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The 2011 Great East Japan Earthquake and Tsunami are one of the most catastrophic natural disasters that took place in Japan. The disaster caused widespread destruction to the estuarine and coastal areas along the northeastern region of Japan. The Abukuma River mouth, among many estuaries in Miyagi Prefecture, suffered major damage from the tsunami in which the sand spit and the sand terrace were severely eroded. The present study aims to provide a better comprehension of the morphology of the Abukuma River mouth within a decade after the 2011 tsunami based on the collected datasets of imagery and offshore bathymetry surveys. The consecutive changes of the sand terrace and sand spit indicate their connection influencing the morphological recovery in the Abukuma River mouth. About a year after the tsunami, the sand terrace re-appeared by a flood event and became the sediment source to recover the sand spit under the intrusion process. When the sand spit almost recovered and achieved a stable configuration, the sediment from the sand terrace into the river was restricted. As a result, more sediment from the sand terrace was expected to be transported to the downdrift area under the dominant wave condition in the Abukuma River mouth. [ABSTRACT FROM AUTHOR]

In most of the current conventional tsunami numerical models, the steady flow friction law has been widely used to access the tsunami-induced bottom shear stress. However, according to our previous theoretical investigation showed that in almost the entire model domain from the tsunami source to the nearshore area, the tsunami-induced bottom boundary layer behaves like a wave boundary layer, rather than a quasi-steady flow boundary layer. The present study aims to develop a numerical method for tsunami simulation that elaborates a wave friction factor by implementing a correction method proposed by authors' previous work into the conventional 1D shallow water equation. As a result, the new method provides similar results to the original Manning method regarding the water level and the tsunami-induced velocity. However, the bottom shear stress estimated by the new method is higher than that from the Manning method over the entire computational domain. The transition locations from the wave friction zone to the steady friction zone are found at the water depth around 5 m under the present computational condition. [ABSTRACT FROM AUTHOR]

This paper presents a detailed investigation on unsteady effects in bottom boundary layer beneath a tsunami. In numerical simulation of tsunami, bottom shear stress has commonly been evaluated by using steady flow friction laws such as Manning equation, simply assuming that long-period wave motion satisfies quasi-steady flow conditions. However, the present study found that the behavior of tsunami-induced bottom boundary layer has an unsteady characteristics and acts similar to that induced by wind-generated waves even under long-period wave motion. As a result, bottom shear stress under tsunami is much larger than the estimation using steady friction coefficient due to a steep velocity gradient in the bottom boundary layer. Surprisingly, the steady flow friction law is not valid in almost the entire computational domain, from the source area to shallow region. This result highly coincides with the field measurement data obtained by Lacy group during the 2010 Chilean Tsunami at the Monterey Bay mouth in U.S. A correction coefficient is proposed to take into account unsteady effects in the conventional estimation method using a steady friction factor; this approach is validated by comparing with numerical simulation results using k-ω turbulence model. [ABSTRACT FROM AUTHOR]

In the 2011 Tohoku Earthquake Tsunami, run-up of the tsunami along rivers expanded the damaged area into more inland areas far from the coast. High water level in rivers was kept for more than several times of the wave period in the 2011 event, while more simple periodical oscillations of the water level superposed on the tidal motion was observed in the 2010 Chile Tsunami event. The difference of behaviors of tsunami in rivers was discussed with related to the magnitude of tsunami and river mouth morphology through two numerical experiments in this paper. In the first numerical experiments, the run-up of historical tsunamis was simulated along Kitakami River. It was found that the discharge of tsunami flow across the river mouth was proportional to third power of water depth if overflow of the sand spit occurred, and that caused unbalance of water volume coming into and going out from the river channel. Through the second numerical experiment with simple model bathymetry and sinusoidal waves, it was found that discharge during both tsunami run-up and run-down across river mouths is reduced by sand spits at the river mouths and the reduction effect is more significant during flow rushing into the rivers than that during return flow. [ABSTRACT FROM AUTHOR]

