The state-of-the-art in depth-averaged mathematical modelling of 3-D c
oastal morphology is described for the medium-term morphodynamic model
type, in which constituent models of waves, currents and sediment tra
nsport based on first physical principles are linked together to descr
ibe the time-evolution of the bed topography. Various aspects of the c
ombined system of equations are discussed, such as its mathematical ch
aracter, its inherent stability and its equilibrium state. The results
of an intercomparison of different models are shown for two test case
s and the potentials and limitations of the model concept are discusse
d.