03/04/1996 Park Board Packet - Laserfiche WebLink

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�,� ll�dr<>carhonsl. and ,ill plumes (�edinlen- citil,n i. all <>f which affect the entire food chain C[ild the species abundance and diver-,in-of aquatic life. including fish (8). The in- tcraction of these problems distinguish urban streams (3). For stormwater management to compre- Riparian Fonts Buffer llensiv ely protect the quality of streams. it needs to address the multiplicity of prob- —RasofiInflow P* lems, rather than solely controlling peak Woody wetlaAd tlo%vs" The interaction of problems offers an water Lerel Opportunity to solve a range of problems si- �\ nlultaneously, as the following descriptions Seu De edt potlrs illustrate. l�efditdm Yrrwhet Lard Sprwder Controlling increases in runoff peaks. Many municipalities have passed ordinances requiring that post-development runoff Figure 4.A"wooded wetland with infiltration trenches"as a functional peaks not exceed pre-development peaks. landscape enhancement. 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 (44 in). After infiltration, 30 cm/yr (12 in/yr) -play it safe," municipalities often set as a of this total reemerges to support the base design standard the control of the 10 to 100 flow of streams. Low density single family year frequency storms with outlet devices so residential development with an impervious- big that they pass, unchecked, smaller ness of 20 percent has been shown to re- storms of two- to five-year frequency, which duce this base flow by 12 percent to 27 are the cause of much of the flood damages cm/yr (10.6 in/yr) while commercial devel- experienced today. opment with an imperviousness of 90 per Even when designed to also control small- cent would reduce it by 90 percent to 3 er storms, detention basins do not reduce cm/yr(1.2 in/yr) (5). the volume of-runo ,ancl only control peak The use of stormwater infiltration devices rates o ow. Most of the thous;m of de- that infiltrate the first 1.25 cm (.5 in) of tention basins found in the areas surround- runoff would maintain 29 cm/yr (11.6 in/yr) ing our cities are constructed as so-called of post development base flow (96 percent) -dry basins" that do not improve the quality for single family residential uses and 22 Of urban runoff. Often unsightly, such basins cm/yr (8.7 in/yr) (72 percent) of the base are usually found in far corners, using left flow on commercial sites (5). As indicated over spaces of subdivisions. Alternatives to earlier, maintaining such base flow is partic- standard dry basins are "extended detention ularly important for headwater streams. basins" and -wet basins" with a permanent Problems with standing water such as expe- pools that offer water quality improvement. rienced in Maryland can be designed out Such urban ponds are an aesthetic enhance- (2). ment and have been found to more than Infiltration provides the added benefit of double adjacent property values (11). They water—quality enhancement as impurities in are a delight for urban residents and provide the "first"mush""of runoff are filtered. Infiltra- , habitat for water related wildlife. A survey in tion can be accomplished through measures Columbia, Maryland, found that 75 percent that enhance the landscape, such as the of homeowners preferred "wet ponds" for woody wetland with infiltration trenches stormwater management while only 17 per- shown in Figure 4. cent liked dry detention basins. Residents Controlling runoff pollution. The Clean overwhelmingly (94 percent) felt that it is Water Act has set goals to make the Nation's desirable to manage future stormwater waters fishable and swimmable. The Nation- basins for fish and wildlife as well as for wide Urban Runoff Program (13) has docu- flood and sediment control (1). mented the severity of the problem, and Preventing losses in infiltration. To models have been developed based on its control the increased volume of runoff leav- findings. This includes a highly practical ing a site, and not lust t e higher peak "Simple Formula" set forth by the Metropoli- tps can be taken to maintain mfltra- tan Washington Council of Governments (7) tion. Figure 3 presents a water bu get- that that permits the quantification of runoff pol- sFi-ows how groundwater recharge is re- lution loading for development sites. duced by impervious surfaces, while runoff The major sources of runoff pollution volumes rise. Annual precipitation in many from urban areas are impervious surfaces sections of Maryland, for example, is 112 cm that collect nitrogen deposited through acid JANUARY-FEBRUARY 1994 17