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<br />EM 1110-2-1416 <br />15 Oct 93 <br /> <br />change in bed forms as discharge increases. A threshold <br />level exists such that when discharge and velocity reach <br />a certain range, the bed changes from dunes 10 a flat bed, <br />thus dramatically decreasing n. A higher discharge can <br />pass at a lower elevation than an earlier, lower, discharge <br />due 10 this phenomena. This "discontinuous" rating <br />curve is a characteristic of many streams. An example is <br />shown in Figure 3-1. References by Chow (1959), <br /> <br />1tl <br /> <br />.... <br /> <br /> .., <br />r-.. <br />..... <br />- <br />'--' <br />Ul <br />:J N <br />"0 <br />0 <br />a:: <br />0 <br />:J <br />0 <br />~ <br />"0 <br />>- <br />J: <br />.-- <br /> 0> <br /> GO <br /> .... <br /> CD <br /> 1 <br /> <br />French (1985), and Barnes (1967) may be used to assist <br />in the estimation of n for a reach of stream. A more <br />complete discussion of loss coefficients is provided in <br />Appendix D. <br /> <br />(2) Equivalent roughness, k. An alternate method of <br />defining Manning's n is by estimating an equivalent <br />roughness coefficient k. This technique is described by <br /> <br /> <br />Rating Curve for Rio Grande <br />near Bernallio. N.M. <br />(Nordin. 1964. Fig. 3) <br /> <br /> <br />o . <br />lb <br />S <br /><:) <br /> <br />. <br /> <br />2 <br /> <br />Transition Zone <br />. <br /> <br />. <br />. <br /> <br />. <br /> <br />. <br /> <br />3 <br /> <br />4 <br /> <br />Mean Velocity (ft/sec) <br /> <br />Figufe 3-1. Discontinuous feting CUfve <br /> <br />3-8 <br /> <br />. <br /> <br />b <br />$0 <br />lb <br />*. b l::' <br />..~-S <br />* ~ <br />.......l::' <br />~~ <br />i.i:. <br /> <br />5 <br /> <br />6 <br /> <br />7 8 <br /> <br />e <br /> <br />. <br />'" <br /> <br />. <br />~ <br /> <br />e <br /> <br />J <br /> <br />.! <br /> <br />e <br />