Assessing the Hydrologic and Water Quality Benefits of a Network of Stormwater Control Measures in a SE U.S. Piedmont Watershed

Abstract

The hydrologic and water quality benefits of an existing engineered stormwater control measures (SCMs) network, along with the alternative stormwater control simulations, were assessed in the rapidly urbanizing Beaverdam Creek watershed located in SE U.S. Piedmont region through the use of distributed Model of Urban Stormwater Improvement Conceptualization stormwater model. When compared with predevelopment conditions, the postdevelopment watershed simulation without SCMs indicated a 2 times increase in total runoff volume, 3 times average increase in peak flow for 1.5‐3.2 cm 6‐h storm events, and 30 times, 12 times, and 3 times higher total suspended solids (TSS), total phosphorous (TP), and total nitrogen (TN) loadings, respectively. The existing SCMs network, in comparison with the postdeveloped watershed without SCMs, reduced the average peak flow rates for 1.5‐3.2 cm 6‐h storm events by 70%, lowered the annual runoff volume by 3%, and lowered TSS, TP, TN annual loads by 57, 51, and 10%, respectively. A backyard rain garden simulation resulted in minimal additional reduction in TSS (1.6%), TP (0.4%), and TN (4%). Model simulations indicate that mandatory 85% TSS and 70% TP annual load reductions in comparison with the predevelopment levels would require the diversion of runoff from at least 70% of the contributing drainage areas runoff into additional offline bioretention basins.

Cumulative Effects of Low Impact Development on Watershed Hydrology in a Mixed Land-Cover System

“…Model simulation results suggest reductions in peak flows and surface runoff, and increases in evapotranspiration and subsurface flow and infiltration, with all spatial configurations of LID at the watershed scale. This is consistent with Gagrani et al [19], Fry and Maxwell [53], and Avellaneda et al [54], who reported similar effects on water balance components and peak flows after the placement of different LID practices in urban watersheds, with 42-55 percent impervious surfaces and drainage areas ranging from 0.2 km 2 to 12 km 2 .…”

“…The results from the subwatershed with smaller LID lots and underdrain connections showed a substantial reduction in peak discharge (up to 33%) and total storm runoff (up to 40%). Additionally, recent field-based research provides evidence of the cumulative watershed scale effects of LID on hydrologic responses, such as peak flows and pollutant loads [17][18][19][20]. Such experimental studies can be resource intensive (e.g., financial, personnel, time) [21]; however, process-based models provide a means to go beyond measured data and explore the projected "what if" LID scenarios using potentially less resources.…”

Modeling Runoff and Nitrogen Loads From a Watershed at Different Levels of Impervious Surface Coverage and Connectivity to Storm Water Control Measures

“…The effects of connectivity to SCMs on runoff ratios were more dramatic in warmer months (May to September) than the rest of the year (Figure a), which supports the assertion that evaporation is the physical process behind these relationships. Other studies have demonstrated the existence of such a relationship between SCM connectivity and runoff volumes, but these relationships are often stronger (Gagrani et al, ; Hale et al, ; Liu et al, ). While monotonic, the relationship appears to be nonlinear, shown in Figure a by the hockey‐stick shape and in Figure a by the increasing distance between vertically stacked points as UI/TI increases, respectively.…”

“…The Beaverdam Creek watershed is an urbanizing watershed near Charlotte, North Carolina, where streamflow and water quality was monitored for 9 years (Figure a; Allan et al, , ; Allan & Diemer, ). The most urbanized subwatershed of the greater Beaverdam Creek watershed, called BD4, is the subject of this study and has been the subject of other recent studies of hydrology and water quality (Bell, McMillan, et al, ; Gagrani et al, ; Jefferson et al, ). BD4 is 1.8 km 2 in area and is built out with a medium‐density residential neighborhood in the lower elevations of the watershed and an interstate exchange and commercial shopping center in the upper watershed (Figure b).…”

“…Retention of water in SCMs decreases the connectivity between urban surfaces and streams and has the potential to reduce peak discharges and increase discharge response times (Hood et al, ; Horner et al, ; Jarden et al, ; Villarreal et al, ). The presence of SCMs has also been shown to reduce total runoff volumes in urban watersheds (Gagrani et al, ; Hale et al, ).…”