Written by Stormwater Engineer Amber Ellis
Retention ponds are the most common tool with which we deal with stormwater runoff in southeastern NC, and they come in all shapes and sizes. Some stormwater ponds are aesthetically designed as focal points of a community. Others are tucked away behind parking lots. Regardless, these ponds operate similarly. When it rains, water runs off of impervious surfaces, such as parking lots, and into the pond. The pond then releases the water slowly over 2-5 days to make storage space for the next rain event. This alleviates localized flooding and erosion within steambanks. While the water is temporarily stored, sediment and other solid pollutants settle to the bottom. While this settling offers some water quality benefit, years of research have shown that there are several retrofits that can improve the extent to which ponds are able to clean the water they send downstream.
The most intensive retrofit option, and the one that provides the greatest pollutant removal benefit, is to convert a stormwater pond into a constructed stormwater wetland. While ponds are typically several feet deep, wetlands are only inches deep, and they include a greater number of plants and a wider variety of species. This increase in plantlife facilitates additional biochemical and microbial processes that remove pollutants such as nitrogen and pathogens. Because this type of conversion significantly alters the pond, a professional engineer should create or verify the design to ensure safety and operation.
Floating treatment wetlands (FTW) are an alternative retrofit that requires no significant changes to the pond itself. Rather, floating mats that support wetland vegetation are added to the existing surface of the pond. These mats allow the roots to grow into the water, while the leaves and flowers remain above the surface. The plants not only take up excess nutrients from the runoff, but they also serve as a site for biofilm growth, a microbial community that accumulates and breaks down nutrients. FTWs also increase the settling of solid pollutants as the water moves throughout the pond, and add visual interest.
Finally, a simple way to maximize the pollutant removal in a stormwater pond is to ensure a robust vegetated shelf at the water’s edge. New ponds are designed to include a 6-foot wide vegetated shelf, planted with at least three native, herbaceous species. However, this type of native plant buffer can also be added to older ponds to help stabilize the banks, attract pollinator species, and improve safety and aesthetics.
NC State’s Stormwater Engineering Research Group offers an array of Extension workshops on stormwater infrastructure design, maintenance, installation, and retrofits. We encourage anyone interested in these topics to attend, from regulators and professional landscapers to homeowners and property managers. Information on upcoming workshops can be found on our website at bae.ncsu.edu/workshops-conferences/.
Amber Ellis is an Extension Associate for NC State’s Stormwater Engineering Research Group in the Biological and Agricultural Engineering Department, with an office at the Arboretum, 6206 Oleander Drive. You can reach her at adellis3@ncsu.edu.
