A numerical computation of the 2011 Tohoku earthquake tsunami was performed to identify fundamental features of the tsunami evolution along the coast of Hokkaido, Japan. Edge waves formed at multiple locations where the refracted tsunami focused, governing local surface oscillations and regional variations in tsunami height along the Pacific coast of Hokkaido. The computation reasonably reproduced the distribution of surveyed tsunami height as well as the time records of surface elevation recorded at ports in Hokkaido. The major features of the frequency spectrum for the 2011 Tohoku tsunami were identical to those for the 2003 Tokachi-oki earthquake tsunami; inherent local properties of surface oscillation caused by the passage of edge waves existed, determined by the local bathymetry.

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On March 11, 2011, the Great East Japan Earthquake generated huge tsunami waves. Then, tsunami propagation occurred and expanded the flooded area. Tsunami propagation distance was collected and analyzed to estimate the area where we need the defense. It was found tsunami propagation distance in river is about 1.2 to 4.5 times as long as land area and, there is a good correlation between propagation distance in rivers and the bed slope. Furthermore, the dissipation coefficient of tsunami height was successfully estimated by applying the empirical equation and the general relation of wave height to distance.

Abstract:It is inevitable to find in studies that shoreline position extracted from unknown capture time aerial photograph or satellite image was not corrected to tidal effect. In this study, an approach is introduced that can estimate the capture time of image from the solar azimuth angle and the length of shadow of a vertical object on the horizontal surface on the earth. The capture time of tze aerial photograph estimated from solar azimuth angle has a much smaller deviation than the one estimated from the length of the shadow. This error is acceptable for tidal correction purposes. The approach was also utilized to estimate the capture time of a set of satellite images on Sendai coast. Therefore, the tidal correction was implemented for shoreline positions extracted from those images.

A lot of coastal dikes located along the Pacific Coast of Tohoku Region were severely damaged by the Great East Japan Earthquake Tsunami. In this study, hydraulic model experiments were conducted to understand characteristics of local scour at a landward toe of a coastal dike, which has been considered as a main cause of the failures of coastal dikes by the 2011 tsunami. A coastal dike model is fixed in a horizontal open channel with 4.5m length. Scouring of sandy bed behind the dike model under constant discharge is recorded by a video camera and temporal variations of sizes of the scour hole are extracted from the video images. In the experiments, two types of flow patterns, flow with hydraulic jump and submerged flow, were observed around the toe and it is found that the different flow types induce completely different processes of the local scouring, resulting in significant differences of scouring depths. Embedment of the landward toe induced the submerged-type flow to form deeper scour holes than those with a basic model of the toe without embedment. On the other hand, sheet-pile structure is found to be an effective measure against local scouring to delay start of suction of sand under the landward slope through the scour hole.

The 2011 tsunami formed concave shoreline around the Nanakita River mouth and Akaiko breaching areas, Sendai Coast, Miyagi Prefecture, Japan. The recovery process of morphology in these areas is much depended on the longshore sediment which is transported from adjacent sandy coasts. Coastal structures on both sides of the concave portion are considered as rigid boundaries. Analytical solutions of one-line model, which describe the evolution of shoreline around the concave portion in cases without and with rigid boundaries, have been discussed. When the dimensionless of concave width to the length of bounded area is getting smaller, the dimensionless recovery time obtained from solution for the case with rigid boundaries is getting larger and asymptotic the one for case without rigid boundaries. An analytical solution for estimating the proportion of backfilling of sediment deposition in the concave portion for case with rigid boundaries is introduced. It is asymptotic to the one without rigid boundaries when the dimensionless of concave width to the length of bounded area is getting smaller. The comparison between theoretical results and measured data of cases of the Nanakita River mouth and the Akaiko breaching areas are presented.

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