Tidal flats are typically rich in fine-grained sediments, with tidal currents serving as the dominant hydrodynamic force. Sediment transport, mainly in the form of suspended load, is especially active in the mid- to low-tidal zones, where silt-rich sediments prevail. In these zones, variations in silt content significantly influence sediment suspension dynamics. To investigate the effect of silt content on the suspension behaviours of sand-silt mixtures under unidirectional flow, a series of experiments has been conducted in an annular flume. Sediment samples collected from silt-enriched tidal flats were remixed into seven types of sediment beds with the silt content ranging from 19 % to 79 %. Results indicate that under steady current conditions, the suspended sediment concentration (SSC) in sand-silt mixtures increased toward equilibrium. Sediment beds with 36 %–60 % silt content exhibited two-stage erosion behaviour, each marked by distinct characteristics. At low to moderate shear stress (0.18–0.74 Pa), smoother bed surfaces and hiding-exposure effects led to higher near-bed velocities and lower equilibrium SSCs. At higher shear stress levels (1.00–1.23 Pa), this behaviour shifted, with prolonged suspension development becoming the key feature. Based on the experimental findings, we further re-calibrated the equations predicting equilibrium SSC, which can be well described by a modified excess shear stress to the power of two. Subsequently, we investigated the boundary conditions that govern sediment suspension to model the observed suspension process. Building on the boundary condition prescribing a constant upward sediment flux, an asymptotic upward flux condition, which represents the delayed erosion response due to the resistance of silty bed structure, has been introduced and demonstrated as the most feasible. Furthermore, the development of SSC towards equilibrium was investigated. This process is well characterised by a time scale, which becomes notably larger for sediment beds with 36 %–60 % silt content at high velocity levels, reflecting their delayed response in suspension. These findings elucidate key mechanisms governing silt-dominated sediment erosion, enhancing predictive capabilities in modelling mixed sediment transport.

A series of laboratory experiments focused on the wind impact on the vertical turbulence structure in shallow water has been carried out. The turbulence characteristics in the mid-lower water column under relatively strong wave conditions are investigated. For different experimental conditions (i.e., waves only, wind only, combinations of wind and waves) in a wind-wave flume, the effects of wind and waves were investigated in detail by decomposing the total energy into different terms (i.e., wind-driven currents, wind waves, wind-induced turbulence). The results show that shallow water waves play a major role in transferring energy by non-zero wave-induced Reynolds stress (u˜w˜‾), turbulent diffusion (u^′w^′‾). The superimposed wind can further modify the energy transference due to its impact on wave asymmetry and skewness as well as through homogenizing the time-average velocity profiles. Subsequently, the impact of wind on turbulence structure was explored in detail. The most important finding is that the wind can directly influence water turbulent diffusion (u^′w^′‾) along with wave-induced turbulence. The vertical turbulence intensity (σ_w) is more sensitive to wind than the horizontal turbulence intensity (σ_u). Furthermore, the major way that wind affects water turbulence is by introducing nonlinear wind and wave interactions, which exhibit a maximum effect (∼60 % compared to the respective effect of wind and wave) at the edge of the bottom boundary layer. This study demonstrates that the wind can transfer momentum downward to mid-lower water columns even under strong waves in shallow waters, which differs from that in deep water systems.

Mangrove degradation and rapid coastline erosion have been widely observed at many locations along the Mekong Delta Coast. These locales frequently display narrow mangrove forests, occasionally spanning as few as 100 meters. This phenomenon of the narrower mangrove forests is supposed to be due to the construction of sea dikes in a search to establish room for agricultural purposes and to hinder the salinity intrusion, referred to as "mangrove squeeze". Within the context of monitoring mangrove forest evolution alongside the shoreline's dynamic processes of erosion and accretion, the hypothesis of a "squeeze mangrove forest" was advanced by Phan (2015) and Truong (2017). This conceptual construct underscores the fundamental importance of "mangrove width" as a critical length scale influencing the sustainable growth of a mangrove forest. The physical interpretation of this length scale is intricately tied to the extent of the mixing layer's intrusion into the vegetative domain (Truong et al., 2019). It is important to note that while the mixing dynamics of estuarine mangroves are primarily regulated by lateral flow events induced by large vortex structures moving along the vegetation edge, the characteristics of incoming waves primarily determine the length to which the mixing layer penetrates within coastal mangroves. The latter is the primary focus of this study.

