The design challenges of offshore wind are multifaceted, comprising techno-socio-enviro-economic aspects. Among the technological design challenges, seabed dynamics and seabed response to wave-induced and structural loading are of significant importance. While a number of studies are focused on the morphodynamics and scour around, for instance, monopolies, much less research has been dedicated to the liquefaction around marine structures and offshore wind installations in particular. Novel ocean structures such as floating offshore wind installations require complex anchoring systems for station keeping; thus, seabed liquefaction needs to be considered during the design phase. This study, together with the companion paper Part I, aims to provide unique insights into seabed liquefaction around a gravity-based, tension leg platform-type floating offshore wind system. To that end, experiments in the large wave-current flume, GWK+, at the Coastal Research Centre, Hannover, Germany, were performed at a very large length scale of 1:15.56. The results show, for the first time, the severity of compound wave-induced and structural loading. Small wave heights in the order of 0.56 m (model scale) trigger seabed liquefaction and lead to significant structural displacement in the order of meters (model scale). In addition, a discussion on model effects and uncertainties provides suggestions for future improvements of the experimental setup.

Helgoland is a German offshore island in the North Sea with about 50 km distance to the coast. The main island is protected by dikes and revetments. Due to climate change-induced sea level rise and anticipated increasing wave loads, the existing coastal protection needs to be revised and adapted in the northeastern part of the island. Many requirements besides the hydraulic performance and stability of the coastal protection system needed to be considered: nature conservation, land use, contaminated sites, tourism, visual landscape and existing buildings or infrastructure. Due to the number and complexity of the individual elements, there is no existing data- or process-based design approach, which considers the effects of the combined elements on the wave overtopping rate and wave forces on the seawall. The obvious lack of knowledge is hence addressed in this work, using a combined experimental approach, using medium-scale 2D and 3D experimental tests.

The Large Wave Flume (GWK) of the Coastal Research Centre, Germany, has been a well-known and extensively used experimental facility for coastal and offshore engineering research. Following an extensive research and design phase which commenced in mid-2017 incorporating funding proposals, international tender and procurement procedures, construction works commenced in September 2020 for a major extension of GWK to build the new GWK+. The Large Wave Current Flume, i.e. GWK+, stipulates unprecedented possibilities for physical modelling efforts on wave-current-structure- soil interactions. The flume went back into operation in September 2023 starting with the first project on soil liquefaction under progressive waves. The unique modelling capabilities of GWK+ are likewise attractive for unique and novel research approaches related to the fields of coastal or ecological engineering. Research on the stability of dune vegetation and structures on dikes are planned to commence in late 2024 until mid-2025. The present manuscript and talk at ICCE2024 will primarily provide insight into the new features and testing potentials of GWK+ and discuss future challenges in the realm of offshore renewable energy and coastal engineering research where physical model experiments in GWK+ are supposed to make a major contribution.

Tsunamis continue to pose an existential threat to life and infrastructure in many coastal areas around the world. One of the risks associated with tsunamis is the formation of deep scour holes around critical infrastructure and other coastal buildings, compromising their structural integrity and stability. Despite its importance, tsunami-induced scour is still given limited and simplified consideration in design guidelines for coastal structures. To further improve the understanding of tsunami-induced scour processes, and thus provide the basis for safer design of coastal structures, novel large-scale laboratory experiments have been conducted. The experiments featured a unique combination of boundary conditions, including a square coastal structure on a sloping and dry sandy beach. Single broken solitary waves were used to simulate tsunami bores. The spatiotemporal scour development directly at the square column was monitored by a high-resolution camera system, allowing a detailed description of the highly dynamic flow and scour process. Differences in the scour process between the wave runup and drawdown phases are described, and maximum and final scour depths are given as a function of inundation depth, wave height, and distance of the column from the shoreline. The scour process is characterized by several distinct phases of varying intensity and scour rate, the sequence of which varies depending on the location on the sides of the column. It is shown that the drawdown phase has a large influence on the overall scour development, adding up to 58% to the scour depth obtained during the wave runup phase. As a result of significant sediment infilling during the drawdown phase, the maximum scour depths achieved during the drawdown phase are up to twice the final scour depths at the end of a test. This discrepancy between final and maximum scour depths is greater than in previous studies using a flat sediment bed. The results of this study therefore help to interpret scour depths measured during field investigations after a tsunami event and provide a basis for extending design guidelines for coastal structures.

