We present a novel process‐based morphodynamic model, which includes transport processes due to both velocity and acceleration skewness and a new formulation for intrawave motions, that successfully ...simulates observations of onshore sandbar migration. Results confirm findings of previous studies, in which each process was considered separately and in which sediment transport was computed from the observed water motion. However, our results indicate that accounting for the joint action of both velocity and acceleration skewnesses causes major improvement of the modeled onshore bar migration and is essential to accurately model the evolution of the entire cross‐shore bottom profile, when compared with observations. We also demonstrate that the morphodynamics in the shoaling zone are dominated by velocity skewness (bed shear stresses), while sediment transport induced by acceleration skewness (pressure gradients) controls the morphodynamics in the inner surf zone.
Key Points
A novel morphodynamic model describes field‐observed onshore sandbar migration
Both wave velocity and acceleration skewness are equally important
Accelerations control the inner surf zone; velocities dominate the shoaling zone
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The dynamics of small‐amplitude perturbations of an otherwise rectilinear coastline due to the wave‐driven alongshore sediment transport is examined at large time and length scales (years and ...kilometers). A linear stability analysis is performed by using an extended one‐line shoreline model with two main improvements: (1) the curvature of the coastline features is accounted for and (2) the coastline features are assumed to extend offshore as a bathymetric perturbation up to a finite distance. For high incidence angles, instability is found in accordance with Ashton et al. (2001). However, it is seen that instability is inhibited by high waves with long periods and gently sloping shorefaces so that in this case the coastline may be stable for any angle. Similarly, there is no instability if the bathymetric perturbation is confined very close to the coast. It is found that the traditional linearized one‐line model (Larson et al., 1987) tends to overpredict the coastline diffusivity. The overprediction is small for the conditions leading to a stable coastline and for moderate incidence angles but can be very dramatic for the conditions favoring instability. An interesting finding is that high‐angle waves instability has a dominant wavelength at the linear regime, which is in the order of 4–15 km, one to two orders of magnitude larger than the length scale of surf zone rhythmic features. Intriguingly, this is roughly the same range of the wavelength of some observed shoreline sand waves and, in particular, those observed along the Dutch coast. A model application to this coast is presented.
This review highlights the important role of the depth‐averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore ...rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface‐connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth‐averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth‐averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite‐amplitude behavior and their decay can also be explored.
Key Points
A methodology to examine interactions between flow and morphology is presented
It is applied to beach cusps, crescentic/transverse bars and inner shelf ridges
The depth‐averaged sediment concentration is crucial in the feedback mechanisms
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense (dangerous) currents flowing seaward ...through rip channels. Understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics, the shape of the coastline, and the exchange of floating material with the shoreface. Although the mechanisms behind the meandering of an originally straight bar have been studied extensively, a clear physical explanation on the crescentic bar straightening was missing. Recent field observations have highlighted that this morphological reset can be due to wave obliquity. By using a two‐dimensional horizontal morphological model, we show that the bar straightening by oblique waves occurs because the rip current is both weakened in intensity and shifted downdrift from the channel deepest section. The technique employed is useful for the study of other types of bed forms.
Key Points
We show why oblique waves induce morphological reset of surf‐zone rhythmic bars
Rip currents are weakened and shifted alongshore for increasing wave obliquity
Crescentic bar straightening is highly sensitive to the offshore wave angle
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The coupling between topography, waves and currents in the surf zone may self-organize to produce the formation of shore-transverse or shore-oblique sand bars on an otherwise alongshore uniform ...beach. In the absence of shore-parallel bars, this has been shown by previous studies of linear stability analysis, but is now extended to the finite-amplitude regime. To this end, a nonlinear model coupling wave transformation and breaking, a shallow-water equations solver, sediment transport and bed updating is developed. The sediment flux consists of a stirring factor multiplied by the depth-averaged current plus a downslope correction. It is found that the cross-shore profile of the ratio of stirring factor to water depth together with the wave incidence angle primarily determine the shape and the type of bars, either transverse or oblique to the shore. In the latter case, they can open an acute angle against the current (up-current oriented) or with the current (down-current oriented). At the initial stages of development, both the intensity of the instability which is responsible for the formation of the bars and the damping due to downslope transport grow at a similar rate with bar amplitude, the former being somewhat stronger. As bars keep on growing, their finite-amplitude shape either enhances downslope transport or weakens the instability mechanism so that an equilibrium between both opposing tendencies occurs, leading to a final saturated amplitude. The overall shape of the saturated bars in plan view is similar to that of the small-amplitude ones. However, the final spacings may be up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller or larger. In the case of alongshore migrating bars, the asymmetry of the longshore sections, the lee being steeper than the stoss, is well reproduced. Complex dynamics with merging and splitting of individual bars sometimes occur. Finally, in the case of shore-normal incidence the rip currents in the troughs between the bars are jet-like while the onshore return flow is wider and weaker as is observed in nature.
