Acoustic observations of Kelvin-Helmholtz billows on an estuarine lutocline

Junbiao Tu1, Daidu Fan2, Qiang Lian3, Zhiyu Liu4, Wei Liu1, Alexis Kaminski5 and William Smyth6, (1)Tongji University, Shanghai, China, (2)Tongji University, State Key Laboratory of Marine Geology, Shanghai, China, (3)Xiamen University, Xiamen, China, (4)Xiamen University, State Key Laboratory of Marine Environmental Science, and Department of Physical Oceanography, College of Ocean and Earth Sciences, Xiamen, China, (5)University of Washington, Seattle, WA, United States, (6)Oregon State University, Corvallis, OR, United States
Abstract:
Kelvin-Helmholtz (KH) instability plays an important role in turbulent mixing in deep oceans and coastal seas. Though widely observed and studied in thermohaline-stratified waters, KH instability is scarcely observed in sediment-stratified environments. For the first time, we present direct observations of KH billows on an estuarine lutocline by combining echosounder images with velocity and density measurements. The interaction between velocity shear and the density stratification induced by suspended sediments initiated shear instabilities near the bed, indicated by gradient Richardson number (Ri) < 0.25 at the early stages of the observed billows. Once formed, the instabilities enhance vertical mixing of momentum, reducing vertical shear and elevating Ri. Linear instability analysis using measured velocity and density profiles well predicts the vertical location and spatial characteristics of the observed billows. These instabilities are believed to contribute to the vertical mixing, entrainment, and transport of estuarine and coastal sediments.
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