Threshold of Sediment Motion in Coastal Environments

The threshold of sediment motion is one of the most fundamental quantities for sediment transport and dynamics. It is especially important in coastal and shelf seas, because the wave and current flows are often close to threshold, so that predictions of sediment transport rates depend strongly on the value of threshold bed shear-stress used. The present research extends the work of Shields and others in the following ways: (a) a simple analytical formula has been fitted to the traditional hand-drawn Shields curve for threshold bed shear-stress; (b) a correction has been applied to make the curve fit the experimental data better for very fine grain sizes; (c) the formula has been tested against a large set of data for not only currents, but also waves and combined waves and currents; (d) the formula has been extended from the horizontal-bed case to account for the effect of longitudinally and/or laterally sloping beds on the threshold bed shear-stress. The result is a unified expression to predict the threshold of motion of sands and gravels in coastal environments influenced by waves and currents on beds of any slope.