Large Wave Flume Research Articles

Large Eddy Simulation of Cross‐Shore Hydrodynamics Under Random Waves in the Inner Surf and Swash Zones

AbstractA 3D large eddy simulation coupled with a free surface tracking scheme was used to simulate cross‐shore hydrodynamics as observed in a large wave flume experiment. The primary objective was to enhance the understanding of wave‐backwash interactions and the implications for observed morphodynamics. Two simulation cases were carried out to elucidate key processes of wave‐backwash interactions across two distinct stages: berm erosion and sandbar formation, during the early portion of a modeled storm. The major difference between the two cases was the bathymetry: one featuring a berm without a sandbar (Case I), and the other, featuring a sandbar without a berm (Case II) at similar water depth. Good agreement (overall Willmott's index of agreement greater than 0.8) between simulations and measured data in free surface elevation, wave spectrum, and flow velocities validated the model skill. The findings indicated that the bottom shear stress, represented by the Shields parameter, was significant in both cases, potentially contributing substantial sediment transport. Notably, the occurrence of intense wave‐backwash interactions were more frequent in the absence of a sandbar. These intense wave‐backwash interactions resulted in a pronounced horizontal pressure gradient, quantified by high Sleath parameters, exceeding the criteria for momentary bed failure. Additionally, a more vigorous turbulence‐bed interaction, characterized by near‐bed turbulent kinetic energy, was observed in the case lacking a sandbar, potentially augmenting sediment suspension. These insights are pivotal in understanding the mechanisms underlying berm erosion and how sandbar formation serves to protect further beach erosion.

Considerations for designing a new wave generator system in an existing flume

Van Halteren Technologies Boxtel B.V. (formerly known as Bosch Rexroth B.V.) has designed, manufactured, installed, and commissioned the new electrical driven wave generator system in the existing Large Wave Flume (GWK) at FZK in Hannover. The aim of the replacement was designing a new wave generator system, with optimized performance in the given space in the existing flume, and replace the hydraulics for an electrically driven system. The width and depth of the flume are five and seven meters respectively. The water depth is variable up to five meters. In order to get the most out of the finite length of the flume the space for the wave maker is limited. Given the limited space for the wave maker we had to apply a dry back wave generator system which has the advantage of a rather compact design compared to a wet back wave generator. This also introduced challenges such as a seal system around the moving wave segment at the side walls and floor as well as a compensator for compensating the static force of the variable water depth. The hydrodynamic forces are provided by the electrically driven system. The wave generator has been provided with an Active Reflection Compensation system (ARC) for compensating the reflected waves traveling to the wave segment. This ARC must be capable to handle variable water depths, even when the water depth is changed in order to simulate high and low water conditions.

LOW-CRESTED AND EMERGENT BREAKWATERS WITH ECO-FRIENDLY ARMOUR UNITS

Artificial coastal defense systems (such as concrete armour units) frequently host less diverse aquatic populations than natural environments and feature higher concentrations of invasive species (Dafforn et al., 2009). Therefore, coastal structures need to be designed or refitted to achieve sustainable goals through the application of ecological engineering solutions applied to coastal defence structures and for the ecosystem, marine habitat, and biodiversity improvement. Ecological engineering, which integrates ecosystems with engineering principles to construct coastal structures that benefit both humans and the ecosystem, is growing as a means of reducing the adverse ecological impacts of coastal infrastructure (Mitsch & Jorgensen, 2003). Thus, creating new eco-friendly breakwater design guidelines is critical and will benefit from the multidisciplinary involvement of marine biologists and ecologists. The purpose of this experimental modelling study was to provide data on the hydraulic performance and stability of low-crested and emergent rubble mound breakwaters (RMBW) constructed using ecologically friendly armour units under various wave conditions. The University of Ottawa, the National Research Council of Canada (NRC), and ECOncrete collaborated to develop and conduct the physical testing program. Several breakwater models were tested to evaluate their performance, as the idea of an eco-friendly breakwater is still a new area of research. Thus, this experimental program is essential to promote environmentally-friendly armour units in the design of new coastal structures (such as Baker et al., 2018), as well as ecological retrofitting of existing coastal structures. The physical tests were conducted between June 2023 and August 2023 in the Large Wave Flume of NRC’s Ocean, Coastal, and River Engineering Research Center in Ottawa, Canada. ECOncrete's Coastalock armour units were tested in various configurations at a 1/15 scale using two-dimensional low-crested and emergent RMBW models. The hydraulic performance and failure mechanisms of these environmentally-friendly breakwater models were tested under severe wave conditions.

