A dense cohesive sediment suspension, which contains primarily clay particles, is a thixotropic non-ideal Bingham fluid with a true yield stress. Its time-dependent rheological behaviour can be described by the structural kinetics theory in which the yield stress is taken as a measure for the structural parameter. This theory allows the construction of a more general equation of state, which is independent of the rate equation. The applicability of the model is demonstrated by examples of the prediction of constant structure curves and of transient behaviour. The thixotropy model is incorporated into a Navier-Stokes solver to stimulate the flow behaviour in a Couette viscometer.