FIELD MEASUREMENTS OF BEDFORMS IN A RIP CHANNEL ON A MACRO-TIDAL BEACH

Antony Thorpe, Jon Miles, Gerd Masselink, Paul Russell, Tim Scott, Martin Austin

Abstract


A Sand Ripple Profiler (SRP) was deployed in a rip channel on a dissipative sandy beach to measure bedform height (∆), length (λ) and migration rate (Mr¬) throughout a macro-tidal cycle with an offshore significant wave height of 0.7 m and peak period of 10 s. At lower tidal elevations in the strong offshore flow of the rip current (maximum = 0.4 m/s) bedforms (∆ = 0.15 m, λ = 3 m) were found to migrate offshore (Mr = 0.21 m/hr). Outside of active rip current conditions (water depth (h) = >~2.5 m) bedforms were found to be of smaller scale (∆ = 0.09 – 0.12 m, λ = 1 – 1.2 m) migrating onshore at a rate of 0.35 m/hr at mid tide (h = 3.3 m) and 0.03 m/hr at high tide (h = 6.3 m). Onshore migration rates were found to increase with increased wave skewness and velocity variance.

Keywords


rip currents; bedforms; megaripples

References


Aagaard, T., Greenwood, B. & Nielsen, J. 1997. Mean currents and sediment transport in a rip channel, Marine Geology, 140 (1-2), 25-45.http://dx.doi.org/10.1016/S0025-3227(97)00025-X

Austin, M., Scott, T., Brown, J., Brown, J., MacMahan, J., Masselink, G. & Russell, P. 2010. Temporal observations of rip current circulation on a macro-tidal beach, Continental Shelf Research, 30 (9), 1149-1165.http://dx.doi.org/10.1016/j.csr.2010.03.005

Brander, R. W. 1999a. Field observations on the morphodynamic evolution of a low-energy rip current system, Marine Geology, 157 (3-4), 199-217.http://dx.doi.org/10.1016/S0025-3227(98)00152-2

Brander, R. W. 1999b. Seiment Transport in Low Energy Rip Current Systems, Journal of Coastal Research, 15 (3), 839-849.

CCO 2012. Channel Coast Observatory, [Online]. Available at: http://www.channelcoast.org/data_management/real_time_data/charts/?chart=76 (Accessed: 23/11/2011).

Elgar, S., Guza, R. T. & Freilich, M. H. 1988. Eulerian Measurements of Horizontal Accelerations in Shoaling Gravity Waves, Journal of Geophysical Research, 93 (C8), 9261-9268.http://dx.doi.org/10.1029/JC093iC08p09261

Gallagher, E. L. 2003. A note on megaripples in the surf zone: evidence for their relation to steady flow dunes, Marine Geology, 193 (3-4), 171-176.http://dx.doi.org/10.1016/S0025-3227(02)00662-X

Gallagher, E. L., Elgar, S. & Thornton, E. B. 1998. Megaripple migration in a natural surf zone, Nature, 394 (6689), 165-168.http://dx.doi.org/10.1038/28139

Greenwood, B. & Davidson-Arnott, G. D. 1979. Sedimentation and equilibrium in wave-formed bars: a review and case study, Journal of Earth Science, 312-332.

Hoekstra, P., Bell, P., van Santen, P., Roode, N., Levoy, F. & Whitehouse, R. 2004. Bedform migration and bedload transport on an intertidal shoal', Continental Shelf Research, 24 (11), 1249-1269.http://dx.doi.org/10.1016/j.csr.2004.03.006

Kachel, N. B. & Sternberg, R. W. 1971. Transport of bedload as ripples during an ebb current', Marine Geology, 10 (4), 229-244.http://dx.doi.org/10.1016/0025-3227(71)90042-9

MacMahan, J. H., Thornton, E. B. & Reniers, A. J. H. M. 2006. Rip current review, Coastal Engineering, 53 (2-3), 191-208.http://dx.doi.org/10.1016/j.coastaleng.2005.10.009

MacMahan, J. H., Thornton, E. B., Stanton, T. P. & Reniers, A. J. H. M. 2005. RIPEX: Observations of a rip current system, Marine Geology, 218 (1-4), 113-134.http://dx.doi.org/10.1016/j.margeo.2005.03.019

Nielsen, P. 1992. Coastal bottom boundary layers and sediment transport. Advanced series on ocean engineering. London: World Scientific.

Schmit, W. B., Woodward, K., Millikan, R., Guza, R. T., Raubenheimer, B. & Elgar, S. 2003. A GPS-tracked surf zone drifter, Journal of Atmospheric and Ocean Technology, 1069-1075.http://dx.doi.org/10.1175/1460.1

Sherman, D. J., Short, A. D. & Takeda, I. 1993. Seiment Mixing-Depth and Bedform Migration in Rip channels, Journal of Coastal Research, 39-48.


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