This study aims to improve the design of scour protection around offshore wind turbine monopiles, as well as future-proofing them against the impacts of climate change. A series of large-scale experiments have been performed in the context of the European HYDRALAB-PLUS PROTEUS (Protection of offshore wind turbine monopiles against scouring) project in the Fast Flow Facility in HR Wallingford. These experiments make use of state of the art optical and acoustic measurement techniques to assess the damage of scour protections under the combined action of waves and currents. These novel PROTEUS tests focus on the study of the grading of the scour protection material as a stabilizing parameter, which has never been done under the combined action of waves and currents at a large scale. Scale effects are reduced and, thus, design risks are minimized. Moreover, the generated data will support the development of future scour protection designs and the validation of numerical models used by researchers worldwide. The testing program objectives are: (i) to compare the performance of single-layer wide-graded material used against scouring with current design practices; (ii) to verify the stability of the scour protection designs under extreme flow conditions; (iii) to provide a benchmark dataset for scour protection stability at large scale; and (iv) to investigate the scale effects on scour protection stability.

Seagrasses are essential marine ecosystems for which restoration has proven challenging due to increased hydrodynamic stress. This study aims to analyze the flow alteration induced by an artificial seagrass (ASG) meadow by characterizing its wake effect through a shelter distance and thus yield guidance for seagrass restoration projects. Here, we define shelter distance as the longitudinal extent behind a meadow, with respect to the flow direction, where seagrass is protected and can hence grow successfully. Flume experiments were conducted for submerged meadows with three different lengths at constant canopy height, shoot density and water depth, and three different cross-section-averaged longitudinal flow velocities measured with state-of-the-art Particle Image Velocimetry (PIV). For the tested meadow morphology and hydrodynamic conditions, meadow length played a less important role regarding shelter distance, while incident flow velocity and effective canopy height governed the wake effect. Incident velocities 2 m behind the meadow, whereas higher velocities led to a reduced shelter distance ranging from 20-40 cm. ASG additionally produced an upwelling effect on the vertical distribution of the velocity profile observed along the wake, regardless of meadow length and incident velocity. Our results suggest that restoration projects should aim for areas of low flow, where currents induced by tidal or wind waves are less pronounced in order to activate larger shelter distances.

Seagrass surrogates are commonly used to mimic the behaviour of seagrasses exposed to currents and their effects on flow fields. The interaction is highly dependent on the chosen mechanic and geometric properties of the surrogates and needs to be understood in order to design artificial meadows. The interaction of single surrogates in unidirectional flow fields is studied by means of physical modelling. Surrogates made of plastic materials with different flexural rigidities, buoyancies and geometries are exposed to varying flow velocities. The instantaneous velocity fields in the vicinity and wake of the surrogates are measured by stereoscopic Particle Image Velocimetry (PIV). All employed surrogates disrupt and interact with the flow field by changing their posture. An empirical relation is derived between the flexural rigidity, buoyancy and characteristic diameter of the surrogates and the imposed differences in the attenuation ratio of flow velocities. Further, the approaching flow velocity and distance behind the surrogate influence the estimated attenuation. The vortex shedding frequency imposed by artificial seagrass is lower than frequencies determined for infinite, rigid cylindrical structures. Three main characterizing properties: the modulus of elasticity, buoyancy and cross-sectional dimensions need to be taken into account for design of artificial seagrass meadows. Our findings advance knowledge of fluid-structure interactions of flexible materials and help to progress proper design of artificial seagrass meadows.

Seagrasses represent an essential part of the coastal environment and are hence the target of many coastal restoration projects. Artificial seagrass (ASG) mats may facilitate seagrass growth, making them a captivating option for restoration projects. However, little is known about the forces occurring on mats deployed in marine environments and especially on how these forces are transmitted to the anchoring points. Here, we present a study of prototype biodegradable coconut-mesh mats as base layer for ASG meadows and investigate the forces that occur at the anchors. We test the performance of three mesh types under wave forcing using two different anchor configurations without ASG and subsequently test ASG mats of one mesh type under wave forcing and a 4-anchor configuration to assess the effect of the ASG on anchor loading as a function of incident orbital velocities. We found that the mat composition plays a more important role than the number of anchors in anchor load reduction. The anchor forces were 2–4 times higher at front anchors compared to rear anchors, relative to wave propagation direction, and were also considerably higher in that direction compared to the opposite direction. With ASG, the forces increased compared to the highest measured forces without ASG. The forces on the anchors were almost fully dominated by the drag on the ASG based on material properties, ASG reconfiguration and flow conditions. We derive a relation between horizontal orbital velocities and expected forcing on the anchor based on ASG properties and the corresponding area of each anchor and discuss relevant criteria for the design of ASG mats. This should help to assess the loading on anchors deployed for restoration under specific site conditions and chosen materials.

Seagrass restoration can be promoted through the use of artificial seagrass (ASG). However, there is no guideline for ASG design, which requires a sound understanding of the inherent hydrodynamics in a submerged environment. Present know-how primarily stems from idealized ASG attached to a fixed bed. To develop accessible field deployment for restoration, anchored prototype scale ASG mats (coconut mesh) were proposed and tested under differing wave conditions. The aim of this study was then to: 1) analyze hydrodynamic interaction of ASG mats; and 2) assess the suitability of contemporary predictive hydrodynamic models. Velocity structure and wave propagation were measured around one and two ASG mats (separated by a 2-m gap). The mats reduced orbital velocities by up to 16% (2 mats), whereby the average reduction of all tested vegetated conditions was low ( $$<10\%$$< 10 % ) compared to the non-vegetated conditions. Velocities increased above the ASG, with the gap enhancing velocity (up to 11%) instead of attenuating it. Wave decay followed an exponential decrease, further enhanced by the second mat. Current models did not capture the induced hydrodynamics for the full range of wave conditions tested, with the second mat increasing uncertainties. Wave decay models generally overestimated wave attenuation (up to 30%), except for longer wave periods. Nevertheless, for the full range of conditions, the models provide accurate insight into the expected magnitude of attenuation under field conditions. It is speculated that mat flexibility affects the surrounding hydrodynamics through inherent motion, with the gap contributing to the uncertainties.

Seagrass restoration requires a sound understanding of the hydrodynamics around established meadows. Present know-how primarily stems from idealized artificial seagrass (ASG) attached to a fixed bed. With the goal of accessible field deployment for restoration, anchored prototype scale ASG mats (coconut mesh) were tested under differing wave conditions to analyze hydrodynamic interaction and assess the suitability of contemporary predictive models. Velocity structure and wave propagation were measured around one and two ASG mats (separated by a 2-m gap). The mats reduced orbital velocities by up to 16% (2 mats), whereby the average reduction was low (< 10%) compared to the non-vegetated conditions. Velocities increased above the ASG, with the gap enhancing velocity (up to 11%) instead of attenuating it. Wave decay followed an exponential decrease, further enhanced by the second mat. Current models did not capture the induced hydrodynamics for the full range of wave conditions tested, with the second mat increasing uncertainties. Wave decay models generally overestimated wave attenuation (up to 30%), except for longer wave periods. Nevertheless, for the full range of conditions, the models provide accurate insight into the expected magnitude of attenuation under field conditions. It is speculated that mat flexibility affects the surrounding hydrodynamics through inherent motion, with the gap contributing to the uncertainties.