Most sediment-laden flows in nature are produced by strong local gradients in terms of a hillslope or a pressure gradient. Describing such complex flows from an engineering perspective (e.g., using macroscopic sediment transport models) requires in-depth knowledge of the rheology of the sheared sediment beds and the fluid-sediment mixture. In this chapter, we review a previously derived macroscopic model that relies on a two-phase flow approach for the fluid and the sediment phase, respectively, as a promising avenue to predict such a complex flow. We apply this model to data generated by means of highly resolved direct numerical simulations of a sediment bed sheared by a pressure-driven viscous flow. We show that the rheology obtained in the numerical results obeys the scaling that has previously been derived for neutrally buoyant particles in rheometer cells. A two-phase flow model based on these scaling relations is therefore able to reproduce the simulation results of sheared sediment beds with high accuracy. Finally, we provide a list of open issues for future research that will be key to improving our understanding of sediment bed rheology: the roles of higher particle inertia, transient behavior of sheared sediment beds, and cohesion.