12/19/2001 Env Bd Packet - Laserfiche WebLink

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On -Lot Infiltration Reduced Lot Grading • Infiltration can be improved by tilling (scarification) of the lots with flatter grading to a depth of approximately 12 to 24 inches before sod is laid. This would also be of general benefit in all residential areas to address the problems associated with soil compaction (loss of recharge potential) which occurs during construction. The incorporation of compost or manure into the soil also increases infiltration. It should be noted that tilling this deep may require special equipment. • In areas where flatter lot grading is implemented, roof leaders that discharge to the surface should extend 6 feet away from the building. Construction • Soil compaction must be avoided wherever possible. For example, vehicles should never be parked on the future lawn during construction. • Mass grading should be avoided to keep native soil profiles intact and to minimize the area of soil compac- tion. • If soils become compacted through construction activities, the soil should be tilled to 18 inches and 6 to 12 inches of organic compost should be incorporated into the soil. Soakaway Pits Description Soakaway pits, also known as downspout infiltration systems, roof leader infiltration systems and dry wells, can be distinguished from infiltration trenches in terms of scale and sophistication of design. Soakaway pits are designed to receive runoff from individual roof leaders, whereas infiltration trenches are used for large -lot appli- cations (see the Infiltration Trenches BMP section for more detail). Soakaway pits are small, excavated pits, backfilled with aggregate, used to infiltrate "good quality" stormwater runoff, such as uncontaminated roof runoff. Rooftop runoff is discharged to the soakaway pit through the roof leader, which extends directly into a stone -filled reservoir. Figures 2 through 4 show examples of soakaway pit designs. The use of soakaway pits is limited by a number of site constraints, including soil type, contributing drainage area, depth to bedrock, and depth to groundwater. Rooftop gutter screens are needed to trap particles, leaves and other debris, and must be cleaned regularly. Soakaway pits for roof leader drainage have been implemented in Toronto, Maryland and Europe. A monitoring study indicated that 60 percent of 25 soakaway pits studied were operating as designed (Lindsey et al., 1992). Design Guidelines If a formal, detailed design is required by local permitting authorities, the design requirements presented in the Infiltration Trench BMP section can be followed for the design of soakaway pits (although no pretreatment other than gutter screens is required of a soakaway pit that receives only roof runoff). Other design considerations include: 3 -144 Minnesota Urban Small Sites BMP Manual