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Three photos of very low flow on the upper chute taken after the chute failure, clearly show a hole on the left side of the spillway approximately at or near the construction joint near station 29+00. The second photo shows flow occurring at very small depths (probably due only to gate leakage).

At this very low flow, what appear to be role waves, or possibly disturbances due to flow over construction joints or cracks are clear. The third photo shows the hole that developed shortly after initial failure. In this photo, the damage to the chute is totally downstream of the construction joint. Later photos show the damage has taken place on the upper side of the construction joint and has migrated to approximately station 29+00.

These photos show that failure was initiated at the hole at the left side of the chute near station 33+00. The failure, likely occurred as a result of high velocity flow (in the range of 85 to 90 feet per second), penetrating under the slab, causing a strong uplift force and causing the slab to lift, eventually causing all or part of the slab to break away.

Subs equent erosion of foundation material caused progressive failure both upstream and downstream. Repairs had been made to the spillway slab several times since its completion in 1968. The most recent documented repair took place in 2009. Locations of numerous existing cracks and spalls were shown in the report. The spalls were probably caused and enlarged by freeze-thaw damage. Some of these holes were quite large and extended as deep as the reinforcing steel. The hole that triggered the failure was probably of the latter type.

Interesting that using Moonbeam Canyon is the fall back position to cover the case of the lower portion of the MS not being complete by Nov 1.