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.

Mangroves can function as a ‘bio-shield’ to protect coastal communities from harsh environments because of their strong ability to attenuate wave energy. However, as mangroves are usually oversimplified as rigid cylinders in antecedent studies, the effects of complex mangrove morphology on wave attenuation have not been well researched. Although increasing attention has been paid to the wave dissipation induced by varying mangrove morphologies, most of them focus on the bottom trunk and root components of mature mangrove trees. There are few investigations about the contributions of the canopies of young saplings and/or short species to wave attenuation. To bridge this knowledge gap, a series of laboratory experiments under regular waves were conducted to examine the hydrodynamic variations affected by varying mangrove morphology configurations. Three water depths were considered to explore the influences of the vertical-varying submerged volume of mangroves when the artificial mangrove models are submerged, nearly emergent, and fully emergent. The mangrove forest model is 2 m long at a 1:10 scale. Three mangrove configurations, i.e. with no canopy, sparse canopy, and dense canopy were applied and compared to isolate the wave attenuation contributed by mangrove canopies. The results highlight the wave energy attenuation attributed to the canopy density. A linear correlation is found between the wave damping factor and a new variable named hydraulic submerged volume index (HSVI). The bulk drag coefficient, including canopy effects, was calculated to characterize mangrove-induced wave attenuation when the mangrove canopy is submerged. The relationships between the bulk drag coefficient C_D and the characteristic hydraulic numbers (i.e., Reynolds number, Keulegan–Carpenter number, Ursell number) are discussed in detail. Consequently, new generic formulas of C_D were deduced considering the effects of the submerged canopy. The employment of new C_D formulas improves the reliability of the prediction of the wave attenuation ability by mangroves since the canopy effects are incorporated.

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%).

The estuarine turbidity maximum (ETM) in the Yangtze Estuary Delta (YED) is muddy by definition and lacks bottom undulations. However, since 2013, a remarkable change has occurred in the YED. Recent images detected by a multibeam echosounder system, SeaBat 7125, for the first time have confirmed widespread regions of subaqueous dunes in the Yangtze ETM channel. This abnormal change is the result of morphodynamic transformation from the combination of an abrupt decline in sediment supply resulting from the construction of the Three Gorges Dam (TGD) and hydrodynamic changes caused by sea level rise. The latter includes anthropogenic-induced sea level rises (from land subsidence and coastal engineering) of 7–37 cm and a climate-induced sea level rise of 8 cm during the past four decades. Obvious evidence of hydrodynamic changes includes tidal amplification, i.e., a 10–28 cm rise in the tidal range, 42–65 cm rise in the lowest tidal level in the dry season, 45–67 cm rise in the highest tidal level in the flood season and 10–30% increase in the amplitude of the major tidal component. These findings will likely have global implications in formulating strategies to combat the superimposed effects of human interventions and climate change on upstream river and downstream coastal developments.

