2017-02-21_REVISION - M1988112 (3)

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likely flows to reach the structure. Only if the PMF estimate is found to be un -conservative would the design basis be exceeded and even then a large portion of any additional flows would go to the impoundment. Tables 3.1(a), (b), and (c) provide a tabulation of summary results for the HEC -RAS system analysis for the PMF, 500 -Year, and 100 -Year flow events, respectively, for some of the key points along the system. Manning's "n" was held the same for all three analyses and only the flow and associated results are different. Figure 3.1 is a plot of the hydraulic profile for the PMF results presented in Table 3.1 (a) with the flow estimated at 3000 cfs entering the system and 1522 cfs reaching the drop structure. A few observations to note in the hydraulic profile are: 1. The steep channel downstream from the rock grade control section at Station 146 and the decreasing depth associated with flow acceleration at that location, preceded upstream by a relatively flat grade to the discharge from the stilling basin at Station 245. 2. The hydraulic jump at the toe of the chute, Station 324 and the increased water level downstream from there to the end sill wall at Station 274. 3. The increased flow depth prior to the start of the chute between Station 503 and Station 625 which will produce a relatively low approach velocity to the chute. 4. The elevated channel at near uniform slope and depth upstream of Station 6+75. 5. The depressed area between Station 1177 and Station 1364 where Area C contributions join the channel — this location will act to settle sediment from the watershed upstream from the drop structure entrance. 6. The overflow section between Station 3000 and Station 3500 where excess water spills to the impoundment. The analysis presented utilized a roughness value for the concrete structure of 0.008, which is at the low end of accepted values. Such a low value (representative of very smooth concrete) is generally used to predict conservatively high velocities used in design of energy dissipation facilities. Higher roughness values (typically "n" of .014) are used for sizing wall heights such as in the chute, Design of Small Dams (USBR 1987). The results of the analysis indicate that, for the assumed channel roughness, the 500 -year flow of 792 cfs would reach the drop structure without bypass of flow to the impoundment and that during the PMF event; a flow of 1522 cfs would reach the drop structure with up to 1478 cfs entering the impoundment. A peak velocity of 60.8 cfs at a Froude number of 8.3 is estimated at the entrance to the stilling basin. Additional information regarding the HEC -RAS model is provided in Appendix C-2. San Luis Project — South Diversion Ditch Drop Structure - Final Design Report 13