Laserfiche WebLink
CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> 5.4.1 Design Storm Hyetograph <br /> All storm event hydrograph methods require the input of a design storm hyetograph,which is a plot of <br /> rainfall depth versus time for a given design storm frequency and duration. The design storm hyetograph is <br /> constructed by multiplying a dimensionless hyetograph(which plots the percent of total rainfall depth <br /> versus time)by the total rainfall depth for the design storm. The design storm hyetograph to be used in the <br /> City of Everett is a SCS Type 1A rainfall distribution resolved to 10-minute time intervals. <br /> Table 5.4 gives the total precipitation for the 6 month,2-year, 10-year,and 100-year,24-hour duration <br /> storms in Everett: <br /> Table 5.4 Design Storm Hyetograph 24-hour Design Storm Totals <br /> Return Frequency Total Precipitation(inches) <br /> 6 month 1.12 <br /> 2 year <br /> 10 year <br /> 25 year 2.60 <br /> 100 year d <br /> 5.4.2 Runoff Parameters <br /> All storm event hydrograph methods require input of parameters which describe physical drainage basin <br /> characteristics. These parameters provide the basis for development of the runoff hydrograph. This section <br /> describes the three key parameters(area,curve number,and time of concentration)used in hydrograph <br /> synthesis. <br /> Area <br /> Homogenous basin areas must be selected to obtain accuracy in hydrograph analysis. Significant <br /> differences in land use within a given drainage basin must be addressed by dividing the basin into subbasin <br /> areas of similar land use and/or runoff characteristics. .Hydrographs are then computed for each subbasin <br /> area and summed to form the total runoff hydrograph for the basin. <br /> For accuracy,all pervious and impervious areas within a given basin or subbasin must be analyzed <br /> separately. This analysis may be done by either 1)computing separate hydrographs for each area and <br /> combining them to form the total runoff hydrograph,or 2)by computing the precipitation excess for each <br /> area and combining the excesses to obtain the total precipitation excess,which is then used to develop the <br /> runoff hydrograph. This procedure is explained further later in this section. <br /> Curve Number <br /> The Soil Conservation Service(SCS)classified over 4,000 soil types into four soil groups. These are: <br /> A. Low runoff potential. Soils having high infiltration rates,even when thoroughly wetted,and <br /> consisting chiefly of deep,well-to-excessively drained sands or gravels. These soils have a <br /> high rate of water transmission. <br /> B. Moderately low runoff potential. Soils having moderate infiltration rates when thoroughly <br /> wetted,and consisting chiefly of moderately fine to moderately coarse textures. These soils <br /> have a moderate rate of water transmission. <br /> Volume I—Minimum Technical Requirements <br /> 5-4 <br />