Mangroves play an important role in sustaining a healthy coastal environment, providing a natural habitat to various species, a stable shoreline and forestry products. However, the extent of mangroves developed along the tidal coast of the Mekong delta in southern Vietnam has faced and still faces the impact from both natural and anthropogenic drivers. Since the area of mangroves in the coastal Mekong delta is not well documented, this study aims to quantitatively document the evolution of the mangrove area over the past 48 years, i.e. between 1973 and 2020. Satellite Landsat images, along with a classification method comprising Iso Cluster and Maximum Likelihood algorithms, have been used for mapping land cover types including mangroves, aquaculture, soils, plants and water surfaces along the coastal districts of the Mekong delta. The study shows that remote sensing and GIS techniques can be applied to obtain mapping of the land cover, as well as detect and analyse spatial and temporal changes caused by e.g. coastal erosion or aquaculture expansion. The findings reveal that the total mangrove area of an estimated 185,800 ha in 1973 decreased significantly to 102,160 ha in 2020. Approximately 2150 ha/yr of the total mangrove loss over 1973–2020 was due to invasion by aquaculture, while roughly 430 ha/yr was lost due to coastal erosion. A slight increase in mangrove area occurred since 2010 as a result of the implementation of a series of projects to protect against coastal erosion and to restore mangroves by the Vietnamese government and international non-governmental and governmental organizations, although the success rates of mangrove restoration are relatively low. The survival of mangrove forests in the Mekong delta is related to the main pressure drivers: pollution, land use conversion, insufficient sediment sources, coastal erosion and coastal mangrove squeeze. Therefore, an integrated mangroves and shrimp farming model is one of the most appropriate approaches to achieve a beneficial balance between both aquaculture and mangroves.

Wooden fences are nature-based supporting structures to restore mangroves in the Mekong Delta. The hydraulic functioning of wooden fences was studied in previous studies. However, the role of bathymetry in the dissipation and damping of waves by wooden fences has not been studied yet. Thus, in this study, a numerical approach is used to find the effect of the position of fences and the foreshore bathymetry, including two particular slopes of 1/200 and 1/500, on wave damping due to wooden fences. The results show that the bottom slope significantly influences the dissipation of incoming waves, the so-called pre-dissipation, before damping by the wooden fences. Differences in pre-dissipation occur between fence locations along the cross-shore slopes. The higher pre-dissipation takes place for wooden fences closer to the land, as the depth-limited wave height at the fence reduces. The efficiency in wave damping of wooden fences is also increasing as the freeboard is becoming larger for the fence located closer landward.

This textbook on Coastal Dynamics focuses on the interrelation between physical wave, flow and sediment transport phenomena and the resulting morphodynamics of a wide variety of coastal systems. The textbook is unique in that it explicitly connects the dynamics of open coasts and tidal basins; not only is the interaction between open coasts and tidal basins of basic importance for the evolution of most coastal systems, but describing the similarities between their physical processes is highly instructive as well. This textbook emphasizes these similarities to the benefit of understanding shared processes such as nonlinearities in flow and sediment transport. Some prior knowledge with respect to the dynamics of flow, waves and sediment transport is recommended.

Mangrove forests, that often act as natural coastal defences, enormously suffered due to ongoing climate change and human disturbances. Thus, it is necessary to have a countermeasure to mitigate the loss of mangroves. Wooden fences are becoming a viable nature-based solution to protect vulnerable replanted mangrove forests. However, the wooden fence's hydraulic characteristics are not yet fully understood due to the complication of branches arrangement. In the present study, a small-scale wave flume modelling of wave damping by a wooden fence was constructed using the inner branches as an inhomogeneous arrangement tested in earlier flow-resistance experiments. The physical model results indicate that the wooden fence is highly effective on wave transmission and that the effectiveness in wave reduction depends on the relative fence thickness, B/Hi. To understand the scale effect on wave transmission further, the numerical model SWASH was used with the laboratory wave data. By applying the prior experiments' drag coefficient on steady flow, the uncalibrated numerical model gave a good agreement with the wave model results, with a root-mean-square error for the total transmitted wave heights of 4.7%. After validation, potential scale effects for small scale tests were determined from scaling simulations at both full scales and the applied 1:5 model scale. These simulations were performed for a fence porosity of 0.81, and different fence thicknesses to understand scale effects between model- and full-scale. Both wave reflection and transmission at model-scale are about 5% higher than full-scale results due to the increased drag coefficient and viscous effects. The effects of fence thickness and porosity were the same in large and small scale, and much larger than the error due to scale effects. Hence testing fence efficiency at physical small scale is regarded as a useful tool, together with numerical modelling.

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.