The understanding of the wave-structure-soil interaction around marine structures is important in order to drive design decisions towards lean, yet resilient marine infrastructure. In particular, in the light of the increasing use of the marine environment for renewable energy generation, analysis of the wave-structure-soil interaction becomes increasingly relevant. While most experimental studies on wave-structure-soil interaction are conducted at small to medium scale, this paper presents the initial results of an experimental test campaign on the wave-structure-soil interaction of a floating offshore wind turbine at large scale. The data of the period averaged pore pressure reveal a significant buildup of pore pressure beyond the onset of liquefaction, which can be attributed to the additional structural loading when comparing results to similar, isolated wave-soil interaction cases in the absence of a structure.

Wave-induced liquefaction results in significant seabed dynamics and can lead to a complete loss of the bearing capacity of the soil. With the increasing use of marine space for energy harnessing, geotechnical challenges move into focus during the development and planning of such installations. To date, a lack of comprehensive modelling tools for wave-induced liquefaction around marine structures has been observed. This paper documents the efforts in the framework of the NuLIMAS project to provide such a modelling tool. In particular, this paper presents a numerical implementation of the hydrogeotechnical processes together with a detailed overview of the calibration and validation strategy employing small- and large-scale experimental data, respectively.

Tsunamis continue to pose an existential threat to lives and infrastructure in many coastal areas around the world. Numerous studies have been conducted in recent decades to better understand the hazards and eventually mitigate risks resulting from tsunamis (Nouri et al., 2010; Palermo et al., 2013; Chock et al., 2013; Goseberg et al., 2013; Nistor et al., 2017; Stolle et al., 2018). One of these hazards is the emergence of deep scour holes around critical infrastructure and other buildings deemed community-essential, which affects their structural integrity and stability, rendering them unusable. Despite its importance, scour is still given limited and simplified consideration in foundation design guidelines related to tsunami hazards (ASCE-7 Chapter 6). This novel experimental study aims to improve the understanding of the time-variant scour process induced by single and consecutive broken solitary waves at large scale.

This paper presents the ISOPE-2022 conference comparative study on the interaction between breaking waves and a vertical wall with a recurved parapet. The experiments, on the basis of which the comparative study has been conducted, were carried out at small scale (1:8) in the Department of Ocean Engineering, IIT Madras, as well as at large scale (1:1) in the Großer Wellenkanal (GWK), Hannover. The paper discusses the qualitative and quantitative comparisons between 10 different numerical solvers from various universities across the world. The numerical solvers presented in this paper are the recent state of the art in the field; some are commercial, and some have been developed in-house by various academic institutes. The participating codes have been benchmarked for their ability to capture interactions between the incident waves and waves reflected from the seawall. The codes have also been benchmarked for their ability to replicate multiple loading cycles, in time domain, evaluated at selected pressure probe locations over the vertical wall and recurved parapet. The same pressure-time histories have also been compared in the frequency domain to evaluate the solvers’ capability to capture the multitude of harmonics characterizing the impact load. Furthermore, the values of peak impact pressure over five loading cycles have been compared to assess the overall robustness of the codes in simulating repeated impact events at model and prototype scales.

Pressure on the coastline is escalating due to the impacts of climate change, this is leading to a rise in sea-levels and intensifying storminess. Consequently, many regions of the coast are at increased risk of erosion and flooding. Therefore coastal protection schemes will increase in cost and scale. In response there is a growing use of nature-based coastal protection which aim to be sustainable, effective and adaptable. An example of a nature-based solution is a dynamic cobble berm revetment: a berm constructed from cobble and other gravel sediments at the high tide wave runup limit. These structures limit wave excursion protecting the hinterland from inundation, stabilise the upper beach and adapt to changes in water level. Recent experiments and field applications have shown the suitability of these structures for coastal protection, however many of the processes and design considerations are poorly understood. This study directly compares two prototype scale laboratory experiments which tested dynamic cobble berm revetments constructed with approximately the same geometry but differing gravel characteristics; well-sorted rounded gravel (DynaRev1) and poorly-sorted angular gravel (DynaRev2). In both cases the structures were tested using identical wave forcing including incrementally increasing water level and erosive wave conditions. The results presented in this paper demonstrate that both designs responded to changing water level and wave conditions by approaching a dynamically stable state, where individual gravel is mobilised under wave action but the geometry remains approximately constant. Further, both structures acted to reduce swash excursions compared to a pure sand beach. However, their morphological behaviour is response to wave action varied considerably. Once overtopping of the designed crest occurred, the poorly-sorted revetment developed a peaked crest which grew in elevation as the water level or wave height increased, further limited overtopping. By comparison, the well-sorted revetment was characterised by a larger volume of submerged gravel and a lower elevation flat crest which responded less well to changes in conditions. This occurred due to two processes: (1) for the poorly-sorted case, gravel sorting processes moved small to medium gravel material (D50 < 70 mm) to the crest and (2) the angular nature of the poorly-sorted gravel material promoted increased interlocking. Both of these processes led to a gravel matrix that is more resistant to wave action and gravitational effects. Both revetments experienced some sinking due to sand erosion beneath the front slope. The rate of sinking for the well-sorted case was larger and continued throughout due to the large pore spaces within the gravel matrix. For the poorly sorted revetment in DynaRev2, sand erosion ceased after approximately 28 h due to the development of a filter layer of small gravel at the sand-gravel interface reducing porosity at this location, hence a larger volume of sand was preserved beneath the structure. Both designs present a low-cost and effective solution for protecting sandy coastlines but from an engineering viewpoint it appears better to avoid well-sorted gravel material and greater gravel angularity has been seen to increase crest stability.

