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r <br /> GW-Well-graded gravels, sandy gravels <br /> Gravel, sandy gravel and GP — Gap-graded or uniform gravels, sandy <br /> 1.63 silty gravels gravels <br /> A GM -Silty gravels, silty sandy gravels <br /> SW-Well-graded, gravelly sands <br /> 0.8 Sand, loamy sand or sandy SP - Gap-graded or uniform sands, gravelly <br /> loam sands <br /> 0.6 Silt loam SM-Silty sands, silty gravelly sands <br /> rB <br /> 0.3 Loam MH — Micaceous silts, diatomaceous silts, <br /> volcanic ash <br /> C 0.2 Sandy clay loam ML-Silts, very fine sands, silty or clayey fine <br /> sands <br /> GC—Clayey gravels, clayey sandy gravels <br /> SC—Clayey sands, clayey gravelly sands <br /> D <0.2 Clay loam, silty clay loam, CL—Low plasticity clays, sandy or silty clays <br /> sandy clay, silty clay or clay OL—Organic silts and clays of low plasticity <br /> CH—Highly plastic clays and sandy clays <br /> OH—Organic silts and clays of high plasticity <br /> *Thirty guidance manuals and many other stormwater references were reviewed to compile recommended <br /> infiltration rates. All of these sources use the following studies as the basis for their recommended infiltration <br /> rates: Rawls, Brakensiek and Saxton(1982); Rawls,Gimenez and Grossman(1998); Bouwer and Rice <br /> (1989);and Urban Hydrology for Small Watersheds(NRCS). The rates presented in this infiltration table use <br /> the information compiled from these sources as well as eight years of infiltration rates collected in various <br /> infiltration practices located in the Twin Cities metropolitan area. <br /> levels of runoff. Care must also be taken when numbers appropriate to AMC II. However, if the <br /> selecting curve numbers for agricultural land as its specific conditions of interest are expected to <br /> use can change considerably annually and even differ, curve numbers appropriate to AMC I or III <br /> over the course of a season. should be used. <br /> Antecedent Moisture Conditions <br /> Antecedent moisture conditions (AMC) describe Infiltration Rates <br /> the moisture already present in the soil at the Infiltration is the process of water entering the <br /> time of the rain event. AMC level I represents dry soil matrix. The rate of infiltration depends on <br /> conditions, level II represents normal conditions, soil properties, vegetation, and the slope of the <br /> and level III represents wet conditions. Normal surface, among other factors. Discussions of <br /> conditions are defined as 1.4 to 2.1 inches of infiltration often include a discussion of hydraulic <br /> rainfall in the growing season in the five days conductivity. Hydraulic conductivity is a measure <br /> preceding the event of interest. Most evaluations of ease with which a fluid flows through the soil, <br /> of expected future site conditions use the curve but it is not the infiltration rate. The infiltration rate <br /> �� - <br />