Nils Goseberg

Seabed liquefaction poses a significant threat to the operational stability of marine structures resting on or embedded within the seabed. Cyclic wave loading of saturated granular soils leads to a reduction in strength and stiffness. Protective surface layers such as cover stones and rip-rap are routinely installed for scour protection, yet their simultaneous role as a geotechnical surcharge that suppresses residual liquefaction is rarely quantified, leaving a keymitigation mechanism unexploited in design. In this study, an OpenFOAM-based Biot-drift-flux model is extended and applied to investigate a range of seabed liquefaction case studies, such as Wave-Soil-Interaction (WSI), Wave-Soil-Structure-Interaction (WSSI), and Wave-Soil-Structure-Cover Stone-Interaction (WSSI-CS). The model uses Biot’s poro-elastic theory to govern the behaviour of non-liquefied soils, while liquefied zones are represented using thedrift-flux model, with the transition between the two defined based on a liquefaction criterion. Through validation against reference experimental data, the developed model is shown to accurately reproduce the onset of residual liquefaction. The results further demonstrate that cover stones are effective in reducing the pore pressure accumulation by increasing the initial mean normal effective stress through a geotechnical surcharge, by 42 % for a single looselayer and up to 129 % for three layers near to the mudline. Furthermore, when the model is applied to WSSI, it has been found that although structural weight can provide localised confinement to the seabed, dynamic mooring force of 600 N significantly accelerates the pore pressure accumulation, resulting in residual liquefaction within 60 s. However, the application of cover stones successfully maintains seabed stability, reducing the maximum normalised pore pressure ratio from a failure state of 1.0 back to approximately 0.12. The novelty lies in resolving progressive wave loading, and granular surface protection concurrently within a single residual liquefaction solver, offering valuable insight for the design and resilience of coastal and offshore infrastructure

Onshore flows and waves rarely kill alone; they ally with debris. Mobile debrissuch as collapsed structures, vehicles, and vegetation pose severe hazards duringextreme hydrodynamic events including storm surges, tsunami bores, andflash floods. While single-object transport has been widely studied, the collectivedynamics of large debris groups remain poorly understood. This studyanalyses the motion of up to 240 debris elements across 91 controlled dambreakexperiments using YOLO-based detection and DeepSORT-trajectory reconstruction.Three robust and novel dynamical features emerge. First, lateralspreading follows a diffusive power law with exponent p ≈ 0.5, consistentwith a random-walk collision process and in clear contrast to deterministicconstant-angle models that impose ballistic spreading. Second, the meangroup streamwise velocity exhibits a distinct S-shaped entrainment profile, arisingfrom a time-varying effective resistance as the bore progressively mobilisesinitially stationary debris, and substantially outperforms existing entrainmentmodels across all tested bore-strength conditions. Third, embedding this entrainmentmodel into a lateral advection–diffusion framework yields a single,time-dependent two-dimensional transport model that reproduces the observeddebris concentration field C(x, y, t). Together, these findings provide a unifiedframework for predicting group-debris transport and offer new guidance for assessingdebris-related hazards and designing resilient coastal infrastructure.

1. The global population in coastal zones has been steadily increasing, with approximately 15% of the world’s population living within 10 km of the sea (Cosby et al. 2024). This growing concentrati...

Predicting hydrodynamic loads on seaweed is difficult because its flexible structure continuously reconfigures under oscillatory flow. This study investigates wave-induced hydrodynamic loading on realistic \textit{Laminaria digitata} surrogates in a wave flume and uniquely compares three Morison-type approaches for deriving hydrodynamic coefficients and predicting forces. Synchronized force, wave-elevation, and video measurements were recorded for sixteen non-breaking regular waves with wave heights of \SIrange{0.2}{0.3}{\meter} and periods of \SIrange{2}{5}{\second}. The experiments showed that wave period and height strongly affected motion and forces. Longer and higher waves produced more coherent bundle motion and pronounced streamlining, whereas shorter waves limited reconfiguration causing more heterogeneous blade behavior. These differences were reflected in the force time-histories as substantial wave-to-wave scatter and intermittent extreme loads, which also transferred to the derived hydrodynamic coefficients. Phase-resolved analysis revealed intra-wave variations in the coefficients, which, together with the inclusion of a time-varying frontal area resolved part of the wave-to-wave scatter. However, across all methods, considerable scatter remained, indicating that further non-deterministic processes such as clashing, tangling, rolling, and fluttering of blades also influenced the loads and especially contributed to the intermittent peak forces. Although these effects cannot be captured by deterministic formulations, prediction errors for the Morison-type approaches became acceptable when forces and coefficients were considered over multiple waves. The Morison equation therefore remains valuable for applications at the system scale, where design-relevant loads are governed by the average of multiple samples rather than by individual seaweed, although continued development of models that explicitly resolve reconfiguration, blade interactions, and turbulence-driven effects remains necessary.

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 k t m / D i 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.