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10 Chapter 4-Site-Specliv/ndustria/ Storm Water BMPs <br />Infiltration Trenches <br />What Are They <br />11 <br />An infiltration trench usually consists of a long, narrow excavation ranging from 3 to 12 feet deep. <br />The trench is filled with stone, which allows for temporary storage of storm water runoff in the <br />open spaces between the stones. The stored storm water infiltrates into the surrounding soil or <br />drains into underground pipes through holes and is then routed to an outflow point. Infiltration <br />trenches are designed to remove both fine sediments and soluble pollutants rather than larger, <br />coarse pollutants. <br />weucao 0o:erratan wen <br />.o a <br />Emergency Overtto.. Berm d;?o•?• o'o° -} <br />p Runoff F ne a Thro qn y <br />o ?' • ??? - ? . 20 Fool w as G aaa B?ffe 51 ?o <br />j J •p•tj 1Oo a O? ... 1.' <br />•Q ' 1 <br />ProUtlive Layer d1 Fitter F.D- 1 <br />>l71 ? '? .?.0 Q F.her Faorlc lmee SM.. 10 <br />.? "? •O ,. w..em Son Comam.nanon <br />e?e d <br />.Q <br />e 11 0? O <br />• Sand Fitter 1612 Feet Deed, <br />or Fit- Eq.-lenr <br />r?\? f \ (? TMOUgh U.d.11-Oed Suowlls <br />FIGURE 4.34 TYPICAL INFILTRATION TRENCH <br />(Modified from MWCOG, 1987) <br />When and Where to Use Them <br />Infiltration trenches should be restricted to areas with certain soil, ground water, slope, area, and <br />pollutant conditions. For example, infiltration trenches will not operate well in soils that have high <br />clay contents, silt/clay soils, sandy/clay loams, or soils that have been compacted. Trenches <br />should not be sited over fill soils because such soils are unstable. Hardened soils are often not <br />suitable for infiltration trenches because these types of soils do not easily absorb water. Infiltration <br />practices in general should not be used to manage contaminated storm water. <br />The drainage area contributing runoff to a single trench should not exceed 5 acres (State of <br />Maryland, 1983). Construction of trenches should not start until after all land-disturbing activities <br />have ceased so that runoff with high levels of sediment does not fill in the structure. <br />If slopes draining into the trench are steeper than 5 percent, the runoff will enter the trench too <br />fast and will overwhelm the infiltration capacity of the soil, causing overflow. The depth from the <br />• bottom of the trench to the bedrock layer and the seasonal high water table must be at least three <br />feet. Infiltration trenches may not be suitable in areas where there are cold winters and deep frost <br />levels. <br />4-107