Legacy stormwater systems primarily designed for efficient drainage have led to channel erosion and sedimentation as well as nutrients and pathogen exports that have degraded and impaired urban streams. Current regulations for most municipalities require stormwater control measures to address the impacts of increased impervious cover. However, developed landscapes can constrain retrofit options as existing infrastructure and available space prevent certain options while limiting the size of other options. Under-sizing stormwater control measures is a possible solution for retrofitting urban watersheds if the scalability of functions can be understood better. Greens Mill Run is a coastal plain stream impaired by excess stormwater runoff from the urban core of Greenville, North Carolina. At nearly 50% impervious, East Carolina University’s (ECU) main campus is a significant contributor of stormwater volumes and nutrient loads. In 2013, a dry detention basin was regraded to construct the Mark Brinson Stormwater Wetland on ECU’s campus. Due to topographic and infrastructure limitations the wetland was only 20% of the typical full design size for the contributing watershed. Flow monitoring and water quality sampling of inflows and discharges were performed between August 2015 and August 2016.
Results/Conclusions
Water quality treatment lagged behind conventional credits with respect to nitrogen, phosphorus, and sediments. Phosphorus exports were likely tied to re-sodding banks of the regraded stormwater wetland. The undersized geometry of the wetland relative to the drainage area limited the residence time. Complete replacement of the pool volume occurred after events of less than 0.25 inches, after which point outflow was comprised primarily of new stormwater runoff. However, other ecosystem valuable ecosystem services were provided by the wetland. The detention effect diminishing the flashiness of runoff from the watershed increased the overall time of concentration at downstream locations, reducing the occurrence of flooding. The watershed provided habitat to a variety of organisms including frogs, dragonflies, and birds. Mosquitoes were a concern in the design phase, however sampling only found larvae within the storm pipes leading into the wetland. It is thought the abundance of dragonflies and frogs controlled the mosquito populations. Finally, the wetland serves as an on-campus educational resource for a wide range of courses.