Changes in the coastal morphology are the consequence of spatial gradients in net sediment transport rates (compare Sect. 1.4.3 and Eq. 1.1). Therefore, this chapter pays attention to sediment transport mechanisms and prediction methods. Unfortunately, the interaction between hydrodynamics and sediment is very complex and poorly un- derstood. It is an often heard anecdote that when Albert Einstein’s son started researching sediment transport, his father warned him strongly of the difficulties in dealing with sediment transport processes (see e.g. Vollmers (1989)). Whether this actually happened is questionable (Gyr & Hoyer, 2006), but the anecdote illustrates the frustration that coastal engineers and scientists must sometimes feel when trying to understand and model sediment transport. In view of the above, the modelling of sediment transport is largely based on empiricism.
Sediment transport can be defined as the movement of sediment particles through a well-defined plane over a certain period of time. The movement of sediment particles depends on the characteristics of the transported material (grain size, fall velocity). These sediment properties are discussed in Sect. 6.2. Next, in Sect. 6.3, initiation of motion is considered. There it is shown that sediment particles will start moving when a so-called critical velocity (or critical shear stress) is exceeded. This bed shear stress is the result of the combined wave-current motion. Section 6.4 discusses the various transport regimes and modes. Generally two transport modes are distinguished: bed load and suspended load. If the particles are moved in bed load mode, they roll, shift or make small jumps over the seabed, but stay close to the bed. Transport in suspended load mode, on the other hand, implies that grains are lifted from the seabed at flows above the critical flow velocity and transported in suspension by the (moving) water. Below a certain flow velocity, the grains settle down again. General bed load and suspended load transport formulations are treated in Sects. 6.5 and 6.6 respectively. In Sect. 6.7 the so-called energetics approach to bed load and suspended load transport is discussed. This approach gives us an easy method to unravel the total sediment transport in contributions of waves and currents separately. Section 6.8 deals with some of the additional complications that can be expected when dealing with fine sediment (mud and the finer sand fractions). Section 6.9 discusses the choice between various transport formulations and schematisations that can be made for specific situations.