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

Abstract

Urbanization and its associated increased impervious footprint lead to stream impairment through erosion, flooding, and augmented pollutant loads. Low Impact Development (LID), an alternative approach to stormwater management, focuses on disconnecting impervious areas, increasing infiltration, and reusing stormwater on site through the use of stormwater control measures (SCMs). These SCMs, such as bioretention, rainwater harvesting, and permeable pavement are used independently or in series to mimic pre-development hydrology. In this study, a traditionally developed centralized commercial site and a nearby infiltration-based LID commercial site in Raleigh, North Carolina, are compared with respect to stormwater quality and quantity. A dry detention basin, which mitigates peak flow rate increases, treats the traditional development. The LID site consists of an 11,700-gallon aboveground cistern, two underground cisterns (15,300 gallons and 16,000 gallons used for landscape irrigation and toilet flushing), and an underground detention system overflowing into a series of infiltration galleries beneath the parking lot of the shopping center. The LID shopping center is intended to mimic pre-development hydrology from a runoff perspective. Data collection is underway at both the LID and traditional developments, with hydrologic measurements to be gathered for one year. Preliminary results of winter data show a runoff reduction of 98.0% at the LID site, and a 47.8% runoff reduction at the traditional site. Flow proportional, composite water quality samples will be analyzed for total nitrogen (TN), total phosphorus (TP), total Kjeldahl nitrogen (TKN), ammonium (NH4-N), nitrite-nitrate (NO2+3-N), orthophosphate (PO4-3) and total suspended solids (TSS). Comparisons will be made between normalized pollutant loads from the LID and traditional developments. Preliminary results of winter data show the LID site nutrient loadings for all species studied to be less than 9% of nutrient loadings of the traditional 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 be feasible due to high land costs and constricted spaces in urban areas. Moreover, whether a site in a clay-based underlying soil can truly meet strict LID hydrologic criteria will be evaluated.