1125 80TH ST SW 2016-01-01 MF Import

extent feasible, in order to minimize the required size of the separator and to maximize it's effectiveness for treatment. City standards call for the oil/water separator vaults to have a minimum design flow determined by using the rational formula with a rainfall intensity of 0.5 inches per hour over the contributing area. The vault is required to provide a 15 minute detention time for the design flow. High flow bypass piping is required for the vault per City standards, unless the vault is sized for the peak flow from the 25 year design flow. If a bypass is preferred, the vault design flow should br: reacted at the point that the water elevation reaches the invert of the high flow by3ass pipe. The flow through the separator should not exceed the maximum process flow (the largest flow the separator can handle before accumulated oil begins to b: Flushed out) when the head needed to pass the 25 year storm is reached in the high flow bypass pipe. The separator's maximum process flow should be available from the manufacturer. If bypass piping is provided, the outlet valve must be provided downstream of where the bypass piping and vault piping re-converge. The following information is required for City review and approval of oil/water separators: r 1) design flow calculations 2) design inlet capacity calculations 3) vault inlet sizing calculations 4) oil/water separator sizing calculations 5) bypass sizing calculations 6) copy of site plan showing the contributing area Infiltration systems: City standards call for at least 3 feet of relatively permeable soil to be present between the bottom of the infiltration system and the seasonal high groundwater elevation, bedrock, hardpan or any impervious layer such as clay. Soil logs are needed to demonstrate that this requirement is met on this site. It is important that the amount of grading that will take place on the site and the estimated depth of the system (typically 34 feet below the final grade) be taken into account when determining the depth of the soil logs. The size of the system is dependent upon the type of soils present on the site. For roof downspout systems, the Soil Survey of Snohomish County (Soil Conservation Service) can be used to determine the textural analysis of the soil. However, the Soil Survey designation needs to be verified on site by a soils expert. The least permeable USDA texture given in the Soil Survey for the soil horizon is to be used in the design analysis. City standards do not allow infiltration in soils with 30% or greater clay content, 40% or greater silt+clay content, or soils with an infiltration rate of less than 0.5 inches per hour.