A Comparison of Runoff Quality and Quantity from a Urban Commercial Infill Low Impact Development and a Conventional Development

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). SCMs, such as bioretention, rainwater harvesting, and permeable pavement, can be used independently or in series to mimic pre-development hydrology. In this study, a conventional development (centralized stormwater management) and a nearby infiltration-based LID commercial site in Raleigh, North Carolina, were compared with respect to stormwater quality and quantity. The conventional development (2.76 ha, 61% directly connected impervious area (DCIA)) and the LID (2.53 ha, 84% DCIA) have underlying 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 intended to mimic pre-development hydrology from a runoff perspective for the 10-year return period, 24-hour duration storm. For the 50 hydrologic storms monitored, a mean runoff reduction of 98.1% at the LID site, and a 48.9% mean runoff reduction at the conventional site, when normalized by DCIA. The conventional development had a 9x higher peak flow value, on average, than the LID site when normalized by DCIA. Flow-proportional, composite water quality samples were analyzed for total nitrogen (TN), total phosphorus (TP), total Kjeldahl nitrogen (TKN), ammonia (NH3-N), nitrite-nitrate (NO2+3-N), orthophosphate (PO4-3) and total suspended solids (TSS). For the 19 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.