Sriram Venkatachalam

The present study aims at generating the fully nonlinear waves based on Finite Element method (FEM) used by SRIRAM et al. (2006). The author simulated the nonlinear waves based on structured mesh by regenerating the mesh at each and every time step using the Mixed Eulerian and Lagrangian (MEL) scheme. In this paper, it has been extended to unstructured mesh. The mesh is adapted at each and every time step by using the spring analogy method instead of regenerating at every time step which makes the above method called as Semi- Arbitary Lagrangian and Eulerian (Semi-ALE/SALE). The simulation has been carried out in a numerical wave tank (NWT) with a surface piercing rectangular object. For such a situation, the diffraction by a surface piercing object becomes relevant in connection to breakwater studies where the primary interest is wave reflection and wave transmission. Regular waves and solitary waves are generated from one end of the tank. The nonlinear wave reflection and transmission characteristics reveals that the transmission is less for regular waves, while in the case of solitary waves the reflected energy is very small and the transmission is more.

The following sections are included: Introduction Tsunami Warning Detection of Tsunamis with Sea-Level Sensors Indian Tsunami Early Warning System (ITEWS) Experimental Studies Summary References.

Destruction of the structures in coastal areas due to an extreme coastal event like a tsunami necessitates the deeper understanding of flow behaviour to improve the design guidelines. The characteristics of inland propagating tsunami surge or bore consist of an initial aerated surge or bore tip followed by the gradual increase in water depth (quasi-steady flow phase). During the bore interaction, the structure initially experiences an impulsive pressure, bore pressure followed by quasi-static pressure (quasi-steady flow phase) depending on the ratio of the structure obstruction width to flow channel width (b/W), incoming bore Froude number (Fr), the shape of the structure, and the orientation of structure to the direction of flow. From field surveys and video observations during the 2004 Indian Ocean Tsunami and the 2011 Tohoku tsunami, the structure failure during the quasi-steady state of tsunami flow is found to be predominant. Also, most of the literature in the past focused on the interaction of bore on a single structure in which the flow channelling effect (b/W) is not considered. Thus, the present experimental study gives a detailed insight into the b/W and Fr effect in the force acting on the rectangular structure during the quasi-steady flow of tsunami-like events. To estimate the hydrodynamic force in the quasi-steady flow phase, we have adopted the hydrostatic force equation by incorporating bore height at the structure front (hf) and bore height at the structure back (hr). A simplified semi-analytical method is proposed based on conservation of mass and momentum to estimate the force on the rectangular structures. This approach of force estimation is showed to reasonably predict experimental force-time history. Since design guidelines use the hydrodynamic drag force equation for force estimation, the present study also provides the drag force coefficient (Cd) for b/W > 0.2 and Fr between 0.6 and 2. Along with the consideration of Fr and b/W, the study attempted to provide a closed-form set of equations to the quasi-static force, which helps designers with a convenient force determination method.

Tsunamis pose a substantial threat to coastal communities around the globe. To counter their effects, several hard and soft mitigation measures are applied, the choice of which essentially depends on regional expectations, historical experiences and economic capabilities. These countermeasures encompass hard measures to physically prevent tsunami impacts such as different types of seawalls or offshore breakwaters, as well as soft measures such as long-term tsunami hazard assessment, tsunami education, evacuation plans, early-warning systems or coastal afforestation. Whist hard countermeasures generally aim at reducing the inundation level and distance, soft countermeasures focus mainly on enhanced resilience and decreased vulnerability or nature-based wave impact mitigation. In this paper, the efficacy of hard countermeasures is evaluated through a comprehensive literature review. The recent large-scale tsunami events facilitate the assessment of performance characteristics of countermeasures and related damaging processes by in-situ observations. An overview and comparison of such damages and dependencies are given and new approaches for mitigating tsunami impacts are presented.

During the tsunami landfall, the structures near the coast perform like a wall of certain obstruction, b/W (b = structure width, W = available flow channel width) to the incoming flow due to abutting adjacent structures. In the previous paper (Harish et al., 2021), the generalized semi-analytical equation for the horizontal forces using the free flow parameter during the quasi-steady flow is provided. The emphasis was on understanding flow characteristics at the structure front. However, the quantitative dependence of the hydrodynamic drag force coefficient (Cd) on the influence of b/W and Froude number (Fr) are not investigated in detail. In the present paper, from the experimental analysis, in addition to show Cd as a function of b/W and Fr, the influence of Fr in Cd variation for various b/W is brought out. Based on this, a generalized empirical equation for Cd is proposed. The experimental analysis on the vertical force shows that the hydrodynamic uplift force (Fu) can be explicitly estimated based on the average water depth at the structure front and backside from the buoyancy force equation (Fub). Further, Fu/Fub is found to be independent of Fr and b/W, unlike the horizontal force. From the practical design perspective, these two explicit expressions are essential, and the physical process involved in obtaining this expression has been discussed in this paper.

The estimation of forces and responses due to the nonlinearities in ocean waves is vital in the design of offshore structures, as these forces and responses would result in the extreme loads. Simulation of such events in a laboratory is quite laborious. Even for the preparation of the driving signals for the wave boards, one needs to resort to numerical models. In order to achieve this task, the two-dimensional time domain nonlinear problem has received considerable attention in recent years, in which a mixed Eulerian and Lagrangian method (MEL) is being used. Most of the conventional methods need the free surface to be smoothed or regridded at a particular/every time step of the simulation due to Lagrangian characteristics of motion even for a short time. This would cause numerical diffusion of energy in the system after a long time. In order to minimize this effect, the present study aims at fitting the free surface using a cubic spline approximation with a finite element approach for discretizing the domain. By doing so, the requirement of smoothing/regridding becomes a minimum. The efficiency of the present simulation procedure is shown for the standing wave problem. The application of this method to the problem of sloshing and wave interaction with a submerged obstacle has been carried out. © 2006 Elsevier Ltd. All rights reserved.

In this article, tsunamis represented as solitary waves was simulated using the fully nonlinear free surface waves based on Finite Element method developed by Sriram et al. (2006). The split up of solitary wave while it propagates over the uneven bottom topography is successfully established. Wave transmission and reflection over a vertical step introduced in the bottom topography is in good agreement with the experimental results from Seabra-Santos et al. (1987). The wave transformation over a continental shelf with different smooth slopes reveals that the solitary wave reflection increases while the continental slope varies from flat to steep. The interaction of the solitary wave with a vertical wall for different wave steepness has been analysed. The reflected shape of the profile is in good agreement with the observation made by Fenton and Rienecker (1982) and an increase in wave celerity is observed.

The following sections are included: Introduction Outline of Damage Caused by Debris Different Types of Debris Boulders as Debris Research on Motion and Impact of Debris Guidelines for Debris Impact Loading Debris Modelling Physical Model Tests Summary References.

Failure of structures during extreme coastal flood events like a tsunami is common due to inadequate structural design, resulting in substantial material loss. Nevertheless, improvements in the design could reduce the global structural failure. Although many studies developed force equations for non-overtopping structures, many critical and non-critical structures still exist along coastlines with low heights. In this study, we investigate the effect of flow overtopping in the case of low-rise structures, typically representing a residential building, using our in-house CFD solver IITM-RANS3D. The numerical performance of IITM-RANS3D for violent impact and over-topping scenarios is first benchmarked against the experiments of Kleefsman et al. [1], wherein a comparison with OpenFOAM is also presented. After validation, structures of different heights are tested with the dam-break bore to replicate the tsunami bore. The flow and force characteristics are compared with the non-overtopping condition to understand the flow overtopping effect. The results presented herein indicate that the horizontal force on the structure and the bore height at the structure front decreases with a decrease in structure height.

To understand tsunami flow interaction with the structure occupying a certain percentage of flow channel width in the field, several authors modelled experiments or numerical simulations with the single structure occupying the same obstruction ratio, denoted by b/W (b = single structure width; W = flow channel width, considered as flume width). This setup resulted from laboratory width constraints or computational time reduction in numerical modelling, leading to the inherent assumption of symmetrical flow conditions at the flume walls. In this study, we compare the flow and force characteristics of the b/W setup with the results from the unsteady flow-structure interaction experiments conducted along with the adjacent structures in the flow choked condition, thereby replicating a realistic flow behaviour (termed as b/W* setup, here W* represents the actual flow width replicating the real field). The results from the experiments indicate that the change in the bore height at the structure front (h*f) is marginal or almost negligible between the two setups, independent of Fr. However, the bore height at the structure backside (h*r) is significantly altered. The existence of the hydrostatic pressure at the structure front and backside still ensured F*≈ρbg(hf*2−hr*2)/2 in flow-choked condition for the b/W* setup. Further, the 'h*f' prediction using the conservation of mass and momentum equations is improved. In regards to the vertical uplift force (F*u), the ratio of F*u/F*ub (F*ub = buoyancy force calculated using the average of h*f and h*r in the b/W* setup) is found to be approximately 1.15 times higher than Fu/Fub in the b/W setup (Harish et al., 2022a).