Under this award the PIs will investigate the origin and significance of rippled scour depressions or sorted bedforms. These are found on the inner continental shelves in relatively coarse sediment, which extend kms in the cross-shore direction and are on the order of 100 m wide. Previously these have been interpreted to indicate concentrated cross-shelf currents, which transport sediment away from shore during storms. However, recent observations strongly suggest that they are associated with sediment transport along-shore. A new hypothesis for the origin of these features begins with the observation that where a shallow seabed is covered by coarse material, that sediment is sculpted into large wave-generated ripples. Wave motions interacting with these large roughness elements will generate large-scale, energetic turbulence. This enhanced turbulence will tend to enhance the entrainment of fine sediment, and to inhibit its redeposition locally. Then, any mean current will tend to advect the fine sediment to a location where the bed is finer, the wave-generated ripples are smaller, and the turbulence is less energetic. Starting from a nearly homogeneous seabed, these interactions will tend to preferentially remove fine sediment where the bed is coarser, and to preferentially deposit it where the bed is finer, producing accumulations of fine material separated by self-perpetuating coarse domains.
Research to test the hypothesis that this feedback and subsequent interactions between sorted features could produce large-scale sorted patterns with the characteristics observed in nature will consist of two linked parts: 1) field measurements of turbulence and suspended sediment concentrations, using tripod-mounted instruments located in both coarse and fine domains on a well-studied shoreface off Wrightsville Beach, North Carolina; and 2) development of a numerical model simulating these processes and the evolution of the seafloor. The field program will rigorously characterize interactions between coarse concentrations and sediment entrainment and deposition. Field data would subsequently be used to direct and constrain model development. This model will investigate what sediment-transport relationships that arise from the small-scale processes, and what environmental conditions, are sufficient to produce bedforms similar to the features observed.
