Overview
Typhoon Hagibis, which occurred at the beginning of October 2019, was one of the largest and most powerful tropical cyclones and is considered the most devastating typhoon to hit Japan in recorded history. Extreme heavy rainfall caused massive impacts to Japan in general and to Marumori Town, Miyagi Prefecture, in particular. This study investigated the detailed flood characteristics at Marumori Town using field observation and numerical simulations.
Model Setup
Data obtained immediately after the flood clearly showed that most levee breaches were caused by water overflow on the river embankment at constriction areas such as tributary junctions and the intersection of river embankments. Numerical simulations were performed to investigate the mechanism of levee breaching, and a sensitivity analysis examined the effect of backwater and phase lag of water level rise between the mainstream and tributaries.
Key Findings
According to the simulation results, flooding water from upstream levee breach locations flowed into the paddy field area and caused the levee to breach at the river embankment intersection in the downstream area. A new levee breach criterion in terms of overflow depth and its duration on the river embankment was proposed. The study outcomes provide useful information to understand how the river embankments of tributaries are vulnerable to water level rise, and to support river authorities in preparing better mitigation plans for future flood disasters.