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The radial sand ridges (denoted as "RSRs" hereafter) in the Southern Yellow Sea, China, are morphologically striking because of the remarkable size and radial planar orientation, standing out as a unique coastal geomorphology among the worldwide sand ridge systems. The formation of this giant fan‐shaped geomorphic feature requires delicate conditions and awaits in‐depth investigation. Using an idealized morphodynamic model, this study unravels the governing factors for the formation of the unique large‐scale RSRs, in comparison with other types of sand ridge systems over the world. The effects of the M2 tidal constituent, the Coriolis forcing, the bed resistance, and the initial water depths on the morphodynamic behavior of the RSRs are explored. Numerical results indicate that the tidal regime, characterized by rotational and progressive current action associated with the tidal bulge, is dependent on the eastern coastline of China as well as latitudinal effects. Through the comparison between the simulated and the measured morphology, this tidal regime is demonstrated to be the key driver in forming and maintaining the present‐day RSRs. The runs with different parameters further suggest that the asymmetric pattern of the RSRs, which shows larger northern sand ridges than the southern ones, results from both the asymmetric distribution of current activity caused by the tidal bulge and unequal sediment supply. Overall, this study highlights the delicate condition, predominantly represented by the particular currents set up by the tidal wave system and the sediment supply, required to shape the striking large‐scale RSRs in the Southern Yellow Sea. Key Points: The unique giant fan‐shaped morphology of the radial sand ridges (RSRs) is numerically simulatedThe formation of the RSRs requires delicate conditions governed by currents originating from the rotational, progressive tidal wave systemThe mechanism underlying the asymmetric spatial pattern of the RSRs is revealed [ABSTRACT FROM AUTHOR]

Abstract: A new process-based approach is introduced for a more efficient computation of the overflow-induced growth of an erosional channel in a non-cohesive homogeneous narrow landmass such as the breach growth in a sand-dike. The approach is easy to incorporate in a 1D/2DV morphodynamic model to compute the channel growth both vertically and laterally. The flow modeling is based on the shallow water equations. For modeling the channel growth, a set of closed equations describing the channel growth in both vertical and lateral direction has been derived in connection with several new morphologic parameters such as the representative channel width and the channel cross-sectional growth index. The approach has been applied to simulate the breach growth in sand-dikes and the morphological development of wave overwash across sand barriers. The computational results bear fairly good resemblance with existing experimental data. [Copyright &y& Elsevier]

Abstract: The phenomenon of overtopping is traditionally studied for well-emerged harbour structures and often focuses on safety and stability. In this paper laboratory tests are presented and analysed to sharpen the hypothesis that overtopping is capable of changing the horizontal circulation pattern around low-crested structures. A unique data set from laboratory experiments was acquired in the wave basin at Delft University of Technology. The experiments were performed using an emerged impermeable low-crested structure (three freeboards and three different wave conditions for each freeboard) and yielded nine different combinations of set-up and overtopping driving forces. Using this information it was possible to quantify the changes in cross-shore and longshore velocity induced by the overtopping and the set-up changes under the different freeboard and wave conditions described. It is found that overtopping enhances the outgoing flows (longshore velocities parallel to the structure) away from the lee side of the structure and dampens the water level gradient driven flow towards the structure. [Copyright &y& Elsevier]

The coastal-tract approach to coastal morphodynamics, described in the companion paper (The Coastal-Tract Part 1), provides a framework for aggregation of process and spatial dimensions in modeling low-order coastal change (i.e., evolution of the shoreline, continental shelf and coastal plain on time scales of 10² to 10³ years). Behavior-oriented, coastal-change models encapsulate aggregate dynamics of the coastal tract. We apply these models in a coastal-tract framework to illustrate the use of the concept, and to explore low-order morphological coupling under different environmental settings. These settings are characterized by data-models that we have constructed from four contrasting continental margins (NW Europe, US Pacific, US Atlantic, and SE Australia). The gross kinematics of the coastal tract are constrained and steered by sediment-mass continuity. The rate of coastal advance or retreat is determined quantitatively by the balance between the change in sediment accommodation-space, caused by sea-level movements, and sediment availability. If the lower shoreface is shallower than required for equilibrium (negative accommodation), then sand is transferred to the upper shoreface (NW Europe, US Pacific, and SE Australian cases modelled) so that the shoreline tends to advance seaward. This tendency also occurs when relative sea level is falling (coastal emergence). Coastal retreat occurs when the lower shoreface is too deep for equilibrium (positive shoreface accommodation). This sediment-sharing between the upper and lower shoreface is an internal coupling that governs first-order coastal change. The upper shoreface and backbarrier (lagoon, estuary or mainland) also are coupled in first-order coastal change. Sediment accommodation-space is generated in the backbarrier by sea-level rise (and reduced by sea-level fall), but the amount of space is also moderated by influx of fine sediments from the coast, or sand and mud from fluvial sources. Remaining space can then be occupied by sand transferred from the upper shoreface causing a retreat of the latter (transgressive phases modelled for NW Europe, US Atlantic, and SE Australian cases). [ABSTRACT FROM AUTHOR]

