Constructed Wetland Design
Shipshewanna Lake, which is located in northeastern Indiana, was plagued by algal blooms, nuisance stands of aquatic vegetation, and excessive accumulations of sediments. High levels of both nutrients and sediments enter the lake from adjacent urban and agricultural areas. The Shipshewanna Lake Improvement Association retained F. X. Browne, Inc. to design a two constructed wetlands for the treatment of urban stormwater and agricultural runoff. Both created wetlands were designed with large sediment settling forebays to accommodate high runoff volumes during storm events. These forebays were designed in order to remove the high concentrations of sediments from incoming water, to detain large volumes of stormwater, and to provide an even flow of water to the created downstream emergent wetland area. Selected wetland vegetation in the emergent wetland area also provides additional treatment by removing finer sediment particles that have passed through the forebay; but more importantly, the downstream emergent wetland area also removes dissolved nutrients such as nitrogen and phosphorus via plant uptake. The Sara Davis Ditch constructed wetland was designed as an in-line system which treated the entire ditch flow during dry weather and wet weather conditions. The watershed upstream of the wetland was primarily agricultural. This wetland was designed to treat up to the 25 year storm. The second constructed wetland that we designed was at the Shipshewanna Elementary School and was designed to treat urban stormwater runoff from the school and an adjacent factory. This wetland was an off-line system that treated stormwater runoff and discharged treated runoff into a branch of the Sara Davis Ditch.
Many design considerations were made to ensure the success of these wetland systems over time. Both created wetlands have access roads into their sediment forebays, thereby allowing for the removal of accumulated sediments. To avoid “wash-out” by larger storm events, the sediment forebay of the created wetland system for agricultural runoff at the Sara Davis Ditch site was designed with an emergency overflow channel. During very large storm events, the overflow channel allows excessive stormflow volumes to by-pass the more sensitive emergent wetland portion of this system.
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