Evaluating Stormwater Retrofit Potential to Reduce Flood Impacts and Improve Water Quality in Urban Coastal Plain Communities

The City of Greenville is situated on the flat coastal plain landscape of eastern North Carolina and experiences pervasive challenges with flooding and stormwater runoff. Although current regulations help to reduce the impacts of new urban development on flooding and water quality, many older areas of the City have no stormwater management facilities or use outdated systems such as dry detention basins, Dry detention basins are typically implemented to provide water quantity control and are generally not designed for stormwater pollutant removal. It is estimated that there may be several hundred dry detention basins in the City, although few of these systems have been mapped and there is limited information on their functionality. This project was implemented due to a growing need to locate and assess unmapped dry detention ponds across the City, with the goal of identifying basins in need of maintenance and candidate basins for stormwater retrofits that can improve water quality and reduce flood impacts, with a specific emphasis on underserved communities.

East Carolina University (ECU) led the development of an approach to identify Greenville’s unmapped dry detention basins using LiDAR data, DEM contouring, a machine learning approach using a random forest model, GIS analysis, and field and aerial verification. In addition, ECU evaluated trash deposition, suspended sediments during storm events, and water levels in a subset of dry detention basins and conducted a range of educational activities to help enhance awareness of stormwater management challenges and potential solutions for Coastal Plain communities. The Center for Watershed Protection (CWP) led a condition and stormwater retrofit assessment for a subset of the identified dry detention basins.

ECU’s analysis identified and mapped a total of 214 dry detention basins throughout the City. Of the ponds evaluated for maintenance needs, only a quarter of them were considered well-maintained, while two required immediate attention. Maintenance concerns included trash deposition, poorly maintained/clogged inlets and/or outlets, damaged inlets and/or outlets, erosion and sedimentation, unmaintained vegetation, tree growth on berms, and shallow groundwater. At the sites assessed for trash, plastic, such as bottles and bags, was the dominant type of trash at most sites. Basins that drained commercial development generally had more trash, with the most severe trash deposition at sites that drained parking lots from malls, fast food restaurants, and hotels. Water level data and observations during field visits indicated that some basins remain wet during periods between rainfall events and in some basins, shallow groundwater has led to a natural conversion towards wetland conditions. CWP’s retrofit assessment identified and prioritized a total of 34 retrofit opportunities, consisting of five primary approaches that would enhance stormwater volume and water quality, including wetland conversion, bioretention, infiltration, wet ponds, and tree planting.

The final project report includes details on the methods, findings, and recommendations for each site. Initially, the City of Greenville planned to develop a citywide stormwater nutrient offset bank, where funding associated with stormwater nutrient treatment credits could be utilized to retrofit selected dry detention basins and provide local improvements in nutrient treatment. However, NC House Bill 600 (2023) recently curtailed the potential for this approach by restricting sales of nutrient offset credits owned by local governments to other government entities. Therefore, other funding approaches will need to be developed for future retrofit implementation.

CWP is continuing its partnership with ECU through the newly established Coastal Stormwater Center of the Southeast, an EPA-funded Center of Excellence that aims to provide data, tools, resources and guidance to assist Southeastern Coastal Plain communities with stormwater challenges. The Coastal Stormwater Center will support further work by ECU to use publicly-available GIS data to fill information gaps to help municipal agencies make informed management decisions more cost-effectively. This upcoming project will use GIS data to estimate the depth of the water table in Greenville that will help to delineate suitability for retrofit and new stormwater control measures based on groundwater depth constraints.

For more information, contact: Lisa Fraley McNeal at lfm@cwp.org.