The effect of suspended sediment on the shape of near-bottom velocity profiles has been investigated theoretically and in the laboratory by a number of authors. However, because the resulting expressions for the profile are generally complicated, such effects are often neglected by engineers analysing measured profiles. We present here a criterion for the conditions of flow velocity and sediment grain-size under which it is permissible to neglect the sediment effects. Where it is not, an estimate of the resulting error in the friction velocity is given. A direct method is presented for obtaining the friction velocity (and hence bed shear-stress) from velocity profiles measured under mobile sediment conditions, which takes account of the suspended sediment effects.
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