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Runoff Coefficient

Runoff Coefficient

Watershed Imperviousness (%)

FIGURE 3.15 A relationship between runoff and impervious surfaces in a watershed. (Adapted from Schueler, T. R. 1995. Watershed Protection Techniques. 2:233-238.)

Watershed Imperviousness (%)

FIGURE 3.15 A relationship between runoff and impervious surfaces in a watershed. (Adapted from Schueler, T. R. 1995. Watershed Protection Techniques. 2:233-238.)

FIGURE 3.16 Comparison of hydrographs from rural (i.e., vegetated soil) and urban (i.e., impervious cover) areas. (Adapted from Ferguson, B. K. 1998. Introduction to Stormwater: Concept, Purpose, Design. John Wiley & Sons, New York.)

the focus is on storms so that units of hours or days are most relevant. Hydrographs provide a wealth of information as noted by Hewlett and Nutter (1969): "A hydrograph tells more about the hydrology of a drainage basin than any other single measure." A hydrograph represents a functional response of a watershed in relation to the water balance, and its shape is determined by two sets of factors: (1) characteristics of the watershed such as imperviousness, and (2) weather factors such as quantity, intensity, and duration of rainfall; distribution of rainfall over the watershed; and temperature (which is important in terms of freezing of soil or melting of snow and ice). Storms strongly influence hydrographs because they release large volumes of rainfall over short periods of time. A storm hydrograph is hump-shaped with a rise and fall of discharge as the stream drains the runoff generated by rainfall. Because urbanized watersheds have more runoff than less developed watersheds, their hydrographs differ in shape (Figure 3.16). The important features of a storm hydrograph from an urbanized watershed are the increased peak discharge (the highest point of the hump) and the shortened duration (the length of time between the rise and fall of the hump). Basically, the shape of the urban storm hydrograph shows that a large amount of water is moving quickly through the watershed over the surface, with consequent impacts of flooding and erosion. In a less developed watershed some of this water would have infiltrated into the ground and entered the stream over a longer time period as baseflow. There are also water quality impacts associated with storms since pollutants are washed into streams with runoff. This is an important type of nonpoint source pollution because the pollutants are advected by runoff moving over the watershed, as opposed to point source pollution that is generated by a discrete outfall such as from a wastewater treatment plant or a factory. Makepeace et al. (1995) provide a review of the pollutants in urban stormwater runoff, and Hopkinson and Day (1980) provide an example of a simulation model that combines urbanization and stormwater.

Stormwater management involves engineering of BMP structures that mitigate and control both the water quantity (flooding and erosion) and quality (nonpoint source pollution) impacts of storms in urban landscapes. Their role is to reduce the peak discharge of urban streams during storms. Stormwater management has a long tradition in civil engineering which has evolved into a kind of "pipe and pond" conventional approach (Urbonas and Stahre, 1993). In this approach, storm runoff is collected into centralized systems and stored temporarily in large detention ponds. Water in the ponds is released over a longer period of time, thus reducing peak discharge. While effective, this conventional approach has a number of problems associated with it, and over time new kinds of BMPs have been developed. These designs include wetlands, infiltration systems, filter strips or buffers, and porous pavement (Schueler, 1987). These designs are growing in diversity and implementation, and a whole new approach to urban stormwater management seems to be emerging. The new approach is very much a kind of ecological engineering, which is referred to by some workers as bioretention (Table 3.2). This is a very different approach compared with traditional stormwater management. The goal is to mimic natural hydrology through use of BMPs that emphasize vegetation. A strong effort is made to integrate BMPs into the site plans of new developments so that they become part of the landscaping rather than large, unattractive, and unsafe structures that create liabilities. Also, new ways of retrofitting stormwater management systems are being devised for sites that are already developed. This is a very creative field where workers must understand and utilize traditional engineering along with hydrology and ecology. The basic philosophy is to apply many small scale BMPs throughout the watershed, dispersing runoff rather than concentrating it. A key is to keep the drainage basin for each individual BMP small so that runoff volumes are more manageable and do not overwhelm the system's ability to function. The emphasis is on infiltration and evapotranspiration rather than drainage, and preliminary results indicate that these systems are less expensive than conventional alternatives. Biore-tention is still a new approach and designs are evolving rapidly, as indicated by reports in such journals as Watershed Protection Techniques from the Center for Watershed Protection in Ellicot City, MD.

Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.

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