2D MODEL FOR FLUVIAL BEDFORM MORPHODYNAMICS: THEORY AND EMPIRICAL EVALUATION USING PROFILING ACOUSTIC DOPPLER VELOCIMETERS

Bedforms are a primary observable topographic feature in most systems in which moving fluid interacts with a mobile, sandy bed. Their behavior is a classic morphodynamic problem in which local stress, transport, and topographic conditions result in complex local growth, migration, and deformation characteristics of forms. Numerous models exist describing individual relations between these conditions. However, models are commonly derived from small scale, limited measurements. Measuring and describing the complete set of variables necessary for constraining the entire system has been hindered by instrumental limitations stemming from fine spatial and temporal scales of variability in all three components of the morphodynamic problem. In this study, we present a morphodynamic model by combining existing descriptions of sediment entrainment/deposition as a function of local skin friction shear stress, a statement of conservation of sediment mass (Exner equation), and particle saltation lengths. This model is compared to data from flume experiments with a natural sand bed using repeat scans with a Nortek Vectrino II profiling acoustic Doppler velocimeter (ADV) mounted to a moving carriage. By utilizing the combination of three-component near bed velocity data coupled with bed topographic evolution along a two-dimensional transect we are able to fully constrain the three components of the morphodynamic system in two-dimensional space. Initial qualitative results show that migration, growth, and decay of bedforms are consistent with expectation relative to the local gradients in near-bed shear stress conditions. The relation between topographic evolution and the local near-bed stress field remains to be quantitatively assessed.