Francesca Ribas

Modeling large scale shoreline sand waves under oblique wave incidence

waves develop with wavelengths between 2 and 5 km, they migrate downdrift at about 0.5 km/yr and they reach amplitudes up to 120 m within 13 years. Larger wave obliquity, higher waves and shorter wave periods strengthen the shoreline instability. Cross-shore transport is essential for the instability and faster cross-shore dynamics leads to a faster growth of the sand waves. Simulations with variable wave incidence angles (alternating between 60 � and 30 � ) show that a large proportion of high angle waves is required for spontaneous sand wave formation (at least 80%). Insight is provided into the physical mechanism behind high angle wave instability and the occurrence of a optimal length scale for sand wave growth. The generic model results are consistent with existing observations of shoreline sand waves, in particular with those along the southwest coast of Africa.

Understanding coastal morphodynamic patterns from depth‐averaged sediment concentration

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.

Assessing the Suitability of Video Imaging for Studying the Dynamics of Nearshore Sandbars in Tideless Beaches

Nearshore sandbars, an important natural defense mechanism of the beaches, can be monitored using shore-based video systems. Before studying bar dynamics with video images, we must establish the relationship between the real bar positions and the videoed bar positions (detected by the preferential wave breaking on the shallows). This analysis becomes essential in the two studied tideless beaches of Barcelona due to the critical differences with respect to the sites studied previously. Bogatell beach is terraced (without a trough) in more than 50% of the profiles. There, the videoed barlines are a good proxy of the terrace edge position. In La Barceloneta beach, with dominance of barred profiles, the videoed barlines better represent the bar crest position. On average, the obtained distances between real and videoed bar positions ¿r are 10-15 m, with the videoed barlines located shoreward. Changes in the bathymetric profile shape and the root-mean-squared wave height H rms induce a variability of ¿r of 16 m in La Barceloneta and 13 m in Bogatell. This apparent variability masks the real changes in bar position and should preferably be reduced before further analysis. As a highly significant correlation between ¿r and H rms is detected in the two beaches, the proposed reduction method consists of sampling at a specific range of H rms. This diminishes the variability by 10% to 14 m in La Barceloneta and 11 m in Bogatell. This paper confirms the suitability of using video systems for monitoring bars and terraces in the Barcelona beaches.

Shoreline Instability due to Very Oblique Wave Incidence: Some Remarks on the Physics

Abstract Previous research has shown that a very oblique wave incidence on a coast may render a rectilinear shoreline unstable. Here, we present some further insight into the physics of such instability. The obliqueness of wave incidence has two effects on the alongshore drift: (i) a direct effect on the relative angle between the wave fronts and the shoreline, and (ii) an indirect effect on the breakers height via the wave energy spreading as the waves refract when they approach the shore. The direct effect turns out to be, in all incidents, stabilizing, and the instability occurs only from the effects of the wave energy spreading, which dominate for large incidence angles. Whereas earlier studies have pointed only to the alongshore drift as the cause of the instability, we show that the instability mechanism involves both the surf and the shoaling zones, so that the link provided by the cross-shore sediment transport is also essential.

Formation mechanisms for self‐organized kilometer‐scale shoreline sand waves

The feedbacks between morphology and waves through sediment transport are investigated as a source of kilometer-scale shoreline sand waves. In particular, the observed sand waves along Srd. Holmslands Tange, Denmark, are examined. We use a linear stability model based on the one-line approximation, linking the bathymetry to the perturbed shoreline. Previous models that consider the link by shifting the equilibrium profile and neglecting the curvature of the depth contours predict a positive feedback only if the offshore wave incidence angle (?c) is above a threshold, ?c?42°. Considering curvilinear depth contours and using a linearly decaying perturbation in bed level, we find that ?c can vary over the range 0–90° depending on the background bathymetric profile and the depth of closure, Dc. Associated to the perturbed wave refraction, there are two sources of instability: the alongshore gradients in wave angle, wave angle mechanism, and the alongshore gradients in wave energy induced by wave crest stretching, wave energy mechanism. The latter are usually destabilizing, but the former are destabilizing only for large enough Dc, steep foreshores, and gently sloping shorefaces. The critical angle comes out from the competition between both mechanisms, but when both are destabilizing, ?c=0. In contrast with earlier studies, the model predicts instability for the Holmslands Tange coast so that the observed sand waves could have emerged from such instability. The key point is considering a larger Dc that is reasonably supported by both observations and wave climate, which brings the wave angle mechanism near the destabilizing threshold.

On the mechanism of wavelength selection of self-organized shoreline sand waves

Sandy shorelines exposed to very oblique wave incidence can be unstable and develop self-organized shoreline sand waves. Different types of models predict the formation of these sand waves with an initially dominant alongshore wavelength in the range 1–10 km, which is quite common in nature. Here we investigate the physical reasons for such wavelength selection with the use of a linear stability model. The existence of a minimum wavelength for sand wave growth is explained by an interplay of three physical effects: (a) largest relative (to the local shoreline) wave angle at the downdrift flank of the sand wave, (b) wave energy concentration at the updrift flank due to less refractive energy dispersion, and (c) wave energy concentration slightly downdrift of the crest due to refractive focusing. For small wavelengths, effects (a) and (c) dominate and cause decay, while for larger wavelengths, effect (b) becomes dominant and causes growth. However, the alongshore gradients in sediment transport decrease for increasing wavelength, making the growth rate diminish. There is therefore a growth rate maximum giving a dominant wavelength, LM. In contrast with previous studies, we show that LM scales with λ0/β (λ0 is the wavelength of the offshore waves and β is the mean shoreface slope, from shore to the wave base), an estimate of the order of magnitude of the distance waves travel to undergo appreciable transformation. Our model investigations show that the proportionality constant between LM and λ0/β is typically in the range 0.1–0.4, depending mainly on the wave incidence angle.

