Effect of drifted houses on inundation region and the washout region of houses at the tsunami caused by the great east Japan earthquake

Abstract

Breaking or washout condition of houses by a tsunami was usually analyzed using fluid force and moment by drag force. However, the condition was not validated in actual large tsunami that could cause catastrophic damage on houses, and produce a large amount of debris including a floating houses or broken houses. Therefore, the objective of this study is to develop a numerical model considering the effect of drifted house on the tsunami inundation distance from the shore line and washout situation of neighboring houses at large tsunami event. Field investigations were conducted at 18 locations in Sendai Plain between April to May after the tsunami caused by the Great East Japan Earthquake. In the field survey, tsunami water depths and situation of broken or washed out houses were investigated. In this study, to elucidate the effect of the drifted house on the damage of neighboring houses, one location was selected. The effects of the drifted house were included into numerical model as an additional resistance. In addition, the numerical model was validated by the observed water depths and washout region of houses at the 2011 Japanese tsunami. The numerical simulation demonstrates that the drag force by debris or floating houses on flow reduces the washout region of houses, but on contrary, it also increases the drag force on the neighboring house and thereby increases the washout region of houses. The washout region of houses was reduced because the effect of the drag force by debris or floating houses on flow are greater than that of the additional drag force on the neighboring house. This result indicates that evaluating the effect of the drag force acting on the debris or floating houses on flow is quite important. Moreover, the simulation could reproduce well the washout region within the reasonable limits. The model enables the analysis of large tsunami that produces large amount of debris in densely populated area.