Diversity of bed evolution at wave and tidal scales on Truc-Vert beach

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...

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Bibliographic Details
Published in:Journal of coastal research 2009-01, Vol.SI56, p.1726-1730
Main Authors: Berni, C., Mignot, E., Michallet, H., Dalla-Costa, C., Grasso, F., Lagauzère, M.
Format: Article
Language:English
Subjects:
Beaches
Bed sheets
Engineering Sciences
Fluid mechanics
Fluids mechanics
Mechanics
Mixing height
Narrative devices
Optical fibers
Physics
Pressure sensors
Sedimentary Processes and Morphodynamics of Sandy Beaches on Short Time Response
Sensors
Tidal waves
Velocity distribution
Waves
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Summary: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 different high tide conditions (Hs/h ∼ 0.6 and Hs/h ∼ 0.3) corresponding to energetic and calm wave forcings. The evolution of the bottom during the two types of tides is very different. When the measurement point remains in surf zone (Hs/h > 0.5), the bed shows a continuous erosion during the rising tide and a symmetric deposition during the ebb tide. In contrast, for weaker wave conditions (Hs/h < 0.4), 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.
ISSN:0749-0208
1551-5036