Page - 137 - in Book of Full Papers - Symposium Hydro Engineering

chute. The wall height was taken as the non-aerated flow depth plus freeboard. The freeboard was calculated using both USACE [10] and USBR [11] guidelines. These guidelines account for flow depths, pier end wave height, slug flow or roll waves, air entrainment, and minimum freeboard above the mean surface elevation. The proposed final wall heights for the FCO Spillway chute are taller than the original wall heights. 6.1.3. Cavitation Index and Air Concentration Analysis Cavitation along the FCO Spillway chute was a concern, considering the high velocities created by the largest releases. Hydraulic engineering analyses assessed the potential for cavitation. The potential of cavitation damage is measured by a cavitation index (σ), which is a function of pressure and velocity; no cavitation damage is expected when the index exceeds 0.2. If the cavitation index falls below the recommended value of 0.2, cavitation damage can be prevented if air concentration in the flow is about 10% [12, 13], or by using high- strength materials. The proposed FCO Spillway chute was designed as a self- aerating spillway [3, 14], thus avoiding cavitation even with the highest-velocity flows. Fig. 5 shows the variation of the cavitation index and entrained air concentration computed with a 1-D model developed by DWR. Near the chute failure location, Station 1021 m (3350 ft), for a release of 1841 m3/s (65000 ft3/s), the air concentration is about 25% and the cavitation index is higher than 0.2. This suggests that cavitation was not a significant factor in the failure of the FCO Spillway chute — an assessment that was confirmed by an independent forensic assessment [15]. The entrained air concentration calculation due to self-aeration is very conservative as it neglects the aeration contribution by the outlet structure piers. 137