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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />baffles are constantly breaking up the flow, preventing supercritical flow. Examination of tailwater <br />conditions is still important for a baffle chute to evaluate riprap and basin layout. <br /> <br />e <br /> <br />To determine the location of the hydraulic jump, a tailwater elevation has to be established by water <br />surface profile analysis that starts from a downstream control point and works upstream to the drop basin. <br />This backwater analysis is based upon entire cross sections for the downstream waterway. The hydraulic <br />jump, in either the low-flow, trickle channel, or the main drop, will begin to form where the unit specific <br />force of the downstream tailwater is greater than the specific force of the supercritical flow below the drop. <br />Special consideration must be given to submerged hydraulic jumps because it is here that reverse rollers <br />are most common. For submerged jumps, the resulting downstream hydraulics should be evaluated <br />(Cotton 1995). <br /> <br />The determination of the jump location is usually accomplished through the comparison of specific force <br />between supercritical inflow and the downstream subcritical flow (i.e., tailwater) conditions: <br /> <br />F=(~J+(~2J <br /> <br />(HS-6) <br /> <br />in which: <br /> <br />F = specific force (ft') <br /> <br />e <br /> <br />q = unit discharge (determined at crest, for low-flow, trickle, and main channel zones) (cfslft) <br /> <br />y = depth at analysis point (ft) <br /> <br />g = acceleration of gravity = 32.2 ft/sec' <br /> <br />The depth, y, for downstream specific energy determination is the tailwater water surface elevation minus <br />the ground elevation at the point of interest, which is typically the main basin elevation or the trickle <br />channel invert (if the jump is to occur in the basin). The depth, for the upstream specific energy <br />(supercritical fiow), is the supercritical flow depth at the point in question. <br /> <br />Note that on low drops, the jump may routinely submerge the crest or may occur on the face of the drop. <br />Refer to Littie and Daniel (1981), Little and Murphey (1982), Chow (1959), USACE (1994), and Peterka <br />(1984) for these cases. <br /> <br />The jump at sloping drops typically begins no further downstream than the drop toe. In vertical drops, the <br />jump should begin where the jet hits the floor of the basin. This is generally accomplished in the main <br />drop zone by depressing the basin to a depth nearly as low as the downstream trickle channel elevation. <br />This will provide drainage for the basin. <br /> <br />e <br /> <br />HS-14 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />