Comparison of Low Impact Development Treatment, Traditional Stormwater Treatment, and No Stormwater Treatment for Commercial Shopping Centers in North Carolina

Abstract

Low impact development (LID) stormwater practices are becoming more popular because of their ability to improve water quality and recharge groundwater. New regulations require water quality treatment of stormwater runoff in addition to reducing peak flows, especially in nutrient sensitive watersheds. Previously, the main focus of traditional stormwater practices had been on mitigating flooding and reducing peak flows; whereas, newer LID practices improve water quality and attempt to restore a site's natural or pre-developed hydrology. This is accomplished by promoting more evapotranspiration and infiltration. Three commercial shopping centers have been monitored from April 2008 to September 2009 to measure the performance of using LID stormwater treatment, traditional stormwater treatment, or no stormwater treatment. All three sites were monitored for water quality and hydrology, and they were located within 70-km of each other. The site with no stormwater treatment and the site with traditional stormwater treatment were located in Raleigh, NC, and the site with LID treatment was located in Nashville, NC. Since the sites did not receive the same precipitation depths for each storm, the hydrology data were normalized per area treated. The LID practices were designed to treat the first flush of runoff or water quality event. The LID site incorporated the use of bioretention, permeable concrete, and constructed wetlands. Seven bioretention cells of varying media depths (0.6-m and 0.9-m) treated the front asphalt parking lot, and permeable concrete treated the rear parking lot. Storage was added beneath the permeable concrete to completely capture a 2.5-cm event. The constructed wetlands treated rooftop runoff, miscellaneous paved areas, and outparcel lots. Each LID practice was monitored as a separate unit and the site was monitored as a whole system. Effluent was monitored from the retention basin at the site with traditional stormwater treatment. A mixture of parking lot and rooftop runoff was monitored at the site with no stormwater controls. In addition to the water quality and hydrology results, much was learned about the construction and implementation of multiple and large scale LID practices at one site. LID practices are typically more sensitive practices, so proper construction oversight, installation, and maintenance are vital to adequate functioning of these stormwater treatment devices. Errors at this site included: undersized bioretention cells, clogged bioretention cells, a continuously flowing bioretention cell due to interception of the water table, and constructed wetlands that remained flooded, resulting in vegetation die off.