Cua Dai beach, located in Hoi An town - Quang Nam province, has experienced serious erosion for the past few years with extremely high speed of shoreline retreat. Despite coastal erosion itself may not be that much problematic, it turns out to be serious problem since there is a large residential area right behind and a huge investment in tourism has been done. Generally, causes of erosion are among unbalance of sediment transport, storm surges and sea level rise. However, the rate of shoreline retreat in Cua Dai beach within the past decade is so aberrant and not likely to be explained by the above phenomenon alone. The goal of this study is to consider effective factors of this coastal erosion based on analysis of the recent progress of erosion using Empirical Orthogonal Function. As a result, the dominant factor for the long-term erosion is concerned to be the change of longshore sediment transport, which was caused by the decrease in sediment supply from Thu Bon River. Therefore, it is recommended to collect more data on sediment transport in the river system and study the relationship between coastal erosion and the change of sediment supply. [ABSTRACT FROM AUTHOR]

This study investigates the changes in coastal topography of the Rikuzen-Takata Coast in Iwate Prefecture, Japan before and after the 2011 off the Pacific coast of Tohoku Earthquake Tsunami and the effects of coastal structures on these changes. The changes in coastal topography were analyzed using bathymetry data and aerial photographs before and after the tsunami in addition to aerial video during the tsunami. The bathymetry data were obtained from 1989 to 2002 during the construction of three submerged breakwaters and from after the 2011 tsunami until 2013. The aerial photographs were acquired from 1947 to 2015, and the aerial videos were acquired during the tsunami run-up and backwash. The results demonstrated that the coast was eroded mainly due to the tsunami backwash, and the submerged breakwaters trapped the seaward transport of sediment from the coast. Erosion was partly prevented in locations where the seawall was not washed away. The coastal structures had significant effects on the behavior of the coastal tsunami and on sediment transport. We also found that the coast did not recover naturally at the desired speed after the tsunami because the coast had been stable before the tsunami. Coastal restoration five years after the 2011 tsunami are also summarized in the Appendix to illustrate the future reconstruction plan for the study coast. [ABSTRACT FROM AUTHOR]

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 and 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. [ABSTRACT FROM AUTHOR]

The 2011 Great East Japan Earthquake and Tsunami caused severe damage along the affected coastal area. This study analyzed seabed subsidence and morphological changes along the Ishinomaki Coast based on bathymetry measured shortly before and after the 2011 event. In general, seabed subsidence ranged from 0.32 m to 0.91 m. In addition, massive erosion on the seabed due to the 2011 tsunami occurred locally, in front of the Naruse River mouth and at the west end of the Ishinomaki Coast. Moreover, the depth of closure generated by the tsunami in these areas was higher than in others. The sandspit in front of the river mouth was severely eroded and the riprap protection at the west end was completely destroyed. Thus, there were no buffers in these areas during the tsunami. Therefore, tsunami-induced bed stresses were higher and may have occurred in deeper areas. Other areas, which were protected by relatively unharmed concrete structures, did not suffer from severe erosions. This suggests that the coastal protection structures protected both the land and the seaward side during the tsunami. [ABSTRACT FROM AUTHOR]

The 2011 Great East Japan Earthquake that occurred on March 11, 2011, had magnitude of 9.0 on the Richter Scale with the epicenter approximately 70 km east of the Oshika Peninsula in Miyagi Prefecture. This earthquake triggered terrible tsunami waves which hit the coast of Japan and propagated around the Pacific Ocean. The earthquake and tsunami caused extensive and severe infrastructural damage, such as damages of coastal protection structures and buildings, and significantly changed coastal and river morphology. This paper presents tsunami-induced coastal and estuarine morphology changes in Miyagi Prefecture, Japan, and subsequent recovery process in each study area. On sandy coasts, discontinuous coastal protection is likely to be severely damaged, resulting in serious erosion in the surrounding sandy coast. Furthermore, severe breaching was observed on sandy coasts where formerly river mouth was located, due to strong return flow from the catchment area. The restoration process of the coast and estuaries is highly dependent on sediment supply availability in the surrounding area. [ABSTRACT FROM AUTHOR]

