Stormwater runoff and export changes with development in a traditional and low impact subdivision
Introduction
Runoff from developed areas continues to be a leading cause of impairments in the nation's waterways (US EPA, 2002). Development continues at a rapid pace throughout the country, with some cities increasing in size by up to 50% in the past 30 years (US EPA, 2001). Several research studies have documented increases in runoff volume (Jennings and Jarnagin, 2002; Waananen, 1969) and peak flow rates (Leopold, 1968) as areas were transformed from undeveloped to urban. Other studies involving computer modeling of future increases in impervious areas have also predicted increased runoff volumes (Hollis, 1977; James, 1965; Pawlow and Nathan, 1977; Sloto, 1988). In addition, numerous studies have documented decreased water quality in urban runoff (Makepeace et al., 1995).
Imperviousness has been recommended as an indicator for stream health (Arnold and Gibbons, 1996). A variety of impacts have been associated with increased impervious cover, including decreased fish species richness and abundance (Wang et al., 2001), channel morphology changes (Booth et al., 2002), decreased benthic organism richness (Roy et al., 2003) and abundance (Klein, 1979), decreased base flow in streams (Ferguson and Suckling, 1990; Wang et al., 2001), and decreased water quality (Carle et al., 2005; Roy et al., 2003). More complex predictors of stream impacts such as the multimetric urban index composed of numerous infrastructure, socioeconomic, and land cover variables have been proposed (Coles et al., 2004). However, total percent impervious area was found to correlate highly (R2=0.96) with the urban index (Coles et al., 2004). This suggests that percent impervious area is valid as a predictor of stream impacts, and it is a simpler indicator to use.
A degradation threshold value at about 10% imperviousness has been cited by several authors (Booth and Reinelt, 1993; Klein, 1979; Schueler, 1994, Schueler, 2003; Wang et al., 2001). Watersheds with low levels of imperviousness may have a broad range of responses due to complex watershed interactions, but highly developed watersheds have uniformly poor conditions (Booth et al., 2004; Wang et al., 2001). Interpretation of threshold values in the literature should be done carefully due to the use of different measurement methods (Brabec et al., 2002). However, a definite relationship appears to exist between impervious area and multiple measures of stream health.
Recent advances in stormwater management, including low impact development (LID) techniques (Prince George's County, 1999), have provided engineers with a variety of tools to use in place of traditional catch basins and detention ponds. The overall goal of LID is to mimic the pre-development hydrology of an area, including the runoff volumes that existed before development. Current stormwater design in most municipalities mitigates peak flow rates, but does not address the increases in stormwater volume associated with development. Cluster designs, grassed swales, rain gardens, and pervious pavements all contribute to a reduced overall impervious footprint, and encourage decentralized treatment and infiltration of stormwater runoff. Research on individual LID practices shows that pollutant attenuation, reduced flow volumes, and reduced peak flow rates can occur (Davis et al., 2001; Dietz and Clausen, 2005, Dietz and Clausen, 2006; US EPA, 2000). However, there is a lack of peer-reviewed studies demonstrating the effectiveness of the use of LID on a watershed scale.
Although some studies have documented increases in runoff volume as an area was developed, much of the recent research relates to the comparison of different watersheds with varying land uses. Although the information provided by such studies is valuable, it is more difficult to establish causality when data from different watersheds are analyzed at a discreet point in time. Other confounding factors such as different monitoring methods, watershed characteristics, and weather variations can make comparisons difficult. Computer modeling studies can also provide insight into potential impacts to water resources, but simplifying assumptions are often made to calibrate models, which can make it difficult to determine the significance of the results. The objective of this study was to compare stormwater runoff volume and pollutant export from adjacent traditional and LID subdivisions, as development occurred, and as impervious surfaces were added in each of the watersheds.
Section snippets
Study area
The project was located in the town of Waterford, CT, in a drainage basin contributing to a small estuary called Jordan Cove, which discharges into the Long Island Sound. The “traditional” site was a 2.0 ha subdivision containing 17 lots (Fig. 1), which was built using current regulations and construction practices. Traditional zoning was used, as was a curb and gutter stormwater collection system. A typical 8.5-m asphalt road was installed. Landscaping and turf are similar to other new
Precipitation
Annual precipitation varied from 14% above normal in 1996 to 24% below normal 1997 (Fig. 3). For other years, variation was 10% or less of the 30-year normal precipitation (123.8 cm).
Stormwater runoff volume
Changes in stormwater volume were found as total impervious area increased in the traditional subdivision (Fig. 4). As impervious area increased from 1% to about 32%, annual runoff increased 49,000% from 0.1 cm to over 50 cm, or more than two orders of magnitude. Since precipitation during this period followed no
Conclusions
A large increase in runoff volume was observed as total impervious area increased through development of a traditional subdivision in Waterford, CT. Runoff coefficients also increased. These relationships were non-linear, indicating that as imperviousness increases, annual stormwater runoff volume increases exponentially. In contrast, annual stormwater runoff volume in the LID subdivision did not change as watershed impervious coverage increased. This lack of change in flow with increased
Acknowledgements
This study was funded in part by the CT DEP through a US EPA nonpoint source grant under Section 319 of the Clean Water Act. The authors would also like to thank John Alexopoulos at the University of Connecticut for creating the subdivision drawings.
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