CS38C - Concurrent Session 38C: Paleoflood Analysis Along the Kootenai River and Improvements to Hydrologic Loading for Libby Dam, Northwestern Montana

Information on the timing and magnitude of large, pre-historic floods on the Kootenai River in northwestern Montana provides improved characterization of rare and extreme hydrologic loadings for Libby Dam. Existing historic and systematic flood records along the Kootenai River contain 112 years of observation. An extreme-flood analysis was completed to lengthen this record and to evaluate peak flow frequencies of rare and extreme events. This analysis yields geologic evidence of two pre-historic late-Holocene floods that occurred about 900 and 2,900 years ago and a geomorphic surface that has not been flooded since its formation more than about 10,500 years ago. A one-dimensional HEC-RAS hydraulic model of the Kootenai River was utilized for developing ranges in possible discharges responsible for these features. This analysis provides discharge magnitude and timing data for refining the dam’s inflow frequency curves and improves confidence in extreme hydrologic loading for the dam safety risk assessment.