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Introduction
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 | | Stormwater drainage from the Woodlawn High School property. | |
Urban development over the last several decades has greatly increased the amount of impervious surfaces, such as streets, rooftops, and parking lots. Rainwater does not soak into these areas like natural soils, but stays on the surface and runs off towards lower areas, often accumulating pollutants along the way, such as motor oil, gasoline, heavy metals and nutrients. Because rainwater cannot soak into the soil, impervious surfaces also generate greater volumes of water leading to increased frequency and depth of flooding. To prevent flooding, drainage ditches have been constructed in such a way as to channel stormwater as far and as fast as possible away from developed areas, often impacting water quality many miles away.
Woodland High School (WHS) has a detention pond located just north of the football field. It receives stormwater runoff from the high school parking lot and a dairy farm located on the opposite side of Jefferson Highway. Stormwater runoff from WHS flows into a channel along the west edge of the parking lot, past the football field, and into the detention pond, and then into another drainage channel going to the Amite River.
In order to remove pollutants from WHS stormwater runoff, the Environmental Protection Agency (EPA) and Louisiana Department of Environmental Quality (LDEQ) provided a grant to Louisiana State University (LSU) researchers to design and construct a wetland in the WHS detention pond. The underlying premise is that as stormwater runoff from the surrounding landscape enters the constructed wetland, water velocity will slow, water residence time will increase, and downstream flooding and pollution transport will be decreased. A meandering wetland channel was designed to be an aesthetically pleasing landscape feature as well as a functioning wetland filter, demonstrating the integration of landscape design with stormwater management and non-point source pollution mitigation.
The project as originally conceived was altered due to new construction plans by the East Baton Rouge Parish School System to build near the area occupied by the original detention pond. This delayed the project for about a year. The construction plans called for a new detention pond to be built nearby, which occurred in the spring of 2008, and the meandering wetland channel described above was constructed in the new detention pond.
The project area facing west as it looked in the Fall of 2008, prior to planting of wetland vegetation
Click here for a detailed description.
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Wetland Filtration
Numerous studies have shown that wetlands efficiently remove nutrients, sediments and other pollutants, such as pesticides, from water that passes through them. Nitrogen, in particular, undergoes numerous chemical transformations in the wetland environment. The ability of wetlands to remove nutrients from inflowing water is primarily dependent on nutrient concentration, water retention time, available wetland area, temperature and the hydrology of the specific wetland site.
Chemical transformations of nitrogen in wetlands
The primary mechanisms by which wetlands improve water quality are physical settling and filtration of sediments, chemical precipitation and adsorption reactions, and biological processes such as storage in vegetation, peat accumulation, and denitrification and volatization. The hydromorphology of a wetland in a large part determines nutrient cycling and plant species diversity. The two main elements of hydromorphology are: 1) hydrological regime (water quantity, level fluctuations, residence time, and groundwater interactions) and; 2) morphological conditions (bottom depth variation and contours of submerged areas). These variables were taken into account in the design of this project.
Click here to access further information on the design of the project.
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Monitoring
Water Quality
Water quality samples are being collected monthly to characterize stormwater runoff running into and out of the project site. Water quality samples are collected in acid-washed 500-1000 mL plastic bottles, immediately stored on ice and transported to LSU Analytical Services Laboratory where the samples are subsampled and frozen until the day of analysis. The samples are analyzed for Total Kjeldahl Nitrogen (TKN), Nitrate+Nitrite (N02+NO3), Ammonia (NH4), Total Phosphorus (TP), Orthophosphate (PO4), and Total Suspended Sediments (TSS). At the same time that discrete water samples are taken, dissolved oxygen, water temperature, and salinity are measured with a handheld probe.
Water Flux
We are monitoring peak flow and volume of stormwater runoff as indicators of pollution load. This proxy is being used because changes in nutrient and sediment concentrations may be difficult to impossible to determine in the next few years due to disturbance caused by the project construction. Since loading of nutrients and sediments is directly related to concentration and water volume, a decrease in water volume is a reliable indicator of a decrease in pollutant loading. Flows and volumes of stormwater coming into the Woodlawn High School detention pond will be compared to that exiting, with success criteria defined as a decrease in peak flow and total water volume.
Hypothetical hydrograph of water flow in and out of the project site
Water level is being measured every 5 minutes using ISCO 3230 area velocity flow meters with liquid level actuators. A stage-discharge relationship was developed at the two sites to determine the volume of runoff based solely on water height using Manning’s equation. At the inlet channel, a fiberglass v-notched weir flume was installed, and at the exiting channel, the open-channel depth was measured every two feet to calculate the cross-sectional area. Discrete water velocity is also being measured during site visits with a Global Water FP101 velocity probe to crosscheck and calibrate our water flux calculations.
Click here for results
An ISCO 3230 area velocity flow meter on top of a v-notched weir at the inlet to the project site
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Wetland Project > Contact Information
Contact info:
Dr. Robert R. Lane
LSU School of Coast & Environment
Email: rlane@lsu.edu
or
Dana Brown
LSU School of Coast & Environment
Email: dbrown@browndanos.com
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