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<br />e <br /> <br />e <br /> <br />e <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />HYDRAUUC STRUCTURES <br /> <br />complexity of specifying two different sizes on the design drawings and in the construction of the <br />structure before deciding. <br /> <br />10. All boulders shall be grouted in accordance with the guidance in the MAJOR DRAINAGE chapter <br />of this Manual. <br /> <br />11. All grouted boulders above the low-flow channel shall be buried with topsoil to a depth of 4 inches <br />above the top of the highest boulder and the surface vegetated with native grasses on the <br />overbank bench and native grasses and dry-land shrubs on the overbank channel's side slopes. <br /> <br />TABLE HS-5 <br /> <br />Boulder Sizes for Various Rock Sizing Parameters <br /> <br />Rock Sizing Parameter, RD Minimum Dimensions of Boulder, DB Boulder Classification <br />Less than 4.50 18 inches 818 <br />4.50 to 4.99 24 inches B24 <br />5.00 to 5.59 30 inches B30 <br />5.60 to 6.39 36 inches B36 <br />6.40 to 6.99 42 inches B42 <br />7.00 to 7.50 48 inches B48 <br /> <br />2.4.4 Vertical Hard Basin DroDs. The vertical hard basin drop is a generalized category that can <br />include a wide variety of structure designs. However, the vertical hard basin drop is to be avoided where <br />practical due to impingement energy, related maintenance, and turbuient hydraulic potential under some <br />flow conditions (ASCE and WEF 1992). A variety of components can be used for both the hard basin and <br />the crest wall. Various contraction effects can be implemented to reduce approach velocities, and <br />different trickle or low-flow channel options can be selected. Maximum drop height across the structure is <br />limited to 3 feet for safety considerations. <br /> <br />The hydraulic phenomenon provided by this type of drop is a jet of water that overflows the crest wall into <br />the basin below. The jet hits the hard basin and is redirected horizontally. With sufficient tailwater, a <br />hydraulic jump is initiated. Otherwise, the flow continues horizontally in a supercritical mode until the <br />specific force of the tailwater is sufficient to force the jump. Energy is dissipated through the turbulence of <br />the hydraulic jump; therefore, the basin is sized to contain the supercritical flow and the erosive turbulent <br /> <br />zone. <br /> <br />Generally, a rough basin is advantageous since increased roughness will result in a shorter, more <br />economical basin. Figure HS-9 shows a vertical drop with a grouted boulder basin. The rock-iined <br />approach length ends abruptly at a structural retaining crest wall that has a neariy rectangular cross <br />section and trickle channel section. <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-25 <br />