Physical Modelling Of The Wave Field Around An Array Of Centrally Controlled Wave Energy Converters

Stepped revetments have shown to be effective in limiting wave overtopping and wave run-up compared to sloped revetments. However, literature on wave impact pressures and comprehensive design guidelines for these structures is scarce. Laboratory experiments support establishing design recommendations. So far, studies for wave impacts at stepped revetments were mainly performed at small scales. Results from these tests are likely to be subjected to scale effects and therefore inaccurately replicate the wave-structure interaction at full scale. This study quantifies scale effects of wave impact characteristics for design cases based on a comparison to small-scale tests (Kerpen et al., 2018). Full-scale flume experiments were studied with a slope of 1:3 and uniform step heights of 0.17 m and 0.50 m in the Large Wave Flume (GWK) in Hannover, Germany. Horizontal and vertical wave impacts were measured at 15 locations in the plunging region of the revetment for a range of wave steepnesses (). The results show that previous small-scale tests underestimate design wave impact pressures by a factor of up to 7.7. Impact loadings occur considerably faster than at small scale with relative peak rising times decreasing by a factor of up to 5.6. Prediction formulae are derived for the vertical distribution of horizontal impact pressures (Figure 1b) as well as for temporal characteristics of these pressures at stepped revetments.

Performance Assessment Of Two Active Absorption Systems In A Large Wave Flume

The quality of most coastal or ocean laboratory experiment depends on the accuracy, uniformity and stability of the generated waves. Generation of waves in the laboratory seems simple, but it should consider the spatio-temporal variability of the wave profile. As they propagate, waves interact with the boundaries, i.e. changes in water depth, lateral walls, test specimens, instrument supporting structures, passive absorbers, etc., as well as with varying hydrodynamic conditions (wave-induced currents, wave-wave interactions, or nonlinear energy transfer, among others). Reflected, scattered and radiated waves may propagate back to the wave generator and reflect back. Re-reflections from the wave generator may be a non-realistic representation of the intended wave field, to the extent that the total energy content in the testing facility may increase significantly and uncontrollably. Theoretically, for weakly nonlinear unidirectional waves propagating over a relatively small number of wavelengths, and as long as the still water depth and facility width does not change, the wave properties should remain constant in space and time, which can be identified as the uniformity and stability of the waves. Under these conditions, nonuniform or unsteady wave properties can be associated to an inadequate selection of the wave theory (Airy, Stokes n-order, Cnoidal, Stream Function, etc., e.g. Fenton 1999), the approximation of the wave generation technique (first or second order, nonlinear - see e.g. Mohtat et al., 2020), formation of wave instabilities (e.g. Benjamin-Feir, 1967), wave reflection, scattering or radiation, and the performance of the active wave absorption system. It can be said that the performance of the active wave absorption control system is one of the responsible factors that can affect directly the quality of the generated waves. Many laboratories have implemented different passive and active wave absorption systems, but the performance on the uniformity and stability of the generated waves, as well as the performance of the active wave absorption system used, have not been assessed in a systematic way, and related published data and information is quite scarce, with few interesting exceptions, e.g. (Lykke-Andersen, 2016; Spinneken 2010; Schäffer and Skourup 1996). However, a direct comparison among the different available active wave absorption control systems is still missing. This paper presents a thorough experimental study on the performance assessment of two different active wave absorption control systems (MTS and Awasys7) as a means to establish the quality of the experiment in terms of the uniformity (space) and stability (time) of generated wave parameters (descriptors) in the laboratory.

