Shinpei KUDO
Investigation of the hydrodynamic force due to tsunami acting on bridge
Takumi SHIMOMURA
On March 11th in 2011, the Tohoku Pacific Ocean Earthquake occurred. Its magnitude was 9.0 and maximum seismic intensity was 7 which is the strongest recorded earthquake in Japan. By the tsunami generated from this earthquake, many bridges were damaged. Loss of bridges lead the delay in rescue and recovery. It is predicted that the same problem will occur at the next Tokai, Tonankai and Nankai earthquakes, which are expected to occur near future. Several studies have been performed to estimate the tsunami force. Many researches focus on the impulsive wave force generated by bore and breaking waves. However, it has been reported that a number of bridges were damaged even if the elevation was enough high to avoid direct attack from breaking wave. Therefore, to clarify the bridge damage mechanisms, it is necessary to examine the hydrodynamic force acting on the bridge.
In this study, we focused on washed away bridges due to the lifting force of the flow after the bore passed. The mechanism how lifting force is generated and acting hydrodynamic forces were examined by computational fluid dynamic analysis. Furthermore, the validity of the analytical results were verified by comparing with the experimental results. Then, a prediction formula for tsunami force was proposed. Verification of our model was performed using dataset of field survey.
Strong eddy was generated on the upper part of the bridge when the water level reached high enough to make water surface flat. This eddy generates lifting forces. This phenominon was confirmed experimentally, and since the dragging force and lifting force of the experimental results coincides with analytical results, analytical results were verified. Furthermore, we confirmed that it is possible to evaluate the lifting force by using the approximate expression which considers velocity, bridge width and the water level as parameters. By using our prediction formula, we estimated whether the bridge will survive or not against 3.11 tsunami. In the case study in Rikuzen Takada area, our predictions were correct in almost all cases.
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