Stéphane Bujan

Rip current system over strong alongshore non-uniformities: on the use of HADCP for model validation

Modeling and understanding topographically-controlled rip currents remains a challenging task. One of the reasons is the lack of intensive, high-spatial resolution, flow field measurements in the rip channel vicinity. During the ECORS (DGA-SHOM) intensive field measurements, an intertidal inner-bar rip channel was instrumented with fixed eulerian current meters. In addition, for the first time in such a system, a Horizontal ADCP (HADCP) was implemented in the vicinity of the rip current, on the sandbar edge, for horizontally profiling wave induced-currents. Results show that the HADCP provides unique information on the shear in the vicinity of the rip neck, which is particularly useful for model calibration. The HADCP data was compared with local flow measurements for various tide and wave conditions, showing a very good agreement at a 5 m range. Restrictions and recommendations for HADCP implementation in the field are pointed out. The use of HADCP for horizontally profiling rip current circulations would benefit from being deployed outside of the breakers to measure the cross section of the rip head where sediment plumes and bubbles are essentially surface dominated. In this rip current system area, which would suffer from acoustic opacity only during high energy conditions, the rip current jet is strongly unstable owing to the current shear. HADCP would provide unique information on the rip current instabilities and vortex shedding in this poorly understood area of the rip current system.

ON THE IMPACT OF A SERIES OF SEVERE STORMS ON A DOUBLE-BARRED SANDY COAST: DUNE EROSION AND MEGACUPS EMBAYMENTS

The winter of 2013/2014 was outstanding in terms of the available energy to move sediment and caused large-scale unprecedented erosion of the Gironde coast (France), comprising the onset of megacusp embayments cutting the dune with a typical alongshore and cross-shore lengthscale of O (100 m) and O(10 m), and a dune erosion scarp height often exceeding 10 m. Megacusps are found to be coupled to the outer crescentic sandbar and triggered by the outstandingly high-energy, long-period and normally-incident storm wave event Hercules on January 6-7. Our observations indicate that both the antecedent outer sandbar morphology and storm wave characteristics, including period and angle of incidence, govern patterns of beach and dune erosion along open multiple-barred sandy coasts during severe storms.