Testing magnetic tracers as indicators of sediment transport in a wave flume experiment

AbstractThe in situ measurement of sediment transport in wave‐dominated environments presents significant challenges and currently often relies upon the use of fluorescent sediment tracers. However, this method is constrained by challenges in conducting unbiased and representative sampling, as well as facing overall logistical complexities and labour‐intensive procedures. Whilst other tracer techniques are available, such as using magnetic tracers, their performance in tracking sediment transport has not been quantified. The objective of this study is to assess the effectiveness of magnetic tracers in evaluating net transport rates and tracer dispersal patterns. Conducted in a controlled large wave flume, the experiments simultaneously employed fluorescent and magnetic tracers, allowing a comprehensive comparison of the tracers' dispersion patterns and the net transport rates. Results show that the dispersal of magnetic and fluorescent tracers displays a high degree of spatial coherence in both horizontal and vertical dimensions. Similarly, net transport rates are comparable (<16% difference), both showing net transport in the direction of the wave propagation (towards onshore) driven by non‐linear and streaming effects. Magnetic tracer recovery rate (49%) was lower than for fluorescent tracers (73%) and is attributed to the loss of magnetic ink from particles; an aspect of the magnetic technique that requires improvement. This study therefore indicates that the use of magnetic tracers to quantify sediment transport is an effective method with the advantages of being significantly less labour‐intensive than using the commonly applied fluorescent sediment tracer method.

Large-scale physical experimental study on the evolution of 3D bedform by irregular waves

A large-scale physical experiment was conducted to analyze the morphological seabed evolution in a large-wave flume at the Tianjin Research Institute for Water and Transport Engineering. The morphologies of the bedforms, ranging from two to three dimensions, were analyzed as wave mobility (represented by ѱw) increased. The sand-ripple morphology underwent four stages: straight, staggered, lunate and flat. Furthermore, the three-dimensional (3D) features and flat patterns were enhanced with increasing ѱw. The seabed ripple was processed using a wavelet transform and filter method to eliminate the effects of noise and dunes. The ripple wavelength followed the gamma and lognormal distributions for irregular waves, whereas the distribution of ripple height transitioned from normal to exponential with increasing ѱw. Furthermore, to predict the ripple-geometry statistics more accurately, a modified predictor was developed based on the acquired large-scale experimental data. The findings of this study can aid the development of innovative methods for evaluating bedform morphologies and statistical parameters.

Importance of Bed Liquefaction‐Induced Erosion During the Winter Wind Storm in the Yellow River Delta, China

AbstractData obtained during a 5‐month field observation of the Yellow River Delta show that the storm wave‐related oscillatory flows are mainly in an intermittently turbulent regime (400 < ReΔ < 1,200). Wave‐induced seabed liquefaction causes changes to the bed erodibility and increases the sediment entrainment rate, resulting in the formation of fluid mud layer (FML) during winter wind events. The strong NE winter winds lead to most of the wave‐induced liquefaction events in the study area. It was found the sediment erosion rate is underestimated when we solely focus on the wave‐induced bottom shear stress. To parameterize wave‐induced excess pore pressure buildup and sediment liquefaction, a series of experiments were conducted in a large wave flume and a new liquefaction‐erosion method is proposed to calculate the sediment erosion flux. Analysis of model results shows that the FML thickness is about 2–12 cm at the storm‐arrival stage during the entire observation period. The thickness of FML significantly increases with the increase of wave height and decrease of water depth. Once the FML is formed, strong currents can remove fluid mud 10 hr after the storm‐arrival stage, while the background currents on calm days can hardly cause suspension. The erosion rate in liquefaction zones can increase 5–10 times compared with the erosion rate in nonliquefaction conditions. The whole Bohai Sea is highly turbid after intense NE winter wind, and the liquefaction zones during winter winds would be the major sediment source. Enhanced sediment erosion can cause the subsequent degradation of the subaqueous Yellow River Delta.

Geomorphic Response of a Coastal Berm to Storm Surge and the Importance of Sheet Flow Dynamics

AbstractDuring a storm, as the beach profile is impacted by increased wave forcing and rapidly changing water levels, sand berms may help mitigate erosion of the backshore. However, the mechanics of berm morphodynamics have not been fully described. In this study, 26 trials were conducted in a large wave flume to explore the response of a near‐prototype berm to scaled storm conditions. Sensors were used to quantify hydrodynamics, sheet flow dynamics, and berm evolution. Results indicate that berm overtopping and offshore sediment transport were key processes causing berm erosion. During the morphological evolution of the beach profile, two sand bars were formed offshore that attenuated subsequent wave energy. The landward extent of that energy was confined to the seaward foreshore, inhibiting inundation of the backshore. Net offshore‐directed transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incident‐band motions on sediment transport was relatively greater in the inner‐surf zone. Near‐bed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the inner‐surf zone. This paper also provides a valuable analysis between morphology‐estimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore cross‐shore sediment processes, constituting the largest portion of the total sediment transport load throughout the berm erosion.

