Hydrodynamic variability on megatidal beaches, Normandy, France

Several experiments aimed at characterising the hydrodynamics of megatidal beaches outside the surf zone were carried out between 1990 and 1994 on the Cotentin coast of the Cherbourg Peninsula in Normandy. The database was es tablished from the records of several electromagnetic current meters and pr essure sensors and from field surveys. The mean spring tidal range on these beaches varies between 9.3 and 11.4 m. The results show the prevalence of strong longshore currents, with velocities up to 0.5 m s(-1), on the low- a nd mid-tidal beach zones. Mostly oriented northward, these currents reflect both a progressive tidal wave and a strong longshore gradient in water lev el between the Channel Islands embayment and the English Channel. While var ying largely during a typical tidal cycle, these longshore velocities are m aximum at high tide, reflecting the progressive nature of the tides. This h igh-tide maximum velocity increases by a factor of 1.5 between the mean tid e and mean spring tide, and between the mid- and low-tidal zones due to bed friction effects. Cross-shore velocities are generally weak (<0.1m s(-1)), but sometimes stronger in smaller water depths. In the low-tidal zone, the y are commonly oriented onshore at the beginning of the rising tide and off shore during the falling tide. This circulation results from a west-east cr oss-shore gradient in water level that is particularly important around the mean water level. Towards high tides. weak offshore steady flows were obse rved in the presence of waves. Site-specific relationships were defined in order to characterise the modul ation of significant wave height by sea level fluctuations both on the shor eface and in the intertidal zone. The water depth variability during the ti dal cycle induces fluctuations in the dissipation by bottom friction, resul ting in wave height changes. The influence of tidal currents on the wave he ight proved to be very small in this context. The tidal fluctuations also influence the instantaneous near-bed currents i nduced by simultaneous action of non-breaking waves and the tides. During s tormy conditions, wave-induced gravity orbital motions dominate the steady flows in the mid-tidal zone, outside the surf zone. At this location. the s hallow water friction effect results in weak steady longshore currents, and low water depths explain strong orbital motions. The opposite conditions p revail in the low-tidal zone, where the steady tidal currents are stronger than gravity orbital velocities during a few hours around high tide. Outsid e this period, with the decrease in water depth and in steady current inten sity due to friction effects, the tidal and gravity wave-induced currents h ave comparable intensities. In both the low- and mid-tidal zones, infragrav ity motions are weak outside the surf zone. The foregoing results show that outside the surf zone, these megatidal beaches are characterised by wave-d ominated mid-tidal zones and tide-dominated low-tidal zones during spring t ides. We suggest the term "mixed wave-tide-dominated'' for these beaches wi th very large tidal ranges. (C) 2001 Elsevier Science Ltd. All rights reser ved.