Sandy beds of shallow tidal seas often exhibit a range of rhythmic patterns, from small-scale ripples a few metres long to large tidal sandbanks with a wavelength of kilometres. For example, on the access route to Rotterdam harbour ships cross a field of sandwaves. The crests of these sandwaves determine the effective navigation depth. To warrant navigability, the North Sea Directorate of the Netherlands Ministry of Transport, Public Works and Water Management continually monitors the bathymetry in the sandwave area, originally using echo sounding. Our analysis of these data has revealed a new rhythmic pattern, in addition to the well-known sandwaves and tidal sandbanks. The wavelength of this new pattern, labeled here as long bed-waves, is three times the one of sandwaves, and the crest orientation is different. Interference of the three modes leads to the rather complex bathymetry revealed by echo soundings. ¿ 2001 American Geophysical Union