Yuta Mitobe

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.

Post-Tsunami Lagoon Morphology Restoration Sendai; Japan

The 2011 Great East Japan Earthquake and Tsunami caused significant changes of morphology of Gamo Lagoon which is located in the northern part of Sendai Coast, Miyagi Prefecture. The investigation on the morphology changes and its recovery process after the tsunami are discussed. Frequently captured aerial photographs are effectively utilized to analyze detailed morphological changes in response to wave action after the tsunami. The lagoon area was greatly changed due to the 2011 tsunami, and a gradual process of river mouth changes and restoration were observed using aerial photographs. As compared with the shoreline position before the tsunami a 40 m retreat of the shoreline can be confirmed even after 3 years from the tsunami. Meanwhile, the water area shows distinct reduction by 40 % as compared with the pre-tsunami situation. Gamo Lagoon is well known not only having important roles in the coastal processes but also valuable brackish water environment, hence further investigation is also largely required to better understand the future state of the devastated coastal environment by the tsunami waves.

Numerical Study on Tsunami Propagation into a River

ABSTRACT Aoyama, Y.; Adityawan, M.B., Widiyanto, W., Mitobe, Y., Komori, D., and Tanaka, H., 2016. Numerical Study on Tsunami Propagation into a River. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1017 - 1021. Coconut Creek (Florida), ISSN 0749-0208. The Tohoku Earthquake and Tsunami in 2011 caused serious damage not only to coastal structures, but also to riverine infrastructure due to long distance of wave propagation into rivers located on the coast facing the Pacific Ocean. Although there have been numerous investigations on tsunami, limited number of studies have been made for tsunami propagation into a river channel. This study investigates tsunami propagation into a river numerically by comparing with laboratory experiment data. The present numerical simulation is based on shallow water equations, which are solved with the MacCormack scheme. Difference between calculated and experimental results are evaluated in terms of root mean square error. It is concluded that the present numerical simulation yields good agreement with experimental data in a wave flume. In addition it is observed that geographical characteristics in the river channel, such as sandbars and estuarine sand spits, highly affect tsunami propagation process in a river, causing lowering water level along with late arrival of tsunami peak.

Impact of the 2011 tsunami on the littoral system around offshore breakwaters on Sendai Coast

After the 2011 Great East Japan Earthquake Tsunami, littoral system on Sendai Coast have been changing due to tsunami-induced highly non-equilibrium condition along the coast. In order to clarify the modification of littoral environment and its subsequent recovery on Sendai Coast, analysis of shoreline change has been carried out. In this study, the shoreline was extracted from frequently captured aerial photographs from 2009 until now, and analysis using an empirical orthogonal function (EOF) method was conducted. It is seen that the shoreline retreated greatly due to tsunami event. In particular, tombolo which existed behind the offshore breakwater completely disappeared due to the tsunami. Total of contribution of the 1st and 2nd EOF components is more than 70%. It is concluded that the 1st component originated from cross-shore sediment movement, while the 2nd component represents longshore sediment transport. Although the 1st component shows only slight modification after the tsunami, the 2nd component resulting from a longshore sediment transport shows distinct change after the tsunami around the offshore breakwaters.

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