Title:

Observations of Shear Instabilities in Hampton Inlet

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Abstract

Simple barotropic linear stability analysis suggests that strong cross-channel shear of tidal currents flowing through a bounded inlet can lead to instabilities in the flow, causing a meandering of the mean along-channel current and potentially the spinoff of large eddies. The resulting variability of the currents and vorticities may affect navigation and the transport of organic and inorganic matter, and cause lateral mixing of momentum across the inlet. To estimate the wavenumbers of shear instabilities within Hampton Inlet, NH, a spatially-lagged array spanning 389 m consisting of seven acoustic Doppler velocimeters and current profilers equipped with pressure sensors were deployed along the ~3 m depth contour for one week during the spring tide in May 2021. Using iterative maximum likelihood estimators, wavenumber-frequency spectra are estimated during 2-4 hour periods with approximately steady currents on both the flood and ebb tide. Dominant wavenumbers (+/- 0.001-0.015 m-1) of the low frequency motions (0.0015-0.01 s-1) are resolved and consistent with motions determined from the linear stability analysis. The instabilities are directed into the inlet on flood tides and out of the inlet on the ebbs, consistent with the expected propagation of unstable modes. The normalized velocity-to-pressure variance ratio, R, at each station shows that the infragravity band is dominated by shear wave motions (R>>1) during both the flood and ebb tide. The lack of breaking wave group modulations within the inlet and the presence of the seaward propagating instabilities on the ebb flow indicate that the presence of the instabilities can be attributed to the shear of the tidal current.

Authors

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Katie Kirk

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Submission Details

Conference GRC
Event Graduate Research Conference
Department Oceanography (GRC)
Group Oral Presentation
Added April 9, 2022, 11:32 a.m.
Updated May 9, 2022, 11:46 a.m.
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