12/19/2001 Env Bd Packet - Laserfiche WebLink

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Infiltration Basins Basin Shape The length and width of the basin will be determined by the characteristics of the site in question (topography, size and shape). A desirable length -to -width ratio for an infiltration basin is 3:1 or greater. Vegetation A key feature of an infiltration basin is its vegetation. It is important to vegetate the bottom of the basin with deep - rooted plants to increase the infiltration capacity of the basin. Roots create small conduits for water to infiltrate. The root penetration and thatch formation of the vegetation maintains and may enhance the original infiltration capacity. Soluble nutrients are taken up during plant growth, improving the pollutant - removal capac- ity of the basin. Dense vegetation will also impede soil erosion and scouring of the basin floor. Immediately following basin construction, the bottom and side slopes of the basin should be stabilized with a dense stand of water - tolerant grass. Use of low- maintenance, rapidly germinating grasses, such as red top (Agrostis alba) can be used. Likewise, vegetative buffers around the perimeter of the basin are recommended for erosion control and additional sediment and nutrient removal. A diversity of plant species should be planted to allow for best survivability. Plants that are tolerant of both wet weather and drought should be used. Plantings in an infiltration basin should be able to withstand periods of ponding and maintain or enhance the pore space in the underlying soils. A list on the last page of this BMP section provides some plant recommendations based on different site conditions (Rozumalski, 2001). Inflow /Bypass If runoff is delivered by a storm drain pipe or along the main conveyance system, the infiltration practice must be designed as an off-line practice. To prevent incoming flow velocities from reaching erosive levels, which can scour the basin floor, inlet channels to the basin should be stabilized. Riprap may be used for this purpose. The riprap should be designed to terminate in a broad apron, which spreads the runoff more evenly over the basin surface to promote better infiltration. A bypass system should be implemented for all infiltration basins. A bypass flow path or pipe should be incorpo- rated in the design of an infiltration basin to convey high flows around the basin. This will necessitate the con- struction of a flow splitter upstream of the basin. The bypass serves several functions. Specifically, the bypass can be used as the normal outlet during 1) stabilization of the site (while the inlet to the basin is blocked off), 2) basin maintenance and 3) winter conditions. Overflow All infiltration basins must have an emergency spillway capable of passing runoff from large storms without damage to the impounding structure. The overland flow path of surface runoff exceeding the capacity of the infiltration system should be evaluated to preclude erosive concentrated flow. If computed flow velocities do not exceed the non - erosive threshold, overflow may be accommodated by natural topography. Groundwater Mounding: Calculations to determine groundwater mounding may be necessary in cases where slope stability is a concern and/or a high water table is encountered. A hydrogeologist should be consulted about the potential for ground- water mounding in these areas. The results from groundwater mounding calculations should be regarded as an indication of the mounding potential rather than as an accurate representation of the actual mounding depth. Metropolitan Council / Barr Engineering Co. 3-163