The determination of the shear stress at which a sediment grain of a given size and density starts to move has been treated with theoretical, experimental and numerical procedures by many authors. The seminal contribution of Shields [7] addresses a relationship for the non-dimensional critical shear stress in terms of the friction Reynolds number for a single particle in a flat bed. This work focusses on the incipient transport of particles for bedforms. The proposed numerical approach to the problem integrates the Discrete Element Method (DEM) [9] with a continuous finite element approximation. The DEM simulates the motion of the landform, defined as an aggregate of rigid discs that interact by contact and friction. The continuous finite element approach predicts the boundary shear stress field coming from the fluid flow over the bed (for basic formulation, see [4] and reference therein). Both methods are coupled through the flow-particle force transmission using drag coefficients. While for single particles (or very simple sets of particles) incipient motion (and consequently, the threshold stress) is clearly defined, for complex forms the use of the concept of incipient transport becomes necessary, and critical shear stress is established in terms of a threshold sediment flux over the bed surface. We present a series of numerical experiments for single particles, showing good agreement with Shields curve for the whole range of Reynolds number. In this communication we show some of these results, in compare with the basic Shields curves for flat bed and single grains.
