Quantifying friction drag mechanism and morphological evolution of combined ripple and dune bedforms using the Jetyak autonomous surface vessel

Peter A. Traykovski , Applied Ocean Physics & Engineering

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2016 COI Funded Project

Peter A.  Traykovski
Applied Ocean Physics and Engineering

Abstract:

The migration of bedforms is a primary sediment transport mechanism controlling the morphology of coastal systems such as tidal inlets.  Onshore migration and reattachment of dunes to the shore can cause rapid shoreline accretion while offshore migration can cause shoreline retreat.  In addition to the sediment carried by bedforms, sub-aqueous bedforms also control the flow in shallow coastal systems.  Flow acceleration over large scale features provide the leading source of current speed spatial variability, while bed roughness due to small and medium scale bedforms controls frictional drag and near-bed turbulence.  This roughness increases mixing near the seabed which in return influences the transport of contaminants and sediment.  In the past decade, the deployment of seafloor mounted frames equipped with bedform imaging sonars has indicated tidally reversing mega-ripples are ubiquitous bedforms in tidal inlets.  These small to medium scale (0.2 to 0.5 m high, 1 to 5 m wavelength) bedforms reverse migration direction and asymmetry on each half tidal cycle and migrate up to several wavelengths per tidal cycle.  In some locations mega-ripples are superimposed on large scale dunes with wavelengths of 100s of meters and heights of several meters.  Conceptual models in the literature suggest that superimposed bedforms may coexist, but do not morphodynamically interact.  Our observations from the imaging sonars suggest an alternate hypothesis, that the convergent or divergent migration of the small scale bedforms directly forces the migration of the large scale bedforms.  Our ability to fully test this hypothesis was limited, however, due to the relatively small (~20 m diameter) footprint of the imaging sonar compared to the 100 m wavelength dunes.  In the proposed work we would use the recently developed Jetyak (jet drive kayak hull) autonomous surface vessel equipped with a high resolution swath bathymetry sensor to measure mega-ripple and dune migration on tidal and subtidal scales.  This would aid in testing the hypothesis that the migration of the small scale bedforms serve as an intermediate step between grain-scale transport processes and large scale dune migration.

In addition to the measurements of morphologic evolution, we will measure hydrodynamic force balances to examine the relative roles of small and large scale bedforms.  The tidal reversal of mega-ripple asymmetry lags the reversal of the flow resulting in the bedforms being in temporal disequilibrium for part of the tidal cycle.  This introduces dynamic variability in the bed friction that is little understood.  The bedforms also influence pressure gradients which has been observed in preliminary studies from the Jetyak phase resolving GPS system.  The flow acceleration over the large dunes results in a step-like decrease in sea surface elevation on the order of several centimeters while a more gradual slope in sea surface elevation occurs over smaller bedforms due to flow dissipation.  In the proposed work, the Jetyak would be used to take extensive surveys of the flow and sea surface elevation.  We would then integrate these measurements with in-situ sensors to facilitate a better comprehension of the combined morphodynamic and hydrodynamic response of coastal systems containing multiple scale bedforms.  The measurements and analysis in this project would be coupled with numerical modeling efforts in other projects to increase our ability to predict the morphological evolution of systems with multiple scale bedforms.