Degrees of Freedom Effects on a Laboratory Scale WEC Point Absorber

The number of rigid bodies and operational degrees of freedom of a wave energy converter (WEC) have significant impacts on power production magnitude and efficiency across wave periods, but this concept is not well studied. Investigating the effect of the number of bodies and degrees of freedom will help WEC developers design devices that most efficiently meet power and cost needs. This paper presents experimental and numerical results for a point absorber WEC across three modes of operation in regular waves. The three modes of operation are ‘one body heave only’, ‘two body heave only’, and ‘two body six degrees of freedom’. The Laboratory Upgrade Point Absorber (LUPA) was used as the test article. This work builds on the 2021 EWTEC paper [1] by completing fabrication and undergoing wave testing. The LUPA experiment took place in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University in Corvallis, Oregon, USA; the installation and a wave test are shown in Figure 1. LUPA is constructed as a 1:20 scale two body point absorber with a buoyancy driven float, and a spar with a heave plate to act as a reactionary body. The spar is 3.7 meters tall and the float has a 1-meter diameter. Figure 1. (Left) Mooring line installation. (Right) Wave tests in the Large Wave Flume. The ‘one body heave only’ mode locks the LUPA spar so only the float is moving, and only in heave (vertical). The ‘two body heave only’ mode allows both the float and spar to move but both bodies are restricted to heaving. The ‘six degrees of freedom’ mode has taut mooring lines connected to the spar and LUPA can translate and rotate in all directions. A damping optimization of the power take off was performed experimentally and numerically by sweeping damping values across a wide range of regular wave periods. The optimal damping value is used to compare the power generation of the three modes of operation. The LUPA project is an open-source, lab-scale wave energy converter that acts as a testing platform for students and researchers. The data from this testing along with numerical models and engineering design files will be publicly available here: https://github.com/PMEC-OSU/LUPA. This paper presents how increasing complexity of WECs through the number of bodies and degrees of freedom affects power production. It also provides a baseline case for LUPA which other researchers can compare against when applying their research to the LUPA project.

Experimental passive and reactive control of a Laboratory Scale WEC Point Absorber

Scaled testing is an important and valuable step in the process of determining best practices of WEC development, validating numerical models of WEC systems, and/or preparing for larger scale testing. Validation and verification of numerical models are very important as there are physical phenomena that are hard to numerically model such as non-linear frictional effects. This paper builds on the 2021 EWTEC paper [1] in performing and evaluating experimental testing of the Laboratory Upgrade Point Absorber (LUPA). Since this last paper, LUPA has been fabricated and deployed and an initial characterization has been performed. Particularly passive (damping), and reactive (damping and stiffness) control methodologies are employed in regular waves to characterize and evaluate the mechanical power extracted from the waves. Reactive control allows us to invest energyin the system to get a greater average energy out as compared to damping control.
 The LUPA project has just finished its first deployment in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University as shown in Figure 1. Three modes of operation were tested, namely one body heave only, two body heave only, and six degrees of freedom. The one body heave only mode restricts the motion to linear and vertical and fixes the spar body so that the float is the only body free to move. The two body heave only maintains the vertical linear motion, but unlocks the float such that the float and spar are free to heave. The six degrees of freedom case is a floating moored mode with no restrictions on motion.
 This paper will focus on comparing passive and reactive control for the one body heave only case and the six degreesof freedom case with preliminary results shown in Figure 2. Regular waves were tested, focusing on a single wave heightand sweeping the wave period. Results will be presented in power (W) units and in capture width (m). Fig. 2. Top shows max power output vs input wave period for one body and two body configurations and just damping and damping and stiffness cases.Take note of differing input wave height. Bottom shows capture width.
 The LUPA project will provide a valuable testing platform for students and researchers. It will also provide a publicly available open source design and dataset for the research community here: https://github.com/PMECOSU/LUPA. This paper will serve as documentation of the initial testing of the system, providing baseline control results to be compared in future testing.

