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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 />discharge to the jump zone and could transfer pressure fluctuations to areas under supercritical flow, <br />pressure fluctuations are of concern. <br /> <br />2.3.7.9 Qyerall Analysis. All of the above forces can be resolved into vertical and horizontal <br />components. The horizontal components are generally small (generally less than 1 psi) and capable of <br />being resisted by the weight of the grout, rock, and reinforced concrete. When problems occur, they are <br />generally the result of a net vertical instability. <br /> <br />The overall (detailed) analysis should include reviews of the specific points along the drop and the overall <br />drop structure geotechnical and structural stability. All steps of this detailed analysis are not necessary <br />for design of drops along modest capacity grass-lined channels, provided that the design is developed <br />using the guidelines and configurations presented in the following simplified analysis approach section <br />and that other District criteria are met. The critical design factors are seepage cutoff and relief and <br />pressure fluctuations associated with the hydraulic jump that can create upward forces greater than the <br />weight of water and structure over the point of interest. Underflow can easily lift a major slab of rock and <br />grout and, depending upon the exposure, the surface flow could cause further weakening, undermining, <br />or displacement. Generally, a 30-pound net downward safety allowance should be provided, and 60 <br />pounds is preferred. A toe drain is generally needed as shown by Figure HS-3. <br /> <br />2.4 SimDlified Desians for District's Grass-Lined Channel DroDs <br /> <br />2.4.1 Introduction and Precautions. As previously mentioned, there is a balance between the crest <br />shape chosen, upstream channel stability, and the configuration of the drop downstream which will result <br />in reasonable or optimal energy dissipation. Further, there is usually a single configuration of drop crest, <br />upstream channel slope, and base width that will result in an acceptable drop structure performance for <br />grass-lined channels designed using the District's criteria described in the MAJOR DRAINAGE chapter. <br /> <br />This subsection presents simplified relationships that provide basic configuration a[ld drop-sizing <br />parameters that may be used when the District's maximum allowable velocity and depth criteria for grass- <br />lined channels are used. <br /> <br />Design guidance presented in this section is developed for channels that operate at the brink of maximum <br /> <br />criteria. They do not consider channel curvature, effects of other hydraulic structures, or unstable beds, <br /> <br />all of which require detailed analysis. They do provide guidelines for initial sizing and reasonableness <br /> <br />checking, but are not a substitute for comprehensive hydraulic analysis in the context of the entire <br /> <br />waterway. <br /> <br />2.4.2 Guidelines for Standard Grass-Lined Channel DroDS. Grouted sloping boulder drops and <br />vertical hard basin drops are the primary types of drops for which a simplified design approach may be <br />utilized for grass-lined channels. Other designs are available, but they are more limited in application and <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-19 <br />