Diversity of bed evolution at wave and tidal scales on Truc-Vert beach
Résumé
The present paper is devoted to the analysis of bottom evolution in the mid intertidal zone of a sandy beach during high tides. The on-site experimental set-up is composed of i) an Acoustic Doppler Velocity Profiler able to measure simultaneously the local velocity field, bed elevation and thickness of the sheet flow layer and ii) a set of optical fibers and pressure sensors aligned vertically throughout the sediment/water interface, able to detect the states of the medium in front of each sensor: low or highly concentrated flow, unstable or stable bed. Measurements were taken during tides with calm and energetic waves (Hs ~ 0.5 m and Hs > 1 m). The evolution of the bottom during the two types of tides is very different. With energetic waves, the bed shows a continuous erosion during the rising tide and a symmetric deposition during the ebb tide. This suggests a notable modification of the beach profile during the tide. Oppositely, during a tide with less energetic waves, the bottom elevation reveals periodic oscillations characteristic of ripples propagation. The detailed bottom evolution is investigated at the time-scale of the wave. The correlation between thickness of the sheet flow and excess pore pressure indicates that in most cases, the excess pore pressure is negative under the wave crest and becomes positive under the trough, promoting the development of the sheet flow. Consequently, exfiltration from the porous bed would be an important mechanism for sheet flow initiation during backrush.