On the behaviour of tidal current directions due to the presence of submarine sand waves

Side-scan sonar records reveal that the crestlines of megaripples in the tr oughs as well as on both sides of the slopes of sand waves form an angle wi th the crestline of the sand waves itself. The existence of such an angle b etween the crestline of megaripples and sand waves indicates that changes o f the (tidal) current direction across sand waves can be expected. Direct e vidence that such (tidal) current direction changes occur above sand waves is derived from measurements of the Air-Sea Interaction Drift Buoy (ASIB) s ystem at the sea surface and from side-scan sonar records of the sea bed. B oth systems were operated from on hoard research vessels during two C-STAR field experiments in the Hoek van Holland study area off the Dutch coast in April 1996 and in April 1997. Measurements performed by electrical resista nce wires on board the ASIB system indicate that variations of the short an d moderate wave direction and the wave directional spread are associated wi th changes of the (tidal) current direction at the sea surface. According t o the results of the measurements derived from the ASIB system and the side -scan sonar records it can be summarised that the variation of the directio n of short and moderate period water waves as well as changes of the (tidal ) current speed and direction across large sand waves must be considered fo r correct modelling of the wave-current interaction mechanism. (C) 2000 Els evier Science B.V. All rights reserved.