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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />At the main channel zone: <br /> <br />e <br /> <br />Lbm =Ldm + Djm + 60% (6Y,J <br /> <br />(HS-11) <br /> <br />At the trickle channel flow zone, without baffles or boulders to break up the jet: <br /> <br />Lbt = Ldt + Djt + 60% (61;.) <br /> <br />(HS-12) <br /> <br />3. Caution is advised regarding the higher unit flow condition in the low-flow zone. Large boulders <br />and meanders in the trickle zone of the basin are shown to help dissipate the jet, and rock is <br />extended downstream along the low-flow channel. When large boulders are used as baffles in <br />the impingement area of the low-flow zone, the low-flow basin length Lbt, may be reduced, but not <br />less than Lbm. Boulders should project into the flow 0.6 to 0.8 times critical depth. They should <br />be located between the point where the nappe hits the basin and no closer than 10 feet from the <br />basin end. <br /> <br />4. The basin floor elevation should be depressed in depth, and variable with drop height. Note that <br />the basin depth adds to the effective tailwater depth for jump control. The basin can be <br />constructed of concrete or grouted rock. Use of either material must be evaluated for hydraulic <br />forces and seepage uplift. <br /> <br />There should be a sill at the basin end to bring the invert elevation to that of the downstream <br />channel and sidewalls extending from the crest wall to the sill. The sill is important in causing the <br />hydraulic jump to form in the basin. Buried riprap should be used downstream of the sill to <br />minimize any local scour caused by the lift over the sill. <br /> <br />e <br /> <br />5. Caution is advised to avoid flow impinging on the channel side slopes of the basin. <br /> <br />6. Crest wall and footer dimensions should be determined by conventional structural methods. <br />Underdrain requirements should be determined from seepage analysis. <br /> <br />7. Seepage analysis should be completed and adequate provisions made for control and <br />weight/size of components (see Section 2.6). <br /> <br />8. Simplified design criteria are provided in Table HS-6 for vertical hard basin (grouted boulder) <br />drops. These criteria are valid only where the channel flow conditions meet the criteria in the <br />MAJOR DRAINAGE chapter of this Manual. Seepage uplift conditions require evaluations for <br />each use. Reinforced concrete basins will have thickness and length parameters that vary <br />somewhat depending upon hydraulic and seepage considerations. <br /> <br />e <br /> <br />HS-28 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />