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Updated Surface Water Management Plan (SWMP) <br />City of Ramsey, Minnesota <br />lateral systems. As the storm water runoff enters the system, the flow time in the <br />storm sewer is then added to the concentration time and compared to the <br />downstream drainage area concentration time. The maximum of these values is <br />used downstream, which results in a longer concentration time and peak runoff <br />rate as the flow moves downstream from the initial design point. <br />5. Land Use Factors in Modeling (Runoff Coefficients) <br />The percentage of rainfall falling on an area that must be collected by a hydraulic <br />facility is dependent on watershed variables such as soil permeability, ground <br />slope, vegetation, surface depressions, type of development and antecedent <br />rainfall. These factors are taken into consideration when selecting a runoff <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 well -graded gravels, sandy gravels, gap -graded <br />or uniform gravels, sandy gravels, silty gravels, silty <br />sandy gravels, well -graded gravely sands, gap -graded or <br />uniform sands, gravelly sands. <br />Group B - Includes silty sands, silty gravelly sands, micaceous silts, <br />diatomaceous silts, volcanic ash. <br />Group C - Includes silts, very fine sands, silty or clayey fine sands. <br />Group D - Includes clayey gravels, clayey sandy gravels, clayey <br />sands, clayey gravelly sands, low plasticity clays, sandy <br />or silty clays, organic silts and clays of low plasticity, <br />highly plastic clays and silty sands, organic silts and clays <br />of high plasticity. <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 final <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 />Section IV <br />October 21, 2015 Page 38 <br />