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Some areas of the California coast have physical conditions—such as local bathymetry, topography, shoreline orientation, and other factors—that increase the risk for damage from tsunami inundation compared with other regions. Technical tsunami hazard analyses can inform decision-makers on regional planning and policy decisions. In this paper, we focus on recent efforts in California to delineate various tsunami hazard zones along the coastline under the Seismic Hazard Mapping Act, wherein structural mitigation measures will be recommended. To define the zone wherein structural mitigation measures are recommended, we present a state-wide tsunami reliability analysis that combines existing probabilistic hazard information and structure fragility curves derived from recent events. We examine design-life chances of significant structural damage for various structure types along all developed coastlines throughout California and use this analysis to show how hazard and risk have different correlations throughout the State. Finally, we outline an approach for creating mitigation zones, which provides state-wide tsunami risk consistency for all structures in the tsunami inundation zone. [ABSTRACT FROM AUTHOR]

Forensic engineering field surveys conducted in the aftermath of large-scale tsunamis documented the presence of deep local scour holes around structures caused by extreme inundation occurring during such events. The mechanisms leading to scour in extreme flows are still not well understood, as several physical phenomena influencing the spatiotemporal extent of scour have not been adequately investigated. The authors have conducted an experimental test program that has employed a large square column in the Large Wave Flume of the Coastal Research Center, Germany, while they also used a state-of-the-art numerical model (FLOW-3D) to numerically reproduce the experimental results. An investigation of the turbulent flow structures observed around the impacted structure showed that these flow structures are largely responsible for the sediment transport during the runup phase, but the turbulent energy was far less intense during the drawdown phase. The weakness of the turbulent structures observed during drawdown indicates that a different physical phenomenon than the one corresponding to the inflow phase is responsible for the sediment transport experienced during inundation drawdown. Due to the rapid lowering of the flow depth during the drawdown phase of tsunami inundations, a loss of excess pressure occurs because of the upward pressure gradient forming within the soil. However, the pore pressure measurements taken inside the soil in the physical experiment indicate no sign of upward pressure gradient on the inshore side of the column, which is an observation that is incongruent with previous similar studies and previous theoretical concepts. This difference was explained by a layer of soil that remained with a low water content throughout the test because the column was installed on dry sand with low permeability, a condition never tested before for pore pressure change caused by tsunami-like waves. [ABSTRACT FROM AUTHOR]

The direct simulation of inundation in developed urban areas presents a much greater challenge than the more common bare earth simulations that use roughness, which are used in many tsunami studies. This study intercompares the performance of four longwave models for tsunami inundation on a detailed topographical model of Kainan, Wakayama, Japan, with laboratory results. All simulations include buildings, which have a large impact on overland flood propagation. Inter-model comparisons yield several apparent characteristics: (1) variations between models were small in areas that are always wet; (2) wetting, drying, and overland propagation increased inter-model variation in the inundation front arrival time, maximum water surface elevation, and overland flow velocities; (3) inundated areas and maximum water surface elevations show lower inter-model variation (V) than inundation front velocity and maximum current velocities. Sources for V appeared to occur from differences in wetting, drying, and detailed code implementation rather than major differences in model physics. Using published tsunami fragility models, V led to significant differences in the predicted damage. Differences were largest for fragilities that used velocity and lower for fragilities that only used maximum inundation depths. Based on these results, inundated areas and water levels from building-resolving simulations might be assigned relatively higher confidence, and all the predicted velocities should be considered to have a greater error and potentially should be considered only when using ensembles. [ABSTRACT FROM AUTHOR]

Vegetation on foreshores in close vicinity to sea dikes may prove beneficial as regulating ecosystem service in the context of coastal defense, dike safety, and flood protection by reducing loads on these defense structures. Predominantly, a decrease in wave heights and bottom shear stresses is hypothesized, which calls for an inclusion in design procedures of coastal defense structures. In contrast to heterogeneous and variable salt marsh vegetation, this study uses surrogate vegetation models for systematic hydraulic experiments in a wave flume, without modeling specific plant species a priori. Froude-scale experiments are performed in order to investigate the effect of salt marsh vegetation on the wave transformation processes on the foreshore and wave run-up at sea dikes. The effect of plant and wave properties on wave transmission, energy dissipation, and wave run-up at a 1:6 sloped smooth dike are presented and discussed, focusing on the wave–vegetation–structure interaction. Vegetated foreshores can contribute to wave attenuation, where an increasing relative vegetation height hv/h results in decreased wave run-up on the dike by up to 16.5% at hv/h = 1.0. [ABSTRACT FROM AUTHOR]

