2017-02-21_REVISION - M1988112 (3)

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For the above conditions, the downstream conjugate depth associated with the hydraulic jump identified as d2 is calculated using the formula: d2=0.5d#I+8F12)1'-1), Hydraulic Design of Stilling Basins and Energy Dissipators, Engineering Monograph No. 25 (USBR 1984) and is as follows: n=0.008: d2 = 18.79 ft. n=0.014: d2 = 17.67 ft. Because the downstream channel is unstable and slopes steeply away from the location of the stilling basin discharge, the required tail water cannot be obtained through simple positioning of the basin floor. Measures to compensate for deficient tail water generally include providing a depressed apron or an elevated section, such as a wall, prior to discharge from the structure. We don't think that a depressed basin is appropriate for this project because the system's infrequent use would result in standing water since a depressed basin would be difficult to make free draining. Accordingly a weir wall (with drainage openings) near the end of the hydraulic jump basin was determined to be the most prudent measure to establish necessary tail water. For the incoming hydraulic conditions noted above, a USBR Type III basin is considered to be appropriate. Such basins are recommended in order to reduce the basin length and are used where incoming maximum velocity is on the order of 50-60 fps, and unit discharges less than 200 cfs. Although the extreme velocity calculated is slightly over 60 fps during the PMF condition with very smooth concrete assumed, for all practical purposes, the predicted conditions for this project are within the recommended range. Guidelines for design of the basin are published based on extensive laboratory testing conducted by the USBR, Hydraulic Design of Stilling Basins and Energy Dissipators, Engineering Monograph No. 25 (USBR 1984). A standard Type III Basin incorporates chute blocks, and impact baffle piers to help dissipate the energy and shorten the basin. It also incorporates an end sill which is typically a short 2:1 sloping wall at the outlet discharge. The standard basin design will be utilized for this project excepting that a vertical weir wall will be substituted for the sloping end sill in order to establish necessary tail water. Incorporating the impact blocks will also be helpful in limiting high velocity flow along the bottom of the stilling basin which could normally occur in a drowned or submerged jump. The USBR determined that the incorporation of chute blocks and baffle piers in the Type III basin provided a 15% to 18% margin of safety in the tail water required for satisfactory performance. However, guidelines still recommend maintenance of full d2 tail water where practical. For this project, end sill wall height was established at 8.5 ft. based on subtracting flow through the openings in the wall from the total flow at the PMF and then subtracting the estimated head over the weir wall for the remaining flow as required to obtain the theoretical d2 values listed above. A weir coefficient of 3.33 is considered to be the most representative of the wall being constructed. However, that coefficient was reduced to 2.70 for the PMF design condition based on an estimate of the submergence caused by the flow below the weir wall. For the 2.70 weir coefficient a wall height of 8.5 was calculated based on the full d2 level for a worst case Manning's n of 0.008. Although erosion of the downstream channel in a PMF event could cause a reduction of the submergence, resulting in an increase in the weir coefficient and a San Luis Project — South Diversion Ditch Drop Structure — Final Design Report 20