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It is widely known that Infragravity (IG) waves induce cross-shore fluid motion in the nearshore, and multiple recent observational studies have identified IG waves as the dominant factor for a range of nearshore processes such as particle drift in the surf zone, transport of suspended sediment and river plume oscillations. While it is clear that the underlying orbital motion linked to IG wave excursions correlates with IG wave periods, the exact relation between the IG wave amplitude and the strength of the cross-shore motion has not been investigated in great detail. In the present contribution, we aim to quantify the cross-shore motion as a function of the IG wave amplitude. Indeed, it is shown that IG waves of even the most minute amplitude induce a large horizontal movement of particles, and the cross-shore movement is often several orders of magnitude larger than the particle movement induced by ordinary gravity waves. The results hold across a number of situations including monochromatic waves, sea states given by a spectrum as well as nonlinear waves with and without strong bathymetric forcing. [ABSTRACT FROM AUTHOR]

It is shown that very steep coastal profiles can give rise to unexpectedly large wave events at the coast. We conduct a statistical analysis of runs from a nearshore Boussinesq-type model to demonstrate that under certain wave conditions, which a casual observer would perceive as calm, the likelihood of large run-up events is uncharacteristically high. The data computed by the Boussinesq-type model show that sea states with lower overall wave steepness favor higher run-up. Under these wave conditions, more of the available wave energy reaches the shore, since less wave breaking occurs, which can create a false sense of security for beach-goers. [ABSTRACT FROM AUTHOR]

Coastal boulders are often indicators of past extreme wave events. In fact, the coastal boulder distribution induced by infragravity‐dominated storm waves (energetic IG waves) may be similar to that induced by tsunamis; however, this assumption is yet to be investigated. We show that the factors responsible for generating energetic IG waves under storm scenarios are not identical to those affecting the boulders' transport distances. Our results indicate that the storm waves typically only transport boulders over short distances as compared to boulders deposited by tsunamis, even when energetic IG waves are being generated. When the dimensionless transport distance of a boulder (=transport distance of a boulder/offshore wave height) is less than 4.0 × 10 over planar topography and 3.0 × 10 over reef topography, both waves can potentially be responsible for the transport distance. In this case, whether a reasonably‐sized storm or tsunami can explain a boulder location in a study area should be investigated through detailed numerical modeling. We found a clear relationship between the dimensionless transport distance of tsunami boulders and the Iribarren number, and it is plausible to directly estimate offshore wave height or wavelength from the tsunami boulder distribution and beach slope without numerical simulation. Plain Language Summary: Coastal boulders transported by the tsunami and storm waves have been used to reconstruct the sizes and recurrence intervals of past extreme waves. Differentiating tsunamis from storm boulders is essential for revealing the characteristics of past coastal disasters. However, as the hydraulic characteristics of the tsunami and infragravity‐dominated storm waves (energetic IG waves) are similar, the distribution characteristics of these boulders are also likely to be similar. This makes identifying tsunami and storm boulders difficult. We reveal that the conditions that can easily generate energetic IG waves are not identical to those affecting the boulders' transport distances. We also show that dimensionless transport distances (=transport distance of a boulder/offshore wave height) shorter than 4.0 x 10 over planar topography and 3.0 x 10 over reef topography are characteristic of storm boulders, even storms dominated by energetic IG waves. Thus, a longer transport distance generally points to tsunami boulders even if energetic IG waves were generated during an event. Additionally, we have demonstrated that the offshore wave height and wavelength can be derived without numerical modeling if the relationship between tsunami boulders' dimensionless transport distance and the Iribarren number was used. Key Points: Even when infragravity waves dominate, coastal boulders are transported to shorter distances compared to transport induced by tsunami wavesSeabed slope angle significantly affects the transport distance of storm bouldersThe Iribarren number can be derived from coastal boulder distribution and then used to estimate offshore tsunami conditions [ABSTRACT FROM AUTHOR]

Numerical 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. [ABSTRACT FROM AUTHOR]

