Streamwise and Cross-Stream Sediment Dynamics Over Fixed, Three-Dimensional Bedforms

Ubiquitous across spatial and temporal scales, subaerial bedforms play a vital role in the conveyance of sediment from source to sink in river systems. Understanding the complex interactions between fluid and sediment over river bedforms is paramount to understanding deposition and erosion over these structures as well as the role that bedforms play in planform river morphology. While streamwise sediment transport dynamics have been studied over two-dimensional bedforms, intra-bedform transport (i.e., transport in the cross-stream direction) has yet to be investigated in detail. Additionally, sediment transport over three-dimensional bedforms has yet to be thoroughly analyzed. How much sediment moves along bedforms in the cross-stream direction? How does this intra-bedform transport affect the transition from 2D to 3D bedforms? Here we present flume experiments in which we observed sediment transport (via high-speed imaging) and fluid dynamics (via Acoustic Doppler Velocimetry) over bedforms with two simple three-dimensional geometries: lobe crests (concave downstream) and saddle crests (concave upstream). Using manual particle tracking techniques, we observe the spatiotemporal changes in grain transport rate over the two different bedform geometries in the streamwise and cross-stream directions, on the sub-bedform and bedform scales. With these experiments we aim to provide a more robust insight into the function of cross-stream migration of sediment in the evolution of bedforms with complex geometries.