Hydro-elastic interactions of layered coastal vegetation incorporating the influence of roots, trunk, and crown for tsunami force reduction: a flume experimental study

Damage to vegetation by tsunami moment and reduction of potential tsunami force are discussed based on a numerical simulation. A numerical model based on two-dimensional nonlinear long-wave equations that include drag forces and turbulence-induced shear force due to the presence of vegetation was developed to estimate tree breaking. The numerical model was then applied to a coastal forest where two dominant tropical vegetation species, Pandanus odoratissimus and Casuarina equisetifolia, were considered. The threshold water depth for tree breaking increased with increasing forest width, and the analysis was consistent with the field investigation results that the critical tsunami water depth for breaking is around 80% of the tree height for P. odoratissimus. C. equisetifolia is stronger than P. odoratissimus against tsunami action, but P. odoratissimus can reduce a greater tsunami force than C. equisetifolia due to its complex of aerial root structures. Even if breakage occurs, P. odoratissimus still has high potential to reduce the tsunami force due to its dense aerial root structures. Previous numerical models that do not include the breaking phenomena may overestimate the vegetation effect for reducing tsunami force. The combination of P. odoratissimus and C. equisetifolia is recommended as a vegetation bioshield to protect coastal areas from tsunami hazards.

Study on tsunami force mitigation of the rear house protected by the front house

Enhancing tsunami resilience for coastal bridge piers through seawall-integrated breakwaters: An experimental study

Improvement of effectiveness of existing Casuarina equisetifolia forests in mitigating tsunami damage

Some of dense costal forests in tropical areas protected several villages from tsunami hazard in 2004 Indian Ocean Tsunami. Several physical and numerical researches to simulate the reduction rate of tsunami force by the forests have been carried out. A few surveys were, however, done to evaluate the prototype effects of the forest in the field devastated in tsunamis but any clear quantitative evidence for the effects was not revealed. A field survey to measure the dense of coastal forest and to validate the simulation results for the tsunami force reduction by coastal vegetation was carried out after the 2008 Solomon Island Earthquake Tsunami. The results demonstrated the validity of the simple calculation method applying the experimentally derived drag force coefficient in dense vertical columns resembling trees.

This experimental study was conducted to idealize the efficacy of sea wall in controlling the tsunami forces on onshore structures. Different types of sea walls were placed in front of the building model. The tsunami forces and the wave heights were measured with and without the sea wall conditions. Types of sea wall, wall height, and wall positions were varied simultaneously to quantify the force reductions. Maximum of 41% forces was reduced by higher sea wall, positioned closer proximity to the model whereas this reduction was about 27% when the wall height was half of the high wall. Experimental investigations revealed that wall with adequate height and placed closer to the structures enables a satisfactory predictor of the force reduction on onshore structures. Another set of tests were performed with perforated wall placing near the building model. Less construction cost makes the provision of perforated sea wall interesting. The overall results showed that the efficacy of perforated wall is almost similar to solid wall. Hence, it can be efficiently used instead of solid wall. Moreover, overtopped water that is stuck behind the wall is readily gone back to the sea through perforations releasing additional forces on the nearby structures.

Smoothed Particle Hydrodynamics (SPH) serves as a numerical technique extensively employed for simulating free surface flow. The computational intricacy of the SPH method arises from the numerous computations of a particle's properties, derived from interactions with surrounding particles. To address this complexity, experts developed DualSPHysics. This study employs the SPH method, specifically the DualSPHysics application, for tsunami modeling. To accurately represent tsunami characteristics, precise numerical parameters are essential for numerical modeling. This research provides valuable insights into optimizing numerical parameters for accurate SPH simulations. Therefore, the research aims to identify the exact values of crucial parameters in DualSPHysics model. Validation of numerical calculations involves comparing the tsunami forces, as simulated by DualSPHysics, with secondary data obtained from physical experiments results. The findings highlight the significance of particle size (dp) as a crucial numerical parameter in DualSPHysics modeling. A smaller particle size contributes to model’s accuracy. The determination of the particle size must account for model’s shortest characteristic (s). According to simulations those have been carried out, it is recommended to set the maximum limit value of dp/s at 1/3.67 to achieve precise calculation. Furthermore, the DualSPHysics simulation demonstrates a reduction in force due to the opening configuration (n).

Coastal forests on the Sendai Plain reduced the force of the 2011 Great East Japan tsunami, but the tsunami also produced driftwood that increased the impact force or additional drag force due to accumulation in front of houses (damming). The advantage of reducing the tsunami force and disadvantage of increasing the force on houses after collisions of driftwood were repeatedly pointed out after the tsunami, but have never been compared quantitatively. Therefore, a driftwood model was developed to evaluate tree washout, motion of the drifting trees, and collisions with houses. Two-dimensional non-linear long wave equations were coupled with the equation for motion of driftwood, and the impact force was solved assuming the collision to be inelastic. Experiments were conducted to clarify the driftwood movement and determine the impact duration in the simulation. The ratio of the impact force of driftwood to the maximum drag force on a house was comparatively small, around 0.01–0.10. On the other hand, when driftwood piled up in front of a house, the increasing water depth caused additional fluid force. The rate of accumulation of driftwood was not large in experiments, but the magnitude of the drag force increase after accumulation was far larger than that of the impact force of collision. However, in comparison with the reduction of fluid force by the coastal forest, the fluid force due to the damming in front of a house was found to be smaller and not a main factor in the washout of houses.

