# 6.5.1: Importance of the Shields parameter

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Bed load transport occurs when the bed shear stress or the bed shear stress velocity \(u_* = \sqrt{\tau_b /\rho}\) exceeds a critical value (initiation of motion, Sect. 6.3). In the bed load layer the turbulent mixing is often assumed to be still small (due to the presence of the bed), so that it only slightly influences the motion of sediment particles. Gravity limits the vertical particle movement. We can then assume that the bed load transport responds instantaneously (without delay) to the bed shear stresses.

Many approaches for bed load transport are based on this reasoning and take the sediment transport to be a direct function of the shear stress on the grains. In such formulas the dimensionless sediment transport is invariably a function of a Shields parameter \(\theta\) (dimensionless shear stress). Many formulations used in coastal engineering practice have been based on already existing formulations used in river engineering. In Fig. 6.11 a comparison is made between various bed-load transport formulas developed for rivers.

Although the formulas seem quite different at first glance, Fig. 6.11 demonstrates that they all represent the dimensionless transport as a function of a Shields parameter. Further note that the predicted transport rates for a certain value of the Shields parameter vary by up to an order of magnitude. This is (unfortunately) quite common for sediment transport predictions and underlines the fact that calibration of the transport formulations for the locations and conditions under consideration is crucial.