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BMP: INFILTRATION SYSTEMS <br />1. METHODS: An infiltration system is a device used to percolate runoff <br />into the soil. A typical system is a rock-filled trench or basin (called a dry <br />well). You should infiltrate runoff from all impervious surfaces, including roof <br />tops, driveways, and areas where the soil has been packed down. <br />An infiltration system should be used: <br />? Below roof drip lines; <br />? Along driveways and walkways; <br />? Along dikes and ditches; <br />? Below sediment trap outlets; and <br />? On flat or gently-sloping ground. <br />The capacity of infiltration trenches decreases as the slope of the trench <br />increases. Don't build infiltration trenches with drain slopes steeper than 15%. <br />Where a roof drip line or driveway exceeds 15% slope, install a lined ditch to <br />convey the runoff to a dry well or lateral infiltration trench located along a slope <br />contour. A french drain is a trench containing a perforated pipe surrounded by <br />gravel. <br />.a-" p?err?RPrr?t? <br />Fire <br />4' <br />MAN. <br />fjfrpyE 1- <br />T0* <br />Where water dripping from a roof will fall on a slope above a house's <br />foundation, install a french drain to convey the roof drip to an infiltration <br />system away from the house. The french drain will prevent water from seeping <br />under the foundation and weakening it. <br />Where an infiltration system is located below a disturbed area, install a <br />sediment barrier to remove the soil before it reaches the system. Removing <br />sediment from the runoff will increase the efficiency of the infiltration system <br />and reduce maintenance costs. <br />The size of the infiltration system depends on soil permeability and runoff area. <br />The system must be able to infiltrate one inch of precipitation per hour (the <br />peak hour of the 25-year storm in the Counties). If you have hired an <br />architect or engineer to prepare the plans for your house, he or she can calculate <br />the size of the infiltration system needed. <br />15 <br />