The process of formation of a rip channel/crescentic bar system on a straight, sandy coast is examined. A short review of earlier studies is presented. A morphodynamic stability model is then ...formulated. The resulting model includes a comprehensive treatment of shoaling and surf zone hydrodynamics, including wave refraction on depth and currents and waves. The sediment transport is modeled using a total load formula. This model is used to study the formation of rip currents and channels on a straight single‐barred coast. It is found that this more comprehensive treatment of the dynamics reveals the basic rip cells predicted in earlier studies for normal incidence. Also as before, cell spacings (λ) scale with shore‐to‐bar crest distance (Xb), while growth rates decrease. The λ increases with offshore wave height (H) up to a saturation value; increasing H also increases instability. Experiments at off‐normal wave incidence (θ > 0) introduce obliquity into the evolving bed forms, as expected, and λ increases approximately linearly. the e‐folding times also increase with θ. At normal incidence, λ increases weakly with wave period, but at oblique angles, λ decreases. Tests also reveal the presence of forced circulation cells nearer to the shoreline, which carve out bed forms there. The dynamics of these forced cells is illustrated and discussed along with the associated shoreline perturbation. Transverse bars are also discovered. Their dynamics are discussed. Model predictions are also compared with field observations. The relevance of the present approach to predictions of fully developed beach states is also discussed.
Sand ridges, with length scales of several km, are prominent features of the seafloor landscape of many sandy continental shelves. Knowledge about the extent to which these ridges influence the ...large‐scale (i.e., decadal and kilometer scales) morphodynamic evolution of the adjacent shoreline and vice versa (shelf‐shoreline morphodynamic coupling) is limited. The present work aims at quantifying this coupling by using a coupled nonlinear shelf‐shoreline model forced by tides and different wave conditions. Model results show that the presence of sand ridges on the shelf creates longshore non‐uniform wave patterns, which act as a forcing template for the morphodynamic development of the shoreline. The shelf‐shoreline coupling primarily works one way, meaning that the morphodynamic evolution of the shelf affects the evolution of the shoreline. When wave propagation is predominantly aligned with the long axis of the shelf ridges, the forced shoreline undulations are so prominent, that they affect the shelf morphology (significant two‐way coupling). Moreover, for those waves, the longshore spacing of the ridges is strongly imprinted on the shoreline morphology. Weaker shoreline undulations develop for waves that propagate more across the ridges and the weakest for time‐varying wave conditions with large variability in their angles of propagation. Model results compare fairly well with observations. Physical mechanisms underlying the different morphodynamic responses of the coupled shelf‐shoreline system to different wave conditions are also given.
Plain Language Summary
Huge sand ridges (scales of 10 kilometers) are observed on the seafloor of many sandy continental shelves. Knowledge about the effects of these ridges on the evolution of the adjacent shoreline is limited. The present study addresses these effects using a coupled shelf‐shoreline model. Model results show that, when waves propagate along the sand ridges, the latter cause the development of large shoreline undulations, which have the same longshore spacing as the ridges. The shoreline undulations are weak for time‐varying wave conditions with large wave directionality (i.e., large variability in their wave angles). Wave climates with a large wave directionality, as is the case of the Belgium coast, are expected to have weak shoreline undulations compared with wave climates with a small wave directionality (e.g., wave climate of Fire Island)
Key Points
Ridges on the shelf provide a forcing template for the large‐scale (decadal and kilometer scales) morphodynamic development of the shoreline
When waves propagate along ridges, strong shoreline undulations develop, which feed back to the shelf morphology (two‐way coupling)
For time‐varying waves with large variability in their angles of propagation, shoreline undulations are small
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Many inner continental shelves are characterized by the presence of large rhythmic bedforms, such as shoreface-connected ridges and the more offshore located sand ridges, which have heights of ...several meters and are spaced several kilometers apart. This study focuses on explaining the observed orientation difference between shoreface-connected sand ridges and the more offshore located ridges. For this, an existing idealized morphodynamic model is used, but modified such that sea level rise simultaneously induces a steepening of the inner shelf and a retreating shoreface. Different settings (rate of sea level rise; landward depth of the inner shelf) are systematically explored. For each setting, the gross characteristics of ridges (growth rate, height, migration, orientation) during their initial formation and long-term evolution are quantified. Model results show that a rising sea level and associated shoreface retreat and shelf steepening lead to new ridges in the shallow area of the inner shelf, which remain active in time (i.e. ongoing growth and downstream migration in time). Old ridges that were already formed in the antecedent area of the shelf and which in the course of time experience deeper water become less active with the rising sea level. In the case that migration of the offshore parts of the ridges vanishes, these parts change orientation to become more shore-parallel compared with the active onshore parts of these ridges. In the case of small landward depths of the inner shelf and a decreasing rate of sea level rise, the active onshore parts migrate too fast, thereby causing the drowned offshore parts to detach and to become inactive. The characteristics of modeled shore-oblique shoreface-connected and more parallel offshore located ridges agree with those of observed sand ridges.
