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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />. Multiply each succeeding precipitation value times the unit hydrograph <br />value and lag them appropriately in the table. Finally, add up all the <br />multiplied values horizontally to obtain the design storm runoff hydrograph. <br /> <br />4.12 Example - One Homogeneous Basin Hydrograph <br /> <br />Given: <br /> <br />A basin that has the following characteristics: <br />Watershed Area (A) = 0.38 square miles = 243 acres <br />Watershed Length (L) = 1.28 miles <br />Distance to Centroid (Lca) = 0.52 miles <br />Impervious area (Ia) = 44% <br />Slope along waterway (S) = 0.0102 ft/ft <br /> <br />Required: A storm hydrograph for the 100-year design storm. <br />Solution: <br /> <br />. <br /> <br />Step 1. Determine Ct using procedure given in 4-9. Using the given <br />percent of impervious cover (i.e. 44%) find from Figure 4-2 the <br />value of Ct' <br /> <br />Ct = 0.091 <br /> <br />Determine t using Equation 4-1. <br />p <br />tp = ct [(L Lca/ S )JO.48 <br /> <br />= (0.091)[1.28 x 0.52/ 0.0102 JO.48 <br /> <br />Step 2. <br /> <br />Step 3. <br /> <br />tp = 0.225 hours (13.5 minutes) <br /> <br />Determine if a 5 minute unit hydrograph needs to be used or a <br />longer unit hydrograph may be used in this case. <br /> <br />tu 1/3 tp (5 minute minimum) <br /> <br />1/3 (13.5) = 4.5 <br /> <br />Use tu = 5 minutes (0.083 hours) <br /> <br />. <br /> <br />5-1-84 <br />