Journal of Fluid Mechanics



Self-organization mechanisms for the formation of nearshore crescentic and transverse sand bars


M. CABALLERIA a1, G. COCO a2, A. FALQUÉS a3 and D. A. HUNTLEY a4
a1 Departament de Física i Matemàtica Aplicades, Universitat de Vic, 08500 Vic, Spain
a2 Complex Systems Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0225, USA
a3 Departament de Física Aplicada, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain
a4 Institute of Marine Studies, University of Plymouth, Plymouth, PL4 8AA, UK

Abstract

The formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow-water equations and on a simplified sediment transport parameterization. By assuming normally approaching waves and a saturated surf zone, rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed. Two types of bedforms appear: one is a crescentic bar pattern centred around the breakpoint and the other, herein modelled for the first time, is a transverse bar pattern. The feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations. Basically, the waves stir up the sediment and keep it in suspension with a certain cross-shore distribution of depth-averaged concentration. Then, a current flowing with (against) the gradient of sediment concentration produces erosion (deposition). It is shown that inside the surf zone, these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms. Numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro-morphodynamic input conditions to which of the patterns develops. It is suggested that crescentic bar growth would be favoured by high-energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment. In intermediate conditions mixed patterns may occur.

(Received December 4 2000)
(Revised November 22 2001)



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