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Updated Storm Water Management Plan (SWMP) <br />City of Ramsey, Minnesota <br />coefficient (C) for the Rational Method or a runoff curve number (CN) for use in <br />SCS methodology. <br />Under ultimate (fully developed) conditions, the values of the coefficient will <br />increase with increases in the amount of impervious surfaces caused by street <br />surfacing, building construction, and grading. <br />The antecedent moisture condition (AMC) relates to the moisture content of the <br />soil prior to a given storm event. Curve numbers based on land use can be <br />adjusted based on an assumed moisture condition. For purposes of the model, <br />normal antecedent moisture condition (AMC II) was assumed. Curve number <br />values can be adjusted for dry conditions (AMC I) or wet conditions (AMC III). <br />Curve numbers are also dependent on the type of soil in a given drainage area. <br />Soil types are classified into four basic hydrologic groups as follows: <br />Group A - Includes soils consisting of deep sand and aggregated <br />silts. <br />Group B - Includes sandy loam soils. <br />Group C - Includes soils that are low in organic content and made up <br />of clay loams and soils high in clay. <br />Group D - Includes soils consisting of heavy plastic type clay soils. <br />Curve numbers that were assumed in the development of the model were based <br />on the hydrologic soil group for each watershed based on the information <br />contained in the County Soil Survey. Development plans shall consider post - <br />development site soil conditions when choosing runoff curve numbers for fmal <br />design. <br />Curve numbers (CN) are given in SCS TR -55. Average CN values for each land <br />use type are used in the design of the storm drainage facilities in undeveloped <br />areas. For the modeling of existing facilities, CN values were determined for <br />each type of development and current zoned land use in each subwatershed. In <br />general, the unpaved, non -wetland areas were modeled with curve numbers that <br />most closely represent the Anoka Sand Plain. The curve numbers were then <br />adjusted to reflect the percentage of impervious surfacing. <br />It should be noted that if land use changes to more or less impervious surfacing <br />than the model, it will affect the model and updates may be needed. <br />Section IV <br />February 20, 2015March 6, 2015 Page 37 <br />