Conceptual modelling of sand bars morphodynamics for a microtidal beach (Sète, France)

Abstract

The sandy littoral of Sète, with its barred shoreface, is one of the three studied sites of the thematic action “Morphodynamics of sandy beaches” of the French Programme National d’Environnement Côtier (PNEC), selected because of its microtidal wave-dominated characteristics and of the long time series of available data. During the PNEC program, the principal results obtained show that the morphological evolution of the sedimentary bars can be synthesized according to two conceptual models, in reaction to hydrodynamic variability. (1) The model of “oscillation around a position of equilibrium” (O.P.E) defines the usual mode of behaviour of the bars, with an alternation of shoreward and seaward movements. The bars migrate seaward during storms and shoreward when the energy conditions decrease. This oscillation is expressed with several rhythms. (i) During the main phases of the bar evolution, which are long periods during which the bars present the same geometrical characters. The passage from one phase to the other occurs when a 4 m significant height storm happens. (ii) At the seasonal scale, which is well illustrated by the behaviour of the inner bar: during summer, when the conditions of agitation are weak, the bar aggregates punctually with the shore; when the conditions of agitation increase in autumn the inner bar is reformed and moved seaward. (2) The model of “Net Offshore Migration” (N.O.M) points to the tendency of the bars to retreat under the effect of paroxysmal events (storms with a 20 to 50 yrs return time) being a prelude to their degeneration. From the former position of equilibrium, the outer bar strongly moves seaward and drops following heavy swell. Instead of moving to the coast according to the O.P.E model, the outer bar loses material that is recovered by the inner bar and degenerates. The inner bar, exposed to the swell, then moves seaward to replace the initial outer bar, a new inner bar being created at the coast. A few years after, the standard pattern is restored. These results are compared with those described in the literature.

Dynamics of a nearshore bar system in the northern Adriatic: A video-based morphological classification

“…5). These types of morphologies are not unusual in the Mediterranean, as they were reported by Certain and Barusseau (2005) at Lido de Sete, on the Languedoc French coast. In this case the authors call them "sinuous" bars, but possibly the original French term "en-echelon" better describes them (R. Certain, personal communication).…”

“…Certain and Barusseau (2005) already discussed in detail this aspect and the reader can find in their paper alternative models to NOM. From a study of a Mediterranean beach at Lido de Sete (France), the authors propose in addition to the NOM behaviour a model of Oscillation around a Position of Equilibrium (OPE) as usual behaviour for the bars, while they suggest that a NOM behaviour is only triggered by extreme storm events (return period 20-50 years).…”

“…Noticeable contributions are those of Barusseau and Saint-Guily (1981), Goldsmith et al (1982), Bowman and Goldsmith (1983), Guillén and Palanques (1993), Certain (2002) and Certain and Barusseau (2005). The studies of beaches in the Mediterranean using Argus technology are so far rare and only focussed on the shoreline variability of beaches protected by structures (e.g.…”

Double bar beach dynamics on the high-energy meso-macrotidal French Aquitanian Coast: A review

“…It differs considerably for Noordwijk (The Netherlands) where crescentic features of the outer bar had a lifetime of 0.5-37months (Van Enckevort and Ruessink, 2003). Certain and Barusseau (2005) also showed that a decadal storm was required to straighten the outer bar at the microtidal Sètes Beach. Truc Vert Beach outer bar lifetime may be of the same order as the latest as crescentic patterns deduced from breakers were still clearly observed on an aerial photograph during high energy conditions with H s ≈ 6 m (Castelle, 2004), suggesting Hs on the order of 10 m (i.e.…”

Analysis of submerged bar nourishment strategies in a wave-dominated environment using a 2DV process-based model

“…During peak storm phases, an important offshore migration of the bars with a change in shape and a shoreline retreat are observed. During waning episodes, bar reshaping is observed due to onshore sediment transport (Certain and Barusseau, 2005). Between storm events, the wave amplitude is almost constant with a low mean significant height (0 b H s b 1 m) and a peak period ranging from 7 to 10 s. These calm weather intervals have no significant impact on the bottom evolution.…”