Removable elevated-hinge wave generator for testing marine energy devices

A new removable elevated-hinge wave generator has been designed and commissioned in the O.H. Hinsdale Wave Research Laboratory (HWRL), at Oregon State University. The wave maker, built by Edinburgh Designs Ltd., is comprised by six electrically actuated dry-back paddles, self-contained in a single steel box and capable of generating mid-scale regular, irregular and user defined waves in a typical range of periods from 0.5 to 4 s at a maximum depth of 4 m. The system is intended to increase the available depth range in the Large Wave Flume (LWF) and satisfy the demand of intermediate to deep water waves at a relatively large scale by the marine energy industry.
 The existing Large Wave Flume at the HWRL, is currently equipped with a piston-type, dry-back wave generator with a 4.2 m maximum stroke hydraulic actuator assembly. The flume is 104.24 m long, 3.66 m wide, and the sidewalls are 4.57 m high. The existing wave maker can generate large regular, random and tsunami-like long waves for the purpose of large-scale model tests, particularly in the area of coastal hazards (storm waves and tsunamis). Currently, the maximum water depth for generation of regular or random waves is 2.7 m, with a maximum wave height of 1.7 m in a wave period range from 4 to 8 s. The maximum depth for tsunami-like waves (solitary waves) in the flume is 2.0 m, with a maximum wave height of 1.4 m. The LWF works as a standard coastal (shallow water) testing facility where the models, bathymetry and instrumentation are typically installed with the facility completely empty. This allows for a full range of testing depths (from practically 0 to the maximum depth corresponding to the wave type to be generated). Correct representation of the bathymetry is necessary in coastal studies since wave propagation, transformation and breaking is part of the intended research. However, intermediate to deep water conditions, required for medium scale marine energy studies, represented a challenge.
 The maximum depth at the flume for wave generation was limited by the structural design of the wave machine. However, the flume wall height is capable of handling a maximum depth in excess of 4 m. To increase the capabilities of the facility and responding to an increasing demand of deeper experimental conditions (particularly from the wave energy industry), the procurement of a specialized wave machine able to generate high-quality waves in deeper water was deemed necessary.
 The uniqueness of the system relies on its flexibility. It was conceived to be, first, removable and can be relocated anywhere along the flume, at the full range of depths (from 1 m to 4 m) and it can be reversed facing both directions along the flume. This flexibility, required by the intention to keep the existing wave machine operational, increases its functionality by making it compatible to the generation of waves and co-linear currents, as well as expanding the available testing section along the flume, with generation on one side and absorption on the opposite.

A Numerical Study of Dam‐Break Driven Swash and Beach Groundwater Interactions

AbstractThe dynamic interaction between swash and beach groundwater is fundamental to understanding wave runup and sediment transport processes. A numerical model that simultaneously resolves free surface and porous media flow for a variably saturated subsurface is presented. The novel numerical model, SedOlaFlow, is developed by merging the existing two‐phase, Eulerian sediment transport model, SedWaveFoam, with the surface wave solver, olaFlow. SedOlaFlow is validated with large wave flume surface and subsurface data for dam‐break driven swash over permeable gravel and sand beaches. Sediment size significantly impacts the swash‐groundwater relationship through infiltration/exfiltration and subsurface processes which modulate runup. Model results demonstrate that vertical infiltration into the upper unsaturated beach leads to a delayed groundwater table response to swash in the sand beach. Pressure fluctuations in the sand beach are not directly indicative of the swash depth or groundwater table and exfiltration may occur even when the swash depth is non‐zero. Groundwater circulation induced by a single swash event is non‐uniform and highly dynamic in the variably saturated vadose zone. Elevated ambient groundwater levels generally increase swash extent and duration. These findings establish the existence of a bi‐directional relationship between swash and groundwater flows and have significant implications on the effects of sea level rise on coastal flooding.

