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<br />Section 3.07. Standard project runoff computation <br /> <br />Where unit hydrographs have been derived from recorded rainfall or <br />snowmelt and runoff data, data inaccuracies and other factors usually <br />result in different unit hydrographs for different storms. Those <br />derived for the larger floods are considered to be the most reliable <br />for standard project use. If no large floods at the location are <br />available for study, some increase in the maximum ordinate and decrease <br />in time to peak of unit hydrographs derived from smaller floods would <br />normally be advisable to account for increase flow velocities in those <br />rivers where larger flood flows can be expected to be accompanied by <br />increased hydraulic efficiency. If future urbanization on channel <br />modifications are anticipated, adjustments should be made in the unit <br />hydrographs to reflect these changes also. <br />Where methods other than the unit hydrograph method are used in <br />runoff computation, adequate adjustments may likewise be necessary in <br />order to assure that severe rainfall-runoff relations are applied. <br />Care must be exercised, where new reservoirs eliminate the retarding <br />effects of channel storage, that such changes are properly accounted for <br />(by reducing the time of concentration or the routing effects). <br />Runoff computations are then straight-forward. Losses are sub- <br />tracted from rainfall and snowmelt for each computation interval, <br />direct runoff is computed from the resulting excess values by use of <br />the unit hydrograph or other method, and base flows are added. <br /> <br />Section 3.08. Standard project flood series <br /> <br />In the design of storage facilities, it is occasionally necessary <br />to examine the operation of the facility over a long period of time. <br />In the case of rainfloods, it is ordinary to compute runoff only for a <br />period of 3 or 4 days, because uncertainties in the rainfall-runoff <br /> <br />3-11 <br />