The rapid growth of offshore wind energy, motivated by the demand for sustainable energy solutions and the aim of achieving greenhouse gas neutrality, has led to increased attention to the impact of marine biofouling on substructures such as monopiles and jacket structures. Although the effects of hard biofouling have been studied, soft biofouling remains underexplored. This study investigates flow dynamics and vorticity patterns around eight cylindrical structures subjected to wave loading, with hard and soft biofouling surrogates. Soft biofouling is further divided into stiff and flexible models. Physical experiments are conducted with slender piles (D/L = 0.07) in a mid-scale wave flume, covering Reynolds numbers of Re = 2 104-2 105 and Keulegan-Carpenter numbers of KC = 2-50. Volumetric flow velocities are measured using Particle-Tracking Velocimetry with the Shake-the-Box method. Results show that biofouling alters flow patterns, creating recirculation zones with reverse flow velocities. Vorticity analysis reveals vortex formation in the wake, expanding with wave period and roughness. For the same fibre lengths, flexible biofouling models allow high levels of vorticity to spread further downstream (up to 133%), while stiff models create distinctive recirculation zones with a 18% larger recirculation length. These findings improve understanding of wave-induced wake development for rough surfaces.

Coastal dunes serve as vital natural defenses against storms, with vegetation playing a key role in sediment stabilization and erosion mitigation. This study examines the effects of planting density, planting strategy, and biomass distribution on dune erosion resistance, using Ammophila arenaria in 1:7 scale flume experiments exposed to wave collision regimes. Tests with whole plants (uncut) and belowground-only biomass (cut) at varying planting densities resulted in erosion volume reductions of up to 31.2% compared to bare dunes. Intermediate densities with well-developed root systems and buried shoots showed the most consistent reductions, while variability indicated the importance of root development and plant health. Belowground biomass alone provided nearly equivalent resistance compared to whole plants. Vegetation also influenced failure mechanisms, promoting notching and slumping with block detachment and deposition at the dune toe. Time-resolved 3D surface data from laser scanning revealed dynamic erosion patterns, while Structure-from-Motion photogrammetry supported detailed end-state analyses.

Coastal zones need further investigations of beach dune erosion to improve storm-surge protection measures in light of climate change. This study presents a hybrid approach investigating the influence of coastal foredunes on storm-induced secondary dune erosion. A data set from 1:7 scaled laboratory flume study was used to calibrate and validate a 1D-XBeach model. Morphological measurements at various time steps, which included two consecutive storm surges, captured the effects of varying hydrodynamic conditions and foredune configurations, thereby facilitating detailed model application and the identification of uncertainties. Overall, model performance improved over time, demonstrating good to excellent agreement after the foredune evolved into a quasi-equilibrium profile, while the secondary dune remained in the collision regime (BSS between 0.72 and 0.85 after first storm surge). A temporal delay in foredune transition was observed, which was most evident in case of the high foredune and likely due to the neglect of individual short waves and associated runup. Based on the identified application limitations, the model was employed for a variant study considering additional hydrodynamic conditions and foredune configurations. An increase in water level, significant wave height, and peak wave period each affects secondary dune erosion roughly linearly. The foredune volume was found to be the most significant factor determining the reduction of the secondary dune erosion volume up to an almost complete reduction for non-flooded foredunes. Furthermore, it was observed that higher significant wave heights lead to increased offshore sediment transport at the foredune, while higher peak wave periods enhance onshore-directed sediment transport.

Marine growth on cylindrical structures alters their hydraulic roughness and thus affects load predictions, such that parameters beyond the traditional k/D ratio must also be taken into account when analyzing the loads and hydrodynamics around these structures. However, for cylinders overgrown by marine biofouling, usually composed of hard and soft biofouling communities, little information exists on the relevant additional geometric roughness parameters. Thus, a method is proposed to retrieve flat-surface geometric roughness parameters from an overgrown cylinder using structure-from-motion (SfM) photogrammetry. Eleven cylinders, exposed to the marine environment at two locations in the Jade Bight, Germany, for up to 29 months are characterized by root-mean-square height, skewness, kurtosis, mean of maximum peak-to-valley height, surface coverage ratio, probability density, autocorrelation, and power spectral density; these data exhibit a comprehensive and novel characterization of the geometric surface roughness of cylinders overgrown by marine biofouling to date. Cylinders located further offshore were overgrown by a higher percentage of hard biofouling and a larger roughness height than the cylinders closer to the shoreline. The cylinders are classified to be overgrown by medium to very large roughness, with ktm/Di ranging from 0.085 to 0.613. The probability density function and the power spectral density of the measurements can be combined for artificial surface generation to conclude upon hydraulic roughness in future research.

