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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. [ABSTRACT FROM AUTHOR]

The Hilbert-Huang transformation (HHT) is a new method for analyzing nonlinear and non-stationary data series. The central idea behind the HHT is the so-called empirical mode decomposition (EMD) that numerically decomposes a time-dependent signal into its own underlying characteristic modes. Applying the Hilbert transformation (HT) to each of these disintegrated intrinsic mode function (IMF) subsequently provides the Hilbert amplitude or energy spectrum--producing more accurate spectra and proposing in all probability entirely new physical insights of nonlinear and nonstationary processes. The present paper describes the application of the HHT concerning the spectral frequency analysis of nonlinear transient water waves.

Extreme natural events, like e.g. tsunamis or earthquakes, regularly lead to catastrophes with dramatic consequences. In recent years natural disasters caused hundreds of thousands of deaths, destruction of infrastructure, disruption of economic activity and loss of billions of dollars worth of property and thus revealed considerable deficits hindering their effective management: Needs for stakeholders, decision-makers as well as for persons concerned include systematic risk identification and evaluation, a way to assess countermeasures, awareness raising and decision support systems to be employed before, during and after crisis situations. The overall goal of this study focuses on interdisciplinary integration of various scientific disciplines to contribute to a tsunami early warning information system. In comparison to most studies our focus is on high-end geometric and thematic analysis to meet the requirements of small-scale, heterogeneous and complex coastal urban systems. Data, methods and results from engineering, remote sensing and social sciences are interlinked and provide comprehensive information for disaster risk assessment, management and reduction. In detail, we combine inundation modeling, urban morphology analysis, population assessment, socio-economic analysis of the population and evacuation modeling. The interdisciplinary results eventually lead to recommendations for mitigation strategies in the fields of spatial planning or coping capacity. [ABSTRACT FROM AUTHOR]

The present study provides indications and underlying drivers of wave-induced transport and retention potential of microplastic particles (MP) in marine vegetation canopies having different densities. The anthropogenic occurrence of MP in coastal waters is well documented in the recent literature. It is acknowledged that coastal vegetation can serve as a sink for MP due to its energy dissipating features, which can mimic a novel ecosystem service. While the transport behavior of MP in vegetation has previously been investigated to some extent for stationary flow conditions, fundamental investigations for unsteady surf zone flow conditions under irregular waves are still lacking. Herein, we demonstrate by means of hydraulic model tests that a vegetation's retention potential of MP in waves increases with the vegetation shoot density, the MP settling velocity and decreasing wave energy. It is found that particles migrating by traction (predominantly in contact with the bed) are trapped in the wake regions around a canopy, whereas suspended particles are able to pass vegetated areas more easily. Very dense canopies can also promote the passage of MP with diameters larger than the plant spacing, as the canopies then show characteristics of a solid sill and avoid particle penetration. The particle migration ability through a marine vegetation canopy is quantified, and the key drivers are described by an empirical expression based on the particle settling velocity, the canopy length and density. The findings of this study may contribute to improved prediction and assessment of MP accumulation hotspots in vegetated coastal areas and, thus, may help in tracing MP sinks. Such knowledge can be considered a prerequisite to develope methods or new technologies to recover plastic pollutants and rehabilitate valuable coastal environments., Competing Interests: Declaration of competing interest David R. Fuhrman reports financial support was provided by Independent Research Fund Denmark. David R. Fuhrman reports a relationship with Independent Research Fund Denmark that includes: funding grants., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

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 ([Formula: see text]) 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., (© 2023. The Author(s).)

The settling velocities of 66 microplastic particle groups, having both regular (58) and irregular (eight) shapes, are measured experimentally. Regular shapes considered include: spheres, cylinders, disks, square plates, cubes, other cuboids (square and rectangular prisms), tetrahedrons, and fibers. The experiments generally consider Reynolds numbers greater than 10 2 , extending the predominant range covered by previous studies. The present data is combined with an extensive data set from the literature, and the settling velocities are systematically analyzed on a shape-by-shape basis. Novel parameterizations and predictive drag coefficient formulations are developed for both regular and irregular particle shapes, properly accounting for preferential settling orientation. These are shown to be more accurate than the best existing predictive formulation from the literature. The developed method for predicting the settling velocity of irregularly-shaped microplastic particles is demonstrated to be equally well suited for natural sediments in the Appendix., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Incipient motion conditions for 57 regular (spheres, cylinders, disks, square plates, cubes, square prisms, rectangular prisms, tetrahedrons, and fibers) and eight irregular microplastic particle groups, having various sizes and densities, are investigated in a circular flume. The present data set is combined with additional data from the literature and systematically analyzed. A new framework is developed for predicting incipient motion conditions for foreign particles, accounting for variations in static friction, hydraulic roughness, and hiding-exposure effects. Via this framework, incipient motion conditions for microplastic particles lying on a sediment bed are, for the first time, reconciled with the classical Shields diagram.