12/19/2001 Env Bd Packet - Laserfiche WebLink

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Infiltration Basins Design • Proposed alternative infiltration rates must be representative of vertical water infiltration through the soil surface (values that include lateral flow are not acceptable). • Values that should not be used as alternative infiltration rates include, but are not limited to, the following: hydraulic conductivity, saturated hydraulic conductivity, transmissivity, permeability and percolation rate. Designers should use their best judgement to determine if the slowest or average measured infiltration rate in the proposed basin area should be used for the design of the basin. Where feasible, larger -scale infiltration measurements are encouraged. Procedures such as the Pilot Infiltration Test are described in the State of Washington's Stormwater Management Manual (Washington State Department of Ecology, 1999). This document is currently available on the internet at www.ecy.wa.gov. This type of proce- dure can minimize some of the error associated with smaller -scale tests and provide an indication of a longer -term infiltration rate that better represents the future conditions of the site. Duration of Ponding: The depth of the infiltration basin should be adjusted so that maximum drain time is 72 hours for the total runoff volume. However, certain types of vegetation (turf, for example) will require a shorter ponding duration to survive storm events. The design ponding duration should be determined by plant inundation tolerances. Average Depth After the infiltration rate of the soil has been determined, the maximum depth of the infiltration basin can be calculated with the following equation: d = (f) * (Tp) Where: d = maximum design depth (inches), f = soil infiltration rate (in/hr), and T = design ponding time (hours). The maximum depth and ponding time of the infiltration area should promote the survival of vegetation. The maximum depth should be no greater than 2 feet; the ponding time no greater than 72 hours. In recent monitoring studies (Galli, 1992) one of the causal factors of failure was noted to be the depth of water retained in the basin. The weight of the water is thought to compact the basin, decreasing its infiltration potential. The depth of storage should be limited to a maximum 2 feet in order to minimize the compaction of the basin. Basin Slopes: The bottom of the basin should be graded as flat as possible to provide uniform ponding and infiltration of the runoff across the floor. The side slopes of the basin should be no steeper than 3H:1 V (flatter slopes are preferred) to allow for proper vegetative stabilization, easier mowing, easier access, and better public safety. Designs for infiltration basins should emphasize accessibility and ease of maintenance. 3 -162 Minnesota Urban Small Sites BMP Manual