Besides the natural protection of coastal dunes against storm surges (Mehrtens et al., 2023), blue-grey-hybrid dune structures play a key role in strengthening the protective mechanism of dunes against erosion, while imitating their natural landform appearance and function (Nordstrom 2019). Although artificial dunes with hybrid core enhancements are widespreadly implemented along many coasts worldwide (Winters et al., 2020), limited work addressed the effectiveness and potential enhancement of specific dune reinforcement strategies in a comparative fashion. An exception to this statement is the work by Figlus et al (2015), who conducted a series of experiments with three incorporated dune cores: a T-wall, a clay levee and an armour stone revetment. Nonetheless, a benchmark study on the efficacy and comparative performance of hybrid dune reinforcement strategies, with an identical parameter frame remains unavailable to date.

In the context of nature-based solutions, there is a growing interest in considering the benefits of ecosystem services towards sustainable coastal protection measures (Sutton- Grier et al. 2015). Coastal dunes serve as a natural barrier at sandy coastlines. While higher elevated dunes protect the adjacent hinterland from flooding during severe storm surges (e.g. Figlus 2022), coastal foredunes can act as a first buffer zone during storm surges by enhancing near- shore wave breaking-induced energy dissipation as a result of the deposition of eroded sand in the foreshore bathymetry (Temmerman et al. 2013). The Eiderstedt peninsula in Northern Germany harbors a complex beach- dune system that, among others, consists of an up to 2 km wide beach, an elongated foredune of varying height and an established secondary dune (Mehrtens et al. 2023). Previous large-scale experiments focused on the influence of a foredune on the erosion volumes of a secondary dune during varying storm conditions following the regimes as described by Sallenger (2000). Results showed a reduction of wave-induced erosion at the secondary dune independently of both the hydrodynamic conditions and the foredune configuration with respect to volume and location. However, due to the natural time limitation in laboratory studies, additional data is beneficial in order to evaluate the protection potential of coastal foredunes more general.

With the ever-increasing demand for renewable energy sources, the significance of both cost-efficiency and lifetime assessment of offshore wind energy plants has been growing substantially (Zhang et al., 2013; Ziegler and Muskulus, 2016). This necessitates the development of more precise design guidelines (Haselibozchaloee et al., 2022). The support structure of an offshore wind turbine marks a key part by accommodating aero- and hydrodynamic loads. During structural design, the evaluation of hydrodynamic loads relies mostly on simplified analytical methodologies such as the MOJS equation (Morison et al., 1950). The involved drag and inertia coefficients are determined based on considerations such as flow regime and roughness. These determinations often stem from rudimentary experimental investigations in small- to mid-scale test facilities prone to scale effects (Jusoh and Wolfram, 1996; Tian et al., 2020).

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.

Coastal dunes area critical natural defense against storm surges and sea level rise, yet their stability is increasingly compromised by intensified hydrodynamic forces. To withstand stronger and more frequent storm surges as a result of climate change, engineered natural coastal barriers play an important role. This study systematically investigates the potential of artificial root system surrogates based on the root structure of Ammophila arenaria to augment dune stability under simulated storm surge conditions. Laboratory experiments were conducted in a 1.0 m wide and 90.0 m long wave flume, replicating the geomorphological characteristics of a dune profile from Sankt Peter-Ording, Germany, at a scale of 1:7. Three surrogate materials (i) coir grid, (ii) basalt grid, and (iii) coir mat were evaluated across three distinct placement configurations (Crest-only, Crest-Slope and Crest-Slope-Foot) under hydrodynamic regimes corresponding to collision, minor overwash, and heavy overwash. High-resolution 3D-lidar scanning provided quantitative, continuous assessments of erosion volumes and dune profile changes. The experimental results indicate that the flexibility of the materials, particularly coir grid and coir mat, substantially mitigates erosion through attenuation of incoming waves and sediment retention, while the relatively stiffer basalt grid exhibits inferior performance. Comparative analyses of small-scale experiments demonstrate that strategically designed artificial root systems can reduce erosion by 13.3 % to 47.6 %, thereby matching or surpassing the 23 % to 40 % reductions documented for natural vegetation. These findings provide critical insights for advancing nature-based coastal defense strategies and highlight the necessity for further large-scale investigations to refine material properties and deployment configurations.

