Numerical analysis of tsunami flow around coastal dyke

Japan has a long stretch of coastal dykes along its shoreline to protect against storm surges, tsunamis and high wind waves. The 2011 Tohoku Earthquake and Tsunami caused serious damage to these coastal dykes. To improve the coastal dykes along the damaged coast and also to reconsider tsunami risks in other parts of the Japanese\ud coast, it is important to understand the effect that a tsunami can have on a coastal dyke. The present paper thus aims to analyze a tsunami flow around a coastal dyke using a LES numerical model. To simulate both an impact phase and an overtopping phase of a tsunami flow, which were observed in the 2011 tsunami, a dam break flow and a pump flow were used to generate a tsunami-like flow in the numerical model. Based on the numerical results, the pressure and the velocity field around a coastal dyke were ascertained. These effects were considered to have a big influence on the dike failure in the 2011 tsunami, and thus it is recommended that the design of countermeasures should include a calculation of both of these parameters

Suspended sand concentration models under breaking waves: Evaluation of new and existing formulations

A total of 7 reference concentration (C₀) models (6 existing and 1 newly proposed) were validated against 119 test cases from 4 recently published datasets collected under the LIP, CROSSTEX, SandT-Pro and SINBAD experimental studies. These models were evaluated for performance in different cross-shore regions: the shoaling zone, breaking (outer surf) zone and inner surf zone, under regular and irregular breaking wave conditions. In almost all existing C₀ models, substantial under-prediction was found particularly around the wave plunging point (point at which breaking wave plunges and surface generated turbulent kinetic energy, TKE, is injected into the water column) where strong localised increases in C₀ were observed. This strong increase in concentration was attributed to the large-scale breaking-generated turbulent vortices invading the wave bottom boundary layer (WBBL) and entraining dense clouds of sediment near the plunging point. Models that were directly or indirectly driven by local wave climate such as the local wave height (H), breaker height (Hb) or local water depth (d), were found to perform quite poorly in the breaking region under regular and irregular plunging breaker waves. Formulations that related C₀ to the sand pickup rate (i.e. depending on exerted bed shear exceeding critical bed shear for entrainment) were adept in regions unaffected by external breaking-induced TKE (e.g. the shoaling zone) but could not account for the high levels of concentration observed at the plunging point. This is because these formulations were based on the implicit assumption that sediment entrainment is only induced by the local TKE generated by bed shear; not taking surface-generated breaking-induced TKE into account. This assumption was addressed in more recent studies, by including breaking-induced TKE into sediment pickup rate or reference concentration formulations. Though latest studies have shown promising relationships between near-bed TKE (kb) and reference concentration/sediment pickup, such formulations also face inherent limitations. These formulations are highly dependent on the accuracy of measured or modelled kb and are also sensitive to the magnitude of k. For example, the magnitude of measured kb was found to vary by a factor of 1.1–1.3 between regular and irregular wave conditions, with kb being smaller under irregular wave conditions. This resulted in varied performance between datasets in kb-driven reference concentration formulations. The Froude-scaled TKE produced smaller deviations in magnitude of TKE between datasets, suggesting that it may be a more suitable driving parameter for reference concentration models than kb. A new reference concentration model, L19, was empirically derived from an inverse relationship observed between d and C₀, and from the roller energy dissipation rate. The newly proposed L19 model shows good agreement with measured C₀ (with RMSE ranging between 0.36 and 1.79 kg/m³ over the different datasets) in regular and irregular wave conditions, even at the plunging point where concentration is highest. The modified concentration profile [C(z)] equation also performs well, generally capturing the vertical concentration profile accurately throughout the whole water column

Corrigendum to “Suspended sand concentration models under breaking waves: Evaluation of new and existing formulations” [Marine Geology 426 (2020) 106197]

Refers to: Lim, G., Jayaratne, R. and Shibayama, T. 2020. Suspended sand concentration models under breaking waves: Evaluation of new and existing formulations. Marine Geology. 246 (Art. 106197). https://doi.org/10.1016/j.margeo.2020.10619

New Suspended Sand Concentration Model for Breaking Waves

Process-based morphodynamic modelling suites (as well as other process-based models) are often considered to be inefficient and unsuitable for simulating medium- to long-term morphodynamics due to the various theoretical (e.g. robustness of sediment transport models) and practical (e.g. computational costs) limitations. In particular, a lack of knowledge of sediment transport processes and how they relate to hydrodynamics makes the application of short-term models to long-term coastal evolution challenging. Even the state-of-the-art coastal area modelling suites (such as Delft3D and MIKE21) consist of relatively simple physics, relying instead on numerous semi-empirical parameterizations, which are often poorly supported by measured data and/or physical process understanding. In particular, suspended sediment transport in the highly turbulent surf zone is poorly modelled under breaking wave conditions. Six existing suspended sand concentration (SSC) models were critically evaluated against four high-resolution datasets with field-scale breaking waves and co-located velocity and concentration measurements over multiple cross-shore zones (shoaling, breaking and inner-surf zones). A new improved concentration model was proposed based on a novel empirical relationship observed between local water depth and reference concentration, as well as latest process understanding and insights

