Page - 129 - in Book of Full Papers - Symposium Hydro Engineering

Reservoir behavior and operation with various operation and potential failure scenarios were simulated to predict future pool elevations and timing of potential Emergency Spillway discharge. For this, reservoir conditions were monitored continuously by onsite DWR staff, reported using all available resources, and used as boundary and initial conditions for the models. Inundation that could result from potential failure of the main dam was predicted and displayed with maps included in the dam’s Emergency Action Plan (EAP), which emergency and incident command teams consulted. These inundation maps were augmented with situation-specific maps prepared from near- real-time forecasts as the incident progressed. Both were used to inform local officials regarding the need for evacuation of downstream residents. For inundation mapping, the HEC-RAS [6] and FLO-2D applications [7] were used; maps were created by overlaying computed flood flow depth grids on feature maps with geographic information system (GIS) software. Terrain data and bathymetric data had been collected by DWR in prior years for analysis of downstream floodplain risk and for preparation of the EAP. Additional hydraulic analyses provided DWR managers with information about potential flow paths along the hill slope downstream of the Emergency Spillway, and with estimates of potential debris and sediment loads in the Feather River downstream. The hill slope was modeled with a 2-D model created with HEC- RAS. Challenges for analyses during the event in February 2017 included: (1) the need for almost immediate turnaround to provide information critical for decision making; (2) the lack of certainty about near-future inflows to the reservoir due to uncertainty about the weather and watershed response; (3) the lack of certainty about behavior of the never-used Emergency Spillway, with unstable terrain; and (4) the risk of misunderstanding or misinterpreting model results as critical decisions were made. Solutions included: (1) the use of cloud computing, with parallel model runs on multiple servers in the Amazon cloud; (2) analysis with ensemble forecasts, coupled with sensitivity analysis with varying inflow hydrograph properties; (3) constant monitoring and reporting to modelers of observed changes to the spillways so model inputs could be adjusted; and (4) careful communication of results by DWR and consultant subject matter experts. 4. RESERVOIR OPERATION ANALYSES DURING RECOVERY DWR’s Oroville Emergency Recovery (OER) team considered options for restoring the FCO Spillway chute and the Emergency Spillway, and selected the optimal configuration. A consideration in this selection was the way Lake Oroville would be operated to protect public safety prior to, during, and following construction — the reservoir operation plan. Through reservoir operation analysis, alternative schemes were assessed and refined. Once the new spillway 129