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9 Chapter 4-Site-SpecAndustria/ Storm Water BP?,Ps <br />f Grassed Swales <br />What Are They <br />Grassed swales are vegetated depressions used to transport, filter, and remove sediments. <br />Grassed swales control high runoff rates by reducing the speed of the runoff and by reducing the <br />volume of the runoff through infiltration of the storm water. Pollutants are removed because runoff <br />travels slowly and infiltrates into the soil and because the vegetation in the grassed swale works as <br />a filter or strainer. <br /> -S <br />I <br />es <br />S <br /> Z <br />t <br />" <br />or less <br /> <br />Swale Slopes _ <br />Padroao T,e <br />as nse to chKa-Oam <br />Ze.o as Drainage Itrt.reases In10trat-1 <br />Wdl Permit <br /> li0 <br />Oense Growth <br />of Grass '' <br />? <br />?'" <br />q <br />• <br /> A <br />? ? <br />? . _ <br /> Weep Mote. <br /> <br />• <br />stow Pra.ents <br />Downstream Scow <br />FIGURE 4.32 GRASSED SWALE WITH RAILROAD TIE CHECK DAM <br />(Modified from MWCOG, 1987) <br />When and Where to Use Them <br />Grassed swales are suitable for most areas where storm water runoff is low. Certain factors will <br />affect the operation of grassed swales, including soil type, land features, and the depth of the soil <br />from the surface to the water table (i.e., the top of the drenched portion of the soil or bedrock <br />layer). The soil must be permeable for runoff to be able to infiltrate well. Sandy soils will not hold <br />vegetation well nor form a stable channel structure. Steep slopes will increase runoff rates and <br />create greater potential for erosion. Storm water flows will not be easily absorbed where the water <br />table is near the surface. Swales are most useful for sites smaller than 10 acres (MWCOG, 1987). <br />Even without highly permeable soils, swales reduce velocity and thus are useful. <br />Grassed swales usually do not work well for construction runoff because the runoff has high <br />sediment loads. <br />What to Consider 1 <br />The channel of the swale should be as level as possible to maximize infiltration. Side slopes in the <br />• swale should be designed to no steeper than 3:1 to minimize channel erosion (MWCOG, 1987). <br />Plans should consider (1) the use of existing topography and existing drainage patterns and (2) the <br />4-103