40% and a phosphorus removal efficiency of 35% by the North Carolina Department of Environment and Natural Resources (NCDENR) Division of Water Quality (2004). Monitoring at both the BES and SSHS stormwater wetlands showed that the water quality treatment exceeded the required removal rates of the state of North Carolina.
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Keywords
runoff, forebay, wet pond, wetland, stormwater
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Degree
MS
Discipline
Biological and Agricultural Engineering