An intermediate water gravity wavelength and wave height measurement inside a large wave flume tank

This paper discusses an intermediate water gravity wavelength and wave height measurement method. Three methods were used to obtain wave height, wavelength, and period. Firstly, conventional methods used two Honeywell pressure sensors mounted at the bottom of the wave tank at two different locations. Secondly, using the transfer function of the flap wavemaker (the relationship between wave height and the wave paddle stroke). Thirdly, the Laser Sheet technique (PIV image processing technique). The significant wave height and period from pressure reading sensors of regular gravity waves were obtained from the Raleigh distribution and Zero up crossing technique. Wavelength was obtained indirectly by using a dispersion relationship that was solved by using the Newton Raphson numerical method from both the pressure sensors and the transfer function of the flap wavemaker. This experiment was focused on getting the direct value of wave measurements by developing an image processing technique in a clear large wave flume tank to replace the conventional methods and eliminate the error that may produce by using the numerical methods. The PIV setup with the CCD camera was used to capture wave images. The image processing technique based on Canny edge detection with constant threshold value was used to detect the edge of the waves. The experimental result showed a good agreement between the results obtained from these three methods, with the percent of error up to 8.683%.

Development, calibration and validation of a phase-averaged model for cross-shore sediment transport and morphodynamics on a barred beach

Simulating cross-shore sediment transport and associated sandbar migration is still a challenging task for phase-averaged coastal morphological models. Numerical studies have mostly relied on beach morphology prediction for calibration and validation, without examining in much detail the underlying hydrodynamics, sediment concentrations and transport rates. This paper reports on a new three-dimensional coastal morphodynamic model based on the hydrodynamic model of Zheng et al. (2017), combined with an advection-diffusion type suspended sediment transport model and the extended SANTOSS near-bed sediment transport formula of Van der A et al. (2013), to represent the key cross-shore transport mechanisms. The model is are calibrated based on comprehensive measurements from a large-scale laboratory experiment involving regular plunging breaking waves over an evolving sandbar, covering detailed comparisons on hydrodynamics, sediment suspension, transport rates, and bed level evolution. Separate validation using large scale wave flume experiments were also conducted to confirm the model's performance on different conditions. Good agreements are obtained between measurements and model results, which demonstrates the model's ability to reproduce cross-shore sediment transport processes under breaking waves correctly, given that the appropriate parameterizations for intra-wave processes are included. Model results also reveal the onshore near-bed transport is related to wave-induced near-bed streaming, wave skewness and asymmetry, and bed slope effects at different locations across the beach surface. Wave breaking-induced turbulence enhances the near-bed transport within the bed boundary layer which needs to be taken into account in order to achieve good model prediction skills.

Transport of heterometric sediments in wave-dominated flows – Tracer experiments

Sediment dynamics is a complex subject, playing an important role in coastal areas. The interaction between wave action and sediment particles is determinant to understand sediment transport processes. The knowledge of sediment transport comprising sand mixtures is relevant as coastal areas, usually, present heterogeneities of sediment particle sizes in the horizontal and vertical directions, denoting the existence of selective transport processes. This study investigates the effects of non-cohesive size-graded sands on sediment transport fluxes to understand selective transport mechanisms under wave-dominated conditions. A laboratory experiment was carried out at the large wave flume (Großer Wellenkanal, GWK) in Hannover, considering distinct sediment mixtures between two well-sorted sands with median grain sizes of 0.21 mm and 0.58 mm. The experiments were carried out for four sediment bed configurations with two different wave energy conditions. A fluorescent sand tracer technique was implemented to measure total and fractional net sediment transport rates and the outcomes are compared with the ones obtained with the mass conservation method. The experimental results show the occurrence of the interaction between different sand fractions: a decrease in the transport of fine sediments and an enhancement of the transport of the coarse sediments as the percentage of coarse sediments in the bed mixture increases. These effects are larger as the coarser fraction in the sand bed increases and they are noticeable for different sediment transport regimes (sheet flow and rippled beds).

Spatial Wave Measurement Based on U-net Convolutional Neural Network in Large Wave Flume

This study proposed a spatial wave measurement method based on a U-net convolutional neural network. First, frame images are extracted from a video collected by a physical model experiment, and a dataset of spatial wave measurements is created and extended using a data enhancement method. A U-net convolutional neural network is built to extract the spatial wave information of the images; evidently, the segmented water level is close to that of the original image. Next, the U-net convolutional neural network is compared with the sensor, pixel recognition, and Canny edge detection methods. Pixel recognition results reveal that the maximum and minimum errors of the U-net convolutional neural network are 3.92% and 1.05%, those of the Canny edge detection are 5.97% and 1.33%, and those of the sensor are 11.8% and 1.6%, respectively. Finally, the nonlinear characteristic quantities of waves are measured using the proposed U-net convolutional neural network. The kurtosis and asymmetry calculated in the spatial domain are slightly larger than those calculated in the time domain, whereas the skewness calculated in the spatial domain is smaller than that calculated in the time domain. The asymmetry and kurtosis increase with an increase in wave height and period, whereas the skewness increases with an increase in wave height but decreases with an increase in period.