•Study impact of sea level rise on evolution of inner shelf sand ridges.•Shoreface retreat and steepening of the inner shelf are accounted for.•Formation of new ridges and drowning of old ridges with increasing sea level.•Migration and orientation difference between new and drowned ridges are related.•Too large migration difference: drowned ridges detach from the new ones.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A morphodynamic model has been applied to explain the characteristics of transverse sandbars observed in the inner surf zone of open beaches. The model describes the feedback between waves, rollers, ...depth‐averaged currents and bed evolution, so that self‐organized processes can develop. The modeled bar characteristics, i.e. wavelength (30–70 m), crest orientation (up‐current) and the e‐folding growth time (about 12 hr) are in good agreement with those of observed transverse bars at Noordwijk beach, the Netherlands, but modeled migration speeds (tens of meters per day), turn out to be a factor 2 larger than those observed. The wavelength increases with the distance between the shoreline and the peak of the longshore current and the migration speed is correlated with the maximum longshore current. The model also explains why transverse bar formation at Noordwijk occurs for obliquely incident waves of intermediate heights. Realistic positive feedback leading to formation of up‐current oriented bars like those observed is only obtained if a term related to the turbulence sediment resuspension created by the rollers is included in the transport formula. In that case, the depth‐averaged sediment concentration decreases seaward across the inner surf zone, enhancing the convergence of sediment transport in the offshore directed flow perturbations that occur over the up‐current bars. This offshore current deflection is mainly caused by frictional torques, but the roller radiation stresses also play an important role.
Key Points
A model is used that reproduces the characteristics of observed transverse bars
Bars are up‐current oriented and they grow for intermediate oblique waves
The physical mechanisms underlying transverse bar formation are revealed
A morphodynamic model has been extended to gain more fundamental knowledge about the formation of nearshore sand bars. The model describes feedbacks between waves, rollers, depth-averaged currents ...and bed evolution, so that self-organized processes can develop. Offshore wave, wind and tide conditions and the bathymetry measured at Egmond site (the Netherlands) are firstly applied to compute the longshore current and wave height profiles. A comparison of the results with field data of that beach reveals that including the roller dynamics and the forcing by wind shear stresses and tide-induced sea level slopes is essential to accurately simulate the longshore current. Several model parameters are calibrated to minimize the root-mean-square errors. Subsequently, the wave and bathymetric conditions measured at Noordwijk site (the Netherlands) are used to compare modeled crescentic bars and up-current oriented finger bars with the nearshore sandbars observed at this site. Positive feedback leading to realistic formation of up-current bars only occurs if a novel term, i.e., the resuspension of sediment due to the turbulence created by the rollers, is included in the model. The modeled wavelength, crest orientation and growth rate agree with those of the observed up-current bars but the model overestimates the migration rate. The wavelength and migration rates of crescentic bars in case of oblique wave incidence are more accurately simulated if the reference longshore current is modeled including the roller dynamics.
► A morphodynamic model is applied to understand the formation of nearshore sandbars. ► The model includes the feedback between waves, rollers, currents and bed evolution. ► Including rollers and wind is crucial to accurately simulate the longshore current. ► Up-current oriented bars can only be modeled if resuspension by rollers is included. ► Crescentic bar features are more accurately modeled if roller dynamics is included.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
PDF
