1010 N BROADWAY EVCC 2018-01-01 MF Import

CITY OF EVERETT STORMWATER MANAGEMENT MANUAL 5.4.1 Design Storm Hyetograph All storm event hydrograph methods require the input of a design storm hyetograph,which is a plot of rainfall depth versus time for a given design storm frequency and duration. The design storm hyetograph is constructed by multiplying a dimensionless hyetograph(which plots the percent of total rainfall depth versus time)by the total rainfall depth for the design storm. The design storm hyetograph to be used in the City of Everett is a SCS Type lA rainfall distribution resolved to 10-minute time intervals. Table 5.4 gives the total precipitation for the 6 month,2-year, 10-year,and 100-year,24-hour duration storms in Everett: Table 5.4 Design Storm Hyetograph 24-hour Design Storm Totals Return Frequency Total Precipitation(inches) 6 month 1.12 2 year 1.55 10 year 2.25 25 year 2.60 100 year 3.25 5.4.2 Runoff Parameters All storm event hydrograph methods require input of parameters which describe physical drainage basin characteristics. These parameters provide the basis for development of the runoff hydrograph. This section describes the three key parameters(area,curve number, and time of concentration)used in hydrograph synthesis. Area Homogenous basin areas must be selected to obtain accuracy in hydrograph analysis. Significant differences in land use within a given drainage basin must be addressed by dividing the basin into subbasin areas of similar land use and/or runoff characteristics. Hydrographs are then computed for each subbasin area and summed to form the total runoff hydrograph for the basin. For accuracy,all pervious and impervious areas within a given basin or subbasin must be analyzed separately. This analysis may be done by either 1)computing separate hydrographs for each area and combining them to form the total runoff hydrograph,or 2)by computing the precipitation excess for each area and combining the excesses to obtain the total precipitation excess,which is then used to develop the runoff hydrograph. This procedure is explained further later in this section. Curve Number The Soil Conservation Service(SCS)classified over 4,000 soil types into four soil groups. These are: A. Low runoff potential. Soils having high infiltration rates,even when thoroughly wetted,and consisting chiefly of deep,well-to-excessively drained sands or gravels. These soils have a high rate of water transmission. B. Moderately low runoff potential. Soils having moderate infiltration rates when thoroughly wetted,and consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission. Volume I—Minimum Technical Requirements 5-4