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<br />3. channel roughness. n. The rougher the channel. the greater the travel <br />time. and the more the peak is reduced. <br /> <br />4. Channel geometry. The wider the channel. the greater the travel time. <br />and the more the peak is reduced. <br /> <br />5. Base time of the failure hydrograph. Tp. Peak reduction is great for <br />failure hydrographs with short base times. <br /> <br />Start by dividing the downstream channel into several typical reaches. Each <br />reach should have qenerally uniform physical characteristics such as <br />cross-sectional geometry. slope (s). and roughness (n). It is not necessary <br />to designate a typical reach for every minor change in stream <br />configuration. For example. several typical reaches over a ten mile study <br />length is usually sufficient for mapping purposes. <br /> <br />Travel time (Th) of the.flood along any typical reach is given by: <br /> <br />Th =...!:.... <br />V <br /> <br />Equation 9 <br /> <br />Where L = Length of the typical reach. in feet. and <br />V = Average flow velocity <br /> <br />Average flow velocity can be computed using Manning's equation. assuming <br />uniform flow: <br /> <br />Q = 1.486 A R2/3 51/2 <br />n <br /> <br />Equation 10 <br /> <br />V=Q <br />A <br /> <br />Equa t ion 11 <br /> <br />Where A is the cross-sectional flow area. R is the hydraulic radius (Alp). <br />and n is the roughness coefficient. Values of n are published for various <br />typical channels.4 However. when the evaluation involves steep gradient <br />natural streams (slopes greater than 0.002). equation 12 should be used to <br />estimate n values:5 <br /> <br />n = 0.39 sO.38R-0.16 <br /> <br />Equation 12 <br /> <br />Floods flowing in steep. natural drainages tend to transport large volumes <br />of sediment and debris (boulders. trees. etc). This effectively increases <br />the roughness coefficient. so that critical velocity is not exceeded, and <br />the flow regime remains subcritical. except for very short stretches. n <br />values. calculated from equation 12 for steep gradient streams can greatly <br />exceed published values for typical channels. with the result that predicted <br />travel times (Th). and predicted flood depths (Di) will be greater. <br /> <br />10 <br />