Infiltration Rates from Design Manuals for Storm Water Sample Clauses

Infiltration Rates from Design Manuals for Storm Water. Infiltration Facilities Two widely recognized references are used to design of storm water infiltration facilities in Washington State. One is the Washington Department of Ecology’s Stormwater Management Manual for Western Washington (WDOE, 2001), and the second is the King County Surface Water Design Manual (King County, 1998). The Department of Ecology manual recommends three methods for estimating long-term infiltration rates. The first method is based on a correlation between USDA soil texture and infiltration rates for homogeneous soils. The infiltration rates that the Department of Ecology has recommended, which are reproduced in Table 1.2, are based on a correlation developed by Xxxxx, et al. (1982) with minor changes based on the Water Environment Federation (1998). The manual indicates that infiltration rates provided through this correlation represent “short-term conservative rates for homogeneous soils.” These estimates do not consider the effects of long-term clogging due to siltation and biomass buildup in the infiltration facility. The Department of Ecology recommends that the short-term infiltration rates be reduced by dividing them by a correction factor of 2 to 4, depending on the soil textural classification. These correction factors, which are included in Table 1.2, can be reduced if approved by the local jurisdiction for sites with little soil variability, sites that will have a high degree of long-term facility maintenance, or sites where pretreatment will be employed to reduce the total suspended solids entering the infiltration facility. The WDOE manual specifies that correction factors must be greater than or equal to 2.0 and that factors higher than those included in Table 1.2 should be considered “for situations where long-term maintenance will be difficult to implement, where little or no pretreatment is anticipated, or where site conditions are highly variable or uncertain.” The second method for estimating infiltration rates included in the Department of Ecology manual is based on soil gradation parameters measured with the ASTM soil gradation procedure. These estimates are based on studies that compare infiltration measurements from full-scale infiltration facilities to soil gradation data developed with the ASTM procedure (ASTM D422). The recommended infiltration rates are included in Table 1.3. These estimates are described as “long-term rates” that represent average conditions regarding site variability, the degree of long-...