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<br />Bed shear stress--The bed shear stress may be computed by the relation <br /> <br />to = (daSo <br /> <br />Where: <br /> <br />( is specific weight of water, <br /> <br />da is mean depth of flow, and <br /> <br />So is the slope of the water surface or the channel bed. <br /> <br />Mean (average) depths of flow may be used in the relation rather than <br /> <br />hydraulic radius of the channel because mean depth is more readily determined. <br /> <br />For most channel shapes, the mean depth and hydraulic radius are approximately <br /> <br />equal and errors introduced by use of mean depth is usually negligible. The <br /> <br />slope of the energy grade line, Sf, is used in the determination of bed shear <br /> <br />stress; however, the channel slope, So, may be substituted as an approximation <br /> <br />for Sf. <br /> <br />12 <br />