The effects of climate change and sea level rise, combined with overpopulation are leading to ever-increasing stress on coastal regions throughout the world. As a result, there is increased interest in sustainable and adaptable methods of coastal protection. Dynamic cobble berm revetments consist of a gravel berm installed close to the high tide shoreline on a sand beach and are designed to mimic naturally occurring composite beaches (dissipative sandy beaches with a gravel berm around the high tide shoreline). Existing approaches to predict wave runup on sand or pure gravel beaches have very poor skill for composite beaches and this restricts the ability of coastal engineers to assess flood risks at existing sites or design new protection structures. This paper presents high-resolution measurements of wave runup from five field and large-scale laboratory experiments investigating composite beaches and dynamic cobble berm revetments. These data demonstrated that as the swash zone transitions from the fronting sand beach to the gravel berm, the short-wave component of significant swash height rapidly increases and can dominate over the infragravity component. When the berm toe is submerged at high tide, it was found that wave runup is strongly controlled by the water depth at the toe of the gravel berm. This is due to the decoupling of the significant wave height at the berm toe from the offshore wave conditions due to the dissipative nature of the fronting sand beach. This insight, combined with new methods to predict wave setup and infragravity wave dissipation on composite beaches is used to develop the first composite beach/dynamic revetment-specific methodologies for predicting wave runup.

This work presents the results of an investigation on how wave overtopping at a near-vertical seawall at the back of a sandy foreshore is influenced by sequences of erosive storms. The experiments were carried out in the Large Wave Flume (GWK) at Leibniz University, Hannover (Germany). The tested layout consisted of a near-vertical 10/1 seawall and a sandy foreshore with an initial 1/15 slope. Three sequences of idealised erosive storms were simulated. Within each storm both the incident wave conditions and still water level were varied in time to represent high and low tide conditions. Each sequence started from a 1/15 configuration and the beach was not restored in between storms. The measurements included waves, beach profile, wave overtopping volumes. The profile of the beach was measured after each sea state tested. Wave overtopping at each stage of the tested storms was significantly influenced by bed changes. This was linked to the measured evolution of the beach. Measurements showed that a barred profile developed quickly at the start of each sequence, and scour developed at the toe of the structure during high water level conditions, while accretion or partial backfilling developed during low water level conditions. Due to these processes, the position of a sea state in the tested sequence is shown to be an important factor in determining the wave overtopping volume. Remarkably, when a weaker idealised storm followed a more energetic one, nearly the same level of overtopping was recorded. This is explained by the foreshore erosion, leading to increased water depths and wave heights at the toe of the structure. This finding allows to quantify and to explain the variability of wave overtopping in storms following one another at intervals shorter than the recovery time of the foreshore.

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.

The development of coastal regions combined with rising sea levels is leading to an increasing risk of coastal flooding caused by wave overtopping of natural beaches and engineered coastal structures. Previous measurements of wave overtopping have been obtained for static coastal structures using fixed current meters and depth sensors or tanks. These are unsuitable for dynamically stable coastal protection structures however, because the geometry of these structures is expected to evolve under wave action. This study investigates the potential to use elevated 2D laser scanners (Lidar) to remotely sense the flow volumes overtopping the time-varying crest of a porous dynamic cobble berm revetment. Two different analysis methods were used to estimate the wave-by-wave overtopping volumes from measurements of the time-varying free surface elevation with good agreement. The results suggest that the commonly used EurOtop parameterisation can be used to estimate overtopping discharge to an acceptable precision. An advantage of the remote sensing approach reported here is that it enables the spatial distribution of overtopping discharge and infiltration rate to be measured. It was found that the overtopping discharge on a porous dynamic revetment decays rapidly landward of the structure crest, and that this has implications for safety and structure design.

