Page - 163 - in Book of Full Papers - Symposium Hydro Engineering

generated by earthquake events strong enough to conceivably cause damage to large civil structures were recorded by instrumentation at the project within the prior 20 years. Principally for these reasons, the IFT concluded that seismic damage was not a likely contributor to failure of the chute slab. 4.2. EMERGENCY SPILLWAY The development of the damage to the emergency spillway discharge channel was closely observed during the incident. Photographs and videographic footage, along with eyewitness reports, provide documentation of the development of the emergency spillway damage. During the incident response, in preparation for the possible use of the emergency spillway, trees had been cleared from the natural hillside downstream of the spillway crest structure before flow over the crest structure occurred. As the emergency spillway discharge flowed over the natural ground downstream of the crest structure, erosion began to occur. Erosion of surficial soil deposits began to develop as expected. However, erosion continued into the underlying weathered bedrock to greater depths than expected. By the afternoon of February 12, concentrated areas of erosion were observed to be rapidly progressing upstream (headcutting) toward the emergency spillway crest structure (see Fig. 11), resulting in the issuance of the evacuation order. Fig. 11 Emergency spillway headcutting on February 12 The principal physical factor contributing to the damage at the emergency spillway was clearly the presence of significant depths of erodible soil and rock in features orientated to allow rapid headcutting toward the crest control structure. Hillside topography and the presence of infrastructure (roads, transmission towers, etc.) may have concentrated flows and increased erosive forces, facilitating headcut formation. Insufficient energy dissipation at the base of the spillway ogee 163