Compared with circular, square and diamond piers, the N60 pier proposed in our previous study has been numerically proven to be effective in reducing tsunami force. The relatively stronger vortices behind the N60 pier are responsible for the not-small-enough tsunami force on the N60 pier. The asymmetry in shape makes the N60 pier fail to reduce flood force because flood propagates in the opposite direction of tsunami bore. A series of new type piers named [Formula: see text] are proposed to further improve the anti-tsunami ability of the N60 by computational fluid dynamics (CFD) method among which the N60-60 pier is proven to be most effective in reducing tsunami force, and its tsunami force mitigation mechanism is analyzed numerically. Further, the physical experiments were conducted to validate the N60 pier and the new type pier N60-60. Results show that compared with circular, square and diamond piers, the N60 pier is indeed capable of reducing tsunami force, and compared with the N60 pier, the new type N60-60 pier is capable of further reducing tsunami force. The order of magnitudes of tsunami forces on piers is: N60-60 [Formula: see text] N60 [Formula: see text] circular [Formula: see text] diamond [Formula: see text] square.

Coastal vegetation can protect people and property from erosion and flooding, potentially providing a solution for conservation and development. Recently, there has been a substantial interest in the ability of natural vegetation to protect people and infrastructure from storm, wind, and wave damage. These ecosystem services provide new and powerful reasons for conservation of coastal habitats and may represent solutions that balance conservation and development. Since the costs of installing hard structures for coastal protection are very high; strong negative public reaction to rock emplacements along the coast often aggravate the problem; research in the field of soft measures of coastal protection is important which highlights the need and importance of a sustainable, environment friendly, and cost efficient solution such as coastal or beach vegetation. This paper tries to bring out the effect of artificial emergent vegetation of meadow widths 1 and 2 m on wave run up through an experimental study. The tests were carried out with emergent vegetation placed on the bed of a 50 m long wave flume. For wave heights ranging from 0.08 to 0.16 m at an interval of 0.02 m and wave periods ranging from 1.4 to 2 s in water depths of 0.40 and 0.45 m, measurements of wave run up over the beach slope were observed.

The 2004 Indian Ocean Tsunami claimed more than 220,000 lives. It was a low-probability high-consequence event. A similar disaster could strike elsewhere, particularly in the Pacific but also in Caribbean, Atlantic, and Mediterranean regions. Unlike in seismic ground shaking, there is usually a short lead-time precedes tsunami attack: from a few minutes for a local source to several hours for a distant source. Because mega-tsunamis are rare and because forewarning of these events is possible, the primary mitigation tactic to date has been evacuation. Hence, most efforts have focused on the development of effective warning systems, inundation maps, and tsunami awareness. This strategy makes sense from the standpoint of saving human lives. However, it does not address the devastating damage to buildings and critical coastal infrastructure, such as major coastal bridges, oil and LNG storage facilities, power plants, and ports and harbors. Failure in critical infrastructure creates enormous economic setbacks and collateral damage. The accelerating construction of critical infrastructure in the coastal zone demands a better understanding of design methodology in building tsunamiresistant structures. In some coastal areas such as low-elevation coastal spits or plains, evacuating people to higher ground may be impractical because they have no time to reach safety. In these situations, the only feasible way to minimize human casualties is to evacuate people to the upper floors of tsunami-resistant buildings. Such buildings must be designed and constructed to survive strong seismic ground shaking and subsequent tsunami impacts. The primary causes of structural failure subject to tsunami attack can be categorized into three groups: 1) hydrodynamic force, 2) impact force by water-born objects, and 3) scour and foundation failure. Tsunami behaviors are quite distinct, however, from other coastal hazards such as storm waves; hence the effects cannot be inferred from common knowledge or intuition. Recent research has addressed tsunami forces acting on coastal structures to develop appropriate design guidelines, and mechanisms leading to tsunamigenerated scour and foundation failures. This special issue is a compilation of 14 papers addressing tsunami effects on buildings and infrastructure. The four main groupings begin with two papers on tsunami force acting on vertical walls. Arikawa experimentally investigates the structural performance of wooden and concrete walls using a large-scale laboratory tank in Japan. Also using a similar large-scale tsunami flume but in the US, Oshnack et al. study force reduction by small onshore seawalls in front of a vertical wall. The second grouping focuses on tsunami force on 3-D structures. Arnason et al. present a basic laboratory study on the hydrodynamics of bore impingement on a vertical column. Fujima et al. examine the two types of formulae for tsunami force evaluation: the one calculated from flow depth alone and the other based on the Euler number. Lukkunaprasit et al. demonstrate the validity of force computation recommended in a recently published design guideline (FEMA P646) by the US Federal Emergency Management Agency. The other two papers look into the specific types of structures: one is for light-frame wood buildings by van de Lindt et al, and the other is for oil storage tanks by Sakakiyama et al. The topic of debris impact force is the focus of the third grouping. Matsutomi summarizes his previous research on impact force by driftwoods, followed by the collision force of shipping containers by Yeom et al. Yim and Zhang numerically simulate tsunami impact on a vertical cylinder; this paper is included in this grouping because their numerical approach is similar to that of Yeom et al. As for the fourth grouping, Shuto presents field observations on foundation failures and scours, and Fujii et al. discuss the erosion processes of soil embankments. There are two more papers: those are the application of fragility analysis to tsunami damage assessment by Koshimura et al. and evaluation of an offshore cabled observatory by Matsumoto and Kaneda. The topics presented here are undoubtedly in progress, and many revisions and improvements are still needed in order to achieve better predictability for tsunami effects on buildings and infrastructure. We hope you find the papers in this issue intriguing and the information useful, and become further interested in this important natural hazard. Lastly, we wish to express our appreciation to the authors for their timely contributions, and to the reviewers for their diligent and time-consuming efforts.