An experimental study on oscillatory characteristics of young mangroves behind a portable reef

ABSTRACT Approximately half of the world’s mangroves are concentrated in Asia, but they have been logged at an alarming rate. To compensate for this, mangrove plantations are being attempted at various sites but with many failures. In this study, we investigated the role of a small portable reef in protecting young mangrove plants from hydrodynamic disturbances caused by short-period waves. To investigate the effectiveness of such a small reef, an experiment using a large wave flume was conducted with two types of real-sized portable reefs (stone and block reefs). A numerical wave model was also constructed to analyze in detail the turbulence around the reef. Our previous study showed that short-period waves can cause resonant oscillations in young mangrove plants. To confirm whether this occurs even behind a reef system, a young mangrove model made of flexible olefin resin was tested with a small wave flume placed behind porous and non-porous reefs, and its oscillation was precisely measured using a high-speed camera. These experiments yielded several new findings. If appropriately designed, small porous reefs can minimize oscillations with adverse effects and provide a favorable environment for the initial growth of mangroves at restoration sites.

Spatiotemporal evolution of excess pore pressures in a silty seabed under progressive waves during residual liquefaction

Both transient and residual pore pressure responses are induced as ocean waves propagate over a silty seabed. In this study, the spatiotemporal evolution of excess pore pressure in a silty seabed under progressive waves was physically modelled in a large wave flume. Three typical stages were identified in the process of residual liquefaction via flume observations, including quasi-elastic, intensive build-up of residual pore pressure, and continuous liquefaction stages. During the initial quasi-elastic stage before the liquefaction of the silt bed, the transient pore pressure can be well predicted by the analytical solution based on poro-elastic theory. After that, the residual pore pressure builds up intensively to its maximum value, even in the case that the transient pore pressure is nearly negligible at deep soil layer. Once the residual liquefaction occurs, the poro-elastic theory becomes invalid for describing the pore pressure response. The residual liquefaction is not simultaneously induced within the entire bed, but gradually progresses downward from the shallow layer of the silt bed to deeper. The pore pressure amplitude is significantly amplified after the silt liquefies, while no amplitude-amplification was observed within the un-liquefied silt. An amplification ratio (ζ) is proposed to characterize the amplification effect and distinguish the onset of residual liquefaction. The value of ζ during the continuous liquefaction stage is found to be one order of magnitude larger than that in the quasi-elastic stage. Comparisons with the existing centrifuge tests further indicate that the critical cyclic stress ratio for the silt bed is much smaller than that for the sand bed, implying the silt bed is more prone to residual liquefaction.

Collapse processes and associated loading of square light-frame timber structures due to bore-type waves

Extreme hydrodynamic events such as hurricanes or tsunamis threaten coastal regions in particular. Such hazards must be assessed and appropriately incorporated into building codes to mitigate casualties and damages to coastal structures. Guidelines are often developed through experimental investigations that assume buildings remain rigid during hydrodynamic loading. To challenge this ‘rigid building paradigm’, test specimens were designed to replicate the deformation characteristics of an idealized light-frame timber structure using Froude-Cauchy similarity. Subsequently, a large-scale experimental study was conducted at the Large Wave Flume of the Coastal Research Center in Hannover. Hydrodynamic loads and load gradients were investigated to describe both the influence of an elasto-plastically modeled test specimen compared to a rigid reference model and the effect of load history on the structural loads. Finally, the collapse sequences of elasto-plastic specimens were extracted from high-speed photographs and classified into three failure mechanisms. In this study, data analyses are presented with the intention to not only inform local authorities for future development of guidelines but also serve as calibration and validation data for improving numerical methods. • A new methodology to replicate light-frame wood structures is provided. • Realistic tsunami on land conditions are resembled in large-scale experiments. • Structural loads due to bore-like flows differ for rigid and non-rigid structures. • Collapse processes are disseminated and categorized in three failure mechanisms. • Failure mechanisms of idealized light-frame wood structures vary significantly.