Laserfiche WebLink
<br />3.2.1. Using Seddon's law, a flood wave velocity can be approximated from the <br />discharge rating curve at a station whose cross section is representative of the routing <br />reach. The slope of the discharge rating curve is equal to dQ/dy. The flood wave velocity, <br />and therefore the travel time K, can be estimated as follows: <br /> <br />v w= 1.J!Q <br />Sdy <br /> <br />(13) <br /> <br />K=.b <br />Vw <br /> <br />(14) <br /> <br />Where: <br /> <br />Vw = flood wave velocity, fils <br />B = top width of the water surface <br />L = length of the routing reach, It <br /> <br />3.2.2. Another means of estimating flood wave velocity is to estimate the average <br />velocity (V) and multiply it by a ratio. The average velocity can be calculated from <br />Manning's equation with a representative discharge and cross section for the routing reach. <br />For various channel shapes, the flood wave velocity has been found to be a direct ratio <br />of the average velocity. <br /> <br />ChannelShaoe <br />Wide rectangular <br />Wide parabolic <br />Triangular <br /> <br />Ratio V.,/V <br />1.67 <br />1.44 <br />1.33 <br /> <br />For natural channels, an average ratio of 1.5 Is suggested. Once the wave speed <br />has been estimated, the travel time (I<) can be calculated with equation 14. <br /> <br />Estimating the Muskingum X parameter In an ungaged situation can be very <br />difficult. X varies between 0.0 and 0.5, with 0.0 providing the maximum amount of <br />hydrograph attenuation and 0.5 no attenuation. Experience has shown that for channels <br />with mild slopes and flows that go out of bank, X will be closer to 0.0. for steeper streams, <br />with well defined channels that do not have flows going out of bank. X will be closer to 0.5. <br />Most natural channels lie somewhere in between these two limits, leaving a lot of room for <br />'engineering judgement.' One equation that can be used to estimate the Muskingum X <br />coefficient in ungaged areas has been developed by Cunge. The equation is taken from <br />the Muskingum-Cunge channel routing methods, which is written as follows: <br /> <br />1 q, <br />X = 2 ( 1 - as ) <br />oCt:>x <br /> <br />(15) <br /> <br />Where: Q = reference flow from the inflow hydrograph <br />o <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />78.7 <br /> <br />a=vv=r <br />