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e e e DRAINAGE CRITERIA MANUAL (V. 2) HYDRAULIC STRUCTURES 2. Define the representative maximum channel design discharge (often the 1 OO-year) and other discharges appropriate for analysis, (e.g., low or trickle flows and other discharges expected to occur on a more frequent basis) which may behave differently. For channels designed for the 5-. 10-, 25-, or 50-year occurrence, the 1 OO-year flow should be used for stability analysis. 3. Approximate the channel dimensions and flow parameters including longitudinal slope. Identify the Drobable range of drop choices and heights with the aid of Figure HS-1. 4. Select drop structure alternatives to be considered for grass-lined or other channel types (see Section 2.2). 5. Decide if channel performance at maximum allowable criteria (i.e., velocity. depths, etc.) for grass-lined channels is practical or desirable. If not, or if the design flow is over 7,500 cfs, go to step 6; otherwise, the simplified design charts in Section 2.3.3 may be used to size the basic configuration of the crest. The designer should review the precautions given and the iimits of application with respect to site conditions. Then the crest section and upstream channel transition will need to be refined for incorporation of the trickle or low-flow channel. This requires review of the upstream water surface profile and the supercritical fiow downstream of the crest through the dissipation zone of the drop. Under conditions of a submerged jump due to a high tailwater elevation, steps to mitigate the reverse roller should be evaluated. If measures are taken to provide baffles or large boulders to break up the jet, then extensive analysis of the trickle zone hydraulics is not necessary. The steps involved are discussed in Section 2.3. Then go to step 7. 6. For refined analysis and optimal design of grass-lined channel drop structures, use the "detaiied" hydraulic analysis in Section 2.3.2. 7. Perform soils and seepage analyses as necessary to obtain foundation design information. 8. In the case of drops for grass-lined channels, comply with the minimum specific criteria and follow the guidelines for the recommended types of drops (baffle chute, vertical hard basin, and grouted sloping boulder) presented in Section 2.3.4. Otherwise, provide a complete hydraulic analysis documenting the performance and design for the type of drop or other type of channel being considered. For channels with alluvial beds that present an erosionldegradation risk, a complete stability and scour analysis should be completed, accompanied by a geotechnical investigation and seepage analysis. 9. Use specific design criteria and guidelines to determine the final drop structure flow characteristics, dimensions, material requirements, and construction methods. 10. Obtain necessary environmental permits, such as a Section 404 permit. 06/2001 Urban Drainage & Flood Control District HS-9