Laserfiche WebLink
<br />is applied to concurrent ordinates The Instantaneous peak flows should be averagod. <br />regardless of differences in lag time and plotteCI at the average lag time. The average unit <br />hydrograph can then be sketched to conform to the slwpe of the graphs. passing tllroUgll <br />the computed average peak. and having a volume of 1 un t of runoff. <br /> <br />By inspection of tile heaviest rainfall and snowmelt excesses that caused tile peal, <br />flows of the flood hydrograph. unit hydrograph ordinates around the peak are first <br />estimated. The remainder of unit hydrograph is then estimated so tllat the volume is equal <br />to 1 unit of runoff. In estimating the overall sllape of tile unit hydrograph, consideration <br />is given to the rate of rise and recession of 'l'le flood hydrograph and the estimated lag <br />time from the heaviest excess period to the peak of the flood hydrograph. This fil'st-trial <br />unit hydrograph is then applied to the excess estimates. and the resulting cor~puted <br />hydrograph is checked against the observed '1lood Ilydrograpfl. The unit hydrograph is <br />then adjusted as needed. and the process IS repeated until tile computed hydrog'apll <br />approximates the observed hydrograph to the desired accuracy <br /> <br />The computation of a hydrograp\1 requires not only developing a unit hydrog'apll <br />but estimating rainfall and ~,nowmelt losses as well. and ordinarily the analysis involves <br />both procedures at the same time. This is normally I'equireej since It is not always obvious <br />whether the unit hydrograp~ or the loss rates should be adjusted to obtain a bdter fit <br />between the computed and observed hydrographs. <br /> <br />The reconstitution of historical flood Ilydrog'aphs involves the estimation of base <br />flow, loss rates, and unit hydrographs; and it oecomes clear tllat many combinations of <br />these three items Can reconstitute the same flood Ilydrograph equally as well. <br />Consequently, a considerable amount of engineerinG! judgment will be required to establish <br />reasonable estimates for each of these items. Some subjectivity Can be eliminated by <br />techniques such as the ba~.e flow separation. However. there is no way to completely <br />eliminate all of the subjectivty involved. Consistency in aporoach Can be gained tllrough <br />the use of computer prograrns such as the HI::C-1 Flood Hydrograph Package, Wtllch will <br />automatically derive "best fit" unit hydrograoh and loss rate coefficients based on <br />optimization techniques to minimize the differences between obselYEld and computecl flows. <br />Care must still be exercised by the engineer, howevel', to evaluate the computElct <br />coefficients for reasonablenlJss. <br /> <br />3. S-CURVE HYDROGRAPHS. <br /> <br />According to the unit hydrograph concept, if tile unit rate 01 rainfall excess ovel' a <br />drainage area should continue indefinitelY Wittl the same areal distribution anej intensity <br />characteristics, successive units of rainfall excess would contribute runoff at rates <br />corresponding to the basic unit hydrograph. An accumulation of runoff ordinates <br />corresponding to a particular time would giVE' the total rate of runoff produced by tile <br />uniform, continuous rate of rainfall excess antecedelt thereto. Al a time equal to the rate <br />of rainfall excess and would remain constant thereaf'ter. The hydroqraph generated in tllis <br />fashion will be referred to herein as an S-curve Hydrograph. The S-curve hydrograph, as <br />defined above, should not be confused with ITass curves of runoff that simply represent <br />accumulative volumes. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-38 <br /> <br />CRtJH <br />