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<br />. <br /> <br />. <br /> <br />. <br /> <br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />4.8 Basin Land Use Consideration <br />^ lumped parameter model such as a CUHP relies on data from basins having <br />relatively uniform land use. It is recommended that basins having zones of <br />differing land use be subdivided into sub-basins having relatively uniform <br />land use. As an example, if a lower half of a watershed has been urbanized <br />and the upper half is to remain as open space, it is best to develop two <br />distinct hydrographs. The upper sub-basin hydrograph will be based on the <br />coefficients for undeveloped land and the lower sub-basin hydrograph will be <br />the result of coefficients for the developed area. <br /> <br />4.9 Determination of Ct and Cp Coefficients <br />The value of ct in Equation 4-1 may be determined using Figure 4-2. Note <br />that the curve in Figure 4-2 can be represented using parabolic equations <br />having the percent impervious (I ) as an independent variable. Three sets of <br />a <br />coefficients that are unique for each of the three ranges of imperviousness <br />describe the curve. The mathematical description of the Ia vs ct curve. was <br />developed so that the CUHP procedure could be computerized. <br />The value of C to be used in Equation 4-3 may be determined using Figure <br />p <br />4-3. The curve in Figure 4-3 is also represented with a parabolic equation. <br />To determine C , first obtain the value of the Peaking Parameter P from Figure <br />p <br />4-3. Then calculate C using Equation 4-6: <br />p <br /> <br /> cp = P C AO.15 (4-6) <br /> t <br />In Which, ct = coefficient from Figure 4-2 <br /> P = peaking parameter from Figure 4-3 <br /> A = bas i n a rea in square miles. <br /> <br />4.10 Unit Hydrograph Shape <br />The shape of the unit hydrograph is a function of the physical <br />characteri sti cs of the watershed. It incorporates the effects of watershed <br />size, shape, degree of development, slope, type and size of drainage system, <br />soils, and many other watershed factors. The shape of the unit hydrograph is <br />also dependent on the temporal and spatial distribution of rainstorms and will <br />vary with each storm event. As a result, a unit hydrograph based on <br />rainfall/runoff data is an approximation that provides the engineer or <br /> <br />5-1-84 <br />