Page - 157 - in Book of Full Papers - Symposium Hydro Engineering

the spillway drains are mystifying but probably not dangerous as the chute is anchored.” The report also states that “inquiry to the designers regarding this aspect of performance would be in order.” The IFT was not able to locate any documents indicating that the designer(s) were consulted at that time. From the design documents, the IFT is confident that the intent of the chute underdrain system was to collect seepage from groundwater beneath the slab, and not to collect large amounts of leakage from flows in the spillway chute. Over the life of the project, reports of flows from the spillway chute underdrain outfalls were sporadic and limited. From an evaluation of the available reports, it can be concluded that, when the spillway was discharging, high chute drain flows consistently occurred, and the data indicate that the drain flows increased with increasing spillway chute flow, though not necessarily proportionally. High drain outfall flows were not limited to times when the spillway gates were open. Rather, significant outfall flows were also observed when the gates were closed and the reservoir level was high enough for water to be against the upstream sides of the service spillway gates. In these circumstances, gate leakage resulted in significant flow in the spillway chute. The IFT agreed with the initial 1969 conclusion that the high drain flows were principally attributable to discharge leakage through the cracks and joints in the service spillway chute slab. This was dramatically demonstrated during post- incident repair work in 2017, when water was observed flowing on the spillway chute from leakage with the gates closed. To facilitate investigation and repair work, sand bags were placed on the spillway chute to direct the leakage flow toward a narrow area near the left chute wall. When the sand bags were placed, drain outfalls on the right side, which had been flowing significantly, stopped flowing, which indicates that the water collected by the drains was not seepage through the foundation, but rather leakage through the slab. 3. CHRONOLOGY OF THE FEBRUARY 2017 SPILLWAY INCIDENT In January and February 2017, the service spillway experienced its first significant discharges since 2011. After some limited service spillway discharges earlier in January, the discharges were ramped up beginning on January 30, in response to an approaching large precipitation event. In the first few days of February, discharges were in the range of about 425 to 708 m3/s (15,000 to 25,000 ft3/s). On February 6, 2017, the spillway discharges were increased to between 1,190 and 1,270 m3/s (42,000 and 45,000 ft3/s) and were held in that range until the morning of February 7. 157