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<br />. <br /> <br />. <br /> <br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />4.2 Basic Assumptions <br />The derivation and appl ication of the unit hydrograph are based on the <br />following assumptions: <br />1. The rainfall intensity is constant during the storm that produces the <br />unit hydrograph. <br />2. The rainfall is uniformly distributed throughout the whole area of the <br />drainage basin. <br />3. The base or time duration of the design runoff due to an effective <br />rainfall of unit duration is constant. <br />4. The ordinates of the design runoff with a common base time are directly <br />proportional to the total amount of direct runoff represented by each <br />hydrograph. <br />5. The effects of all physical characteristics of a given drainage basin, <br />including shape, slope, detention, infiltration, drainage pattern, <br />channel storage, etc., are reflected in the shape of the unit hydrograph <br />for that basin. <br /> <br />4.3 Equations <br />There are four basic equations used in defining the limits of the <br />synthetic unit hydrograph. The first equation defines the lag time of the <br />basin in terms of time to peak, t , which, for the cUHP Method, is defined as <br />p <br />the time from the center of the unit storm duration to the peak of the unit <br />hydrograph as shown in Figure 4-6. <br /> <br />t = C [(L L /n d.48 <br />p t ca <br /> <br />(4-1) <br /> <br />In which tp = time to peak of the unit hydrograph from midpoint of <br /> unit rainfall in hours. <br /> L = length along stream from study point to upstream <br /> limits of the basin in miles <br /> Lca = length along stream from study point to a point <br /> along stream adjacent to the centroid of the basin <br /> in miles. <br /> S = weighted average slope of basin along the stream to <br />. upstream limits of the basin in feet per foot. <br />Ct = coefficient reflecting time to peak. <br />5-1-84 <br />