Measurements and Dynamics of Multiple Scale Bedforms in Tidally Energetic Environments

Katie Jones, S.M., 2018
Peter Traykovski, Advisor

The presence of superimposed bedforms, where smaller bedforms exist on larger bedforms, is ubiquitous to energetic tidal environments. Due to their wide range in scale, it is difficult to simultaneously observe these features over tidal timescales. Using novel instrumentation and platform systems, the morphological response of superimposed bedforms to a tidally reversing flow is analyzed. A method is developed to expand the functionality of low-mounted sidescan sonars by utilizing sonar shadows to estimate bedform height and asymmetry. This method is applied to data at Wasque Shoals which reveal small bedforms reversing their asymmetry with the flow while the larger bedforms remain oriented in the dominant flow direction. Similar bedform dynamics are observed at Nauset Inlet using an autonomous jet-powered kayak, the Jetyak, equipped with a bathymetric swath sonar. The response time of the bedforms at both locations agree with a geometric model with larger bedforms and slower flows resulting in longer lag times.  Finally, the migration of superimposed bedforms at Wasque Shoals is analyzed. The net convergence of megaripples on the tidally dominate lee face of the dune suggests that the smaller bedforms serve as an intermediate step between grain-scale transport processes and larger scale dune migration