The tsunami force acting on a land structure generated by the run-up tsunami was generally evaluated from the maximum inundation depth of the passing run-up tsunami, which is estimated on the supposition that the structure does not exist. However, the maximum depth does not always accompany the maximum tsunami force, e.g., in the case that the inundation depth gradually increase. Therefore the time series evaluation of the tsunami force not with the maximum depth is required. In this study, the physical model experiment to examine the tsunami forces acting on a wall structure in time series was conducted. It is found that the time series of the tsunami force was expressed with the inundation depth and the velocity of the passing tsunami at the same moment.

Using coastal buildings for the emergency evacuation process in an unexpected tsunami event is an effective measure in flat widespread coastal areas. This is especially true in countries such as Japan located adjacent to the faults of tectonic plates, and having only a very limited time frame for evacuation following a tsunami warning. Well-constructed tsunami evacuation buildings can play a vital role under such circumstances. This makes proper understanding of tsunami force and its variation in different building configurations vital in the engineering design of such buildings. In this study, we assessed tsunami force estimation methods for buildings consisting of openings. We also discuss the influence of their internal configurations and orientations for incoming tsunami flow based on physical model experiments and numerical simulation and analysis. Results indicated that the arrangement of internal building configurations with large openings is important in estimating tsunami force, and that building orientation related to the direction in which the tsunami approaches also affects tsunami force, mainly due to the change in effective area directly facing the tsunami.

This paper presents new experimental data and predictive equations for the reduction of the tsunami inundation force provided by finite-length seawalls. The hydraulic model experiments were conducted in a rectangular basin equipped with a large-stroke piston-type wavemaker to produce a transient pulse on the basis of an error function to best simulate the initial phases of tsunami inundation. The bathymetry had a mild slope constant in the cross-shore direction and was followed by a flat section raised above the mean water line. The tsunami force, pressure, and run-up were measured on an instrumented specimen located on the flat section, and a seawall was placed between the specimen and the shoreline. The incident wave conditions, seawall positions, and seawall heights were varied systematically to quantify the reduction of the maximum and average force relative to the baseline conditions without a seawall. Reduction factors ranged from 1.0 (no reduction) to 0.10 (90% reduction). Two empirical formulas were derived to predict the reduction factors for the maximum and average force using as input the incident (unbroken) tsunami height, the bore height at the seawall (in the absence of the wall), the seawall height, and the position of the seawall relative to the shoreline and design structure. The equations predicted the data for which they were calibrated with R2 values of 0.86 and 0.83 for the maximum and average forces, respectively. The equations were used to predict two other laboratory data sets: one conducted in the same facility under the same conditions with the specimen replaced by a column-supported structure, and a second, previous study in a two-dimensional wave flume at larger scale.

Macroscopic conditions of the damage to RC building and coastal black pine tree in the 2011 Off the Pacific Coast of Tohoku Earthquake Tsunami are discussed through field surveys and field tests. Effects of RC building's location and arrangement, submerged vertical section area in the tsunami inundation flow direction, ratio of the area of submerged windows and/or doorways to the area of submerged vertical wall on the side hit by the inundation flow (so-called, aperture ratio), and foundation piles on the damage condition of the building are examined. The damage condition of the coastal tree is also discussed from the viewpoints of the drag force and moment assessed using inundation flow velocity estimated by a simple method. Moreover, effects and limits of coastal woods on the tsunami energy and force reductions are illustrated through the field surveys.