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Onshore sandbar migration at Tairua Beach (New Zealand): numerical simulations and field measurements

Onshore sandbar migration at Tairua Beach (New Zealand): numerical simulations and field measurements
Onshore sandbar migration at Tairua Beach (New Zealand): numerical simulations and field measurements
We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.
0025-3227
99-106
van Maanen, B.
47cb6ae2-9baf-4f37-a138-067d72966597
de Ruiter, P.J.
01207ebb-f4a2-4f22-80e6-8dd38c92c235
Coco, G.
2fd53078-aedb-4f12-bb28-d69b74d8ad64
Bryan, K.R.
02d42071-7100-4ef5-9bba-4b14bdbd9277
Ruessink, B.G.
5c18a796-5a3d-4599-8d9c-39ab37b76fa2
van Maanen, B.
47cb6ae2-9baf-4f37-a138-067d72966597
de Ruiter, P.J.
01207ebb-f4a2-4f22-80e6-8dd38c92c235
Coco, G.
2fd53078-aedb-4f12-bb28-d69b74d8ad64
Bryan, K.R.
02d42071-7100-4ef5-9bba-4b14bdbd9277
Ruessink, B.G.
5c18a796-5a3d-4599-8d9c-39ab37b76fa2

van Maanen, B., de Ruiter, P.J., Coco, G., Bryan, K.R. and Ruessink, B.G. (2008) Onshore sandbar migration at Tairua Beach (New Zealand): numerical simulations and field measurements. Marine Geology, 253 (3-4), 99-106. (doi:10.1016/j.margeo.2008.05.007).

Record type: Article

Abstract

We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.

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More information

Published date: 2008
Organisations: Energy & Climate Change Group

Identifiers

Local EPrints ID: 354897
URI: http://eprints.soton.ac.uk/id/eprint/354897
ISSN: 0025-3227
PURE UUID: affda55c-e112-4ed6-9a6b-84f44fddc74b

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Date deposited: 29 Jul 2013 16:33
Last modified: 16 Jul 2019 21:27

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Contributors

Author: B. van Maanen
Author: P.J. de Ruiter
Author: G. Coco
Author: K.R. Bryan
Author: B.G. Ruessink

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