To estimate the fragility of wooden buildings (houses) in relation to the tsunami water depth and hydrodynamic force during the 2011 Great East Japan tsunami, a numerical simulation was conducted in three different regions using a nonlinear long wave equation model that includes the breaking or washout condition of trees and houses. Fragility curves as a function of water depth, fluid force (FF) and moment by fluid force (MF) were analyzed for different level of house damages. Some differences exist especially for the fragility curve of washing out houses, but the curve where coastal forest was thin and breaching of embankment was not occurred can be utilized because of the high accuracy of the washout situation of houses in numerical simulation. The differences in place were small in case of a collapsed or major/moderate damage level for FF and MF. Although the simulation includes the washout condition defined by the critical moment index, the applicability of FF was better than MF for high Froude numbers near the coast. The efficacy of selecting the smaller curve was proposed within two water depth curves for 50% collapsed or major/moderate damage level based on Froude numbers.
The effects of a seawall, a coastal dune, and the combination of both structures on the arrival time and inundation depth of tsunami waves running inland is determined experimentally in the laboratory. Seven structural cases are examined, acting tsunami with heights of the range from 2.2cm to 5.7cm. As a baseline, an experiment is carried out without any structures placed on the flume. The measured effects are seen in the inundation depths, reflected and transmitted tsunami heights. The results show that the combination of the seawall and coastal dune proves to be more effective and can be considered as a counter-measure against tsunami waves.
The growth rate of Corbicula japonica at the mouth of the Natori River, Miyagi Prefecture, was estimated in the field by a caging method. There were significant differences in growth rate among stations and the results suggest that lower growth rates are caused by reduced feeding in conditions of high salinity. The growth rate in shell length of C. japonica observed in summer (214±42.4μm/day) was higher than that of other areas in previous studies: within a month, shell length increased from a mean of 9 mm to a mean of 15 mm (minimum harvesting size).
Since wave force becomes small when a coastal dike or a seawall is in the area shallower enough than the breaking water depth of large waves, the concrete covering of the dike or the seawall is not destroyed simply. However, if the scour in the front progresses owing to small waves acting repeatedly and the lowest edge of the front slope comes to be visible, since incident waves penetrate the body of the dike or the seawall from the lowest edge and the return flow by incident waves come to suck back-filling materials out of the body, the cavity in the body is enlarged by the suction, and the concrete covering come to be broken easily by weak waves. In this research, the formula of Ioroi et al. to estimate suction rates for the dike and the seawall covered the front slope and the crown part with concrete is improved for handling simplification and expansion of the applicable extent. Next, diagrams to estimate easily front scour depths required for calculating suction rates are also improved. Furthermore, one method for estimating suction rates in the case that front water depths at the time of storm surges are unknown values is proposed. Then, the destructive mechanism of the dike and the seawall, and countermeasures to the suction of backfilling materials from those are also considered.
In the Great East Japan Earthquake, many coastal levees collapsed due to tsunami overflows. In there, concrete-covered levees showed a specific failure mode where back slope blocks and ground were washed out. Although several hydraulic experiments were conducted in order to understand the failure mechanism of concrete-covered levees, it has not yet been clarified especially from the geotechnical viewpoint. The present study examined the failure behaviour of levees due to overflow considering the ground behaviour by the ground model tests. Usage of the centrifuge technique was the key point of this study to reproduce the prototype ground stress and water pressure. The test results showed that the levees collapsed owing to water-flow over the back slope block combined with groundwater seepage.
Destruction mechanisms of coastal structures due to the 2011 Tohoku Tsunami were investigated on the basis of field surveys in Fukushima Prefecture. Severe destruction appeared to be developed by the action of breaking bores. Laboratory experiments demonstrated that the angle of the tsunami front was an essential parameter for the generation of breaking bores. Larger wave force was observed as the angle of the tsunami front became steeper. Numerical simulation revealed that such a steep tsunami was developed in the central part of Fukushima Prefecture, where the reflection of the preceding tsunami by coastal cliff enhanced the steepness of the largest tsunami.
Field observations and indoor experiments were performed for quantitative elucidation of the effect of suspended particulate matter on seawater salinity based on electrical resistivity. Seawater salinity was found to decline with increasing suspended particulate matter concentration and with increasing sensor penetration into seafloor sediment. The results of the indoor experiments show that nominal salinity decreases linearly with rising suspended particulate matter concentration and that the rate of decrease increases with increasing salinity of filtered seawater. The results of experiments using montmorillonite and kaolinite indicate that this is caused by a decrease in seawater volume in the salinity sensor and by clay particle adsorption. The characteristic decrease of salinity by suspended particulate matter found in the indoor experiments and the vertical distribution of suspended particulate matter concentration near the seafloor estimated from salinity measurements using a multiparameter water quality sensor clearly show that a layer approximately 2 cm thick containing fluid mud at approximately 15,000 mg/L forms on the seafloor.
This study investigated the capability of reproduction for soliton fission propagating over shallow water reef by the extended OpenFOAM. For the three cases with different wave amplitudes and periods, the numerical results by OpenFOAM have achieved very good agreement with experimental measurements. These results have shown that the present OpenFOAM model used in the study has strong ability to simulate tsunami waves and their propagation over shallow water reef. The comparisons with COULWAVE and FUNWAVE simulations have clearly shown that OpenFOAM has achieved much better agreement with experiments. especially for the generation and development of dispersive soliton waves and the reflected short waves.