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3.0 HYDRAULICS DESIGN <br />3.1 General Design Basis and Description <br />General Description: <br />The Drop Structure includes the following elements: <br />• approach section <br />• chute <br />• stilling basin <br />• discharge channel <br />Select drawings from the construction package are provided in Appendix C-1 to illustrate the <br />layout of the Drop Structure. <br />3.2 Drop Structure Hydraulics Design <br />The basic design parameters for the Drop Structure are as follows: <br />Design Flow: <br />• 1522 cfs PMF - Design Flood - for an extreme event with minimal to no freeboard <br />• 792 cfs 500 -Year -for freeboard basis of drop structure and channels <br />Approach: <br />The general approach followed, based on direction from BMRI, was to provide a <br />design for the structure that would pass the 500 -year flow with standard freeboard <br />considerations and that would pass the very extreme flow associated with the PMF <br />with minimal freeboard and still be protective of the dam, but would likely require <br />maintenance of the downstream channel. A further objective was that minimal to no <br />flow would enter the impoundment during the 500 -year event. The new structure <br />was to be designed as an open channel to greatly reduce risk of blocking of the <br />structure by debris during a flood event, which was a major concern with the existing <br />conduit drop structure. <br />3.2.1 HEC -RAS System Modeling <br />A HEC -RAS Model of the system from its discharge point below the rock grade control, <br />upstream through the discharge channel, the drop structure, and the upstream channel was <br />completed to facilitate overall system design. That model was also used to identify the flow split <br />at the upstream diversion structure as well as to confirm the design of the channels and drop <br />structure. <br />A relatively low channel roughness factor (Manning's "n" of 0.025) was used to estimate the <br />flow reaching the drop structure. That "n" assumes a relatively clean and maintained channel. If <br />the channel is allowed to develop greater vegetation after construction is completed, the <br />roughness would increase and the flow reaching the drop structure would decrease, with a <br />greater percentage bypassing to the impoundment. Therefore, the flow used as a design basis for <br />the drop structure and channel should represent a reasonably conservative (high) estimate of <br />San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 12 <br />