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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 />regard to calculating water impact (drag force), which generaily will cover other types of impact force. <br />Specialty situations, where impact force may be significant, must be considered on an individual basis. <br /> <br />2.3.7.4 Turnina Force. A turning force impacts the basin as a function of slope change. Essentially, this <br />is a positive force countering uplift and causes no great stress in the grouted rock or reinforced concrete. <br />This force can be estimated as the momentum force of the projected jet area of water flowing down the <br />slope onto the horizontal base and calculating the force required to turn the jet. <br /> <br />2.3.7.5 Friction. With net vertical weight, it follows that there would be a horizontal force resisting <br />motion. If a friction coefficient of 0.5 is used and multiplied by the net weight, the friction force to resist <br />sliding can be estimated. <br /> <br />2.3.7.6 Frost Heaye. This value is not typically computed for the smaller drops anticipated herein. <br /> <br />However, the designer should not allow frost heave to damage the structure, and, therefore, frost heave <br /> <br />should be avoided andlor mitigated. In reinforced concrete, frost blankets, structural reinforcing, and <br /> <br />anchors are sometimes utilized for cases where frost heave is a problem. If gravel blankets are used, <br /> <br />then the seepage and transmission of pressure fluctuations from the hydraulic jump are critical. <br /> <br />2.3.7.7 SeeDaae UDlift Pressure. As explained previously, uplift pressure and seepage relief <br />considerations are extremely important to structural stability and usually of greater concern than the <br />forces described above. There can be troublesome pressure differentials from either the upstream or <br />downstream direction when there is shallow supercriticai flow on the drop slope or in the basin. One may <br />consider an upstream cutoff to mitigate this problem. Weep locations with proper seepage control may <br />be provided. For high drops (i.e., > 5 feet), more than one row of weep holes may be necessary. <br /> <br />A prudent approach is to use a flow net or other type of computerized seepage analysis to estimate <br />seepage pressures and flows under a structure. <br /> <br />2.3.7.8 Dynamic Pressure Fluctuations. Laboratory testing (Toso 1986; Bowers and Toso 1988) has <br /> <br />documented that the severe turbulence in a hydraulic jump can pose special problems often ignored in <br /> <br />hydraulic structures. This turbulence can cause significant positive and negative pressure fluctuations <br /> <br />along a structure. <br /> <br />A good example of the problem can be envisioned by a situation in which the entire sloping face of the <br /> <br />drop is underlain by a gravel seepage blanket. The gravel could be drained to the bottom of the basin or <br /> <br />other locations where the jump will occur. In such a case, the positive pressure fluctuations could be <br /> <br />transmitted directly to the area under the sloping face, which then could destabilize the structure since <br /> <br />there would not be sufficient weight of water over the structure in the area of shallow supercritical flow. <br /> <br />The key parameter is the coefficient of maximum pressure fluctuation, Cp-mm. which is in terms of the <br />velocity head of the supercritical flow just upstream of the jump: <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-17 <br />