Modeling large scale shoreline sand waves under oblique wave incidence

Berg, Niels van den; Falqués, Albert; Ribas, Francesca

doi:10.1029/2011jf002177

Cited by 41 publications

(59 citation statements)

References 38 publications

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“…On the location, large scale as well as shorter undulations (with a length of approximately 6 km) have been observed, see also van den Berg et al (2012). This might suggest that shorter undulations formed by less high waves forms one kind of undulations, while the larger waves are associated with the larger scale.…”

Section: Stability Analysismentioning

confidence: 91%

Numerical modeling of shoreline undulations part 2: Varying wave climate and comparison with observations

Kærgaard

Fredsøe

2013

Coastal Engineering

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a b s t r a c t a r t i c l e i n f oThe present work applies the shoreline model from part 1 to a real environment. In part 1, a numerical shoreline model which could handle the development of arbitrarily shaped shorelines was applied to consider the development of shoreline undulations on an unstable shoreline exposed to incoming waves with a directional spreading. In this paper, these findings are extended to firstly include the effect of a varying wave climate on the shoreline morphology and secondly, to tune the model to two naturally occurring shorelines. It is found that the effect of a variable wave climate is to slow down the development of the morphology and in some cases to inhibit the formation of shore-parallel spits at the crest of the undulations. On one of the natural shorelines, the west coast of Namibia, the shore is exposed to very obliquely waves from one main direction. Here, the shoreline model is able to describe the observed shoreline features qualitatively and quantitatively. The model slightly over-predicts the scale of the feature and, associated with this, slightly under-predicts the migration speeds of the features. On the second shoreline, the west coast of Denmark, the shore is exposed to waves with an angle close to the critical around 45°, and here the existence of undulations is discussed in detail.

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Section: Stability Analysismentioning

confidence: 91%

Numerical modeling of shoreline undulations part 2: Varying wave climate and comparison with observations

Kærgaard

Fredsøe

2013

Coastal Engineering

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“…All modeling studies [ Ashton et al , ; Falqués and Calvete , ; Ashton and Murray , ; van den Berg et al , ; Kaergaard and Fredsoe , ] with the exception of Idier et al [] have found the existence of a critical angle for HAWI, and, indeed, observations suggest that high‐angle wave climates correlate with sand waves existence [ Ashton et al , ; Ashton and Murray , ; Medellín et al , ; Idier and Falqués , ; Kaergaard and Fredsoe , ]. However, to our best knowledge, the value of the critical angle has only been tested in the spit of Long Point (Lake Erie, Canada) by Ashton and Murray [].…”

Section: Discussionmentioning

confidence: 99%

“…We choose a maximum value of A = 0.3 m 1/3 based on the Dean [] relationship between the fall velocity and A , which for coarse sand of 2 mm gives A = 0.25 m 1/3 . As a comparison, existing shoreline sand wave studies using a Dean profile [ Falqués and Calvete , ; Kaergaard and Fredsoe , , ; Uguccioni et al , ; van den Berg et al , ; Idier et al , ] considered A coefficients falling in the range 0.08–0.2 m 1/3 . For the maximum value of β s , a value of 0.2 would be justified according to the literature [e.g., Wright and Short , ].…”

Section: Model and Methodsmentioning

confidence: 99%

“…In case of shoreline sand waves, it has been hypothesized that they could emerge from a feedback between the morphology and the wave field involving (i) the wave‐driven longshore sediment transport and (ii) the cross‐shore sediment exchange between the surf and shoaling zones that is responsible for the cross‐shore equilibrium profile. This feedback mechanism was proposed by Ashton et al [] and later confirmed and refined in a number of modeling studies [ Falqués and Calvete , ; Ashton and Murray , ; van den Berg et al , ; Kaergaard and Fredsoe , ]. These studies show that sand waves develop for (deep water) wave angle with respect to shore normal larger than a certain threshold, θ c , with θ c ≥ θ c 0 and θ c 0 ∼42°.…”

