Numerical modeling of flood flow after small earthen dam failure: a case study from the 2011 Tohoku earthquake

The 2011 Tohoku earthquake damaged many small earthen dams in Japan, causing flood damage and human casualties. Since then, the necessity for developing accurate inundation hazard maps for areas at risk has been highlighted. We constructed a numerical model using two-dimensional shallow-water equations with the finite element method to simulate such flood flows and tested this approach by simulating the 2011 breach of the Aotashin-ike dam and subsequent flow into the downslope Ohya-ike dam. In addition, we investigated the effect of different inflow hydrographs at a collapsed dam’s breaching point on simulated flood flow comparing three different peak timings. While the spatial extent of the simulated inundation area was not affected, the timing of flood arrival and the simulated water depth in the drainage channel were moderately affected by the inflow hydrograph; the maximum water level in the drainage channel increased with a peak timing delay. The methodology presented here can determine further efforts to better define inundation risk for Japan’s small earthen dams and provides practical procedures to account for downstream topographic features such as spillways and drainage channels.