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A similar approach was taken to establishing risk relative to design capacity for the straight drop <br />basin as to that for the hydraulic jump basin. If the basin were sized to accommodate the full <br />PMF it would have required a length of 43 ft. (nearly as great as the hydraulic jump basin) which <br />is not considered warranted for the situation. That conclusion is based on the remote likelihood <br />of the event, the limited potential damage from exceeding the size selected and the approach of <br />BMRI to accept the potential maintenance associated with that risk. Larger rip rap than required <br />for the calculated velocity is being placed immediately downstream of the basin discharge to <br />address residual turbulence from the basin and overtopping of the end sill wall. Should <br />downstream degradation develop, the tail water would decrease and the flows in the basin would <br />impact closer to the location of the baffle blocks, theoretically decreasing the required basin <br />length. The wall height required to fully contain the PMF flow without freeboard is estimated at <br />between 13.1 and 13.8 ft. A wall height of 15.5 ft. was selected to provide some freeboard <br />during the PMF and also to eliminate the need for railing based on adjacent grading. Grouted rip <br />rap would be placed alongside the basin walls similar to the hydraulic jump basin to address the <br />potential for minor splash and surge overtopping during the PMF. Freeboard during the 500 -year <br />event is estimated at 5.5 and 6.0 ft. <br />Calculations for the stilling basin are provided in Appendix C-3. <br />3.2.5 Discharge Channel Design <br />A short relatively flat section of channel is designed downstream of the stilling basin to provide a <br />controlled tail water for the discharge. To provide maximum tail water, the channel is designed <br />with the smallest cross section that is considered practical for construction and stability (5 ft. <br />bottom width and 2H: IV trapezoidal channel). Calculations for the hydraulic design of the <br />channel are included in Section 3.3.1 under discussion of HEC -RAS modeling. <br />The channel is designed to pass the PMF design flow and provide a limited backwater for the <br />stilling basin at that flow. However, due to practical limitations related to protecting against <br />potential degradation, armoring for the channel is anticipated to provide protection only to <br />around the 100 -year flow event. <br />Downstream of the basin discharge, the channel slopes at a 0.20% grade for a distance of <br />approximately 100 ft. before resuming the historic slope of approximately 10%. The short <br />section of low -slope channel was made possible by the vertical placement of the stilling basin. <br />The channel is excavated into the hill slope opposite the stilling basin in what is anticipated to be <br />the most competent rock available. Nevertheless, the bedrock is anticipated to be erodible. <br />To partially address this situation, a rock grade control section is provided at the grade break <br />between the 0.20% slope and the 10.0% slope to retard potential head cutting within the bedrock. <br />It is anticipated that the steep downstream channel will erode over time and cause a portion of <br />the rock within the control section to slough into the eroded channel, retarding the progress of <br />head cutting until maintenance on the channel and control section can be affected. The grade <br />control section was sized based on stable operation at a 10% grade and flows during a 500 -year <br />event; however, as the downstream slope face starts to increase due to sloughing, increasing <br />instability would be anticipated. <br />----------------------------- ----------------------------------- <br />San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 22 <br />