Physical modeling of three-dimensional intermediate beach morphodynamics

• Published in: Journal of geophysical research. Earth surface Vol. 118; no. 2; pp. 1045 - 1059
• Main Authors: Michallet, H., Castelle, B., Barthélemy, E., Berni, C., Bonneton, P.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-06-2013; American Geophysical Union/Wiley
• Subjects: Beaches; Channel morphology; Channels; Classification; Earth Sciences; Engineering Sciences; Evolution; Fluids mechanics; Geophysics; Marine; Mechanics; Morphology; nearshore patterns; Ocean floor; Positive feedback; rip currents; Sand bars; Sciences of the Universe; surf zone; Tanks; velocity skewness; wave asymmetry; wave breaking
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1002/jgrf.20078

Experiments have been performed in a large wave tank in order to study the morphodynamics of rip current systems. Both accretive and erosive shore‐normal wave conditions were applied, the beach evolving through all the states within the intermediate beach classification, under the so‐called down‐state (accretive) and up‐state (erosive) morphological transitions. Results show that any prescribed change in the wave conditions drastically increases the rate at which the morphology changes. The surf zone morphology tends toward a steady state when running a given wave climate for a long duration. We quantitatively describe a full down‐state sequence characterized by the progressive evolution of an alongshore‐uniform bar successively into a crescentic plan shape, a bar and rip channel morphology, and a terrace. From the analysis of a large data set of dense Eulerian measurements and bathymetric surveys, we depict several feedback mechanisms associated with wave‐driven rip current circulation, wave nonlinearities and the seabed evolution. At first, a positive feedback mechanism drives a rapid increase in the rate of morphological change, beach three‐dimensionality, and rip intensity. By the time the sandbar evolves into a bar and rip morphology, a negative feedback mechanism, characterized by a decaying beach change rate and an increasing beach alongshore uniformity, overwhelms the former mechanism. An erosive sequence characterized by both an overall offshore bar migration and an increase in beach three‐dimensionality is also described.
Key Points
The morphodynamics of rip current systems is reproduced in the laboratory
Beach morphodynamics is driven by negative and positive feedback mechanisms
Our results feed the existing intermediate beach classification schemes