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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 />There are two levels of analysis possible. The first involves detailed analysis of all hydraulic conditions <br />and leads to an optimal design for each structure. The concepts involved are described herein, and <br />numerous references are available for more detailed information. The second level of analysis is a <br />simpler approach that is based on configurations that will be adequate at the limits of permissible grass- <br />lined channel criteria as described in Section 2.4. <br /> <br />There are two general categories of drops: sloping and vertical. For safety reasons, vertical drops <br />should be avoided under typical urban conditions. Performance of vertical or smooth sloping drops into a <br />hard basin is relatively well documented. Their hydraulic analysis is briefly described herein. The design <br />criteria for other drops such as vertical plunge pools and baffle chutes is based on empirical data and <br />model studies. <br /> <br />2.3.2 Crest and UDstream HYdraulics. After preliminary channel layout has indicated probable drop <br />location and heights (see the MAJOR DRAINAGE chapter for guidance, including the design spreadsheet <br />UD-Channels), analysis and design begins with review of the crest section at the top of the drop. As flow <br />passes through critical depth near the crest, upstream hydraulics are separated from downstream. <br />Usually, the key task here is to determine critical depth at the crest based on the entire section. The <br />critical flow state needs to be verified to ascertain that the downstream tailwater does not submerge the <br />crest and effectively controls the hydraulics above the crest. If the downstream tailwater controls, then <br />the structure must still be evaluated as a check for the peak discharge and as a drop at lower flows, if <br />appropriate. <br /> <br />With control at the drop crest, water surface profile computations are used to establish the upstream <br />abutment and bank heights. Computations should include a transition head loss, typically ranging from <br />0.3 (modest transitions in grass-lined channels) to 0.5 (channels approaching abrupt constrictions) times <br />the change in velocity head across the transition (see Section 5.2), and allowance for the end contraction <br />where the flow may effectively separate from the abutment end walls. Refer to Section 5.0 and standard <br />hydraulic references for guidance (Chow 1959, Rouse 1949, and USACE 1994). <br /> <br />2.3.3 Water Surface Profile Downstream of the Crest. Although this discussion concerns the <br />hydraulics below the drop crest, the fundamental analysis is established by the crest conditions. Main, <br />low-flow and trickle channel regions are considered separately. Although the actual location of critical <br />depth can vary according to the channel, transition, and drop geometry, the assumption is made that <br />critical depth occurs at the crest, in a horizontal straight line across the crest section. <br /> <br />The assumption of critical flow conditions across the crest is illustrated conceptually by the diagrams in <br />Figures HS-2a and HS-2b. At any point across the crest, the velocity is a function of the critical depth at <br />that point. This results in a higher unit discharge applied to the trickle channel zone than across the main <br />channel flow area. Figure HS-2a shows the corresponding energy level across the section. Figure HS- <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-11 <br />