Climate change is and will continue altering the world's coasts, which are the most densely populated and economically active areas on earth and home for highly valuable ecosystems. While there is considerable relevant research, in the authors' experience this problem remains challenging for coastal engineering. This paper reviews important challenges in this respect and identifies three key actions to address them: (a) refocusing traditional practice towards more climate-aware approaches; (b) developing more comprehensive risk frameworks that include the multi-dimensionality and non-stationarity of their components and consideration of uncertainty; and (c) building bridges between risk assessment and adaptation theory and practice. We conclude that the way forward includes numerous activities including increased observations; the attribution of coastal impacts to their drivers; enhanced climate projections and their integration into impact models; more impact assessments at the local scale; dynamic projections of spatially-distributed exposure and vulnerability; and the exploration of inherently adaptive options. Given the complexity of the possible solutions, more practical guidance is required.

Although commonly used for the validation of morphological predictions, point-wise accuracy metrics, such as the root-mean-squared error (RMSE), are not well suited to demonstrate the quality of a high-variability prediction; in the presence of (often inevitable) location errors, the comparison of depth values per grid point tends to favour predictions that underestimate variability. In order to overcome this limitation, this paper presents a novel diagnostic tool that defines the distance between predicted and observed morphological fields in terms of an optimal sediment transport field, which moves the misplaced sediment from the predicted to the observed morphology. This optimal corrective transport field has the “cheapest” quadratic transportation cost and is relatively easily found through a parameter-free and symmetric solution procedure solving an elliptic partial differential equation. Our method, which we named effective transport difference (ETD), is a variation to a partial differential equation approach to the Monge–Kantorovich L² optimal transport problem. As a new error metric, we propose the root-mean-squared transport error (RMSTE) as the root-mean-squared value of the optimal transport field. We illustrate the advantages of the RMSTE for simple 1D and 2D cases as well as for more realistic morphological fields, generated with Delft3D, for an idealized case of a tidal inlet developing from an initially highly schematized geometry. The results show that by accounting for the spatial structure of morphological fields, the RMSTE, as opposed to the RMSE, is able to discriminate between predictions that differ in the misplacement distance of predicted morphological features, and avoids the consistent favouring of the underprediction of morphological variability that the RMSE is prone to.

Over the last few decades, the Mekong Delta Coast has undergone many physical changes that have increased its vulnerability. Issues that have grown in importance are erosion, human occupation of coastal and estuarine mangroves, decreased sediment supply by the Mekong River and subsidence due to groundwater extraction. These issues have led to the loss of coastal and estuarine land and mangroves, increasing flood vulnerability and salinity intrusion. Recently, young and promising Vietnamese researchers have undertaken a number of in-depth studies to increase our understanding of the above issues. The objective of the present paper is to give a concise description of their work and place it into a broader context. The topics concerned are satellite mapping of coastal landuse changes, numerical simulation of the tide and wave climate and of coastal erosion, coastal and estuarine mangrove squeeze, wave and current damping in mangroves and wave transmission through bamboo fences. The main findings are that (1) coastal landuse has changed significantly over the last decades with the largest change due to conversion of mangroves to aquaculture and a modest change due to coastal erosion, (2) the understanding of the tide and wave climate and of the erosion has increased due to successful numerical modelling, (3) the role of mangrove squeeze along the coast and along the estuaries has been assessed, (4) the understanding of wave and current damping in mangroves and of wave transmission through bamboo fences has increased through the combined effort of laboratory and numerical modelling.