This paper explores the influence of onshore structures' orientations and arrangements during a tsunami impact using the numerical method of smoothed particle hydrodynamics (SPH). Observations from previous tsunami events often reveal variation in the damage and survivability of impacted similar structures. Such variation can be due to shielding effects and other interactions that occur when the tsunami wave incident occurs in an urbanized location. The SPH model used in this work was first validated against previous experimental results and was then used to explore the resulting hydrodynamic behavior to a level of detail hitherto unobtainable from physical experiments. Groups of three and five structures were modeled with varying spatial separation and orientation to the incoming tsunami bore, characterized by the wake clearance angle (A) and the rotation angle (R), respectively. The results reveal significant reductions in total force on a structure can be made via strategic spatial positioning and orientation. Such reductions may mean the difference between superficial damage and wholesale structural collapse and allow the development of more resilient structures in tsunami-prone regions. [ABSTRACT FROM AUTHOR]

Tsunami waves are often very energetic and therefore pose a significant threat to coastal structures. However, most coastal defenses such as detached breakwaters are designed to protect against storm waves, with the consequence that tsunamis often lead to a catastrophic failure of these structures. This numerical study was inspired by the wave scenario, which occurred at Soma Port in Japan, during the 2011 Tohoku tsunami where a combination of tsunami-type waves and shorter period undular bores caused severe damage to a detached caisson breakwater. The objective herein is to analyze the role that undular bores play on breakwater stability in the case of a tsunami propagating over a gentle slope into shallow water. Two complementary wave models, one of Boussinesq type and the other of Reynolds-averaged Navier-Stokes (RANS) volume of fluid (VOF) type, are used to compute the propagation of a representative tsunami composed of multiple waves components and their impact on a detached breakwater. The presence of an undular bore is controlled by the balance between wave nonlinearity and dispersion. As the draw-down from the first wave reduces the water depth around the breakwater and leaves the structure almost completely exposed, only the second wave transforms into an undular bore. This causes wave breaking far offshore from the breakwater where a large amount of energy is dissipated ultimately resulting in much less-destructive wave impact at the structure. The undular bore influences the wave loading on the breakwater, though the bulk of the wave loads and the excessive bearing pressures were computed for the long-lasting overtopping process of the first wave. These findings facilitate the damage assessment of detached breakwaters as they relate some specific tsunami features to structural stability and failure mechanisms. The results also provide information regarding the applicability and usefulness of different numerical modeling approach for the analysis of the stability of detached breakwaters under tsunamis. For this particular case, even depth-integrated numerical models provide conclusive solutions. To the best of the authors' knowledge, this is the first numerical study addressing breakwater stability with respect to the impact of undular bores under tsunami conditions. [ABSTRACT FROM AUTHOR]

The September 28, 2018 earthquake and tsunami, which occurred north of Palu City, Indonesia, attracted widespread interest from the scientific community due to the unusually large tsunami that occurred after a strike-slip earthquake with a relatively small moment magnitude (MW = 7.5). To understand the structural performance of buildings and infrastructure under hydrodynamic loads and their associated effects, the authors conducted field surveys in Palu City. Light wooden frame constructions and masonry infill walls were common in the area, some of which were severely damaged by the earthquake and tsunami. Reinforced concrete structures remained predominantly intact, although they suffered soil-related issues such as scour around rigid building members. Local structural failures caused by the loss of supporting soil were also observed during the field survey, resulting in an overall reduction in the stability of the inspected structures. Based on the observations made, knowledge gaps and research needs concerning coastal and structural scouring are discussed. These are tied into the latest community research activities and put in the context of a published ASCE standard chapter that discusses tsunami design. [ABSTRACT FROM AUTHOR]

