2017-02-21_REVISION - M1988112 (3)

Home Browse Search

calculated accordingly for that reach varied from 4.3 to 4.7 feet. The wall height required to contain the PMF with minimal freeboard was found to be less than 5 feet. From Station 4+65.44 to 4+95 the wall height was increased to 7.5 ft. Calculated wall height based on 792 cfs plus recommended freeboard for that reach ranged between 4.7 ft. and 6.2 feet. However, because actual flow depth was approaching 6 ft. during the PMF, the wall height was increased to 7.5 ft. to provide some freeboard. Wall height upstream of 4+95 was based on containment of the PMF with a minimal but reasonable freeboard while avoiding frequent changes in wall height. The upstream end of the chute includes a 10 ft. vertical curve to transition from the incoming slope of 0.20 percent to the chute slope of 3H to 1V. The vertical curve was designed according to established procedures to maintain a positive pressure on the chute floor throughout the slope change, Design of Small Dams (USBR 1987). Relative calculations for the chute are provided in Appendix C-3. 3.2.4 Stilling Basin The use of a chute and hydraulic jump stilling basin drop structure concept for this project was requested by BMRI. Incorporation of those facilities at this site requires consideration of specific features associated with the placement and discharge from the drop structure. Proper formation of a hydraulic jump is associated with maintenance of a downstream water depth (d2) that is the conjugate depth for subcritical flow associated with the incoming upstream supercritical flow conditions at depth (di), velocity (v1), and Froude no (Ft). For a hydraulic jump stilling basin the maximum energy dissipation (ideal condition) occurs when the jump forms near the toe of slope or where the supercritical pre jump depth is at a minimum. In situations where the tail water lies below the downstream depth (d2) associated with the upstream supercritical flow conditions the hydraulic jump may partially or fully sweep out of the basin. This condition may prove dangerous from a safety point of view for the stilling basin due to erosion in the downstream channel at the increased velocities. Because of downstream conditions below the stilling basin at this site (steep, erodible channel causing inadequate and unreliable tail water), successful application of a hydraulic jump stilling basin requires incorporation of additional measures to establish the necessary tail water. Those features will be described where they are incorporated in the design. The stilling basin was located to found it on competent bedrock, and to provide a floor elevation that would enable providing a short section of channel below the site that could be excavated in bedrock at a reasonably flat grade thereby providing tail water at normal discharges from the basin. See subsequent discussion of Discharge Channel. USBR Type III Hydraulic Jump Stilling Basin. For the range of assumed roughness (n=.008 to n=0.014) pre jump hydraulic conditions entering the stilling basin are estimated from the HEC -RAS analysis as: n=0.008: v1=60.8, Ft= 8.3, d1=1.67 n=0.014: v1=54.8, F1= 7.1, d1=1.85 Where v1 is incoming velocity (fps), Ft is incoming Froude no., and d1 is the incoming depth (ft.), for a discharge rate of 1522 cfs. ----- -- - - -- - --- - - ------ -- - -------- ----- - ------------------- San ------------San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 19