Adapting to climate change and sea level rise is challenging on small islands. False adaptation can lead to adverse impacts on natural and societal dynamics. Therefore, an interdisciplinary perspective on the interaction of natural dynamics, societal demands, and political decisions is crucial. In this sense, this study scrutinizes coastal processes and socio-political dimensions of erosion on the reef island Fuvahmulah, the Maldives. The national government and Fuvahmulah's population have an opposed perception and attribution of the drivers and processes behind Fuvahmulah's most pressing coastal issue – coastal erosion. To review these perceptions, natural dynamics are recreated with process-based methods and discussed regarding present and projected marine pressures. Population surveys and interviews with actors in coastal development complement the physical insights into erosion on Fuvahmulah and describe the socio-political dimension of climate change adaptation on small islands. This interdisciplinary approach demonstrates how small-islands' adaptive capacities are typically impaired and disclose the potential of local knowledge to overcome maldevelopment. Climate change adaptation and sea level rise pose challenges for both natural and societal dynamics. Here the authors analyse coastal processes and socio-political dimensions of erosion, leading to maldevelopment on Fuvahmulah in the Maldives. [ABSTRACT FROM AUTHOR]

Tsunami 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. Key Points: Computational identification of parameters capturing the key processes affecting the distribution of tsunami and storm depositsStorm deposits are generally much smaller by their volume than tsunami deposits. This helps to identify the origin of the depositsDimensionless numbers show that distributions of sediment deposits can be reconstructed based on wave and topographic conditions [ABSTRACT FROM AUTHOR]

In November 2013, Super Typhoon Haiyan (Yolanda) hit the Philippines. It caused heavy loss of lives and extensive damages to buildings and infrastructure. When collapsed buildings are focused on, it is interesting to find that these buildings did not collapse for the same reasons after the landfall of the typhoon and storm surge. The objective of this study is to develop a statistical model for building damage due to Super Typhoon Haiyan and its storm surge. The data were collected in collaboration with Tanauan Municipality, the Philippines. The data for the inundation map were obtained by field surveys conducted on-site to determine the cause of the damages inferred from satellite data. The maximum wind speed was derived from the Holland parametric hurricane model based on the Japan Meteorological Agency (JMA) typhoon track data and the inundation depth of storm surge was calculated using the MIKE model. Multinomial logistic regression was used to develop a model to identify the significant factors influencing the damage to buildings. The result of this work is expected to be used to prepare urban plans for preventing damage from future storms. [ABSTRACT FROM AUTHOR]

The subduction zone along the northern Tonga Trench has the highest plate convergence rate in the world, but limited records of its seismic and tsunamigenic activities. In 2009, a tsunami generated by an MW 8.1 earthquake doublet caused severe impacts in the region including damage and loss of life on the south shores of Upolu and Savaii Islands, Samoa. Here, we use numerical modeling aided by recorded data and eyewitness accounts to evaluate and identify the published source models in the Tonga Trench region that most suitably represent the 2009 event for use in hazard assessment around Samoa. We show that only a few of the published sources are suitable to reproduce the large inundation observed in Samoa, and none reproduces runup as high as observed in the areas that were most severely impacted on the southeast Upolu coast. The distribution and intensity of runup and inundation are dependent on source model, local topographic and bathymetric features, regional coastal geomorphology, and trapping of short‐period waves over the reef flats. The computed results are also influenced by model approximation and grid resolution especially for areas with extreme runup records. Comparison of the relative contributions from the doublet to the southeast Upolu inundation indicates that the initial intraplate normal faulting dominated the east‐northeastward tsunami propagation and inundation compared with the subsequent interplate thrust faulting. Overall, two key source models are discussed and identified for future refinement. Plain Language Summary: This study models the September 29, 2009 tsunami in Samoa and compares model outputs with tsunami water level observations, field data of runup and inundation as well as eye‐witness accounts collected after the event occurred. Whilst several studies have reconstructed the runup and inundation of this event in American Samoa, this is the first time such work has been undertaken in Samoa which lies to the west (formerly known as Western Samoa). We discuss the effects of the complex earthquake sequence that triggered the tsunami event, as well as influence of nearshore coastal characteristics on the pattern and behavior of tsunami impacts observed onshore. Key Points: The tsunami water level produced by the 2009 earthquake doublet is simulated for Samoa and compared to observed runup on southeast UpoluGeometry of the intraplate normal faulting near the northern Tonga Trench terminus influences the extreme runup on southeast UpoluThe runup on southeast Upolu is also dependent on fine‐scale topographic features and wave excitation over the reef flats [ABSTRACT FROM AUTHOR]

