Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the
Massachusetts Institute of Technology
and the
Woods Hole Oceanographic Institution
June 2018
Description:
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. This thesis examines the
morphological response of superimposed bedforms to a tidally reversing flow using novel
instrumentation and platform systems. A method is outlined in chapter 2 to expand the
functionality of low-mounted sidescan sonars by utilizing sonar shadows to estimate bedform
height and asymmetry. Empirical models are generated to account for realistic variability
in the seabed and the method is validated with bathymetric observations of wave-orbital
ripples and tidally reversing megaripples. Given the high temporal and spatial resolution of
seafloor frame mounted rotary sidescan sonars, the dynamics and evolution of the bedforms
over an approximately 40 m x 40 m area can be resolved. In chapter 3 the method is applied
to data of superimposed bedforms at Wasque Shoals, an ebb delta off the southeast corner of
Martha’s Vineyard, MA. These data reveal the small, superimposed bedforms reversing their
asymmetry with the flow while the larger bedforms on which they reside remain oriented in
the direction of the dominant flow. Similar bedform dynamics are observed at Nauset Inlet,
a dynamic inlet system, on Cape Cod, MA using an autonomous jet-powered kayak, the
Jetyak, equipped with a bathymetric swath sonar. The time needed for bedform asymmetry
to reverse in the presence of a tidal flow was estimated with a geometric bedform model
that incorporates an empirical sediment transport rate. The morphological lag time from
the observations agree well with the geometric model with larger bedforms and slower flows
resulting in a longer lag time. Finally, the migration of these superimposed bedforms is
considered in chapter 4. Data from the rotary sidescan sonar at Wasque Shoals capture
the interaction of smaller bedforms, or megaripples, with a dune. 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.
Description:
Financial support was provided by the National Science Foundation through a Graduate
Research Fellowship (NSF GRFP), the Massachusetts Institute of Technology Energy
Initiative Fellowship, the National Science Foundation under grants OCE-1634481 and OCE-
1635151, the Office of Naval Research under grants N00014-06-10329 and N00014-131-0364,
and the Strategic Environmental Research and Development Program (SERDP) through
project #W912HQ17C0012.
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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