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<br />In the hydraulic analysis of a site for design of riprap to prevent scour, <br />the evaluation generally assumes uniform or gradually varied flow conditions. <br />Uniform flow has a constant depth for all cross sections in a reach. Varied <br />flow conditions occur if the depth of flow changes along the length of channel. <br />The evaluation of a site is generally based on a design discharge and uniform <br />flow conditions. A large magnitude change in channel size or gradient may cause <br />the state of flow to change, The depth is less and the velocity greater in <br />supercritical flow than in subcritical flow. Abrupt changes may cause hydraulic <br />drops or jumps. A hydraulic drop will occur where flow changes abruptly from <br />subcritical to supercritical, and a hydraulic jump will occur where flow changes <br />from supercritical to subcritical. Severe turbulence accompanies a hydraulic <br />jump. If supercritical flow occurs in a reach between two reaches of sub- <br />critical flow, a hydraulic drop may occur at the upstream end of the critical <br />reach and a hydraulic jump may occur at the downstream end, <br /> <br />In several design procedures, the shear stress (also referred to as the <br />tractive force, Chow, 1959) is used as a quantitative indicator of the forces <br />acting on the channel bed and banks. The magnitude of shear stress is dependent <br />on the depth of flow and channel gradient; therefore, values of shear stress in <br />a reach with supercritical conditions may be less than those for subcritical <br />flow. <br /> <br />Shear Stress Related to Permissible Flow Velocity <br /> <br />The maximum permissible velocity is the highest mean velocity that will not <br />cause erosion of the boundary. Procedures for design of channels based on <br />permissible velocity are described in EM-1601, HEC-ll, and USBR-EM-25, Chow <br />(1959) presents a summary of several design procedures that are based on maximum <br />permissible (mean) velocity for channels with vegetative linings. Figure 2 <br />(adapted from EM-1601) shows a comparison of design curves used to relate <br />permissible velocity to stone size. The Isbash (USBR) procedure gives the <br />largest stone size for a given velocity. The curve for an isolated cube, which <br />is not generally used for rip rap design, is given for comparison. <br /> <br />5 <br />