1,721,197 research outputs found
Elucidation of wave pressure acting on a wave-cut notch beneath a coastal cliff based on laboratory experiments and numerical modeling
No full textTo investigate past extreme waves which deposited cliff-top boulders, it is important to elucidate the wave force acting on wave-cut notches beneath coastal cliffs. Our laboratory experiments have revealed that extreme waves over coastal cliffs generated both sustained and impulsive forces, and that sustained forces can be estimated from buoyancy, because velocities are rather small when sustained forces are generated. Thus, boulders at coastal cliffs are likely moved by impulsive forces but not by sustained forces. We have also proposed coefficients of the formula which can reproduce impulsive and sustained wave forces acting on the notch based on observed and simulated wave heights and velocities. Using the proposed formula, wave heights and velocities that are necessary to move boulders up coastal cliffs can be estimated without numerical simulations. In our future investigation, we will account for the scaling effect of impulsive forces acting on coastal cliffs.Submitted/Accepted versionThis work was supported by a Fukada Grant-in-Aid FY2020 from the Fukada Geological Institute to Masashi Watanabe and JSPS KAKENHI Grant Number 21H00631, 22K14455. This work was also supported by a Sasakawa Scientific Research Grant from the Japan Science Society. This work was also supported by Association for disaster prevention research
Modelling emplacement of the world’s largest tsunami boulder
No full textLarge tsunami waves have struck the Sakishima Islands of Japan repeatedly. The latest was the 1771 Meiwa tsunami (> 25 m run-up height), which was generated by an earthquake along the Ryukyu Trench. Numerous tsunami boulders lie along the coastlines of these islands. The largest tsunami boulder of the Sakishima Islands, called “Obi-iwa”, is located on the west coast of Shimoji Island on a ca. 12 m high cliff. Historical documents imply that it was transported by the Meiwa tsunami. Nevertheless, whether this boulder could have been transported by the Meiwa tsunami has not been evaluated definitively. Furthermore, the possibility exists that it was transported by a larger and older tsunami generated along the Ryukyu Trench or by an unknown large tsunami originating from the Okinawa Trough. For this study, we conducted 3D measuring of this boulder using LiDAR equipment and used boulder transport models to evaluate whether the Meiwa tsunami could have transported it. From 3D measurements, the weight of this boulder was estimated as approximately 3400 tons: the world’s heaviest tsunami boulder reported to date. The boulder transport calculations suggest that the Meiwa tsunami could have transported this boulder, which implies that the assumption of some unknown large tsunami event is unnecessary to explain this boulder’s deposition.</p
Are inundation limit and maximum extent of sand useful for differentiating tsunamis and storms?: An example from sediment transport simulations on the Sendai Plain, Japan
No full textWe examined the quantitative difference in the distribution of tsunami and storm deposits based on numerical simulations of inundation and sediment transport due to tsunami and storm events on the Sendai Plain, Japan. The calculated distance from the shoreline inundated by the 2011 Tohoku-oki tsunami was smaller than that inundated by storm surges from hypothetical typhoon events. Previous studies have assumed that deposits observed farther inland than the possible inundation limit of storm waves and storm surge were tsunami deposits. However, confirming only the extent of inundation is insufficient to distinguish tsunami and storm deposits, because the inundation limit of storm surges may be farther inland than that of tsunamis in the case of gently sloping coastal topography such as on the Sendai Plain. In other locations, where coastal topography is steep, the maximum inland inundation extent of storm surges may be only several hundred meters, so marine-sourced deposits that are distributed several km inland can be identified as tsunami deposits by default. Over both gentle and steep slopes, another difference between tsunami and storm deposits is the total volume deposited, as flow speed over land during a tsunami is faster than during a storm surge. Therefore, the total deposit volume could also be a useful proxy to differentiate tsunami and storm deposits.Hydraulic Structures and Flood Ris
Sensitivity analysis of the physics options in the Weather Research and Forecasting model for typhoon forecasting in Japan and its impacts on storm surge simulations
No full textWeather Research and Forecasting (WRF) model is useful for forecasting typhoons as an external force of storm surge forecasts. This study examines the variation in typhoon forecasts caused by different choices of arbitrary physics options in WRF and their influence on storm surge forecasts. Eight frequently used combinations of cloud microphysics and planetary boundary layers were extracted via a review of previous studies. Subsequently, sensitivity analyses of these physics options were performed, targeting nine typhoons that landed in Japan during 2015–2019. Additionally, we conducted case studies of storm surge ensemble forecasts in Tokyo Bay and Osaka Bay using WRF-simulated typhoons generated in the sensitivity analysis. As a result, the ensemble mean of the forecasts was comparable to the storm surge reanalysis simulation results obtained using an empirical typhoon model wherein the best track data is integrated to reproduce atmospheric fields. This may be attributed to the fact that the typhoon parameters (intensity, size, approaching angle, and velocity) obtained from the best track at landfall were generally within the range of the parameters that were simulated using WRF.</p
