Bedform interaction and preservation

The shape and motion of bedforms are envisioned to arise as a consequence of a hierarchical cascade of interactions that occur within a bedform field. Interactions within the field are driven by conditions external to the bedform field, called boundary conditions. One such boundary condition is transport regime, the annual cycle of sediment transporting winds that drive uneven movement and deformation of bedforms. Paradoxically, uneven bedform movement results in geometric organization, as bedform crests align to wind regime. High resolution mapping of bedform crest movement reveals stochastic crest motion that acts to maintain the geometric organization of the bedform field. Stochastic motion of bedforms is driven by the interaction of bedform topography with sediment transporting flows. One such interaction occurs when oblique bedform crests create a wake containing trailing helical vortices. Each trailing helical vortex may preferentially route sediment to or away from a downstream bedform. This routing causes deformation or ablation of the downstream bedform. Either way, a section of the downstream bedform may deform to an oblique orientation. The oblique section may also trail a helical wake and further propagate stochastic bedform motion downstream. Bedform field boundary conditions provide an external influence on stochastic bedform motion. A numerical model of dynamic bedform topography is fueled by four sets of boundary conditions: a uni-modal and bi-modal wind regime each investigated with and without the restriction of a linear bedform source area. The chosen boundary conditions are demonstrated to influence the shape and stochastic motion of bedforms within the field the rate at which the bedforms grow and mature. Fields of bedforms, driven by boundary conditions may leave a stratigraphic record of their stochastic motion. Given sufficient time, boundary conditions are transient. A numerical experiment is performed to investigate the preservation of signals from both stochastic dune motion and a transient boundary condition, the strengthening and weakening of sediment transport with time. The numerical experiment demonstrates that stochastic dune motion and transient boundary conditions influence the preservation of bedform topography in the resulting stratigraphic record.