At 14:46 JST on March 11, 2011 a magnitude 9.0 earthquake (2011 Tohoku Earthquake and Tsunami) occurred off the Pacific Coast of Miyagi Prefecture. This study investigated the extensive changes in beach morphology due to the earthquake and tsunami along the 15 km Northern Sendai Coast using remotely sensed data. The remote sensing analysis on the beach topography and coastal forest demonstrated the following notable characteristics of beach morphological change: erosion of the northern barrier at the mouths of the Nanakitagawa and Natorigawa Rivers; erosion at an old river channel; scour landward of the seawalls in the longshore direction; erosion and deposition in beach areas with detached breakwaters; and deposition in coastal forest areas. Linkage of the deposition in the forest areas with the damage type of coastal forests was observed. The impact of the earthquake and tsunami on the beach morphology was serious; roughly 60% of the study area was degraded by 0.2-0.5 m in elevation mainly due to land subsidence, and a total of 0.4 km2 of beach area was eroded mainly due to erosion of the northern barrier at the mouths of the Nanakitagawa and Natorigawa Rivers. This study explores the geographical changes brought on by a tremendous earthquake and tsunami, which will help to elucidate the mechanisms of coastal forest destruction, beach erosion, and their interaction during tsunami events. [ABSTRACT FROM AUTHOR]

Field observation of water quality changes was performed during summer in 2001 and during autumn in 2002 in Nomi Bay in Kochi Prefecture, Japan, where extensive aquaculture has been carried out. Large-scale internal tides were observed during summer, and DO (Dissolved Oxygen) was sufficiently high even at the bottom. Meanwhile, a phenomenon called "mizu-domari" (slack water) and anoxic water formation were observed during autumn. It is generally well known that anoxic water forms in summer due to strong stratification in vertical direction and it disappears in autumn. Contrary to this general expectation, however, the water quality in Nomi Bay was worse in autumn than that in summer. Thus, the role of the internal tide is quantitatively investigated using a numerical model. It is concluded that internal tides enhance the exchange of seawater, being helpful for maintaining the good water quality of the inner area in Nomi Bay. [ABSTRACT FROM AUTHOR]

At the Shiribetsu River mouth located in Hokkaido, Japan, a jetty has been constructed on the right hand side of the mouth from July to October 1999. In this study, the effectiveness of the jetty construction is studied by analyzing field data of river mouth morphology and water level variation. Before the jetty construction, water level rise due to wave set-up has frequently been observed every year from October to March. It is found that the height of the wave set-up increased in winter with the development of sand spit at the river mouth. After the construction, it is confirmed that the interruption of longshore sediment movement due to the construction of the jetty led to the stability of the river mouth opening. Furthermore, it reduces the wave set-up in the river mouth. [ABSTRACT FROM AUTHOR]

An experimental study is carried out to investigate the transformation of oscillatory boundary layer from ordinary to depth-limited one by using an oscillating tunnel with triangular roughness pasted on the top and bottom surfaces. The velocity is measured by one-component LDV. It is proved that only longer period of oscillation is not sufficient to induce depth-limited properties. Instead, the ratio of boundary layer thickness to the distance from the bottom to the free surface must be used in order to distinguish a depthlimited oscillatory boundary layer from an ordinary one. The experimental data for mean velocity, phase difference, fluctuating velocity, friction coefficient and boundary layer thickness are presented in order to elaborate the structure of depth-limited oscillatory boundary layers on a rough bottom. On the basis of the present experiments, a modification has been made in an existing criterion to distinguish between ordinary and depth-limited wave boundary layers. A typical field situation is discussed to show the usefulness of the proposed criterion. [ABSTRACT FROM AUTHOR]

No abstract received. [ABSTRACT FROM AUTHOR]