Evolution of coastal morphology over centuries to millennia (low-order coastal change) is relevant to chronic problems in coastal management (e.g., systematic shoreline erosion). This type of coastal change involves parts of the coast normally ignored in predictions required for management of coastal morphology: i.e., shoreline evolution linked to behavior of the continental shelf and coastal plain. We therefore introduce a meta-morphology, the coastal tract, defined as the morphological composite comprising the lower shoreface, upper shoreface and backbarrier (where present). It is the first order-system within a cascade hierarchy that provides a framework for aggregation of processes in modeling low-order coastal change. We use this framework in defining boundary conditions and internal dynamics to separate low-order from higher-order coastal behavior for site-specific cases. This procedure involves preparation of a data-model by templating site data into a structure that complies with scale-specific properties of any given predictive model. Each level of the coastal-tract cascade is distinguished as a system that shares sediments internally. This sediment sharing constrains morphological responses of the system on a given scale. The internal dynamics of these responses involve morphological coupling of the upper shoreface to the backbarrier and to the lower shoreface. The coupling mechanisms govern systematic lateral displacements of the shoreface, and therefore determine trends in shoreline advance and retreat. These changes manifest as the most fundamental modes of coastal evolution upon which higherorder (shorter-term) changes are superimposed. We illustrate the principles in a companion paper (The Coastal Tract: Part 2). [ABSTRACT FROM AUTHOR]

This paper focuses on a specific form of groins, Permeable Pile Groins (PPGs), consisting of a single or double rows of wooden piles. With only few experiments and simulations available to study the hydraulic functioning of PPGs, the correlation between the effectiveness and the characteristics of the groin system has yet to be fully understood. This paper presents the application of SWASH, a non-hydrostatic wave-flow model to simulate flow fields affected by PPGs. The SWASH model was calibrated to correctly reproduce longshore current fields in the laboratory. Then, introducing PPGs in the model, the simulation results are compared with available experimental measurements to investigate current-PPG interaction. The simulation results, which generally agree well with the measurements, show that PPGs hardly attenuate wave energy but considerably retard longshore currents within the groin fields. Rip currents are predicted to develop at both flank sides of each pile groin, due to local positive water level gradients toward the pile groins. • Different aspects of the functioning of pile groins are revisited. • Numerical simulations are calibrated and validated by laboratory experimental data sets. • The effectiveness of pile groins is summarised. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the ability of the numerical phase resolving model SWASH (Simulating WAves till SHore) to hindcast wave-induced longshore currents is evaluated. Using default settings for all processes modelled, highly accurate results are found for wave heights, mean water levels and longshore currents. While wave current interaction is intrinsically modelled, insights into the spatial variation of wave driven longshore currents are found. Additionally, vertical variations of modelled longshore currents have been compared. Depth uniform profiles of longshore current within surf zone are noted on plane beaches under regular waves, except for minor deviations near the shoreline. The apparent validity of a depth-uniform longshore current encourages the use of a depth-averaged moment balance equation to compute the longshore current. A simpler model is shown to also be able to predict a proper magnitude of longshore current, although the cross-shore distribution – in contrast with SWASH - needs tuning for the eddy viscosity and the bottom friction coefficient, since the distribution of the wave-induced longshore current heavily depends on lateral mixing. Highlights • The ability of SWASH modelling longshore currents is fairly well validated by laboratory experimental data sets. • Robust estimations are obtained by SWASH model with a high resolution and default free parameters. • SWASH model has comparable results with the calibrated 1D model. [ABSTRACT FROM AUTHOR]

