Impervious Cover Variability in Urban Watersheds
An urban area inventory for watershed development conditions should be part of any comprehensive stormwater management plan, when the goal is to understand the sources of pollution and the magnitude of the expected runoff. The watershed inventory would, therefore, assist in the selection of the most beneficial stormwater control practices. The type of urban development in an area can have a major impact on the local hydrology and water environment. This inventory can therefore be used to support many decision making activities and to increase the success of local stormwater monitoring. Past studies (Schueler 1994; USEPA 1993; Arnold and Gibbons 1996; Booth and Jackson 1997) have demonstrated the importance of knowing the areas of the different land covers in each land use category and their storm drainage characteristics (grass swales, curb and gutters, and the roof drains). Increasing levels of impervious surfaces associated with urbanization result in higher volumes of runoff with higher peak discharges, shorter travel times, and more severe pollutant loadings. Urban imperviousness is an important indicator for urban watersheds in measuring the impact of land development on drainage systems and aquatic life (Schueler 1994). However, there are many different types of impervious surfaces, and their direct connectivity to the drainage system is an important attribute affecting stormwater runoff. The purpose of this chapter is to show the measured variability associated with land surface covers for different land uses in a large urban area in the state of Alabama.
There are a lot of assumptions about impervious area characteristics for different types of land uses, but very little data has been available to validate these assumptions. As part of this research, the Little Shades Creek watershed (Jefferson County, near Birmingham, Alabama) and five highly urbanized drainage areas situated in Jefferson County (in and near the city of Birmingham) were surveyed in detail to determine the actual development characteristics and their variability. Jefferson County is the largest county by population and fifth by size (NACo 2001) in the state of Alabama, having Birmingham as the county seat. This was historically a heavily industrialized area. Rainfall occurs year around, although it is generally driest in the fall. About 55 in. (1400 mm) of rainfall occurs in a normal year, but recent years have been marked by a significant drought. With the decreased rainfall, more attention is being placed on regional water resources, including stormwater.
To determine how land development variability affects the quantity and quality of runoff, different land surfaces (roofs, streets, landscaped areas, parking lots, etc.) for different land uses (residential, commercial, industrial, institutional, etc.) were directly measured. Surface cover of 125 neighborhoods located in the Little Shades Creek watershed and 40 neighborhoods located in five of Jefferson County’s highly urbanized drainage areas were analyzed to determine the actual development characteristics and their variabilities. The locally verified Source Loading and Management Model (WinSLAMM) (Pitt and Voorhees 1995; 2002) was then used to determine the sources of runoff, and to predict its volume. Statistical analyses were conducted at several levels to establish the quantitative and qualitative runoff sensitivity associated with variations of site characteristics.
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