2017-02-21_REVISION - M1988112 (3)

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A 36 -inch thick layer of riprap is being placed for a short distance of 13-20 ft. downstream of the discharge. This riprap is incorporated to provide protection from turbulence at the basin discharge at higher flows and from end sill wall overtopping at lesser flows, until such time as downstream tail water develops to provide a water cushion. Calculations associated with the discharge channel are provided in Appendix C-4. 3.3 South Diversion Ditch Channel Hydraulics Design Analysis of the South Diversion Ditch revealed that relatively minimal "reshaping" and regrading of the existing channel geometry would increase the conveyance capacity of the ditch to convey runoff events through the PMF. Specifically, the existing channel bottom has somewhat rounded corners, and much of the ditch profile is at an adverse grade (i.e., the ditch doesn't freely drain towards the drop structure); the reshaped geometry is described in Table 3.3. Table 3.3. South Ditch - Reshape Channel Geometry Feature Measure Channel Slope 0.20% Bottom Width 15 ft Left Side Slope (looking up -channel) 2H:1V Right Side Slope (looking up -channel) 1.5H:1V (or flatter) Channel Roughness (n) 0.025 The Drop Structure has been designed to pass a maximum discharge of 1,522 cfs. Basins A and B comprise approximately 88% of the area contributing runoff to the South Diversion Ditch. Limiting the flow in the South Diversion Ditch below Basin B (approximately channel Station 28+50) to a maximum of approximately 1,600 cfs will protect the drop structure from flows that exceed its design capacity. Consequently, the left bank (impoundment side) of the upper reach of the ditch will be graded down to serve as an overflow section that allows excess runoff to be diverted to the reclaimed area of the tailing impoundment, to be stored behind the dam. At the confluence of runoff from Basin A and Basin B (approximately Station 28+50), there is a "catchment" area where flow from Basin A and Basin B will "pool" to provide attenuating detention storage during the design event. Below this catchment area, the channel geometry becomes uniform, and the flow through the ditch will approach "normal depth". The design invert of the channel at Station 28+50 is 8619.15 ft; normal depth below Station 28+50 for the design discharge of 1,600 cfs is 7.59 ft, producing a water surface elevation upstream from Station 28+50 of 8626.74 ft. Upstream from the catchment area, the left bank of the upper 500 ft of the ditch (station 32+25 to station 37+25) will be graded down to elevation 8625.5 ft and serve as the overflow section allowing runoff in excess of 1,600 cfs to overflow from the ditch into the reclaimed area. During the PMF event, with 1,600 cfs flowing down the ditch, there will be 1.24 ft of water over the left bank, resulting in approximately 1,800 cfs (approximately 41 ac -ft total volume) flowing over ------------------------- ---- ---------------------------------------------------------- -- --- San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 23