Evaluation of a Commercial Low Impact Development and an Adjacent Traditional Development in Raleigh, NC for Hydrology and Water Quality

Abstract

Abstract. Urbanization and its associated increased impervious footprint lead to stream impairment through erosion, flooding, and augmented pollutant loads. Low Impact Development (LID) focuses on disconnecting impervious areas, increasing infiltration and evapotranspiration, and reusing stormwater on site through the use of stormwater control measures (SCMs). In this study, a conventional development (centralized stormwater management) and an adjoining infiltration-based LID commercial site in Raleigh, North Carolina, were compared with respect to hydrology and water quality. The conventional development (2.76 ha, 61% directly connected impervious area (DCIA)) and the LID (2.53 ha, 84% DCIA) have hydrologic soil group B soils. A dry detention basin, designed to mitigate peak flow rate, was the conventional development SCM. The LID site consisted of a 44,300-liter aboveground cistern used for indoor toilet flushing, two underground cisterns (57,900 liters and 60,600 liters, used for landscape irrigation), and an underground detention system, which overflowed into a series of infiltration galleries beneath the parking lot of the shopping center. The LID shopping center was designed to mimic pre-development hydrology for the 10-year return period, 24-hour duration storm. All hydrologic analyses were normalized by DCIA. For the 47 hydrologic storms monitored, runoff coefficients of 0.02 at the LID site and 0.49 at the conventional site were observed. The conventional development had an 8.7 times higher peak flow rate for the median storm than the LID. For the three storms more intense than the 10-year storm, the conventional site averaged a 7.7 times higher peak flow than the LID. Flow proportional, composite water quality samples were analyzed for total nitrogen (TN), total phosphorus (TP), total Kjeldahl nitrogen (TKN), ammonia (NH 3 -N), nitrite-nitrate (NO 2+3 -N), orthophosphate (PO 4 -3 ) and total suspended solids (TSS). For the 20 water quality storms sampled, the LID site pollutant loadings for all species studied were less than 9% of pollutant loadings of the conventional site. Results from this innovative combined detention, stormwater reuse, and infiltration LID system will provide space-saving solutions for areas where aboveground SCMs, such as bioretention and constructed stormwater wetlands, are not feasible due to high land costs and constricted spaces.