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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />Figure HS-3 illustrates the forces involved for a grouted sloping boulder drop, which is similar to other <br />sloping concrete drops or baffle chutes. Five location points are of concern. Point 1 is downstream of the <br />toe, at a location far enough downstream to be beyond the point where the deflection (turning) force of <br />the surface flow occurs. Point 2 is at the toe where the turning force is encountered. Point 3 is variable in <br />location to reflect alternative drain locations. When a horizontal drain is used, Point 3 is at a location <br />where the drain intercepts the subgrade of the structure. Point 4 is approximately 50% of the distance <br />along the drop slope. Point 5 is at a point underneath the grout layer at the crest and downstream of the <br />cutoff wall. <br /> <br />e <br /> <br />Point 3 is usually the critical pressure location, regardless of the drain orientation. In some cases, Point 1 <br /> <br />may also experience a low safety factor when shallow supercritical flow occurs, such as when the jump <br /> <br />washes downstream. <br /> <br />Seepage uplift is often an important force controlling structure stability. Weep drains, the weight of the <br />structure, and the water on top of the structure counteract uplift. The weight of water is a function of the <br />depth of flow. Thus, the greater the roughness, the deeper the flow condition and the greater the weight. <br /> <br />2.3.7.1 Shear Stress. The normal shear stress equation is transformed for unit width and the actual <br />water surface profile by substituting So. the energy grade line slope for So> and the drop slope. <br /> <br />r='JYS, <br /> <br />(HS-7) <br /> <br />e <br /> <br />in which: <br /> <br />1: = shear stress (lbs/ft2) <br /> <br />r = specific weight of water (Ibslft') <br /> <br />y = depth of water at analysis point (ft) <br /> <br />2.3.7.2 Buoyant Weiaht of Structure. Each design should take into consideration the volume of grout <br />and rock or reinforced concrete and the density of each. In the case of reinforced concrete, 150 pounds <br />per cubic foot can be used as the specific weight (or 88 pounds per cubic foot net buoyant weight). <br />Specific weight of rock is variable depending on the nature of the material. <br /> <br />2.3.7.3 ImDact and Draa Forces. Water flowing down the drop will directly impact any abrupt rock faces <br /> <br />or concrete structure projections into the flow. Technically, this is considered as a type of drag force, <br /> <br />which can be estimated by equations found in various references. Also, the user should compare <br /> <br />calculated drag force results with the forces shown later for baffle chute blocks (Section 2.5). Impact <br /> <br />force caused by debris or rock is more difficult to estimate because of the unknown size, mass, and time <br /> <br />elapsed while contact is made. Therefore, it is reoommended that a conservative approach be taken with <br /> <br />e <br /> <br />HS-16 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />