The subject of hydrodynamics inevitably plays a key role in NCK research. Indeed, like the previous theme, it cuts across all other NCK programmes. Many activities in the area of hydrodynamics take place in the context of morphodynamic and ecological models.

Hydrodynamics is a 'cascade' item avant la lettre: not only in the scheduling of the hydrodynamic processes (from 1D to 3D modelling), but also in the procedures used (e.g. multiple scale expansions) and, of course, in the physical processes that are studied. These include turbulence, waves (sea and swell, long waves), tidal flows and residual long-term currents. Significant effort is directed at 3D numerical modelling of unsteady, turbulent, free-surface flows including salinity and/or temperature induced density variations and suspended sediment. Research issues are the appropriate subgrid (non-isotropic) modelling of turbulence, including buoyancy effects, and the further development of suitable numerical schemes. The development towards inclusion of nonhydrostatic effects is pursued further, even into the domain of relatively short surface waves including breaking waves. This appears to have great potential for studies of wave-current interactions and interaction with sedimentary beds.

In recent years, large effort has been invested in the modelling of water motion in the coastal zone on the temporal and spatial scales of wave groups ('low-frequency' motions, also collectively referred to as 'surf beat'). This work is ongoing. The resulting velocity patterns are used as input for a sediment transport model, which is capable of simulating the initial development and evolution of rip current systems.

Finally, work is being done on the improved modelling of wind-generated waves within the class of phase- and group-averaged spectral wave energy models, particularly through the SWAN model. At the moment, special attention is being paid to the propagation of waves in shallow water.