Section: Introductionmentioning

confidence: 99%

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Self‐organized kilometer‐scale shoreline sand wave generation: Sensitivity to model and physical parameters

et al. 2017

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The instability mechanisms for self‐organized kilometer‐scale shoreline sand waves have been extensively explored by modeling. However, while the assumed bathymetric perturbation associated with the sand wave controls the feedback between morphology and waves, its effect on the instability onset has not been explored. In addition, no systematic investigation of the effect of the physical parameters has been done yet. Using a linear stability model, we investigate the effect of wave conditions, cross‐shore profile, closure depth, and two perturbation shapes (P1: cross‐shore bathymetric profile shift, and P2: bed level perturbation linearly decreasing offshore). For a P1 perturbation, no instability occurs below an absolute critical angle θc0≈ 40–50°. For a P2 perturbation, there is no absolute critical angle: sand waves can develop also for low‐angle waves. In fact, the bathymetric perturbation shape plays a key role in low‐angle wave instability: such instability only develops if the curvature of the depth contours offshore the breaking zone is larger than the shoreline one. This can occur for the P2 perturbation but not for P1. The analysis of bathymetric data suggests that both curvature configurations could exist in nature. For both perturbation types, large wave angle, small wave period, and large closure depth strongly favor instability. The cross‐shore profile has almost no effect with a P1 perturbation, whereas large surf zone slope and gently sloping shoreface strongly enhance instability under low‐angle waves for a P2 perturbation. Finally, predictive statistical models are set up to identify sites prone to exhibit either a critical angle close to θc0 or low‐angle wave instability.

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“…An outline of the model is given in this section omitting the details, which can be found in van den Berg et al (2012). A Cartesian frame with horizontal coordinates x and y and vertical coordinate z is used, where y runs along the initial mean shoreline orientation, x increases seaward and z increases upward.…”

Section: Modelmentioning

confidence: 99%

Modeling shoreline sand waves on the coasts of Namibia and Angola

Ribas

Falqués

Berg

et al. 2013

International Journal of Sediment Research

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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, T p , 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 T p = 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 T p or steeper profiles, the coast is stable but sand waves originated by other mechanisms can propagate downdrift with little decay.Key Words: Shoreline evolution, Shoreline sand waves, High angle wave instability, Longshore sediment transport IntroductionShoreline sand waves are undulations of the shoreline that can occur on worldwide coasts, typically showing length and time scales of kilometres and years, i.e., larger than the typical scales of rhythmic surf zone bars , and references therein, for detailed information on surf zone bars). The undulations do not only occur in the shoreline position but the bathymetric contours also undulate with decreasing amplitude up to a certain depth. Shoreline sand waves are episodically or persistently found along various sandy coasts around the world (Bruun, 1954, Verhagen, 1989Inman et al., 1992;Thevenot and Kraus, 1995;Gravens, 1999;Guillén et al., 1999;Stive et al., 2002;Ruessink and Jeuken, 2002;Davidson-Arnott and van Heyningen, 2003; Medellin et al., 2008; Alves, 2009).Shoreline sand waves can be triggered by different physical mechanisms, including forcing by offshore bathymetric anomalies (Gravens, 1999) or periodic input of large quantities of sand due to inlets and rivers (Thevenot and Kraus, 1995). However, sand waves can also emerge from small irregularities of an otherwise rectilinear coast in absence of any forcing at its wavelength. This can occur if the wave climate is dominated by high-angle waves, i.e., waves with a high incidence angle relative to the shore normal, because the rectilinear coast becomes unstable (Ashton et al., 2001, Ashton andMurray 2006a;Falqués et al., 2011). We will hereinafter refer to the mechanism as high angle wave instability and to the resulting shoreline...

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Modeling large scale shoreline sand waves under oblique wave incidence

Cited by 41 publications

References 38 publications

Numerical modeling of shoreline undulations part 2: Varying wave climate and comparison with observations

Numerical modeling of shoreline undulations part 2: Varying wave climate and comparison with observations

Self‐organized kilometer‐scale shoreline sand wave generation: Sensitivity to model and physical parameters

Modeling shoreline sand waves on the coasts of Namibia and Angola

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