Typhoons and associated storm waves in the northwestern Pacific Ocean commonly cause coastal disasters. The possibility remains that an even stronger typhoon than the strongest one observed to date might have occurred before. The development of a method to estimate a maximum intensity of past typhoons over thousands of years is important for paleoclimatology, paleoceanography and disaster prevention. Numerous storm wave boulders exist on reefs in the Ryukyu Islands, Japan, which have been deposited to their present position by the cumulative effects of the past storm waves. These boulders can be used as proxies for the hydrodynamic conditions of the largest waves from past events. Here, we present numerical computations for storm waves and boulder transport with the boulder distribution as a constraint factor to estimate the maximum intensities of storm waves and their causative typhoon events over the past 3500 years. Though the intensities of the maximum estimated waves and associated typhoon events were slightly stronger than those recorded over the past ~70 years in the Ryukyu Islands, our results suggest that no abnormally intense typhoon has struck the Ryukyu Islands in the past 3500 years. The potential impact from tsunamis remains uncertain; however, our results are meteorologically reasonable. [ABSTRACT FROM AUTHOR]

In this work, a detailed description of the internal flow field in a collapsing bore generated on a slope in a wave flume is given. It is found that in the case at hand, just prior to breaking, the shape of the free surface and the flow field below are dominated by capillary effects. While numerical approximations are able to predict the development of the free surface as it shoals on the laboratory beach, the internal flow field is poorly predicted by standard numerical models. [ABSTRACT FROM AUTHOR]

The 2015 magnitude 7.8 Gorkha earthquake and its aftershocks weakened mountain slopes in Nepal. Co- and postseismic landsliding and the formation of landslide-dammed lakes along steeply dissected valleys were widespread, among them a landslide that dammed the Kali Gandaki River. Overtopping of the landslide dam resulted in a flash flood downstream, though casualties were prevented because of timely evacuation of low-lying areas. We hindcast the flood using the BREACH physically based dam-break model for upstream hydrograph generation, and compared the resulting maximum flow rate with those resulting from various empirical formulas and a simplified hydrograph based on published observations. Subsequent modeling of downstream flood propagation was compromised by a coarse-resolution digital elevation model with several artifacts. Thus, we used a digital-elevation-model preprocessing technique that combined carving and smoothing to derive topographic data. We then applied the 1-dimensional HEC-RAS model for downstream flood routing, and compared it to the 2-dimensional Delft-FLOW model. Simulations were validated using rectified frames of a video recorded by a resident during the flood in the village of Beni, allowing estimation of maximum flow depth and speed. Results show that hydrological smoothing is necessary when using coarse topographic data (such as SRTM or ASTER), as using raw topography underestimates flow depth and speed and overestimates flood wave arrival lag time. Results also show that the 2-dimensional model produces more accurate results than the 1-dimensional model but the 1-dimensional model generates a more conservative result and can be run in a much shorter time. Therefore, a 2-dimensional model is recommended for hazard assessment and planning, whereas a 1-dimensional model would facilitate real-time warning declaration. [ABSTRACT FROM AUTHOR]

The 2011 Great East Japan Tsunami exposed many hidden weaknesses in Japan's tsunami countermeasures. Since then, many improvements have been made in both structural measures (numerical simulations, coastal defense structures, building damage assessment and control forests) and nonstructural measures (warning/observation and evacuation). This review summarizes the lessons and improvements in the five-year time period after the 2011 event. After five years, most of the lessons from the 2011 tsunami have been applied, including more realistic tsunami simulations using very fine grids, methods to strengthen coastal defense structures, building evacuations and coastal forests, improved warning content and key points to improve evacuation measures. Nevertheless, large future challenges remain, such as an advanced simulation technique and system for real-time hazard and risk prediction, implementation of coastal defense structures/multilayer countermeasures and encouraging evacuation. In addition, among papers presented at the coastal engineering conference in Japan, the proportion of tsunami-related research in Japan increased from 15% to 35% because of the 2011 tsunami, and approximately 65-70% of tsunami-related studies involve numerical simulation, coastal structures and building damage. These results show the impact of the 2011 tsunami on coastal engineering related to academic institutions and consulting industries in Japan as well as the interest in each tsunami countermeasure. [ABSTRACT FROM AUTHOR]

