Unlike the central Holland coast, the north (Wadden Sea) and south-west (Delta region) coast can be characterized as tidal basins/inlets and estuarine systems. They are highly dynamic systems with tidal shoals, channels and sometimes entire islands migrating at time scales of only a few decades.

The morphological evolution of the Wadden Sea on this timescale is driven by both tides and waves. More sheltered embayments and estuaries are found in the south-west (Eastern and Western Scheldt) and to the north-east (Ems estuary). In all these areas, strong interactions are observed between the water motion and the erodible bed, resulting in large sediment transport rates and highly dynamic morphological patterns (channels and shoals) exhibiting complex behaviour in both time and space. This yields - even more than in the case of the beach barrier coastal system - a large variety of morphodynamic evolutions over a cascade of spatio-temporal scales. Resarch on coastal inlet and tidal basin systems is therefore likewise structured on the basis of a cascade of scales.

Because of the important economic and ecological functions of tidal inlet systems and estuaries, there is an increasing need to develop reliable simulation models for water motion, transport processes and morphological changes in such areas. Despite the significant progress that has been made in this area over recent decades, many processes are still poorly understood. Issues include:

• identification of the dominant hydrodynamic and morphodynamic phenomena that occur in tidal inlet systems and estuaries and of their main characteristics;
• development of appropriate descriptions to quantify net sediment fluxes in such systems, due to tides, waves and three-dimensional water motion;
• devising ways of modelling feedback from the morphology to the water motion where it takes place on a time scale much longer than the hydrodynamic time scales;
• identification of the relevant feedback between morphology and ecology and devising ways to model it.

Research efforts focus on the modelling of long-term morphological changes (time scales of months to years, spatial scales of 100 m and more) and the interaction between ecological and abiotic processes.