Floating structures play a crucial role in transitioning from fossil fuels to renewable energies, either in floating offshore wind, floating solar, or wave energy applications. However, an accurate and efficient determination of the floating structures’ motion in waves remains a challenging task, particularly in mid-fidelity applications or applications with the aim of real-time capabilities: In particular, evaluating viscous effects and their importance for hydrodynamic damping is crucial. The wave radiation damping is often deemed dominant for vertically oscillating structures, but viscous effects cannot be neglected. The underlying physical processes of such viscous effects are not fully understood due to a lack of high-quality three-dimensional experimental flow field data. The present study investigates structures under forced vertical oscillation and aims at enhancing the physical understanding of the relation between structural shapes and features, the hydrodynamic parameters and the flow field using 3D time-resolved flow field measurements from a particle tracking velocimetry system. These investigations lead to a benchmark dataset for the development and validation of sophisticated numerical models which is published alongside this study. Three different generic box-type shapes are investigated: (i) a sharp-edged box, (ii) a round-edged box, and (iii) a box with a heave plate at the bottom. The experimental results show strong shape effects on the hydrodynamic coefficients, wave radiation, and the flow field. The radiated wave height can differ up to 50% between the three structures. Viscous effects from vortex structures and separation lead to a KC-related increase of damping values for structures with sharp edges or heave plates. Significant shape effects on the vorticity are reported with a factor of up to 500% between the structures. Additionally to structural shape and features, the vorticity depends on KC and oscillation frequency, while the vortex size solely depends on KC and much less on structural shape (with a factor of up to 70%) and frequency. A comparison with potential flow simulations yields qualitatively good agreement to predict shape effects in added mass and radiation damping; however, a comparison between radiated wave heights from experiments and potential flow simulations indicates the necessity to validate potential flow results for quantitatively correct results.

Vegetated coastal dunes are a common landform along sandy coastlines worldwide which often provide essential contributions to coastal protection. In the context of climate change and sea level rise, it becomes more relevant for established dune systems to ensure a sufficient level of protection for adjacent communities and infrastructure (Mehrtens et al., 2023). During storm surges, extreme water level and wave attack cause severe dune erosion while overtopping may potentially lead to destabilization and failure. Established coastal dune forests -previously overlooked- have recently gained increasing scientific attention in this context, as they can cover extensive dune systems, either leading to stabilizing or de-stabilizing effects on the dune erosion process when subjected to storm surges. Which tendency of trees falling into the active erosive zone prevails is currently subject to speculations, as hard scientific evidence is entirely lacking. The novel aspect of this study is to present Froude scaled experimental research where dune-tree models are tested and multiple forest configurations are employed to extend the current understanding of the underlying, relevant physical processes of dune erosion dynamics in a wave- loading regime.

As increasingly complex objectives gain scientific attention in physical experiments on coastal dune erosion, novel and more demanding requirements arise, such as reliable high-resolution methods for measuring erosion processes. This work presents a technical assessment of four distinct laboratory survey methods that were employed for the sake of comparison in 1:7, reduced-scale dune erosion tests. In addition to conventional measurements with terrestrial laser scanning, mechanical bed profile sensing, and lateral camera recordings, novel solid-state laser scanning technology was applied. By utilizing a Blickfeld Cube 1 LiDAR, measurements with a sampling frequency of 0.68 Hz and a minimum spatial resolution of 7.0 cm were conducted, enabling targeted investigations such as (1) the detection of subaerial notching and dune face slumping or (2) the derivation of spatiotemporal erosion volume development. Based on the acquired findings, specific application recommendations are provided regarding all four methods deployed.

German coastal areas are often protected from flood events by a primary sea dike line of more than 1,200 km. Many transition areas, such as the change of surface covering materials and other dike elements such as stairs, fences, or ramps at intermittent locations, characterize the stretch of this sea dike line. During storm surges and wave overtopping, the onset of damage, especially dike cover erosion, is often initiated at these transitions due to locally disturbed flow characteristics, increased loads, and reduced strength at the interface. An in-depth understanding of damage initiation and building stock conditions along coastlines as a foundational element of a flood cycle is essential in order to accurately assess existing defense structures, both deterministically and probabilistically. Thus, the present study is motivated to examine the variety of transition areas on the sea dikes along the German coasts, for further assessment of probability of their damage and failure. A novel remote inventory was elaborated manually, based on satellite images for a length of 998 km along the German North Sea and 123 km along the German Baltic Sea coast and estuaries, and it shows the spatial distribution and frequency of such transitions on sea dikes. During additional on-site investigations at different locations at the coast, detailed information about design variants of dike elements as well as damage to transitions were recorded and reported systematically. The results of the on-site investigations allow the development of a damage catalog in relation to transitions and the validation and verification of the remote inventory. By categorizing and spatially analyzing a large number of transitions (n ≈ 18,300) and damages along the coast, particularly vulnerable transitions and hot spots of loading can be further investigated regarding the flow-structure-soil interaction. Through this, structural layouts and material combinations can be optimized for the design of sea dikes.

