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<br />the bed forms are classified as lower, transition, and upper regime (Simons <br />and Richardson, 1966). Bed forms of lower regime flows consist of plane bed, <br />ripples, and dunes. Bed forms of upper regime flows consist of moving plane <br />bed, standing waves, and antidunes. <br /> <br />The roughness coefficients for the lower and transition regimes depend on <br />grain size and bed-form roughness at a particular time and are very difficult <br />to assess. Generally, as the flow increases dune formation greatly increases, <br />and the roughness is much greater than the upper-regime flow roughness. <br /> <br />Manning n values for upper-regime flow are given in table 2. This table <br />indicates that n values in sand channels increase directly with particle size. <br />To determine the roughness of a sandbed stream, an initial n value based on <br />the median particle size is selected from table 2. <br /> <br />After the hydraulic properties of the channel section are computed using <br />the Manning equation, the flow regime is checked by computing the stream power <br />to determine the reliability of the assigned n value. Stream power is com- <br />puted as 62RS V, where 62 is the approximate specific weight of water, in <br />pounds per cu~ic foot; R is the hydraulic radius, in feet; S is the water- <br />surface slope, in foot per foot; and V is the mean velocity,Win feet per <br />second. <br /> <br />'I <br /> <br />Table 2.--8ase values of the Manning's n for natural channels <br /> <br />Channel type <br />and bed <br />material <br /> <br />Sand channels (Upper-regime flow only): <br />0.2 -------- <br />.3 -------- <br />.4 -------- <br />.5 -------- <br />.6 -------- <br />.8 -------- <br />1.0 -------- <br /> <br />Median size of bed material <br />Millimeters Inches <br /> <br />Base n value <br />Benson and Chow <br />Dalrymple (1959)2 <br />(1967)1 <br /> <br />0.012 <br />.017 <br />.020 <br />.022 <br />.023 <br />.025 <br />.026 <br /> <br />Stable channels: <br />Firm earth-------- <br />Coarse sand------- <br />Fine gravel------- <br />Gravel------------ <br />Coarse gravel----- <br />Cobble------------ <br />Boulder----------- <br /> <br />1-2 <br /> <br />0.025-0.032 <br />.026- .035 <br /> <br />0.020 <br /> <br />.024 <br /> <br />2-64 <br /> <br />0.08-2.5 <br /> <br />.028- .035 <br /> <br />.028 <br /> <br />64-256 <br />>256 <br /> <br />2.5-10.5 <br />>10 <br /> <br />.030- .050 <br />.040- .070 <br /> <br />1Straight uniform channel. <br />2Smoothest channel attributable in indicated material. <br /> <br />20 <br />