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Observations of a tidal intrusion front in a tidal channel

Observations of a tidal intrusion front in a tidal channel
Observations of a tidal intrusion front in a tidal channel
A visible front indicated by a surface colour change, and sometimes associated with foam or debris lines, was observed in a tidal channel during neap tide. This is an example of a tidal intrusion front occurring in the absence of sudden topographical changes or reversing flows, typically reported to be associated with such fronts. Detailed Acoustic Doppler Current Profiler and conductivity/temperature/depth measurements were taken on repeated transects both with fronts apparent and with fronts absent. The results indicated that the front occurred as a result of stratification, which was sustained by the buoyancy flux and the weak tide-induced mixing during neap ebb tide. The stronger tide-induced mixing during spring tide restrained stratification, leading to the absence of a front. The mechanism of the frontogenesis was the density gradient between the stratified water formed during neap ebb tide, and the more mixed seawater during neap flood tide; thus, the water on the landward (southwestern) side of the front was stratified, and that on the seaward side (northeastern) of the front was vertically well mixed. Gradient Richardson number estimates suggest that the flow between the stratified and mixed water was near the threshold 0.25 for shear instability. Meanwhile, the density gradient would provide an initial baroclinic contribution to velocity convergence, which is indicated by the accumulation of buoyant matter such as foam, grass, and debris into a sharply defined line along the surface. The front migrates with the flood current, with a local maximum towards the eastern side of the channel, leading to an asymmetrical shape with the eastern side of the front driven further into the Tiaozhoumen tidal channel.
0272-7714
12-20
Lu, Shasha
d31cc856-0ea7-40e6-a651-b5cfbccbf415
Xia, Xiaoming
19f48c88-c7fd-4904-9b37-d87a585df4cb
Thompson, Charlie E.L.
2a304aa6-761e-4d99-b227-cedb67129bfb
Cao, Zhenyi
260a5d0d-869e-4f49-b58b-9dd030670b04
Liu, Yifei
51e7f9a5-5c9a-4426-be46-34a3c39d2cb6
Lu, Shasha
d31cc856-0ea7-40e6-a651-b5cfbccbf415
Xia, Xiaoming
19f48c88-c7fd-4904-9b37-d87a585df4cb
Thompson, Charlie E.L.
2a304aa6-761e-4d99-b227-cedb67129bfb
Cao, Zhenyi
260a5d0d-869e-4f49-b58b-9dd030670b04
Liu, Yifei
51e7f9a5-5c9a-4426-be46-34a3c39d2cb6

Lu, Shasha, Xia, Xiaoming, Thompson, Charlie E.L., Cao, Zhenyi and Liu, Yifei (2017) Observations of a tidal intrusion front in a tidal channel. Estuarine, Coastal and Shelf Science, 198 (A), 12-20. (doi:10.1016/j.ecss.2017.08.030).

Record type: Article

Abstract

A visible front indicated by a surface colour change, and sometimes associated with foam or debris lines, was observed in a tidal channel during neap tide. This is an example of a tidal intrusion front occurring in the absence of sudden topographical changes or reversing flows, typically reported to be associated with such fronts. Detailed Acoustic Doppler Current Profiler and conductivity/temperature/depth measurements were taken on repeated transects both with fronts apparent and with fronts absent. The results indicated that the front occurred as a result of stratification, which was sustained by the buoyancy flux and the weak tide-induced mixing during neap ebb tide. The stronger tide-induced mixing during spring tide restrained stratification, leading to the absence of a front. The mechanism of the frontogenesis was the density gradient between the stratified water formed during neap ebb tide, and the more mixed seawater during neap flood tide; thus, the water on the landward (southwestern) side of the front was stratified, and that on the seaward side (northeastern) of the front was vertically well mixed. Gradient Richardson number estimates suggest that the flow between the stratified and mixed water was near the threshold 0.25 for shear instability. Meanwhile, the density gradient would provide an initial baroclinic contribution to velocity convergence, which is indicated by the accumulation of buoyant matter such as foam, grass, and debris into a sharply defined line along the surface. The front migrates with the flood current, with a local maximum towards the eastern side of the channel, leading to an asymmetrical shape with the eastern side of the front driven further into the Tiaozhoumen tidal channel.

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Accepted/In Press date: 18 August 2017
e-pub ahead of print date: 21 August 2017
Published date: 5 November 2017

Identifiers

Local EPrints ID: 414217
URI: http://eprints.soton.ac.uk/id/eprint/414217
ISSN: 0272-7714
PURE UUID: 1e072d15-c3c9-49c5-bf72-14451c120f24

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Date deposited: 19 Sep 2017 16:31
Last modified: 07 Oct 2020 04:23

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