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High-resolution observations of a shear front in a tidal channel

High-resolution observations of a shear front in a tidal channel
High-resolution observations of a shear front in a tidal channel
The characteristics of a shear front system in a tidal channel have been investigated through a field study. High-resolution sampling, including three-dimensional measurements of current, temperature, salinity, and turbidity, was performed on repeated transects across the frontal zone both with fronts apparent and with fronts absent. The density measured using conductivity/temperature/depth sensors provides a field proof that the shear front is present whether a density gradient exists. It is distinguishable from conventional buoyant shear fronts, which require a strong along-channel density gradient. The velocities measured using an Acoustic Doppler Current Profiler (ADCP) and trapping Global Position System (GPS) drifters indicate that shear front evolution has a four-stage dynamic process in a tidal cycle: (1) a directional shear front of shoal-flood and channel-ebb during the slack before flood and early flood, (2) a speed shear front of shoal-quick and channel-slow after the early flood, (3) a directional shear front of shoal-ebb and channel-flood during the slack before ebb and early ebb, and (4) a speed shear front of shoal-slow and channel-quick ebb during the late ebb. Thus, the velocity shear, a variation in the current speed and direction on either side of the front, is the generation mechanism of the observed shear front system. It is caused by the unequal tidal durations between deep channels and shallow shoals in the channel discussed herein. The Coriolis effect is found to be negligible through the momentum balance analysis. The unequal tidal durations can be caused by the following alternative mechanisms, according to the analysis and model simulation: (1) differential friction, owing to the steep bathymetry between the shallow shoal and the deep channel, and (2) circulation, owing to the coastline bend of the shallow shoal.
Shear front, tidal channel, density gradient, velocity shear
0272-7714
Lu, Shasha
d31cc856-0ea7-40e6-a651-b5cfbccbf415
Xia, Xiaoming
19f48c88-c7fd-4904-9b37-d87a585df4cb
Cao, Zhenyi
260a5d0d-869e-4f49-b58b-9dd030670b04
Liu, Jingui
972608b2-4dd9-4b22-b39c-2b8fcb7757e9
Thompson, Charlotte
2a304aa6-761e-4d99-b227-cedb67129bfb
Cai, Tinglu
229971fd-173f-41ca-bfb5-62b884b41317
Lu, Shasha
d31cc856-0ea7-40e6-a651-b5cfbccbf415
Xia, Xiaoming
19f48c88-c7fd-4904-9b37-d87a585df4cb
Cao, Zhenyi
260a5d0d-869e-4f49-b58b-9dd030670b04
Liu, Jingui
972608b2-4dd9-4b22-b39c-2b8fcb7757e9
Thompson, Charlotte
2a304aa6-761e-4d99-b227-cedb67129bfb
Cai, Tinglu
229971fd-173f-41ca-bfb5-62b884b41317

Lu, Shasha, Xia, Xiaoming, Cao, Zhenyi, Liu, Jingui, Thompson, Charlotte and Cai, Tinglu (2020) High-resolution observations of a shear front in a tidal channel. Estuarine, Coastal and Shelf Science, 232, [106521]. (doi:10.1016/j.ecss.2019.106521).

Record type: Article

Abstract

The characteristics of a shear front system in a tidal channel have been investigated through a field study. High-resolution sampling, including three-dimensional measurements of current, temperature, salinity, and turbidity, was performed on repeated transects across the frontal zone both with fronts apparent and with fronts absent. The density measured using conductivity/temperature/depth sensors provides a field proof that the shear front is present whether a density gradient exists. It is distinguishable from conventional buoyant shear fronts, which require a strong along-channel density gradient. The velocities measured using an Acoustic Doppler Current Profiler (ADCP) and trapping Global Position System (GPS) drifters indicate that shear front evolution has a four-stage dynamic process in a tidal cycle: (1) a directional shear front of shoal-flood and channel-ebb during the slack before flood and early flood, (2) a speed shear front of shoal-quick and channel-slow after the early flood, (3) a directional shear front of shoal-ebb and channel-flood during the slack before ebb and early ebb, and (4) a speed shear front of shoal-slow and channel-quick ebb during the late ebb. Thus, the velocity shear, a variation in the current speed and direction on either side of the front, is the generation mechanism of the observed shear front system. It is caused by the unequal tidal durations between deep channels and shallow shoals in the channel discussed herein. The Coriolis effect is found to be negligible through the momentum balance analysis. The unequal tidal durations can be caused by the following alternative mechanisms, according to the analysis and model simulation: (1) differential friction, owing to the steep bathymetry between the shallow shoal and the deep channel, and (2) circulation, owing to the coastline bend of the shallow shoal.

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High-resolution observations-of-a-shear-front-in-a-tidal-channel - Accepted Manuscript
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Accepted/In Press date: 2 December 2019
e-pub ahead of print date: 3 December 2019
Published date: 5 January 2020
Keywords: Shear front, tidal channel, density gradient, velocity shear

Identifiers

Local EPrints ID: 437034
URI: http://eprints.soton.ac.uk/id/eprint/437034
ISSN: 0272-7714
PURE UUID: 1290c848-f7ea-44e4-8720-9e071ba57c85

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Date deposited: 15 Jan 2020 17:32
Last modified: 03 Dec 2020 05:01

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