The Aquaduct - Winter 2012
Water Quality Treatment Best Management Practice Design: A Case study for Highway Improvements
By Becky Lyne, Environmental Specialist
The New Jersey Department of Transportation (NJDOT) Route 52 Causeway Replacement Project is one of the largest transportation infrastructure improvement projects ever undertaken in the State. The project involves reconstruction of approximately three miles of State Route 52 including replacement of the existing bridges and causeway over two miles of the Great Egg Harbor Bay. The Causeway crosses Great Egg Harbor Bay connecting the coastal communities of Ocean City and Somers Point and serves as the principal evacuation route. The project consists of replacing the 2.2 mile causeway, including 4 bridges as well as reconstructing the approach roadways. The Great Egg Harbor Bay is a shallow, tidally controlled bay composed of large expanses of open water and scattered islands. The bay is valued for its commercial shellfish resources, wetlands, wildlife habitat, and important recreational and commercial fisheries. Since the project involves significant increase in impervious area and roadway runoff, the potential for adverse impacts to numerous environmentally sensitive natural resources has made storm water runoff quality a critical design issue. Therefore, measures to provide water quality treatment of stormwater runoff have been incorporated into the project to the greatest extent possible while balancing the need to protect sensitive resources.
Bio-retention basins and swales have proven to be very effective storm water best management practices (BMPs) providing the greatest relative treatment credit applicable to linear development projects under most circumstances. However, there are some inherent limitations with bio-retention that will often make it a difficult BMP to employ, especially on roadway projects. The most significant limiting factors for the use of surface BMPs are the requirement for sufficient space within the right-of-way as well as regulatory design guidance constraints. In an urban situation where there is no median and the available right-of-way adjacent to the roadway is fully developed, implementing surface BMPs is a significant challenge. In spite of the many design challenges, the Route 52 project demonstrates that bio-retention can be an effective part of an overall strategy to address the quality of storm water runoff from roadways in an urban coastal setting.
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