Wooden fences are applied as a nature-based solution to support mangrove restoration along mangrove coasts in general and the Mekong Delta coast in particular. The simple structure uses vertical bamboo poles as a frame to store horizontal bamboo and tree branches (brushwood). Fence resistance is quantitatively determined by the drag coefficient exerted by the fence material on the flow; however, the behaviour of drag is predictable only when the arrangement of the cylinders is homogeneous. Therefore, for more arbitrary arrangements, the Darcy-Forchheimer equations need to be considered. In this study, the law of fluid flow was applied by forcing a constant flow of water through the fence material and measuring the loss of hydraulic pressure over a fence thickness. Fences, mainly using bamboo sticks, were installed with model-scale and full-scale diameters applying two main arrangements, inhomogeneous and staggered. Our empirical findings led to several conclusions. The bulk drag coefficient (C_D) is influenced by the flow regime represented by Reynolds number. The drag coefficient decreases with the increase of the porosity, which strongly depends on fence arrangements. Finally, the Forchheimer coefficients can be linked to the drag coefficient through a related porosity parameter at high turbulent conditions. The staggered arrangement is well-predicted by the Ergun-relations for the Darcy-Forchheimer coefficients when an inhomogeneous arrangement with equal porosity and diameter leads to a large drag and flow resistance.

The significant defensive role of vegetation in general and mangroves in particular for coastal and estuarine regions has been increasingly recognized. However, understanding the shallow flow field in and around the region of vegetation is still limited. In order to gain more insight, a laboratory experiment of a vegetated compound channel was conducted. The emerged circular cylinders are a representative model for the emergent mangrove forest. Scenarios of different widths and densities of vegetation were considered. Data acquired from Acoustic Doppler Velocimetry (ADV) and force sensors have been analysed. The influences of vegetation on the shallow flow field were clarified by comparing different scenarios with and without vegetation. Furthermore, the results confirm the presence of the large horizontal coherent structures (LHCSs). The large coherent structures formed in the mixing layer at the vegetation interface emerge, promoting the transverse momentum exchange toward the forests. The LHCSs has not only boosted flow penetration into the vegetated floodplain area but also strongly disturbed the flow inside the forest. As the LHCSs move, they cause the fluctuation of the force on the cylinders. The fluctuations are largest at the vegetation edge. Negative values of stream wise forces were also recorded.

Tidal currents belong to the main driving forces shaping the bathymetry of marginal seas. A globally unique radial sand ridge field exists in the South Yellow Sea off the central Jiangsu coast, China. Its formation is related to the distinctive “radial tidal current” pattern at that location. A generally accepted hypothesis is that the “radial tidal current” is a consequence of the interference between the northern amphidromic tidal wave system and the southern incoming tidal wave. In this study, a schematized numerical tidal model was designed to investigate the tidal current system and the factors of influence in the South Yellow Sea. Concepts of the tidal current amphidromic point (CAP) and the tidal current inclination angle are utilized to analyze the inherent structure of the tidal current system. By conducting a series of numerical experiments, it is found that the Poincaré modes are necessary for the existence of “radial tidal current,” and the e-folding decay length should be smaller than the basin length. In the Yellow Sea, cross-basin phase differences due to lateral depth differences as well as open boundary conditions favor the emergence of the “radial tidal current.” Further analyses indicate that the CAP system (i.e., the co-inclination lines, the CAPs, and the tidal ellipticity) deepens the understanding on the dynamic structure of a tidal current system, and therefore, it deserves more attention in future studies.

As the third largest fresh water lake in China, Taihu Lake is suffering from serious eutrophication, where nutrient loading from tributary and surrounding river networks is one of the main contributors. In this study, water age is used to investigate the impacts of tributary discharge and wind influence on nutrient status in Taihu Lake, quantitatively. On the base of sub-basins of upstream catchments and boundary conditions of the lake, multiple inflow tributaries are categorized into three groups. For each group, the water age has been computed accordingly. A well-calibrated and validated three-dimensional Delft3D model is used to investigate both spatial and temporal heterogeneity of water age. Changes in wind direction lead to changes in both the average value and spatial pattern of water age, while the impact of wind speed differs in each tributary group. Water age decreases with higher inflow discharge from tributaries; however, discharge effects are less significant than that of wind. Wind speed decline, such as that induced by climate change, has negative effects on both internal and external nutrient source release, and results in water quality deterioration. Water age is proved to be an effective indicator of water exchange efficiency, which may help decision-makers to carry out integrated water management at a complex basin scale.