Observations and modeling of surf zone transverse finger bars at the Gold Coast, Australia

The occurrence and characteristics of transverse finger bars at Surfers Paradise (Gold Coast, Australia) have been quantified with 4 years of time-exposure video images. These bars are attached to the inner terrace and have an oblique orientation with respect to the coastline. They are observed during 24 % of the study period, in patches up to 15 bars, with an average lifetime of 5 days and a mean wavelength of 32 m. The bars are observed during obliquely incident waves of intermediate heights. Bar crests typically point toward the incoming wave direction, i.e., they are up-current oriented. The most frequent beach state when bars are present (43 % of the time) is a rhythmic low-tide terrace and an undulating outer bar. A morphodynamic model, which describes the feedback between waves, currents, and bed evolution, has been applied to study the mechanisms for finger bar formation. Realistic positive feedback leading to the formation of the observed bars only occurs if the sediment resuspension due to roller-induced turbulence is included. This causes the depth-averaged sediment concentration to decrease in the seaward direction, enhancing the convergence of sediment transport in the offshore-directed flow perturbations that occur over the up-current bars. The longshore current strength also plays an important role; the offshore root-mean-square wave height and angle must be larger than some critical values (0.5 m and 20∘, respectively, at 18-m depth). Model-data comparison indicates that the modeled bar shape characteristics (up-current orientation) and the wave conditions leading to the bar formation agree with data, while the modeled wavelengths and migration rates are larger than the observed ones. The discrepancies might be because in the model we neglect the influence of the large-scale beach configuration.

Observation and Modeling of Crescentic Bars in Barcelona Embayed Beaches

Two events of crescentic bar formation in La Barceloneta beach are analyzed (Mediterranean coast of Spain). They occurred on October 2003 and on December 2005, during Eastern storms with offshore root-mean-square heights between 1.5 m and 2 m, peak periods of about 9 s and angles of incidence relative to shore-normal between 15 and 30. The final alongshore spacings are about 300 m in the first event and about 100-200 m in the second. A nonlinear morphodynamic model is then applied to La Barceloneta conditions to reproduce these two events. The model is able to simulate the final shape of the crescentic bars, reproducing the observed spacings.

Modeling shoreline sand waves on the coasts of Namibia and Angola

The southwestern (SW) coast of Africa (Namibia and Angola) features long sandy beaches and a wave climate dominated by energetic swells from the Southsouthwest (SSW), therefore approaching the coast with a very high obliquity. Satellite images reveal that along that coast there are many shoreline sand waves with wavelengths ranging from 2 to 8 km. A more detailed study, including a Fourier analysis of the shoreline position, yields the wavelengths (among this range) with the highest spectral density concentration. Also, it becomes apparent that at least some of the sand waves are dynamically active rather than being controlled by the geological setting. A morphodynamic model is used to test the hypothesis that these sand waves could emerge as free morphodynamic instabilities of the coastline due to the obliquity in wave incidence. It is found that the period of the incident water waves, Tp, is crucial to establish the tendency to stability or instability, instability increasing for decreasing period, whilst there is some discrepancy in the observed periods. Model results for Tp = 7-8 s clearly show the tendency for the coast to develop free sand waves at about 4 km wavelength within a few years, which migrate to the north at rates of 0.2-0.6 km yr-1. For larger Tp or steeper profiles, the coast is stable but sand waves originated by other mechanisms can propagate downdrift with little decay.

Normal Mode Analysis of the Surf Zone Morphodynamics

The nearshore zone in front of sandy beaches shows sometimes quite regular morphological patterns at large length scales. This paper presents a morphodynamic instability model which describes the growth of shoreattached transverse/oblique bars. Surprisingly to some extent, the results show that once the offshore incoming wave field is kept fixed, the hydrodynamics, and hence the sediment flux, turn out to be slaved to the morphological systems and a feedback proces can be established. Depending on the sediment transport mode, several topographic patterns may grow. Some of them reproduce quite well the initial formation of such features in the field. 1 I n t r o d u c t i o n In spite of the complex behaviour in space and time of the surf zone dynamics, relatively regular patterns dominate quite often the topography at length scales well above the length scale of incident wind or swell waves. Some of these morphological features are rhythmic along the coast. Well known examples are beach cusps, crescentic longshore bars, ridge and runnel systems and shore-attached transverse/oblique bar systems (see references in Falquds et al., 2000 and Falqu~s et al., 1996). They have been related to the effect of edge waves on sediment transport (see, for instance, Holman and Bowen, 1982). However, an alternative explanation lies on the concept of morphodynamic self-organization process. This line of thinking assumes a steady equilibrium configuration of the beach without the patterns (i.e. alongshore uniform). A small topographic perturbation is assumed and its effect on the hydrodynamics and on the sediment transport is 1Departament de Fisica Aplicada, Universitat Polit~cnica de Catalunya, c/Jordi Girona 1-3, 08034 Barcelona, Spain; phone: 34 934 016 889; e-mail: cesca@fa.upc.es, falques@fa.upc.es 2Departament de Fisica i Enginyeria Nuclear, Universitat Polit~cnica de Catalunya, c/Jordi Girona 1-3, 08034, Barcelona, Spain; e-mail: amadeo.montoto@upc.es