Cause and Consequence of the Fujinuma Dam Failure during the 2011 Tohoku Earthquake

ABSTRACT ONLY - It has been 10 years since the 2011 Tohoku earthquake (M 9.0) triggered a massive tsunami and caused incredible damage along the eastern coast of Japan. The shaking also damaged many dams and levees, including Fujinuma Dam located roughly 45 miles from the coast in the foothills. The Dam Failures and Incidents Committee, Decade Dam Failure series highlights notable dam failures to present lessons learned to the dam safety community. Fujinuma Dam is a rare example of a seismic induced dam failure. The earthfill dam and a saddle dam were built in layers from 1937 to 1949 for the purpose of storing irrigation water. Approximately 25 minutes after the earthquake, the dam was observed overtopping, eventually leading to full breach and release of the reservoir. The flood filled the downstream channel with debris, destroyed homes and bridges, and left 8 people dead. Observations and analyses concluded the dam had poor compaction and foundation treatment, likely resulting in a seismically induced slope failure that lowered the crest below the full reservoir level. The saddle dam and slopes around the reservoir rim likely failed due to rapid drawdown. The flood was confined to canyons for roughly 2 miles resulting in a relatively small area of impact but resulted in intense damages due to high local velocities. Over the last ten years, engineers have analyzed the possible failure modes and rebuilt the dam with more robust methods. This presentation will provide a timeline of the failure, a summary of observations and analyses, and lessons incorporated into the new dam.