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Wave breaking patterns control rip current flow regimes and surfzone retention

Wave breaking patterns control rip current flow regimes and surfzone retention
Wave breaking patterns control rip current flow regimes and surfzone retention
Recent research into rip currents has revealed the existence of multiple circulation patterns, meaning that no single escape strategy is appropriate in all situations. Rip circulation is influenced by surfzone morphology, which can be inferred from wave breaking patterns in video imagery. Wave breaking often occurs over the bars adjacent to the rip channel, with little breaking over the seaward end of the rip. However, under varying wave and tide conditions, breaking can also occur at the seaward extent of rip channels. Here, we use this difference as a novel wave dissipation parameter to classify a rip channel as either â??open??? or ???closed??? in terms of rip-head wave breaking. A 4-day field study provided Lagrangian rip current data at a macrotidal, dissipative beach monitored by a coastal imaging system. Using this new parameter, rip channels that were identified as closed exhibited a 31% increase in current speeds and 43% increase in horizontal vorticity compared to open channels. The transition between open and closed channels occurred over a single tidal cycle, which altered surfzone retention rates. Closed channels promoted surfzone retention, with < 25% of drifters exiting the surfzone. In comparison, open channels were more conducive to exchange, with exit rates up to 91%. Analysis of the Royal National Lifeboat Institution lifeguard rip incident database showed that open rips were disproportionately represented in the occurrence of rescue events, and calculated here to be twice as dangerous as closed rips. The use of this new open/closed parameter could be used by surf lifesaving organisations, and may have implications for the cross-shore exchange of sediment and pollutants.
Video imagery, Beach state, Rip current, Beach rescue, Wave breaking, Perranporth
0025-3227
176-190
Pitman, Sebastian
7657cca5-521d-4731-801a-efb3d9a9efc2
Gallop, Shari L.
be66589c-137a-4e23-81d2-8e3b901a5307
Haigh, Ivan D,
945ff20a-589c-47b7-b06f-61804367eb2d
Masselink, Gerd
c56ad11b-b88a-48cd-86aa-0c6cec5759f3
Ranasinghe, Roshanka
595401f8-1450-454d-b9ec-1ea3de9eebb4
Pitman, Sebastian
7657cca5-521d-4731-801a-efb3d9a9efc2
Gallop, Shari L.
be66589c-137a-4e23-81d2-8e3b901a5307
Haigh, Ivan D,
945ff20a-589c-47b7-b06f-61804367eb2d
Masselink, Gerd
c56ad11b-b88a-48cd-86aa-0c6cec5759f3
Ranasinghe, Roshanka
595401f8-1450-454d-b9ec-1ea3de9eebb4

Pitman, Sebastian, Gallop, Shari L., Haigh, Ivan D,, Masselink, Gerd and Ranasinghe, Roshanka (2016) Wave breaking patterns control rip current flow regimes and surfzone retention. Marine Geology, 382, 176-190. (doi:10.1016/j.margeo.2016.10.016).

Record type: Article

Abstract

Recent research into rip currents has revealed the existence of multiple circulation patterns, meaning that no single escape strategy is appropriate in all situations. Rip circulation is influenced by surfzone morphology, which can be inferred from wave breaking patterns in video imagery. Wave breaking often occurs over the bars adjacent to the rip channel, with little breaking over the seaward end of the rip. However, under varying wave and tide conditions, breaking can also occur at the seaward extent of rip channels. Here, we use this difference as a novel wave dissipation parameter to classify a rip channel as either â??open??? or ???closed??? in terms of rip-head wave breaking. A 4-day field study provided Lagrangian rip current data at a macrotidal, dissipative beach monitored by a coastal imaging system. Using this new parameter, rip channels that were identified as closed exhibited a 31% increase in current speeds and 43% increase in horizontal vorticity compared to open channels. The transition between open and closed channels occurred over a single tidal cycle, which altered surfzone retention rates. Closed channels promoted surfzone retention, with < 25% of drifters exiting the surfzone. In comparison, open channels were more conducive to exchange, with exit rates up to 91%. Analysis of the Royal National Lifeboat Institution lifeguard rip incident database showed that open rips were disproportionately represented in the occurrence of rescue events, and calculated here to be twice as dangerous as closed rips. The use of this new open/closed parameter could be used by surf lifesaving organisations, and may have implications for the cross-shore exchange of sediment and pollutants.

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1-s2.0-S0025322716302821-main.pdf - Accepted Manuscript
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Accepted/In Press date: 28 October 2016
Published date: 1 December 2016
Keywords: Video imagery, Beach state, Rip current, Beach rescue, Wave breaking, Perranporth
Organisations: Geology & Geophysics, Physical Oceanography

Identifiers

Local EPrints ID: 402225
URI: http://eprints.soton.ac.uk/id/eprint/402225
ISSN: 0025-3227
PURE UUID: 1ef45d9b-7422-4d38-b792-61968ae12432
ORCID for Ivan D, Haigh: ORCID iD orcid.org/0000-0002-9722-3061

Catalogue record

Date deposited: 02 Nov 2016 10:44
Last modified: 15 Mar 2024 06:02

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Contributors

Author: Sebastian Pitman
Author: Shari L. Gallop
Author: Ivan D, Haigh ORCID iD
Author: Gerd Masselink
Author: Roshanka Ranasinghe

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