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<br />. <br /> <br />. <br /> <br />. <br /> <br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />4.5 Basin Size Limits <br />The rainfall/runoff data used in the development of the 1982 version of <br />the CUHP was obtained primarily from small basins. Basin sizes ranged from <br />0.15 square miles to 3.08 square miles. Although some extrapolation is <br />justified, unlimited extrapolation of how the basin responds to rainfall is <br />not. It is recommended that the maximum size of a basin to be analyzed with a <br />single unit hydrograph be limited to 10-square miles. Whenever a larger basin <br />needs to be studied, it is suggested it be subdivided into sub-basins of <br />10-square miles or less and individual sub-basin storm hydrograph be routed <br />downstream us i ng appropriate channel routing procedures. The routed <br />hydrographs then need to be added to develop a single composite storm <br />hydrograph. <br />Because of the way a unit hydrograph responds, it is also suggested the <br />minimum basin size be 90 acres. The 5-minute unit hydrograph procedure may be <br />used for a smaller basin provided tp is greater than 10 minutes. <br /> <br />4.6 Basin Shape Limits <br />The basin shape can have a profound effect on the final results and, in <br />some instances, can result in underestimates of peak flows. Experience with <br />the 1982 version of the cUHP has shown that whenever basin length is increased <br />faster than basin area, the storm hydrograph peak will tend to decrease. <br />Although hydrologic routing is an integral part of runoff analysis, the data <br />used to develop the 1982 version of the CUHP is insufficient to say that the <br />observed CUHP response with disproportionately increasing basin length is <br />valid. For this reason, it is important to subdivide irregularly shaped or <br />very long basins (i.e., basin length to width ration of 4 or more) into more <br />regularly shaped sub-basins. A composite basin storm hydrograph can be <br />developed using appropriate routing and by adding of the individual sub-basin <br />storm hydrographs. <br /> <br />4.7 Basin Slope Limits and Considerations <br />The 1982 version of the cUHP was developed using data from basins having <br />a range of major drainageway slopes between 0.005 ft/ft and 0.037 ft/ft. <br />Caution needs to be used when extrapolating beyond this range. <br />Natural and Grass Lined Waterways: <br />In natural and grass lined drainageways, channels become unstable when a <br /> <br />5-1-84 <br />