The coastal states and territories of the United States (US) are vulnerable to devastating tsunamis from near-field or far-field coseismic and underwater/subaerial landslide sources. Following the catastrophic 2004 Indian Ocean tsunami, the National Tsunami Hazard Mitigation Program (NTHMP) accelerated the development of public safety products for the mitigation of these hazards. In response to this initiative, US coastal states and territories speeded up the process of developing/enhancing/adopting tsunami models that can be used for developing inundation maps and evacuation plans. One of NTHMP's requirements is that all operational and inundation-based numerical (O&I) models used for such purposes be properly validated against established standards to ensure the reliability of tsunami inundation maps as well as to achieve a basic level of consistency between parallel efforts. The validation of several O&I models was considered during a workshop held in 2011 at Texas A&M University (Galveston). This validation was performed based on the existing standard (OAR-PMEL-135), which provides a list of benchmark problems (BPs) covering various tsunami processes that models must meet to be deemed acceptable. Here, we summarize key approaches followed, results, and conclusions of the workshop. Eight distinct tsunami models were validated and cross-compared by using a subset of the BPs listed in the OAR-PMEL-135 standard. Of the several BPs available, only two based on laboratory experiments are detailed here for sake of brevity; since they are considered as sufficiently comprehensive. Average relative errors associated with expected parameters values such as maximum surface amplitude/runup are estimated. The level of agreement with the reference data, reasons for discrepancies between model results, and some of the limitations are discussed. In general, dispersive models were found to perform better than nondispersive models, but differences were relatively small, in part because the BPs mostly featured long waves, such as solitary waves. The largest error found (e.g., the laboratory experiment case of a solitary wave on a simple beach) was 10 % for non-breaking wave conditions and 12 % for breaking conditions; these errors are equal or smaller than the thresholds (10 % and 20 %, respectively) defined by the OAR-PMEL-135 for predicting the surface profile; hence, all models examined here are deemed acceptable for inundation mapping purposes. [ABSTRACT FROM AUTHOR]

Accurate wave runup estimations are of great interest for coastal risk assessment and engineering design. Phase-resolving depth-integrated numerical models offer a promising alternative to commonly used empirical formulae at relatively low computational cost. Several operational models are currently freely available and have been extensively used in recent years for the computation of nearshore wave transformations and runup. However, recommendations for best practices on how to correctly utilize these models in computations of runup processes are still sparse. In this work, the Boussinesq-type model BOSZ is applied to calculate runup from irregular waves on intermediate and reflective beaches. The results are compared to an extensive laboratory data set of LiDAR measurements from wave transformation and shoreline elevation oscillations. The physical processes within the surf and swash zones such as the transfer from gravity to infragravity energy and dissipation are accurately accounted for. In addition, time series of the shoreline oscillations are well captured by the model. Comparisons of statistical values such as R 2 % show relative errors of less than 6 % . The sensitivity of the results to various model parameters is investigated to allow for recommendations of best practices for modeling runup with phase-resolving depth-integrated models. While the breaking index is not found to be a key parameter for the examined cases, the grid size and the threshold depth, at which the runup is computed, are found to have significant influence on the results. The use of a time series, which includes both amplitude and phase information, is required for an accurate modeling of swash processes, as shown by computations with different sets of random waves, displaying a high variability and decreasing the agreement between the experiment and the model results substantially. The infragravity swash S I G is found to be sensitive to the initial phase distribution, likely because it is related to the short wave envelope. [ABSTRACT FROM AUTHOR]

Wave resource assessment is the first step toward the installation of a wave energy converter (WEC). To support initiatives for wave energy development in the southwest of France, a coastal wave database is built from a 44-year hindcast simulation with the spectral wave model SWAN (Simulating WAve Nearshore) run on a high-resolution unstructured grid. The simulation includes shallow-water processes such as refraction, shoaling, and breaking. The model is validated against a five-year coastal wave buoy recording. The study shows that most of the resource is provided by sea states with wave heights ranging from 2 to 5 m, with wave periods from 10 and 15 s, and coming from a very narrow angular sector. The long hindcast duration and the refined unstructured grid used for the simulation allow assessment of the spatiotemporal distribution of wave energy across the coastal area. On the one hand, large longshore variations of the resource caused by steep bathymetric gradients such as the Capbreton submarine canyon are underlined. On the other hand, the study highlights that no specific long-term trend can be extracted regarding the coastal wave energy resource evolution. The provided downscaled local wave resource information may be used to optimize the location and design of a future WEC that could be deployed in the region. [ABSTRACT FROM AUTHOR]