Reconstruction of transport modes and flow parameters from coastal boulders
No full textCoastal boulders potentially provide very useful information to reconstruct hydraulic characteristics of extreme waves such as tsunamis or storm waves that struck shores in historical and prehistoric eras. Boulder transport models, which are strong tools to reconstruct the hydraulic characteristics during boulder transport, can be classified into inverse and forward models for identification and size estimation of tsunami or storm wave boulders. An inverse model can estimate the minimum wave height necessary to move a boulder and the minimum wave velocity necessary to slide, rotate, or saltate the boulder. A forward model can estimate precise hydraulic parameters such as the maximum wave velocity or wave runup height by reproducing the boulder transport distance. While some models are useful for practical purposes, few parameters are included in these models because they have been developed by simplification of actual phenomena. Therefore, it is noteworthy that hydraulic parameters estimated from the boulder transport model still include large error and uncertainty: the models need to be improved. Future work must be conducted to estimate the tsunami source or the storm size based on the tsunami or storm wave boulder distribution. Such estimation results are expected to be useful for coastal risk assessments.</p
Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island, Japan
No full textTsunami boulders deposited along the coast constitute important geological evidence for paleotsunami activity. However, boulders can also be deposited by large storm waves. Although several sedimentological and theoretical methods have been proposed to differentiate tsunami and storm wave affected boulders, no appropriate numerical method exists for their differentiation. Therefore, we developed a new numerical scheme to differentiate tsunami and storm wave boulders for coastal boulders on Ishigaki Island, Japan. In this area, tsunami and storm waves have emplaced numerous boulders on the reef and the coast. By conducting numerical calculations of storm waves in this region, we estimated the size of a storm wave that can explain the maximum clast size distribution of boulders on the reef. Consequently, we showed that a wave with a combination of 8 m in initial wave height and 10 s period can satisfy the above conditions when we assume mean sea level. In contrast to the boulders on the reef, all boulders deposited along the shore are heavier than the calculated possible maximum clast size distribution by the storm wave. Therefore, we confirmed these boulders as being of tsunami origin. Results of previous studies showed that they were most likely deposited or reworked by the 1771 Meiwa tsunami. Then, using the tsunami boulders, we numerically estimated the wave period and amplitude of the 1771 Meiwa tsunami, which should have had a 4–5 min period and 5.6–5.9, 6.3–7.0 m amplitude, respectively. Using the proposed scheme, it is possible to differentiate tsunami and storm wave boulders and estimate the size of past storm waves and tsunami waves, although it is noteworthy that there are exceptions for which the scheme cannot be applied.</p
Derivation, validation, and numerical implementation of a two-dimensional boulder transport formulation by coastal waves
No full textNumerical computations for boulder transport have become a state-of-the-art tool for hindcasting the hydraulic processes associated with past storm wave and tsunami events. Since most previously developed two-dimensional formulations cater to boulders with symmetric outlines, they can consequently reproduce the transport distance and the velocity of boulders of cubic shape or similar structured geometries reasonably well. However, the formulations exhibit limitations when applied to rectangular- and flat-shaped boulders. The presently available formulations have difficulties reproducing the variations of frictional drag force due to the changes of the boulders' contact time with the ground. We have developed an extended boulder transport formulation and derived a new empirical roughness coefficient by considering the shape of boulders that accounts for the changes of the boulders' contact time with the ground. In comparison to other existing transport formulations, the present method provides superior accuracy of block velocity and transport distance in most cases - especially for boulders of rectangular geometry. Even by neglecting the full three-dimensional processes, numerical computations extended with the proposed boulder transport formulation can help explaining historic wave regimes, which were responsible for the transport of a variety of coastal boulders reported around the world.Submitted/Accepted versionThis research was financially supported by a Grant-in-Aid for JSPS fellows (project number 16J01953)