Offshore structures become colonized by marine orga-nisms after a short period of time, whose common benthic communities depend among others on geographic location, water depth, water temperature, food supply, salinity and oxygen content of the water (Kröncke and Bergfeld, 2003; Shi et al., 2012). While biofouling can be categorized in biological terms as bivalves, kelp, algae, barnacles, tubeworms and other species (van der Stap et al., 2016; Wilhelmsson and Malm, 2008), engineers mostly distinguish between hard and soft marine growth based on the strength of their outer shell alone (Shi et al., 2012; Skaugset and Baarholm, 2008). Due to an increasing demand for sustainable energy, the offshore renewables industry experiences significant growth. However, many uncertainties persist in the consideration of biofouling, specifically when calculating loads accor-ding to the Morison concept, the influence of marine fouling on fatigue reassessment, on the flow velocities around cylinders and the vortex formation under waves. For the first time, the flow around cylinders with different artificial marine biofouling was recorded and analyzed in an extensive experimental study using a comprehensive 4D particle tracking velocimetry (PTV) system.

A large part of the Baltic Sea coastline in the north-eastern provinces of Germany (Mecklenburg-Vorpommern and Schleswig-Holstein) is protected by coastal protection dunes, which are constructed and maintained to preserve a quasi-natural beach environment. The latest IPCC report predicts a significant sea level rise even in the Baltic, together with an increase in storm surges within the next decades. Sand dune constructions are particularly affected by these developments, in close connection to the increasing problems of sediment mining for beach nourishment and dune maintenance. In the BMBF research project PADO (Processes and Implications of Dune Breaching at the German Baltic Sea Coast), scientists from Rostock University and RWTH Aachen University investigated together with various partners how constructed sand dunes would fail during a storm surge, focusing on the erosion and the initiation and development of a breach. Therefore, a large-scale field experiment was installed at the beach of Rostock-Warnemünde, which was instrumented and surveyed in high resolution during the storm surge in November 2018, leading to the collapse of the dune, using innovative measurement systems, including terrestrial laser scanning and photogrammetry. The measurements were used to run a numerical model (XBeach) to simulate dune breaching for three different sections of existing sand dunes at the coast of Mecklenburg-Vorpommern. Additional investigations were made to analyse the filling and emptying of the polder areas behind the coastal protection dunes and the consequences of the floods with respect to saltwater intrusion. In this paper, the field experiment and the results from measurements and simulations are compared to one of the real reference dunes, and the transfer of results to a dune breach incident near Rostock in 2019 is discussed.

This paper presents and validates a novel root model which accounts for the effect of belowground biomass on dune erosion volumes in XBeach, based on a small-scale wave flume experiment that was translated to a larger scale. A 1D-XBeach model was calibrated by using control runs considering a dune without vegetation. Despite calibration, a general model–data mismatch was observed in terms of overestimated erosion volumes around the waterline. Furthermore, the prediction of overwash had to be induced by increasing the maximum nearshore wave height within the XBeach simulation. Subsequently, applying the root model resulted in a good agreement with the belowground biomass cases, and the consideration of spatially varying rooting depths further improved the results. Predictions of the root model while using locally increased friction coefficients were in line with the aboveground and belowground biomass cases. However, the effect of the root model on the erosion predictions varied among the hydrodynamic conditions, so further improvements are required. Therefore, future research should focus on quantifying the effects of land-based biomass and individual plant characteristics, such as root density, on dune erodibility at large scales, along with their influences on the temporal evolution of dune scarping and avalanching.

This study aims at improving the XBeach model in predicting dune erosion in the presence of belowground (land-based) biomass under varying hydrodynamic conditions. In this regard, the XBeach model was extended by a literature-derived root model, which increases the critical velocity for erosion in user-defined areas due to additional root cohesion. The model was validated by using the results of a small-scale wave flume experiment as a basis, where the presence of belowground biomass reduced the measured dune erosion. Control runs considering an exclusively sand-based dune, which served as a comparison in the physical experiment, were used to setup and calibrate a one-dimensional (1D) XBeach model. Due to the application to small-scale, default parameters related to sediment transport were scaled within the scope of calibration. Results showed that the XBeach model is not capable to reproduce the observed control dune profiles satisfactorily for all hydrodynamic conditions. Subsequently, the calibrated model was applied to the physical model runs with belowground biomass. Regardless of the hydrodynamic conditions, applying the root model led to a decrease in the mean sediment concentrations and, in turn, the model-predicted erosion in the vegetated area was mainly driven by avalanching. As a result, erosion volumes were reduced in the vegetated area and a higher agreement with the measurements was achieved. At this stage, the root model is highly simplified and only validated for collision at small-scale. In this regard, the translation of the model setup to field scale would eliminate the need for parameter scaling and could result in an overall better performance of the default model. Further research should address the influence of belowground biomass on the time evolution of dune failure and avalanching.