The motion of suspended sand particles is studied by means of a new perturbation solution to the equation of motion. In the first approximation, fluid accelerations are neglected so that the relative velocity between sand and water is everywhere equal to the still water settling velocity w↘0. It turns out that some of the most important mechanisms of sediment suspension, such as trapping in vortices, can be derived from this ''zero order solution'' when the flow structure is given proper consideration. The next level of solutions takes into account the effect of fluid acceleration by including terms of order of magnitude &egr;=1/g‖du↘/dt‖. As a main result it is shown that pure wave motion does not cause a net reduction of settling velocity of this order of magnitude. Finally the effect of drag nonlinearity is studied for the case of an oscillatory flow. It is shown that this effect is of the order of magnitude &egr;2 and without practical importance for sediment suspension by waves.