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Coastal permeable groins have been used to protect beaches from erosion for centuries. However, the hydraulic functioning of permeable groins has not been fully understood and their design heavily depends on engineering experiences. In this study, numerical experiments were executed to investigate the effects of layout configurations of a permeable groin system on longshore currents. The non-hydrostatic SWASH (Simulating WAve till SHore) model was employed to carry out the numerical simulations. Two data sets obtained from physical laboratory experiments with different permeable groin layouts on different slopes are used to validate the accuracy of the model. Then, the longshore current reduction by the permeable groin system with varying configuration parameters (e.g., groin spacing, groin length) was numerically investigated under different environmental conditions (e.g., a slight or a moderate wave climate). From the calculation results of numerical experiments, it is indicated that permeable groins function efficiently to reduce the maximal longshore current velocity under the condition that the groin length ranges from 84% and 109% of the wave breaker zone width. The longshore current reduction rate monotonously decreases with the increase in groin spacing; permeable pile groin functions best to reduce longshore current with the minimal groin spacing-groin length ratio 1:1 among the range between 1:1 and 2:1. When the groin spacing–groin length ratios are 1:1 and 1.5:1, the longshore current reduction is not sensitive to the investigated wave conditions in this study. When the spatial ratio is 2:1, the permeable pile groin system functions worse under a moderate wave climate than under a slight wave climate, from the view of longshore current reduction. [ABSTRACT FROM AUTHOR]

The erosion threshold, beyond which bed sediments start to move, is a key parameter describing sediment transport processes. For silt‐dominated mixtures, in which the grain size is between sand and clay, existing experimental studies exhibit contradictory observations. That is, the erosion was either sand‐like or clay‐like, suggesting transitional erosion behavior. To explore the underlying mechanism of the transitional erosion behavior of silt‐sized sediment, we revisited the topic of the erosion threshold of sand‐silt mixtures by carrying out a series of erosion experiments for different bed compositions. The results suggest that there exists a critical silt content of approximately 35%, separating two domains. Below this critical value, the critical bed shear stress follows the Shields criterion, whereas above this value, the erosion threshold of a mixed bed increases abruptly and remains relatively constant with a further increase in silt content. By combining with existing data, we found that the proposed critical silt content acts as a tipping point, beyond which the mixed bed shifts from a sand‐dominated to a silt‐dominated domain. For the silt‐dominated domain, a stable silt skeleton can be formed by attraction forces that resist erosion. However, the attraction forces are too weak to form a stable silt skeleton when the silt content is too small. Based on this finding, a modified critical bed shear stress formula is proposed for silt‐dominated mixtures, which results in a better agreement with experimental data (an averaged bias of 10%), performing better than existing formulas (larger than 30%). Plain Language Summary: When exposed to a certain flow velocity, sediment particles are dislodged from the seabed, namely, the erosion threshold. Attributed to sizes, shapes, minerals, etc., sediments of different grain sizes behave differently, resulting in various erosion behaviors. Sand particles (62.5–2,000 μm) are not sticky and erode particle by particle, whereas clay particles (<4 μm) are sticky and collectively erode as chunks. Silt (4–62.5 μm), of which the grain size is between sand and clay, exhibits either sand‐like or clay‐like behavior, suggesting transitional erosion behavior. Coastal sediments are usually mixtures of clay, silt, and fine sand. Different bed compositions result in different erosion behaviors, leading to a variety of bed forms, morphological patterns, etc. Therefore, a thorough understanding of the erosion behavior of sediment mixtures is of great significance to the topic of coastal sediment transport, which further benefits coastal geomorphology, ecology, etc. This study indicates that, for sand‐silt mixtures, a critical silt contents exists, beyond which the bed mixtures shift from a sand‐like erosion behavior to a clay‐like erosion behavior. The widely adopted parameterized expression of the erosion threshold (namely, the Shields curve) is modified to mimic the transitional erosion behavior of sand‐silt mixtures by considering the effects of silt content. Key Points: Silt content is an important factor controlling the erosion of sand‐silt mixtures in regards to noncohesive or cohesive propertiesThere exists a critical silt content of 35%, beyond which a stable silt skeleton is formed that increases the erosion thresholdA modified critical bed shear stress is proposed to mimic the transitional erosion behavior of sand‐silt mixtures [ABSTRACT FROM AUTHOR]

Byline: Roshanka Ranasinghe (1,2), Marcel J. F. Stive (1) Author Affiliation: (1) Department of Hydraulic Engineering, Delft University of Technology, PO Box 5048, 2600 GA, Delft, The Netherlands (2) Department of Water Engineering, UNESCO-IHE, PO Box 3015, 2601 DA, Delft, The Netherlands Article History: Registration Date: 09/04/2009 Received Date: 26/08/2008 Accepted Date: 13/03/2009 Online Date: 16/05/2009

Instantaneous bed-shear stresses in shoaling waves are studied. The shear stress, or in nondimensional form the Shields parameter, is a classical basic forcing parameter in coastal engineering to calculate sediment transport due to wave motion. Based on our recent work on incipient motion of panicles under shoaling waves, we became interested in the effects on the shear stress if [u.sub.[infinity]] is determined by a more arbitrary wave shape, such as when waves are shoaling. We therefore present a derivation of the Shields parameter [[theta].sub.wf] as a function of the waveform parameter [PHI], which is a further generalization of Nielsen's studies. The results indicate that shear stress distributions are modified depending on the waveform parameter. Based on laboratory observations, we are able to validate some of our findings. The use and quantification of [[theta].sub.wf] should be helpful in the formulation of sediment transport due to shoaling waves confirming the role of wave skewness and asymmetry. CE Database subject headings: Parameters; Shear stress; Shoaling; Acceleration; Skewness.

Coastal vegetation has been increasingly recognized as an effective buffer against wind waves. Recent laboratory studies have considered realistic vegetation traits and hydrodynamic conditions, which advanced our understanding of the wave dissipation process in vegetation (WDV) in field conditions. In intertidal environments, waves commonly propagate into vegetation fields with underlying tidal currents, which may alter the WDV process. A number of experiments addressed WDV with following currents, but relatively few experiments have been conducted to assess WDV with opposing currents. Additionally, while the vegetation drag coefficient is a key factor influencing WDV, it is rarely reported for combined wave–current flows. Relevant WDV and drag coefficient data are not openly available for theory or model development. This paper reports a unique dataset of two flume experiments. Both experiments use stiff rods to mimic mangrove canopies. The first experiment assessed WDV and drag coefficients with and without following currents, whereas the second experiment included complementary tests with opposing currents. These two experiments included 668 tests covering various settings of water depth, wave height, wave period, current velocity and vegetation density. A variety of data, including wave height, drag coefficient, in-canopy velocity and acting force on mimic vegetation stem, are recorded. This dataset is expected to assist future theoretical advancement on WDV, which may ultimately lead to a more accurate prediction of wave dissipation capacity of natural coastal wetlands. The dataset is available from figshare with clear instructions for reuse (10.6084/m9.figshare.13026530.v2, Hu et al., 2020). The current dataset will expand with additional WDV data from ongoing and planned observation in natural mangrove wetlands. [ABSTRACT FROM AUTHOR]