  The movement of subaqueous sediment in laminar shearing flow is numerically
investigated by the coupled lattice Boltzmann and discrete element methods.
First, the numerical method is validated by comparing the phase diagram
proposed by Ouriemi et al. ({\it J. Fluid Mech}., vol. 636, 2009, pp. 321-336).
Second, a detailed study on sediment movement is performed for sediment with
varying solid volume fractions, and a nonlinear relationship between the
normalised thickness of the mobile layer and the normalised fluid flow rate is
observed for a densely-packed sediment. Third, an independent investigation on
the effective viscosity and friction coefficient of the sediment under
different fluid flow rates is conducted in a shear cell; and substitution of
these two critical parameters into a theoretical expression proposed by
Aussillous et al. ({\it J. Fluid Mech}., vol. 736, 2013, pp. 594-615) provides
consistent predictions of bedload thickness with the simulation results of
sediment movement. Therefore, we conclude that the non-Newtonian behaviour of
densely-packed sediment leads to the nonlinear relationship between the
normalised thickness of the mobile layer and the normalised fluid flow rate.
