The understanding of the morphodynamics of harmonic bed forms on the seabed is essential for modeling marine sediment transport and coastal morphologic development. Previous research has mainly focused on the type and distribution of bed forms, but areally extensive data and time series of seabed features are scarce. Multibeam and side-scan sonar data from four expeditions reveal the contrasts between a coastal site with asymmetric and flattened, three-dimensional (3-D) compound sand waves on a shoreface-connected ridge and an offshore site with asymmetric and sharp-crested, 2-D compound sand waves. Migration rates of the coastal sand waves are 6.5--20 m yr-1, while migration rates of the offshore sand waves are -3.6 to 10 m yr-1. This contrasting morphology and dynamic behavior of compound sand waves at the two North Sea sites is explained by differences in the relative importance of tidal currents and wave activity near the bed. These new field data provide parameters and boundary conditions for sand transport models, while the empirically derived behavior of sand waves may be used to validate sand transport and sand wave models.