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<br />e <br /> <br />e <br /> <br />e <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />HYDRAULIC STRUCTURES <br /> <br />Y3/2 1/2 <br />q = , g <br /> <br />(HS-5) <br /> <br />where g is the acceleration of gravity, and each parameter would have the subscript (m) or (,) as <br /> <br />appropriate for the main, trickle, or low-flow channel zone. <br /> <br />Water surface profiles of the drop down the slope and through the basin may be accomplished by the <br />"Standard Step Method" (Chow 1959), or any equivalent method suitable for unit discharge computations. <br />For baffle chutes and vertical drops, individual methods are given in later subsections. It is necessary to <br />plot the energy grade line to assure calculations are reasonable. <br /> <br />Depending on the type of materials and the relative depth, the appropriate roughness parameters should <br />be used in computations. Table HS-2 gives generally recommended Manning's roughness values. Chow <br />(1959), Henderson (1966), Barnes (1967), Bathurst, Li and Simons (1979), and others provide further <br />reference information. Normai equations for riprap do not apply to grouted rock because the voids are <br />partially filled. Only by creating obstructions into the flow (steps, ungrouted rock protrusions and other <br />variations) can the grouted rock have a significantly greater roughness. Stepped rock placement and <br />random large boulders grouted to 50% of their height are recommended to increase roughness and <br />reduce the velocity. <br /> <br />Low Froude number hydraulic jumps with longer areas of hydraulic instability are common in grass-lined <br /> <br />channel applications. Baffles and rock placements that create turbulence and dissipate energy are <br /> <br />recommended to help counteract the low Froude number jumps and the associated tendency to carry <br /> <br />residual energy and waves downstream. <br /> <br />TABLE HS-2 <br /> <br />Suggested Approximate Manning's Roughness Parameter at Design Discharge <br />for Sloping Drops <br /> <br />Smooth concrete <br />Ste ed concrete where ste hei hts e ual 25% of na e de th <br />Grouted rock with variations above grouted plane between Yz- to <br />1-foot <br />Grouted rock with variations above grouted plane between 1- to 2- <br />feet <br /> <br />* These assume approach channel depth of 5 feet. Values would be higher at flow depths of 1 <br />foot to 3 feet <br /> <br />0.015 <br />0.025* <br />0.042* <br /> <br />0.045* <br /> <br />2.3.4 HYdraulic JumD Location. The water surface profile analysis starts at the crest and works <br />downstream to analyze supercritical flow. Separate analysis for the low-flow, trickle, and main channels <br />includes the review of hydraulic jumps. In the case of a baffle chute, no jump will occur because the <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-13 <br />