Submarine landslides have destroyed many ocean engineering facilities and induced catastrophic tsunamis, causing considerable loss of lives and properties. However, the evolution process of submarine landslides is highly complicated and difficult to describe with a unified mechanical framework. For fluidized landslides with wider influence ranges, faster migration velocities, and stronger impact forces, it is necessary not only to describe the constitutive model but also to determine the flow state. First, based on the essential rheological test for determining constitutive models, the relevant method and principle are thoroughly analyzed, and the formula for calculating the Reynolds number of fluidized submarine landslides in the entire shear process is derived. Taking the critical Reynolds number as the evaluation standard, a methodology to distinguish the flow state (laminar or turbulent) of fluidized landslides is quantitatively proposed. Second, through the low-temperature rheological test, the proposed formula and methodology are briefly applied to the natural submarine sediment samples in the South China Sea. In addition, the point of contraflexure of the rheological curve is consistent with the critical Reynolds number calculated by the formula, and the validity of this methodology is further verified. Finally, the mechanism of the flow-state transition is systematically analyzed by the internal evolution of the structure of the fluidized sediment samples in the shear process. This study provides important support for understanding the migration of fluidized submarine landslides. [ABSTRACT FROM AUTHOR]

Since the 2006 Kuril Islands tsunami, California small craft marinas have sustained over $100 million in total damage from tsunami events. Surveys conducted after the 2006 Kuril Islands and 2011 Japan tsunamis indicated that the mooring systems (e.g., cleats and pile guides) responsible for keeping the vessels and floating docks in place during an event are susceptible to failure. The aim of this paper is to present a risk framework that can be used by decision makers to assess future tsunami risks to small craft marinas. Here, the coupling of high-resolution numerical modeling and an existing statistical framework is extended to include observed damage states for structural elements. When applied to one small craft marina (in Noyo River Harbor), our methodology was able to replicate likely failure, which occurred well below previously identified damage thresholds. The results suggest infrastructure age and condition, in addition to the hazardous tsunami phenomenon, can contribute to cleat and pile guide failure. [ABSTRACT FROM AUTHOR]

The damage caused by tsunamis to coastal communities is often not only the result of the flowing water itself, but also of transported debris. Although there have been efforts to characterize forces from single-debris impacts, a more general scenario of multiple-debris impacts has not been studied experimentally. To address this need, experimental studies were performed, considering the effect of debris orientation and the number of debris field components on peak impact and damming forces on coastal structures. These forces are evaluated both qualitatively and quantitatively to provide insight into the nature of these impacts. For a few selected cases, a number of trials were conducted and an initial statistical study of the debris field collision phenomena performed. Observed forces and results are presented in terms of normalized quantities using analytical equations commonly used to calculate impact or drag forces. The lessons from this work provide an initial data set that can be used to guide further experimental and numerical studies on debris-laden tsunami flows. [ABSTRACT FROM AUTHOR]

Fluid impact forces on a bridge superstructure in the horizontal and vertical directions due to tsunami inundation, represented by a solitary wave, were investigated through a large-scale laboratory experiment and numerical simulations. The experiment was conducted in a two-dimensional large wave flume using a 1:5 scale steel girder and concrete slab bridge model, tested under shallow water waves at two water depths. Wave forces measured in the experiment were used to validate and assess the numerical predictive capability of a nonlinear hydrodynamic model. A numerical parametric study was conducted to determine the effect of variations in bridge elevation and wave height on the magnitude of the horizontal and vertical forces and overturning moment. Finally, an improved simplified design procedure based on the experimental and numerical results is proposed. [ABSTRACT FROM AUTHOR]

Data for approximately 20,000 small marine vessels damaged by the 2011 Great East Japan tsunami, including information on motor types and tonnage, were collected and used to develop loss functions. The observed maximum tsunami heights from the field survey were used, and the maximum tsunami flow velocities from numerical simulation were obtained. Damage ratios were calculated, and loss functions were fit using linear regression analysis and log-normal distributions. The damage probability was significantly increased when the tsunami height was more than 2 m or when the flow velocity was more than . The results show that small vessels (weighing less than 5 t) with outboard motors were the most vulnerable. In addition, vessels at locations farther from the tsunami source had less damage because they were hit by smaller tsunamis with slower arrival times, which most likely gave them a chance to evacuate to deep sea. The results of this study, including the loss functions, will be useful for macroscale tsunami hazard and loss predictions involving small marine vessels. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]