Abstract: A process-based morphodynamic model is used to simulate the evolution of tidal inlets towards equilibrium when subjected to a range of tide and wave conditions. Specific attention is given to the evolution of the cross-sectional area of the inlet channel. Equilibrium values of cross-sectional areas are found by approaching the equilibrium from two initial conditions, i.e. by starting an experiment with an initial value of the cross-sectional area that is smaller than the estimated equilibrium value and an initial value that is larger than the estimated equilibrium value. Similar to nature a good correlation between cross-sectional area and tidal prism is found. This suggests that the physical–mathematical formulations in the model, used to describe the hydrodynamic and sedimentary process, are a good representation of the morphodynamics in the real world and that the conceptual idea of Escoffier on the cross-sectional stability of inlets is valid. [Copyright &y& Elsevier]

Abstract: This paper attempts to highlight issues that are relevant in the application of the Parabolic Bay Shape Equation (PBSE) to a non-equilibrium bay. For this case, the bay of Imbituba in southern Brazil was chosen. The construction of a breakwater to shelter the port of Imbituba in the south of the bay was accompanied by an increase in sedimentation to the port area from an eroding downdrift beach. Superimposed plots of the coastline of the Bay of Imbituba from different years confirm a general trend of accretion of the southern part of the bay accompanied with a retreat of the coastline in the northern part. After the application of the PBSE it became clear that the breakwater caused a change in the equilibrium state of the bay. Between 1947 and 2001 the Bay of Imbituba has changed from a dynamic equilibrium to a natural beach reshaping or self-reshaping. The tendency of the sedimentation of the southern part of the bay can be explained by the SEP associated with the new updrift diffraction point (tip of the breakwater): The seaward position of the SEP predicts a need for additional sediment in order to achieve a stable plan form. [Copyright &y& Elsevier]

Abstract: This paper aims to quantify the uncertainty in the application of the Parabolic Bay Shape Equation (PBSE) for a bay in static equilibrium. By means of expert elicitation, a database was generated consisting of the position of the control points that determine the Static Equilibrium Planform (SEP). The elicitation consisted of two parts. In Part 1, 22 expert volunteers were asked to place the three control points that determine the SEP on a vertical aerial photograph of Taquaras–Taquarinhas Bay, a morphologically stable bay in the south of Brazil, approximately 1800m in width and with an indentation of 750m. The distribution of the location of the SEP along four evenly spaced (200m) profiles in the southern part of the bay was determined. The overall bias of the location of the SEP calculated over the four profiles was in the order of 41m (landward) and the average standard deviation was 116m. These parameters increased when moving alongshore toward the curved section of the bay. This means that the uncertainty in the application of the PBSE is dependent on the particular point of interest along the bay. In Part 2 of the elicitation, 30 volunteers participated. This time the consequence of the placement of the control points (the corresponding SEP) was visible. Comparing the results from Parts 1 and 2, it was observed that when volunteers were directly confronted with the result of the placement of the control points (a plotted SEP) a much smaller variation in the position of the SEP occurred. This implies that when feedback on the result is provided the PBSE is a more robust method. [Copyright &y& Elsevier]

Abstract: Incipient motion of coarse particles under regular shoaling waves is examined. Experiments are performed to investigate the effects of bed fluid acceleration on coarse particle stability. By varying wave height, wave period and water depth, combinations of similar peak orbital velocities and weak to strong intra-wave accelerations were created. The particles used in these experiments have two different sizes both of a centimeter order-of-magnitude. The data confirm that acceleration is important for the initiation of motion, since combinations of similar orbital velocity and varying acceleration magnitude resulted in no motion, some motion and motion as acceleration increased. Qualitatively we found that initiation of motion occurs at or is very close to the maximum shear stress due to the combined effects of drag/lift and acceleration as introduced by Nielsen and Callaghan [Nielsen, P. and Callaghan, D.P. (2003), Shear stress and sediment transport calculations for swash zone modelling. Coastal Engineering, 47, pp. 347–354]. However, quantitatively their formulation does not lead to convincing discrimination between motion and no-motion. We expect this to be due to the assumption that the coefficients for drag/lift and acceleration in their formulation are taken equal and constant. From literature and from plotting our data against the Keulegan–Carpenter number we expect that these coefficients strongly vary due to flow separation effects. To arrive at a more convincing discrimination between motion and no-motion we introduce a new fluid acceleration descriptor for nonlinear shoaling waves. The combination of this descriptor with a Reynolds number Re g more clearly delineates the regions with particle motion and without particle motion and has the potential to serve as a descriptor of the incipient motion of coarse particles under nonlinear and skewed, regular waves. [Copyright &y& Elsevier]

