Use of a resistive rods network to monitor bathymetric evolution in the surf/swash zone.

The depth of sediment activation by wave action in the surf and swash zones has been studied at different levels in recent years. Most methods used so far rely on measurements carried out at low tide, which ignore the processes occurring during a tidal period. New techniques have been developed recently to get a real time monitoring. We describe here a method based on local electrical resistivity measurements along a vertical pole half buried into the sediment. The apparatus is able to characterize in situ, over several tidal cycles on a beach submitted to energetic waves, the time evolution of local bed level changes, with an acquisition frequency of 10 Hz. The electrical resistivity contrast between sea water and the sediment layer makes this differentiation possible. Considering different layers characterized by their conductivity in the near bed, the bed level detection method is described and has been applied on a data set of a field experiment (ECORS-2008, France). Five instruments were deployed along a cross shore transect on a meso/macro tidal beach. Wave by wave frequency time series of bed level change over 10 tidal cycles were recorded and analysed. The method is first validated using daily low tide topographic surveys. Then, the sediment activation depth distribution along the cross shore transect during tidal events is analysed.