Search

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]

The 2011 Tohoku tsunami entered the Kitakami river and propagated there as a train of shock waves, recorded with a 1-min interval at water level stations at Fukuchi, Iino, and the weir 17.2 km from the mouth, where the bulk of the wave was reflected back. The records showed that each bore kept its shape and identity as it traveled a 10.9-km-path Fukuchi-Iino-weir-Iino. Shock handling based on the cross-river integrated classical shock conditions was applied to reconstruct the flow velocity time histories at the measurement sites, to estimate inflow into the river at each site, to evaluate the wave heights of incident and reflected tsunami bores near the weir, and to estimate propagation speed of the individual bores. Theoretical predictions are verified against the measurements. We discuss experiences of exercising the shock conditions with actual tsunami measurements in the Kitakami river, and test applicability of the shallow-water approximation for describing tsunami bores with heights ranging from 0.3 to 4 m in a river segment with a depth of 3-4 m. [ABSTRACT FROM AUTHOR]

Adityawan, M.B.; Tanaka, D., Mitobe, Y., and Widiyanto, W., 2016. A new computation method of bottom shear stress under tsunami waves, 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. 1247 - 1251. Coconut Creek (Florida), ISSN 0749-0208. The tsunami propagation in the river as well as sediment transport is highly affected by the bed stress beneath the wave. Unfortunately, most numerical models use an empirical method such as Manning's n, which was developed for steady flow. A new method in numerical modelling to assess bed stress and sediment transport induced by wave is developed based on the boundary layer approach. The boundary layer has a significant effect on the bed stress under unsteady flow. Bed stress under unsteady flow may exhibit phase shift and sign change to the free stream velocity. Recently, Simultaneous Coupling Method (SCM) was developed and verified by combining Shallow Water Equation (SWE) and k-ω model. The simplicity of SWE was maintained while k-ω model was used to assess the bed stress from the boundary layer. The velocity from SWE is assumed as the free stream velocity, which is used as the boundary condition for k-ω. The calculated bed stress from k-ω is used in SWE to obtain water depth and velocity. In this study, SCM is further developed by simulating a tsunami wave propagation around the Kitakami River mouth and verified using a laboratory experiment. The results show good comparison between the numerical and experimental. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The relations of river morphology and tsunami propagation in rivers were studied at several rivers in the Tohoku region during The Great Chilean Tsunami of 2010 and The Great East Japan Tsunami of 2011. It was found that river mouth morphological features play an important role in the intrusion of low magnitude tsunamis in which the geological and geographical conditions are an important factor. Nevertheless, the effects of these features were not found in the case of an extreme tsunami wave. As the wave enters the river, the propagation depends on other factors. It was found that the intrusion distance correlates well to the riverbed slope. The measurements of water level and riverbed slope were analyzed to propose an empirical method for estimating the damping coefficient for the tsunami propagation in rivers based on the tsunami of 2011. The proposed empirical method was used to approximate the length of the tsunami intrusion into a river by assuming that the furthest distance is given for the ratio of local tsunami wave height to the tsunami wave height at the river entrance of 0.05 (5 %). The estimated intrusion length from the proposed method in this study shows a good comparison with measurement data. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the morphological changes near the Nanakita River mouth in Japan. The morphology of the river mouth was greatly influenced during the Great East Japan Tsunami of 2011. The gradual morphological changes at the river mouth were investigated using two sets of data, of which, one was the continuous water level measurement data in the river entrance and in the sea, and the other being the intermittent aerial-photographs. The statistical parameters, viz., the correlation coefficient and the linear gradient between the two water level data, were analyzed, which formed as the basic for understanding the behavior of the river mouth, like river mouth closure or opening detected by the analysis of the water level variation. The proposed method is found efficient and effective in the evaluation of the morphological changes near a river mouth. [Copyright &y& Elsevier]

The Great East Japan tsunami, which occurred on 11 March 2011 (the 2011 tsunami), caused significant changes to the coastal and estuarine morphology along the Sendai coast, Miyagi prefecture, Japan. The recovery process, which took place subsequently after the tsunami, is investigated using aerial photographs, bathymetry data, and Empirical Orthogonal Function (EOF) method. Results from the aerial photograph analysis indicate that the shoreline was in the dynamic equilibrium condition in the period before the tsunami (Period 1). The recovery of the shoreline was very fast in the period of two years post event (Period 2). However, it was more stable for the period of the next 4.5 years (Period 3). The analysis on the bathymetry data indicates that, besides the sediment supply from adjacent coasts, eroded sediment of the offshore seabed was transported onshore and led to a mass recovery of the shoreline at the lagoon areas. The littoral system of the Sendai coast also changed significantly between the periods before and after the tsunami, as well as between Periods 2 and 3. The sediment deposited inland and in the lagoon areas is perceived as the main reason that caused the retreat inwardly of about 70 m–80 m of the shoreline after the tsunami. Regarding the EOF analysis, the reflected coastal processes of the first three dominant components from the EOF analysis for two shoreline data sets after the tsunami can be explained. In Period 2, the morphological recovery at severely damaged areas (lagoons) was the most dominant process, whilst the second one was the cross-shore variety of shoreline which was the most dominant process preceding tsunami. The contribution rate of the first and second components in Period 2 is about 4 times different. In the period of the next 4.5 years (Period 3), the cross-shore variety of shoreline which was induced by high waves came back to be the most dominant process, whilst the second component reflects the longshore sediment transport. These two dominant components are similar to the ones before the tsunami. The contribution rates of the first two components before and after the tsunami are relatively close as well. That confirms that the coastal processes have returned back to the conditions before the tsunami. This is an important finding which is deduced from this study because a strong tsunami such as the 2011 tsunami could have been a major disturbance, tipping the "normal" cross-, long-shore variability of shoreface water interface towards a different regime. [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]

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]

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]

Iida, T., Kure, S., Udo, K., Mano, A., Tanaka, H., 2014. Breach process simulation of coastal levees broken by 2011 tsunami. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 302-307, ISSN 0749-0208. On March 11, 2011, a tsunami hit the Sendai Bay Coast and broke 80% of the coastal levees. The areas around the breached levees were scoured deeply, causing serious adverse effects on post-disaster reconstruction efforts. This motivated us to analyze the mechanism of the local scour in the areas around breached levees. We collected and investigated various data such as aerial photographs and tsunami videos. As the results of the investigation, we found the breach points progressing in two steps. Firstly, the brittle structure was destroyed by the first leading wave. Secondly, the concentration of the return flow eroded and breached levees with the broken points from the first step. In order to understand the details of the second step, a numerical simulation was conducted using a finite volume method. From the result of the simulation, the scouring mechanism around breached levees was analyzed and discussed. [ABSTRACT FROM AUTHOR]