Comparison of Runoff Quality and Quantity from a Commercial Low-Impact and Conventional Development in Raleigh, North Carolina

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 treating storm water on site through the use of storm water control measures (SCMs). In this study, a conventional development (centralized storm-water 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) had underlying hydrologic soil group B soils. The LID was treated by a mix of green (aboveground) and grey (underground) infrastructure including an underground detention chamber and infiltration gallery, underground and aboveground cisterns, and aboveground swales and bioretention; the conventional development was treated with a dry detention basin and swales. Inflow and outflow runoff volumes and peak flows 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 recorded. The conventional development had an 11-fold higher median peak flow rate than the LID site. For the three storms more intense than the 10-year, 5-min average recurrence interval (ARI) event, the conventional site Qp was an average of 7.7 times higher than that of the LID. Flow proportional, composite water-quality samples were analyzed for total nitrogen (TN), total phosphorus (TP), total Kjeldahl nitrogen (TKN), total ammoniacal nitrogen (TAN), nitrite-nitrate nitrogen (NOX), organic nitrogen (ON), orthophosphate (Ortho-P), and total suspended solids (TSS). Generally, no significant difference in pollutant event mean concentrations (EMCs) was observed between sites. For the 20 water-quality storms sampled, the LID site produced pollutant loadings that were less than 5% of those at the conventional site for all species studied. Results demonstrated highly effective and space-saving solutions when green and grey infrastructure are merged, which is often the case when constructing on high land-cost properties. The exceptional results from this LID were due to (1) an overdesigned system capable of capturing the 77-mm storm, rather than a typical 25-mm storm, and (2) the high infiltration capacity of the type B soils coupled with a high driving head (∼3 m).