Experimental flume simulation of sandbar dynamics
Résumé
Sandbar dynamics is a key feature of coastal hydrodynamics and plays an important role in natural shore protection. It is generally recognised that moderate wave forcing can lead to bar formation whereas storms may induce offshore bar migration. Field experiments nevertheless do not usually allow a detailed description of the evolutions due to the poor temporal resolution between topographic surveys. Here, experiments are carried out in a 36 m long flume equipped with a piston wave generator. The sloping bottom consists of a loose material of low density (1.19 g/cm3) with a median diameter d50=0.6 mm. The Shields and Rouse numbers for the experiments are of the same magnitude as those of natural environments. Time and length scales ratios are roughly 1/3 and 1/10. Irregular waves are generated according to a JONSWAP spectrum. The process of bar formation and evolution at several positions on the profile is described. On the one hand for constant wave climates of very long durations (tens of hours), bars become pitched forward and onshore migration is observed. The bar eventually merges to the berm. On the other hand, a succession of increasing, energetic and decreasing wave conditions that are characteristic of the different phases of storm events are applied. An offshore migration of mega-ripples is observed during the most energetic phase, feeding a large offshore bar. The waning conditions smooth the profile and lead to shore-face accretion. Shoreline positions during the different phases are also discussed.