2017-02-21_REVISION - M1988112 (3)

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3.2.2 Approach Section The approach to the drop structure is designed to smoothly transition flow from the incoming diversion channel to the chute. To accomplish that objective required consideration of two components as follows: Rip Rap Drop Section: A sloping section, which lowers the incoming channel invert by approximately 2.5 feet, is provided between Station 6+25 and 6+75. This feature is needed to avoid backing flow up within the incoming channel during extreme events. It is placed upstream of the bend into the drop structure to assure that low velocities are attained prior to the curve (and the transition section) at the entrance to the Drop Structure. The height of the drop was established to coordinate the design for the overflow to the impoundment with the control section at the top of the chute. Rip rap is placed on the drop section to provide erosion protection, and energy dissipation for flows and depths when sufficient backwater to prevent erosive velocities is not present. Analysis was completed over the range of flows to determine the most critical condition and rip rap sized and located accordingly. The worst case scenario was found to be at a flow of 625 cfs, which resulted in a velocity of 12.0 fps. Inlet Transition Section: A concrete inlet section was designed to transition flow from the 15 ft. wide trapezoidal channel with 2:1 horizontal to vertical side slopes to the 15 ft. wide chute section with vertical side walls. The transition section was designed hydraulically to minimize potential wave formation within the downstream chute. The transition section design was generally based on guidelines for convergence through gradually increasing the design flow Froude number throughout the convergence, Hydraulic Design of Spillways EM 1110-2-1603 (USACE,1990). The concrete transition section has a freeboard of approximately 1.65 ft. at the upstream end during the PMF and about 0.65 ft. at the adjacent backfill grade. The upstream end of the Drop Structure where it meets up with the incoming channel is keyed into bedrock to retard the potential introduction of seepage underneath the structure. Calculations for the facilities within approach section are included in Appendix C-3. 3.2.3 Chute The chute is a 15 ft. wide rectangular concrete section extending from the downstream end of the inlet transition at a 3:1 slope to the entrance to the stilling basin. It drops the flow a total of 58 feet from an invert of 8612.0 at the end of the transition to 8554.0 at the stilling basin floor. Velocities within the chute were calculated using HEC -RAS as previously noted and could reach upward of 60 fps at the downstream end of the chute. The chute was generally positioned to match the approximate grade of the existing slope with consideration given to anticipated bedrock location. Wall Heights: From Station 3+24 to Station 3+68.19 the top of wall elevation of 8574 as established at the stilling basin was maintained. This wall height was retained due to the potential formation of a jump upstream of the chute toe at some flows and to accommodate grading. A minimum wall height for the chute of 5 ft. perpendicular to the slope was selected for the section between Station 3+68.19 and Station 4+65.44, based on a flow of 792 cfs and generally accepted procedures for determining freeboard in a chute as defined by the equation Freeboard (in feet) = 2.0 + 0.025vd113, Design of Small Dams (USBR 1987). The wall height ----------------------------------------- -- ---- -------------------------------- --- San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 18