Measurements of bed level oscillation cycles in the surf zone of a sandy beach Ga¨ el Arnaud, Mathieu Mory, St´ ephane Abadie, and Denis Morichon Laboratoire SIAME Universit´ e de Pau et des Pays de l’Adour UFR-Sciences et Techniques Cˆ ote Basque Anglet 64600 Email: gael.arnaud@univ-pau.fr Abstract—High frequency bed level measurements have been realized during several tide cycles, on the beach face of a meso- macro tidal sand beach on the Aquitanian coast (France). A network of seven instruments, using a resistive method to monitor bed level changes, recorded up to 57 tidal cycles. Data are analysed and interpreted regarding wave and tide conditions recorded with pressure sensors deployed in the same area. Bed level evolution is analysed and compared with disturbance depth. The comparison demonstrates the ability of the system to characterize real-time response of bed level to wave forcing in the surf and swash zones under energetic waves and shows different patterns of bed level response depending on tide phase and location on the beach face. I. I NTRODUCTION Nearshore processes of sandy beaches result in highly dy- namical bedforms mainly caused by the dissipation of incident wave energy. The morphological response of the large range of beach types has been investigated, focusing on specific beach- types and/or specific conditions that affect the beach response. Probably the most dynamic part of the nearshore domain is the one that extends from the shoreline through the swash zone and into the inner-most part of the surf zone [1]. On intermedi- ate meso-macrotidal beaches, the daily, or twice daily, sweep of the tide over the intertidal domain induces transient flows resulting in a transient morphodynamic response that migrates along the tide run. It is commonly accepted that changes in morphological features along the intertidal domain occur mainly as a modification of the beach shape by the formation of secondary morphological features [2]. The distribution of these features are mainly governed by wave induced and tide induced currents. The tidal stage determines where the processes is effective and the translation rate implies for how long these processes operate. To quantify these tidal influence, Masselink [3] proposed the relative tide range (RTR) as the ratio of the tidal range (TR) to the wave height (H b ): RTR= TR/H b . The larger the relative tide range, the more important are tidal effects relative to wave effects. Beside the oscillation of water level that induces bedforms migration, the dynamics of beach groundwater is also contributing to the migration of sediment reworking zones. The interaction of the tide with the oscillating water table leads to characteristic patterns of both profile adjustment and equilibrium morphology. According to Masselink and Turner [4], the relative contribution of both to resulting beach morphology is inseparable. When the tide range exceed 2m, the relative elevation of the water level and the groundwater exit point define two contrasting intertidal zones. One upper zone alternatively saturated/unsaturated and a lower zone that remains always saturated. The intersection of these two zones is expressed by a point of divergent sediment transport. This location explains the break in beach slope generally observed on meso-macrotidal beach [5], [6]. The interaction of the swash and water table level has been interpreted to explain cyclic pattern of beach cut and fill associated with rise and fall of the tide [7]. A. Erosion/deposition cycles These cyclic erosion/deposition patterns have been widely observed [7], [8]. Eliot and Clark [9] described the tidal migration of the sea-level and water-table effluent-line across the beach face that establishes the location of the zones of erosion and deposition. Beach face degradation began with the tide rise and terminated shortly after mid-ebb tide. Maximum degradation occurred when the beach face was most saturated (1-2 h after high tide). Above the exit point, unsaturated con- ditions enhanced deposition. It results in an erosion/deposition cycle superimposed on the tide cycle. Measurements of these cyclic changes have been realized with numerous techniques and methods. Probably the most common one [10], [11], [12], [13] employs a vertical rod partly stuck inside the soil. The displacement of a loose-fitting washer placed over the rod allows determining the depth of scour and the net erosion or aggradation which occurred over a tidal period. This technique determines the depth of the soil layer affected by hydrodynamic processes during a tidal cycle, a thickness that is called in the literature ”disturbance depth”, ”depth of activity” or again ”activation depth” [14]. Although the tide is a naturally relevant time scale for de- termining the activation depth, some studies have investigated other particular events with different time spans. For example,