Identification of Coastal Sand Deposits From Tsunamis and Storm Waves Based on Numerical Computations
No full textTsunami and storm deposits can be utilized for estimating inundation zones and recurrence intervals of extreme waves in modern, historic, and prehistoric times. However, the distribution of these deposits is extremely complex and affected by various factors such as the size of the waves, topography, bathymetry, and the supply of sediment and its properties. Here, we use numerical computations to identify the key factors affecting the inundation extent of tsunamis and storm waves, and which subsequently govern the distribution of the corresponding coastal sand deposits. The results demonstrate that the overall topography slope has the most significant impact on the inundation extent of tsunamis and storm waves and subsequently the inland distribution distance of the transported deposits. The existence of onshore sediment sources is crucial for estimating the maximum extent of storm deposits because only a limited amount of sediment is carried inland by the storm waves. In contrast, the presence of onshore sediment sources is less critical for the delineation of the maximum distribution envelope of tsunami deposits compared to other parameters. The parameters that mainly control the sediment deposit volume over land under tsunami and storm wave conditions are the grain size and wave height, respectively. Our computed results are summarized using the Dean number, Shields number, and Iribarren number showing an inter-connectivity between topography, input wave characteristic, and onshore distributions of both types of deposits. Despite some simplifications, this approach can efficiently lead to an identification and reconstruction of past catastrophic wave events.</p
Can Mud Deposits Indicate Inundation Extent of Paleotsunamis? Insights From Sediment‐Transport Simulations for Sand and Mud
No full textField surveys following the 2011 Tohoku-oki tsunami showed that mud tsunami deposits reached close to the tsunami inundation limit. However, the factors controlling the distribution of mud tsunami deposits remained unclear. We investigated these influencing factors by numerically simulating sand and mud transport after validating the tsunami inundation and distributions of sand and mud deposits during the 2011 Tohoku-oki tsunami based on our sensitivity analysis of parameters used in the mud and sand sediment simulations. We have revealed that when the source of mud sediments is only on the seafloor (i.e., no terrestrial source), mud is deposited along less than 10% of the inundation distance. In contrast, if a terrestrial source of mud is present, mud deposits can cover 100% of the inundation distance. We have also revealed that mud sediments are not formed when topographic slopes are steep (1/20–1/500), irrespective of a terrestrial mud source, because flow stagnation does not occur. Therefore, to reproduce past inundation ranges of tsunamis from the distribution of mud deposits, two conditions are required: (a) regions with onshore mud sediments and (b) a gentle topographic slope (around 1/1,000) to allow for long-time (more than 100 min) flow stagnation.Published versionThis research was financially supported by JSPS KAKENHI Grant 22K14455
Large tsunamis reset growth of massive corals
No full textAbstract Corals at Ishigaki Island, Japan, are characterized by their high species diversity. Not only are they struck by storm waves generated annually by typhoons, the corals, especially the massive ones, in the fringing reef were buffeted by huge tsunami waves with a run-up height of ca. 30 m in 1771 Meiwa tsunami and its predecessors at few hundred-year intervals. We present field survey and numerical results demonstrating that such near-field large tsunamis could have reset the growth of massive corals, a phenomenon which large typhoons have not caused. Our field survey revealed that the massive corals in the lagoon are not attached to the bedrock but are instead located on the sandy sea bottom. Therefore, those are movable of sufficiently large wave inundated in the lagoon. Our numerical results further showed that the maximum velocity of the tsunami at the reef edge, calculable as < 21.2 m/s at the study area, is still high in the shallow lagoon, perhaps generating sufficiently strong hydrodynamic force to devastate the massive corals in the shallow lagoon entirely, as well as some presumed damages on tabular and branching corals on the reef crest and reef slope. This numerical result is consistent with the observed fact that even a 9-m long Porites boulder (about 220 t) was cast ashore by the 1771 tsunami. The sizes of the presently living massive corals of Porites spp. are consistent with our hypothesis that they started to grow after the latest 1771 tsunami event. At the coral reefs of high tsunami-risk countries, severe destruction of corals by large tsunami waves should be considered for their growth history because, depending on the bathymetry, coral characteristics, and tsunami hydrodynamic features, tsunamis can radically alter coral habitats
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