Building Foundation Instability Induced by Tsunami Scour

Understanding the role of tsunami-induced scour in building foundation instability can allow for the proper design of buildings located in areas prone to tsunami events. The process of tsunami scour around building foundations reduces the bearing capacity of the soil to support loading, lateral resistance and loss of soil- foundation friction (i.e. piles). Scour can cause loss of material around a foundation, due to increased pore pressure within the soil and removal of the soil during the tsunami, resulting in reduced bearing capacity of the soil (Macabuag et al., 2018). During the 2004 Indian Ocean Tsunami and the 2011 Great East Japan Earthquake and Tsunami, three similar failure modes of building foundations were experienced, namely overturning, sliding and bearing (scour) failure (Macabuag et al., 2018). According to Wright (2015), shallow foundations such as strip, slab or pad are vulnerable to erosion of surrounding soil causing scour during a tsunami. The present paper discusses the application of the scour depth predictive model of Nicholas et al. (2016) and the development of a Relative Risk Index for future design of building foundations accounting for tsunamis

Present and future tsunami and storm surge protections in Tokyo and Sagami bays

On March 11, 2011, a large earthquake that occurred offshore the northeast coast of Japan generated a large tsunami which devastated extensive areas of the Tohoku coastline and large casualties were recorded. Based on the experiences, coastal protection works in Japan are now in the process of modifications. In the present paper, Tokyo and Kanagawa are taken as examples and new methodologies are explained in the area. For the case of storm surge, a new model is applied to predict the future behavior of storm surge. For the case of tsunami, Genroku Kanto Earthquake (1703), Keicho Earthquake (1605) and Meiou Tokai Earthquake (1498) were mainly discussed in the numerical analysis, since tsunamis caused by these three earthquakes gave strong damages to coastal area of Kamakura, and left influences to Yokohama and Tokyo. New tsunami flood maps over coastal land area based on numerical simulations were presented to the residents of coastal region on April 2012 in Kanagawa prefecture. For Kamakura area, Keicho Earthqueke takes 90 minutes to reach the Kamakura coast and the height is over 12 m. But for the case of Genroku Kanto earthquake it takes 25 minutes and the height is 8 m. It appears that there are two different types of risk, 1) high wave comes but we have time for evacuation and 2) relatively small wave comes quickly and time is limited for evacuation. New countermeasures including soft and hard techniques are also required

Modelling of Krakatoa Tsunami Wave Propagation and Community Engagement Based on SWOT Analysis in Southern Lampung, Indonesia

The eruption of Krakatoa which occurred on the 22 December 2018 caused an avalanche from the Gunung Anak Krakatau (GAK) body into the sea, causing a tsunami in the Sunda Strait. The tsunami affected Lampung (Sumatra) and Banten (Java) provinces in Indonesia. Based on the field observations made by Takabatake et al. (2019) in the southern part of Lampung, it was identified that there were severely damaged areas in Lampung; i.e. East Way Muli, Central Way Muli, and Kunjir villages. A numerical model was developed to simulate past and future tsunami wave propagation scenarios. In addition, the strategic planning technique of SWOT analysis was carried out in order to make recommendations for the resilience of local coastal communities for future tsunami events in Southern Lampung

Analysis of Archery Shooting Techniques by Means of Electromyography

The interaction between the processes of excitation and inhibition plays a major role in the mechanisms of motor coordination. In the inhibitory phenomenon, from Hoffmann (1920) cited in Hoff et al. (1934) to Abraham et al. (1987), it has been shown that an electromyographic silent period is evident just before the voluntary movement following the preparatory phase, The present authors (Nishizono et al. 1984, Nishizono et al. 1987) demonstrated the inhibition prior to skilled voluntary movement. In the movement of shooting an arrow, there is a large amount of neuromuscular involvement in the «simple act». Neurophysiologically, the movement of shooting an arrow is the stable posture in a typical tonic neck reflex. To get a good record in an archery competition, one requires a well-balanced and highly reproducible release during the shooting. The stages for archery shooting, such as Bow Hold, Drawing, Full Draw, Aiming, Release and Follow-Through, are the stable sequence of movements and are suitable for studying the motor control and skill-aquiring processes of the voluntary movement. In the present study, first, the shooting techniques of world class archers were analysed compared with middle-class and beginner archers by means of EMGs, and second, archers of three shill levels were employed to measure the EMGs during archery shooting, to clarify the differences of the releasing movement