The scientific sediment world is renowned for its segregation: cohesives and non-cohesives are entirely separate fields of study. The various formulae commonly used for water/bed exchange, settling velocity, etc. are applicable to the transport and fate of either cohesive or non-cohesive sediment. However, nature is generally less discriminating and natural sediment suspensions and deposits often consist of a mixture of cohesive and non-cohesive sediments. Such mixtures may behave entirely differently from their components in isolation; for instance, a small addition of fines to a sandy bed may considerably increase its erosion resistance. Hence, progress in our understanding of natural sediments and in our ability to model them requires studies on the behaviour of the entire mixture.

Sediments mixtures also need to be studies from an ecological point of view. Sediment composition, especially the sand-mud ratio, is an important parameter in the characterization of marine habitats and the evaluation of the health and productivity of natural environments. To further complicate the issue, sediment composition is controlled not only by physical (abiotic) processes, but also by chemical and biological processes. This makes it essential that the study of sand-mud mixtures be undertaken within a multi-disciplinary framework, such as offered by the NCK.

Research within the Sand and Mud theme consists of theoretical studies, laboratory experiments, field surveys, and mathematical modelling. Research activities cover processes in the water column, within the bed and at the bed-water interface, and include subjects such as flocculation, segregation, consolidation and swell, erosion and liquefaction, adsorption and desorption, etc. With respect to the behaviour of sand-mud mixtures, three scales can be distinguished within the micro- and meso-scale ranges:

• on the micro-scale, the erodibility of the bed as a function of the bed composition (sand-mud ratio, stratification, etc) plays a role. The skin friction is also determined on this scale;
• also on the micro-scale, the bed forms (flat bed, ripples, dunes, etc) determine the effective hydraulic resistance of the flow, thus affecting velocity profiles and even flow rates;
• on an intermediate-scale, horizontal sorting effect play an important role; settling times, flow velocity and transport path determine the fate of the various sediment fractions;
• and finally, on an seasonal scale the composition of the bed will vary considerably: in the winter/storm season mud will be eroded and transported, sometimes being exported from the system, while in the summer period the mud content of the tidal flats will be build up again; biological effects increase this process considerably.

It is evident that processes on these various scales are strongly interrelated: erodibility plays a role in the availability of the various sediment, but is itself governed by the sediment fractions depositing at the particular location. Erodibility also governs bed formation, and the bed forms affect the large-scale transport of the various sediment fractions. The gaps in our knowledge occur in particular on the micro- and meso-scale, i.e. in relation to bed formation and erodibility, and the interaction of the various scales. Sand and mud processes are closely related to the morphological development of estuaries and coastal systems. Such development is a function of bed forms and sediment availability, and hence of bed composition. In this way, the scales identified above are closely related to morphodynamic scales, though the time scales may differ.