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<br />hydrographs, between the hydrograph peaks, or between midpoints of the rising limbs. <br />After K has been estimated, a value of X can be obtained through trail and error. <br />Assume a value for X, and then route the inflow hydrograph with these parameters. <br />Compare the routed hydrograph with the observed outflow hydrograph. Make <br />adjustments to X in order to obtain the desired fit. Adjustments to the original <br />estimate of K may also be necessary to obtain the best overall fit between computed <br />and observed hydrographs. <br /> <br />In an ungaged situation a value for K can be estimated as the travel time <br />of the flood wave through the routing reach. The flood wave velocity (V) is greater <br />than the average velocity at a given cross section, for a given discharge. The flood <br />wave velocity can be estimated by a number of different techniques: <br /> <br />3.2.1. Using Seddon's law. a flood wave veloCity can be approximated <br />from the discharge rating curve at a station whose cross section is representative of <br />the routing reach. The slope of the discharge rating curve is equal to dQ/dy. The <br />flood wave velocity, and therefore the travel time K, can be estimated as follows: <br /> <br />v = ldQ <br />w - <br /> <br />(13) <br /> <br />B dy <br /> <br />K=!: <br /> <br />(14) <br /> <br />Vw <br /> <br />Where: <br /> <br />v w = flood wave velocity. ft/s <br />B = top width of the water surface <br />L = length of the routing reach. ft <br /> <br />3.2.2. Another means of estimating flood wave velocity is to estimate <br />the average velocity (V) and multiply it by a ratio. The average velocity can be <br />calculated from Manning's equation with a representative discharge and cross section <br />for the routing reach. For various channel shapes, the flood wave velocity has been <br />found to be a direct ratio. of the average velocity. <br /> <br />Channel ShaDe Ratio V wDL <br />Wide rectangular 1.67 <br />Wide parabolic 1.44 <br />Triangular 1.33 <br /> <br />For natural channels, an average ratio of 1.5 is suggested. Once the <br />wave speed has been estimated. the travel time (K) can be calculated with equation <br />14. <br /> <br />7-44 <br />