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 effects of varying vertical morphologies of mangroves on wave attenuation were analyzed. • The new parameter hydraulic submerged volume index (HSVI) can linearly predict wave decay factor well. • The empirical formulas of the bulk drag coefficient were modified to incorporate the effects of submerged canopies. [ABSTRACT FROM AUTHOR]

Abstract In the present study a new multi-fractional, depth-averaged sediment transport module was developed and embedded into a morphodynamic model for a sand-silt mixed shallow water environment. Subsequently, the model was applied to the case of the Jiangsu coast, which features a silt enriched sedimentary environment bordered by two large-scale geomorphological units: the Old Yellow River Delta (OYRD) in the north and the Radial Sand Ridge Field (RSRF) in the south. Based on this case, the predictive abilities of the present model are assessed on both the short-term and the long-term. Comparisons with measurements over two successive tidal cycles indicate that the present model produces very good results on short-time scales. The model performance is extended and further validated by comparing the overall annual Suspended Sediment Concentration (SSC) pattern, the annual morphological changes, the annual sediment budget and the evolution trend of the bed composition. Also, these long-term results agree well with existing observations over the past several decades. Hence, an essential feature of the present modelling approach is the ability to simulate sediment transport and morphological changes over a relatively long time span (i.e., time scale of years) in a sand-silt mixed sedimentary environment, based on its validated short-term performance. Highlights • A multi-fractional sediment model is developed for a silt-enriched environment. • The model is validated for its ability on successively simulate sediment transport over period from days to years. • The simulated long-term sediment dynamics is consistent with observation-based estimates. [ABSTRACT FROM AUTHOR]

Sand nourishments are presently widely applied to maintain or enhance coastal safety and beach width. Over the last decades, global sand nourishment volumes have increased greatly, and the demand for nourishments is anticipated to increase further in coming decades due to sea level rise. With the increase in nourishment size and the request for more complex nourishment shapes, an adequate prediction of the morphodynamic evolution is of major importance. Yet, neither the skill of current state-of-the-art models for such predictions nor the primary drivers that control the evolution are known. This article presents the results of a detailed numerical modelling study undertaken to examine the model skill and the processes governing the initial morphological response of the Sand Engine and the adjacent coastline. The process-based model Delft3D is used to hindcast the first year after completion of the mega-nourishment. The model reproduces measured water levels, velocities and nearshore waves well. The prediction of the morphological evolution is consistent with the measured evolution during the study period, with Brier Skill Scores in the ‘Excellent’ range. The model results clearly indicate that the sand eroded from the main peninsular section of the Sand Engine is deposited along adjacent north and south coastlines, accreting up to 6 km of coastline within just one year. Analysis of model results further show that the erosional behaviour of the Sand Engine is linearly dependent on the cumulative wave energy of individual high energy wave events, with the duration of a storm event being more dominant than the maximum wave height occurring during the storm. The integrated erosion volume due to the 12 events with the highest cumulative wave energy density accounts for about 60% of the total eroded volume of the peninsula, indicating that the less energetic wave events, with a higher probability of occurrence, are also important for the initial response of the Sand Engine. A structured model experiment using the verified Delft3D model indicates that wave forcing dominates the initial morphological response of the Sand Engine, accounting for approximately 75% of the total erosion volume in the first year. The vertical tide is the second most important factor accounting for nearly 17% of the total erosion volume, with surge, wind and horizontal tide playing only a minor role. [ABSTRACT FROM AUTHOR]

The Optical Backscatter Sensor (OBS) has been widely used to measure suspended sediment concentration in both field and laboratory conditions, even though it is very sensitive to many factors. The most significant factor suggested is the grain size. In order to enhance the quality of OBS data, an improved approach is proposed, based on the “mixture of linear component response” method (Green and Boon III, 1993) to account for the effect of grain size. In addition to an original sediment sample, which commonly serves as a single calibration material, an accompanying sediment sample is necessary to calibrate OBS sensors. A multi-fraction sediment model is used to predict the grain size distribution in suspension. Compared with existing methods, the improved approach does not require a sieving procedure nor the assumption that the sediment fractions of the calibrated sediment sample exhibit the same sensitivities as those of the suspended sediments. The applicability of our method has been verified by a series of laboratory experiments over silt–sand mixtures. The results show that this method successfully yields continuous concentration profiles, which agree well with the measurements using a suction method. The converted concentrations of the time-averaged OBS measurements by traditional method and by the improved method, respectively, are compared with the suction measurements. The results of linear regression analyses show that the coefficient of determination increases (e.g. from 0.55 to 0.92 for wave–current conditions) and the Root Mean Square Error decreases (e.g. from 0.97 to 0.39 for wave–current conditions). It demonstrates that improved method enhances the quality of OBS conversion. Furthermore, suggestions on selecting the accompanying sediment sample (i.e. on grain size and composition) are given. [ABSTRACT FROM AUTHOR]