The liquefaction around marine structures can lead to severe structural failure and the susceptibility of seabed soil to liquefaction at a specific installation site of, e.g., floating offshore wind turbines should be included within the design process and site evaluation. To that end, advanced prediction tools based on numerical modelling can provide valuable insight into the hydro-geotechnical processes. However, due to the complex interaction of the underlying physics, developing a holistic modelling framework for seabed liquefaction is a challenging task. The NuLIMAS research project (Numerical modelling of seabed liquefaction around marine structures) aims at the development of such a numerical model of seabed liquefaction implemented in the OpenFOAM® framework. This paper provides an overview of the NuLIMAS project, laying out the current state of the art of experimental and numerical modelling approaches for seabed liquefaction and presenting some initial results.

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. (C) 2022 American Society of Civil Engineers.

Beach nourishments are a widely used method to mitigate erosion along sandy shorelines. In contrast to hard coastal protection structures, nourishments are considered as soft engineering, although little is known about the cumulative, long-term environmental effects of both marine sediment extraction and nourishment activities. Recent endeavours to sustain the marine ecosystem and research results on the environmental impact of sediment extraction and nourishment activities are driving the need for a comprehensive up-to-date review of beach nourishment practice, and to evaluate the physical and ecological sustainability of these activities. While existing reviews of nourishment practice have focused on the general design (motivation, techniques and methods, international overview of sites and volumes) as well as legal and financial aspects, this study reviews and compares not only nourishment practice but also the accompanying assessment and monitoring of environmental impacts in a number of developed countries around the world. For the study, we reviewed 205 openly-accessible coastal management strategies, legal texts, guidelines, EIA documents, websites, project reports, press releases and research publications about beach nourishments in several developed countries around the world (Germany, Denmark, the Netherlands, Belgium, Spain, UK, USA and Australia). Where information was not openly available, the responsible authorities were contacted directly. The study elaborates on the differences in coastal management strategies and legislation as well as the large dissimilarities in the EIA procedure (where applicable) for both marine sediment extraction and nourishment activities. The spatial disturbance of the marine environment that is considered a significant impact, a factor which determines the need for an Environmental Impact Assessment, varies substantially between the countries covered in this study. Combined with the large uncertainties of the long-term ecological and geomorphological impacts, these results underline the need to reconsider the sustainability of nourishments as “soft” coastal protection measures.

A Correction to this paper has been published: https://doi.org/10.1038/s41597-021-00874-2.

The breaking wave interactions on a vertical wall with three different recurved parapets were examined in two different scales (1:1 and 1:8) to study the scale effects. A similar testing condition with that of the large scale was adopted to identify a method for scaling up the small-scale laboratory measurements that involve impact pressure or forces. In the present study, two approaches for scaling up the small-scale studies, namely the Froude and Cuomo methods are adopted. By comparing the scaled-up results from small-scale studies with large-scale studies using the two approaches, a combined approach was suggested to estimate the force by integrating the pressure. In the proposed approach (named Cuomo-Froude method), the Cuomo method was adopted in the impact zone, and the Froude method was adopted in the nonimpact region for upscaling the small-scale experiments. The proposed approach shows a reasonable comparison with large results for pulsating and impulsive breaking conditions with a vertical wall having a different exit angle. Finally, the paper concludes that the pressure impulse follows the same scale for wave impacts as pressure.

This work presents a comprehensive numerical study of the impact of aeration and hydroelasticity on slamming loads and structural response of elastic plates during a water entry event. A numerical tool is developed with OpenFOAM and validated against experimental data from available benchmark tests. An extensive parameter investigation revealed that the structural flexibility of a plate exerts a noticeable effect on slamming loads for pure water entry cases, which almost completely disappears when the water is aerated. The effect of aeration on slamming loads is quite significant. With only 0.5 % air fraction, aeration can reduce substantially the peak slamming forces, but as the load duration increases at the same time the force impulse remains almost constant. The structural response, in terms of strain rates, reacts directly on the hydrodynamic loads for stiff plates, and exhibits resonating effects and less influence on aeration levels at higher flexibilities. This suggests that the structural performance in a slamming event must be carefully considered, and is only directly related to loads for very stiff structures. For this purpose, a new functional relation between peak impact forces/pressures and impact velocity in the presence of aeration is suggested within the present study.