5:15 PM - 6:45 PM
[HDS11-P10] Simulated coastal seafloor exposure during the 2011 Tohoku tsunami
Keywords:The 2011 Tohoku tsunami, numerical simulation, seafloor exposure
This study reveals the actual state of seafloor exposure during the 2011 Tohoku tsunami based on numerical simulation. We also attempted to understand the causes of the differences due to the seafloor exposure and the location.
We conducted tsunami simulations based on the nonlinear long-wave equations. This simulation confirmed the reproducibility of seafloor exposure by a few testimonies of an eyewitness. Noteworthy results of this study are mainly the following three.
1. Actual state of seafloor exposure
Seafloor exposure remarkably occurred in the south of Sanriku-bay. The average depth of the exposed area is about 6 meters, and the average distance from the shore is about 500 meters. At the Kinkasan cannel, Ishinomaki city, Miyagi prefecture, there is largest seafloor exposure, and the width of the exposed area is 2.4km. At the Samenoura bay, Ishinomaki city, Miyagi prefecture, the maximum depth of the exposed area is about 25 meters. The average maximum velocity in the exposed area is 4.8m/s and average of the 10% from the fastest is 10.3 m/s.
2.Maximum Velocity
In the exposed area, the maximum flow velocity tends to be observed at the arrival of the second wave. At similar sea depth, the Froude number tends to be greater at exposed site. The average of the Froude number based on the still water surface at the exposed site is 0.89.
Regarding the 1., at Samenoura bay and Kinkasan cannel, the tsunami behaved like an edge wave due to reflection and refraction. This is assumed to amplify the water level changes and cause a large sea level drop. At the Samenoura bay, sea level drops easier to spread than sea level rise due to refraction at the bay mouth. Therefore, sea level drop is greater than sea level rise.
Regarding the 2., the slope of the front side becomes steeper than that of the back side during the propagation and the velocity of that also becomes faster. At the exposed site, the second wave arrives without being cancelled by the return flow of the first wave. As a result, the maximum velocity is observed at the arrival of the second wave. In addition, because the second wave arrives at an extremely shallow depth, the water thickness becomes thin. From the above, it is considered that the Froude number was greater due to the flow with thin water thickness and high velocity.
We conducted tsunami simulations based on the nonlinear long-wave equations. This simulation confirmed the reproducibility of seafloor exposure by a few testimonies of an eyewitness. Noteworthy results of this study are mainly the following three.
1. Actual state of seafloor exposure
Seafloor exposure remarkably occurred in the south of Sanriku-bay. The average depth of the exposed area is about 6 meters, and the average distance from the shore is about 500 meters. At the Kinkasan cannel, Ishinomaki city, Miyagi prefecture, there is largest seafloor exposure, and the width of the exposed area is 2.4km. At the Samenoura bay, Ishinomaki city, Miyagi prefecture, the maximum depth of the exposed area is about 25 meters. The average maximum velocity in the exposed area is 4.8m/s and average of the 10% from the fastest is 10.3 m/s.
2.Maximum Velocity
In the exposed area, the maximum flow velocity tends to be observed at the arrival of the second wave. At similar sea depth, the Froude number tends to be greater at exposed site. The average of the Froude number based on the still water surface at the exposed site is 0.89.
Regarding the 1., at Samenoura bay and Kinkasan cannel, the tsunami behaved like an edge wave due to reflection and refraction. This is assumed to amplify the water level changes and cause a large sea level drop. At the Samenoura bay, sea level drops easier to spread than sea level rise due to refraction at the bay mouth. Therefore, sea level drop is greater than sea level rise.
Regarding the 2., the slope of the front side becomes steeper than that of the back side during the propagation and the velocity of that also becomes faster. At the exposed site, the second wave arrives without being cancelled by the return flow of the first wave. As a result, the maximum velocity is observed at the arrival of the second wave. In addition, because the second wave arrives at an extremely shallow depth, the water thickness becomes thin. From the above, it is considered that the Froude number was greater due to the flow with thin water thickness and high velocity.