Sand nourishments are a widely applied technique to increase beach width for recreation or coastal safety. As the size of these nourishments increases, new questions arise on the adaptation of the coastal system after such large unnatural shapes have been implemented. This paper presents the initial morphological evolution after implementation of a mega-nourishment project at the Dutch coast intended to feed the surrounding beaches. In total 21.5 million m 3 dredged material was used for two shoreface nourishments and a large sandy peninsula. The Sand Engine peninsula, a highly concentrated nourishment of 17 million m 3 of sand in the shape of a sandy hook and protruding 1 km from shore, was measured intensively on a monthly scale in the first 18 months after completion. We examine the rapid bathymetric evolution with concurrent offshore wave forcing to investigate the feeding behaviour of the nourishment to the adjacent coast. Our observations show a large shoreline retreat of O (150 m) along the outer perimeter of the peninsula, with locally up to 300 m retreat. The majority (72%) of the volumetric losses in sediment on the peninsula (1.8 million m 3 ) were compensated by accretion on adjacent coastal sections and dunes, confirming the feeding property of the mega nourishment. Further analyses show that the morphological changes were most pronounced in the first 6 months while the planform curvature reduced and the surf zone slope flattened to pre-nourishment values. In the following 12 months the changes were more moderate. Overall, the feeding property was strongly correlated to incident wave forcing, such that months with high incoming waves resulted in more alongshore spreading. Months with small wave heights resulted in minimal change in sediment distribution alongshore and mostly cross-shore movement of sediment. [ABSTRACT FROM AUTHOR]

A series of flume experiments has been conducted to investigate sediment transport of sand–silt mixtures in both wave-only and wave-with-current conditions. Two types of sediments collected from a typical silty tidal flat were used: a silt-sized mixture with median grain size of 46 μm, and a very fine sand-sized mixture with median grain size of 88 μm. A high concentration layer (HCL) was observed near the bottom together with ripples under wave-only conditions. Sediment concentrations inside the HCL are quasi-stationary with the bulk Richardson number approaching a constant value. The thickness of the HCL can be scaled with approximately two times the damped wave boundary layer thickness. For the concentration profiles, we find that the vertical profile of the silt concentration appears different from the profile of the sand concentration, since the silt concentration decreases logarithmically within HCL, while homogeneously distributes outside the HCL. Finally, the reference concentration formulation of van Rijn (2007b) was recalibrated for the silt classes and applied in a multi-fraction model to predict the vertical concentration profile for silt and sand classes. The results show a promising agreement with the measurements, for both wave-only and wave-with-current conditions. [ABSTRACT FROM AUTHOR]

Abstract: Primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology, based on many field observations. However, detailed mechanistic models were restricted to the river-dominated delta class. Analogous to the concept of morphological conditioning, here we show that antecedent stratigraphy controls morphometrics of prograding river-dominated delta distributary networks under steady sea level. Further, we use coupled hydrodynamic-morphodynamic-stratigraphic detailed numerical modelling to assess the influence of wind-generated waves and tides on clastic river delta formation. Our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth-bar induced flow bifurcation and upstream channel shifting. Windwave action suppresses sequestration of fine sediments on the developing delta plain, entailing relatively smooth shorelines, perturbed by a limited number of distributary channels. In contrast, tide-influenced river deltas are found to prograde mainly via lengthening of initially-formed, relatively stable distributaries, as well as being characterized by cyclicity in deposits (interbedding of sands and silts). These results provide a framework for physics-based river delta modelling under various environmental conditions. Particularly, our findings suggest that relatively low-energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas. [ABSTRACT FROM AUTHOR]

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. • Reviews the challenges of climate change risks and adaptation in coastal areas. • Suggests refocusing traditional practice towards more climate-aware approaches. • Proposes multi-dimensional and non-stationary risk frameworks. • Emphasises the need to consider uncertainty. • Explores bridges between risk assessment and adaptation theory and practice. [ABSTRACT FROM AUTHOR]