12/19/2001 Env Bd Packet - Laserfiche WebLink

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Infiltration Basins Sequencing Care should be taken during construction to minimize the risk of premature failure of the infiltration basin. This failure is caused by the deposition of sediments from disturbed, unstabilized areas. This can be minimized or avoided by proper sequencing. • Ideally, construction of the infiltration basin should take place after the site has been stabilized. • No runoff should enter the infiltration basin prior to completion of construction and the complete stabilization of the tributary areas. • Diversion berms or silt fence should be placed around the perimeter of the infiltration basin during all phases of construction. Sediment and erosion controls should be used to keep runoff and sediment away from the infiltration basin. • Initial excavation of the basin should be carried out to within one foot of the final grade of the basin floor. Final excavation of the basin floor should be delayed until all disturbed areas in the drainage area are stabi- lized. All excavation should be performed by equipment with tracks exerting relatively light pressures. This will prevent compacting of the basin floor, which would reduce the infiltration capacity. • In order to avoid soil compaction, absolutely no equipment should be driven in the area of the basin before and after its construction. • After final grading, the basin floor should be tilled to a depth of at least 6 inches to provide a well - aerated, porous surface texture. Six inches of compost should be tilled in at this time if soils are even the slightest bit compacted. This will help to facilitate infiltration and root growth. • During and after excavation, all excavated materials should be placed downstream, away from the infiltration basin, to prevent redepositing during runoff events. • Immediately following basin construction, the bottom and side slopes of the basin should be stabilized with a dense stand of appropriate plants. Construction Experience has shown that the longevity of infiltration practices is strongly influenced by the care taken during construction. The construction sequence and specifications for each infiltration practice must be precisely fol- lowed. • Infiltration basins should not be used as temporary sediment traps during construction. • Infiltration basins will operate as designed only if they are constructed properly. There are three main rules that must be followed during the construction of an infiltration basin: — Basins should be constructed at the end of development construction — Smearing of the soil at the interface with the basin floor must be avoided and/or corrected by raking or rototilling — Compaction of the basin during construction must be minimized 3-166 Minnesota Urban Small Sites BMP Manual