Min Roh

INVESTIGATION OF TSUNAMI PROPAGATION CHARACTERISTICS IN RIVER AND ON LAND INDUCED BY THE GREAT EAST JAPAN EARTHQUAKE 2011

The Great East Japan Earthquake of March 11, 2011 generated a massive tsunami wave that severely damaged coastal areas of Japan. Furthermore, the wave propagated into rivers, causing damage upstream far from shore. Videos recorded during this tsunami event were collected and analyzed to estimate the celerity of tsunami propagation in river and on the land. The result shows good comparison with estimation based on theoretical approaches that use water level measurement data. It was found that the tsunami celerity in river is approximately 25–30 km/h, and moved upstream with gradual deceleration. However, wave celerity on land was decreased significantly due to debris and dominant ground friction. The propagation trend in river mainstream and floodplain behaves differently in the location where the mainstream is not parallel to the embankment. Tsunami discharge and velocity in a river induced by tsunami wave were estimated based on continuity equation using the measured water level variation along the Sunaoshi River. The maximum estimated discharge is approximately 152 m3/s with the maximum velocity of 1.4 m/s.

Tsunami Observations in Rivers from a Perspective of Tsunami Interaction with Tide and Riverine Flow

The observations of the 2011 Tohoku tsunami and the 2010 Chilean tsunami in several rivers in Japan and in the Columbia River in the USA are analyzed for patterns of tsunami behavior in river environments. Tsunamis in rivers exhibit actions very different from those observed on an open coast, but very similar among different rivers, though the action scale in different rivers varies greatly. We describe two tsunami effects in rivers as observed in field data. First, the river tide modulates the tsunami wave in a very specific way common to all rivers. Second, a strong near-field tsunami can cause significant prolonged water accumulation in lower river reaches. Both effects are inherent in tidal river environments, and have been reproduced numerically in a simplified 1-D river using a non-linear, shallow-water model with bottom friction. The numerical experiments highlight the indispensable role of a tsunami’s interaction with tide and riverine flow.

Experimental Study on Embankment Reinforcement by Steel Sheet Pile Structure Against Tsunami Overflow

This study proposes a new embankment reinforcement using steel sheet piles against tsunami overflow, which has been known as the main cause of the failures of the embankments by the 2011 Tohoku Earthquake Tsunami. Effectiveness of the proposed technique was discussed through a hydraulic experiment. A model of embankment was set in a horizontal open channel, and one or two steel plates are installed into the embankment from the top as vertical walls inside. Temporal variations of the shapes of the embankment and the sheet pile structures were obtained from video images. In most of the cases, the sheet pile structures started to rotate after the erosion of the landward slope of the embankment. However the rotation stopped at about 30° and 10° from the initial location with the single- and double-wall cases. Height of the embankment after overflow was less than 20% with no reinforcement, while more than 70% and 95% of the height were kept with the single- and double-wall structures, respectively. The performance of the embankment with the reinforcement was also discussed in terms of tsunami energy reduction with an additional fixed-bed experiment.

Morphological Characteristics of River Mouths After the 2011 Tohoku Tsunami in Miyagi Prefecture

Coastlines and river mouths along the Tohoku Region in Japan are in the process of steady recovery following the 2011 Tsunami. The affected sandy coastlines and river mouths have undergone more significant changes than the corresponding cliffs, rocky beaches, and hard structures. Analyses of aerial photographs and topographic data related to pre-tsunami and post-tsunami conditions together with the estimated minimum width and positioning changes of river mouths describe how they have changed as well as the differences in their recovery processes. In many cases, there is an indication of stable recovery although the behavioral tendency differs, as in the case of the Natori and Naruse rivers. This study shows that the temporal topographic changes and the relationship with the tidal prism in the mouths of these river mouths differ, resulting in different sediment deposition and restoration processes. Sediment supply is intricately associated with the morphological changes in river mouth morphological changes, which in turn reflect on its recovery. The morphological changes in a river’s mouth present practical river management and maintenance problems in ports and harbors; consequently, continuous monitoring is essential.

EXPERIMENTS ON EMBANKMENT REINFORCEMENT USING STEEL SHEET PILES AGAINST TSUNAMI OVERFLOW

In this study, new reinforcement technique of coastal embankment against tsunami overflow is presented, and its performance is discussed through a hydraulic experiment. One or two steel sheet piles are installed into a coastal embankment to have a single or double-wall inside and composite structure of the sheet piles and ground around them is expected to keep the height of the embankment even with severe erosion of landward slope induced by overflow of a huge tsunami. In the hydraulic experiment with 1/50 scale model, the single-wall structure kept about 80 % of the height of the embankment and almost no reduction of the height was observed with the double-wall structure. Introduction Coastal embankments located along the Pacific Coast of Japan were severely damaged by the 2011 Great East Japan Earthquake Tsunami. Local scour behind the embankments induced by tsunami overflow was observed in many places just after the tsunami, and it is considered as one of the main cause of the failures of the embankments (Kato et al. 2012; Tokida and Tanimoto 2012; Mitobe et al. 2014). For designs of new embankments, new reinforcement structures are required to be developed in order to ensure the performance of embankments after overflow of huge tsunamis. In this study, double-wall structure using sheet piles is presented as a new reinforcement structure against tsunami overflow. This structure has been considered as liquefaction countermeasures of embankments (Fujiwara et al. 2013). Composite structure of the sheet piles and ground between them can keep height of the embankment after liquefaction. This structure is expected to keep the height of the 180 16-3-2015