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hw 1jrc'carbons). and silt plumes t•edimen- tation ).` all of which affect the entire food chain and the species abundance and diver- .it }' of aquatic life. including fish (8 . The in- teraction of these problems distinguish urban stream., (3). For stormwater management to compre- hensively protect the quality of streams. it needs to address the multiplicity of prob- lems. rather than solely controlling peak flows. The interaction of problems offers an opportunity to solve a range of problems si- multaneously, as the following descriptions illustrate. Controlling increases in runoff peaks. Many municipalities have passed ordinances requiring that post - development runoff peaks not exceed pre - development peaks. This is accomplished by detaining and then releasing accumulated runoff through a basin outlet device sized to pass flows at pre - development rates. In an attempt to -play it safe," municipalities often set as a design standard the control of the 10 to 100 year frequency storms with outlet devices so big that they pass, unchecked, smaller storms of two- to five -year frequency, which are the cause of much of the flood damages experienced today. Even when designed to also control small- er storms, detention basins do not reduce the volume of -runoff, and only control peak `aces & flow. Most of the thousan .sd; of de- tenti o basins found in the areas surround- ing our cities are constructed as so- called -dry basins" that do not improve the quality of urban runoff. Often unsightly, such basins are usually found in far comers, using left over spaces of subdivisions. Alternatives to standard dry basins are "extended detention basins" and -wet basins" with a permanent pools that offer water quality improvement. Such urban ponds are an aesthetic enhance- ment and have been found to more than double adjacent property values (11). They are a delight for urban residents and provide habitat for water related wildlife. A survey in Columbia, Maryland, found that 75 percent of homeowners preferred "wet ponds" for stormwater management while only 17 per- cent liked dry detention basins. Residents overwhelmingly (94 percent) felt that it is desirable to manage future stormwater basins for fish and wildlife as well as for flood and sediment control (1). Preventing losses in infiltration. To control the increased volume of runoff leav- ing a site, and not lust the higher peak flt,ws, ste s can be taken to maintain infiltra- tion'. ure 3 presents a Nvater budget that sFiows how groundwater recharge is re- duced by impervious surfaces, while runoff volumes rise. Annual precipitation in many sections of Maryland, for example, is 112 cm Riparian Forest Buffer Figure 4. A "wooded wetland with infiltration trenches" as a functional landscape enhancement. (44 in). After infiltration, 30 cm/yr (12 in/yr) of this total reemerges to support the base flow of streams. Low density single family residential development with an irnpervious- ness of 20 percent has been shown to re- duce this base flow by 12 percent to 27 cm/yr (10.6 in/yr) while commercial devel- opment with an imperviousness of 90 per- cent would reduce it by 90 percent to 3 cm/yr (1.2 in/yr) (5). The use of stormwater infiltration devices that infiltrate the first 1.25 cm (.5 in) of runoff would maintain 29 cm/yr (11.6 in/yr) of post development base flow (96 percent) for single family residential uses and 22 cm/yr (8.7 in/yr) (72 percent) of the base flow on commercial sites (5). As indicated earlier, maintaining such base flow is partic- ularly important for headwater streams. Problems with standing water such as expe- rienced in Maryland can be designed out (2). Infiltration rovides the added benefit of water qu iry enhancement as impurities in the "firsh" of runoff are filtered. Infiltra- tion can be accomplished through measures that enhance the landscape, such as the woody wetland with infiltration trenches shown in Figure 4. Controlling runoff pollution. The Clean Water Act has set goals to make the Nation's waters fishable and swimmable. The Nation- wide Urban Runoff Program (13) has docu- mented the severity of the problem, and models have been developed based on its findings. This includes a highly practical "Simple Formula" set forth by the Metropoli- tan Washington Council of Governments (7) that permits the quantification of runoff pol- lution loading for development sites. The major sources of runoff pollution from urban areas are impervious surfaces that collect nitrogen deposited through acid JANUARY- FEBRUARY 1994 17