Probable Maximum Flood Elevation Determination

In response to NRC inquiries regarding worst-case credible flooding on the Missouri River, Nebraska Public Power District’s (NPPD) Nuclear Station contracted with FTN to perform a study to evaluate the Corps of Engineers “Upper Mississippi River System Flood Frequency Study,” as it related to the probable maximum flood (PMF) water surface elevations at the station.

FTN acquired a hydraulic model of the Missouri River from the Federal Emergency Management Agency and used that model to establish a range of PMF elevations. The range was based on different modeling assumptions and techniques. The range could not be narrowed due to limitations of the hydraulic model used in the study.

NPPD then contracted with FTN to develop a more detailed hydrodynamic model for the purpose of refining the Missouri River water surface elevations at the nuclear station during extreme flood events. This model was based on Lidar data and channel data from the FEMA study.

FTN modeled the existing nuclear station design-basis PMF flow hydrograph as well as the 500-year peak flow from the 2004 Corps study and the Fort Randall Dam failure hydrograph developed by the Corps.