Modelling the impact of a time‐varying wave angle on the nonlinear evolution of sand bars in the surf zone

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Summary Sandy beaches typically have one or more shore‐parallel bars with superimposed smaller‐scale three‐dimensional (3D) bars. Knowledge of their morphodynamic behaviour under more realistic wave conditions is limited. This study investigates the response of beaches with two shore‐parallel bars to sinusoidally time‐varying angles of incidence, using a non‐linear morphodynamic model. Different periods and amplitudes of this sinusoidal variation are considered, as well as different time‐mean wave angles. For time‐invariant and normally incident waves, results show that alongshore rhythmic 3D bars form in the domains of inner and outer shore‐parallel bars. The 3D bars in the inner domain are coupled at half the outer‐bars wavelength. This phase coupling breaks up when the wave angle varies in time. Initially, regular 3D bars form in the inner domain (free behaviour), which become irregular when 3D bars develop in the outer domain (forced behaviour). The heights of the 3D bars oscillate with time, reaching maximum values when the forcing period is comparable to the system adjustment time scale (∼ 10–20 days). For a time‐varying wave angle around an oblique mean, alongshore migrating 3D bars emerge in both inner and outer domains. In contrast, for an oblique (constant) wave angle, 3D bars only form in the inner domain and they hardly migrate alongshore. For any forcing period, the dominant response period of the oscillating bar heights is at half the forcing period when waves are (on average) normally incident, and it equals the forcing period when waves are on average obliquely incident. Compared with time‐invariant angles, heights of inner and outer 3D bars are (on average) smaller and larger, respectively, when the angle varies with time, particularly for forcing periods in the order of the system adjustment time scale. Increasing the amplitude of the time‐varying wave angle weakens bar growth. Explanations of these results are also provided.

“…These results are similar to those of Nnafie et al. (2020) in the case of one single shore‐parallel bar. This halving of the response period can be explained based on Figure 14.…”

Section: Discussionsupporting

confidence: 91%

Section: Discussionmentioning

confidence: 71%

Section: Discussionmentioning

confidence: 99%

“…Nnafie et al. (2020) argued that the use of the spectral wave model SWAN by Castelle and Ruessink (2011) could explain these differences. This aspect requires further study.…”

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Modelling the response of a double‐barred sandy beach system to time‐varying wave angles

Nnafie

Driessen

Swart

et al. 2021

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Special issue: Marine and river bedforms dynamics, MARID VI

Lefebvre

2021

Marine and river bedforms are rhythmic features driven by unidirectional or reversing currents and waves. They are ubiquitous on the floors of rivers, estuaries, coastal and marine settings. Despite a considerable history of dedicated studies, many aspects of their origin, development and dynamics are still the subject of scientific debate in various disciplines. The MARID conferences series hosts experts from geosciences, physics, engineering and other disciplines to provide a platform for the interdisciplinary exchange of fundamental and applied knowledge of marine and river dune dynamics. MARID VI took place in April 2019 in Bremen, Germany. Related scientific contributions are presented in this special issue of Earth Surface Processes and Landforms. An overview of the conference series and the specifics of MARID VI, as well as considerations in the context of equity, diversity and inclusion, are given. This introduction highlights the progress made with the papers published in the special issue.

The role of bathymetry and directional wave conditions on observed crescentic bar dynamics

Swart

Ribas

Simarro

et al. 2021

Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore uniform (shore-parallel) to alongshore rhythmic (crescentic).Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetchlimited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100-700 m), alongshore migration speeds (0-50 m/day) and cross-shore amplitudes (5-20 m). Wavelengths increased for larger bar-shoreline distances and the alongshore migration speeds were strongly correlated with the alongshore component of the radiation stresses. Crescentic patterns typically developed during low-medium energetic waves with limited obliquity (θ ≲ 20 at 10 m depth), while bar straightening occurred during medium-high energetic waves with strong oblique angles of incidence (θ ≳ 15 ). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration.