Emergence and Migration of a Nearshore Bar: Sediment Flux and Morphological Change on a Multi-Barred Beach in the Great Lakes

Author:

Greenwood Brian1,Permanand-Schwartz Allana2,Houser Christopher A.3

Affiliation:

1. Scarborough College Coastal Research Group, Department of Physical and Environmental Sciences, University of Toronto, Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada

2. University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada

3. Department of Environmental Studies, University of West Florida, 11000 University Parkway, Pensacola, Florida 32514-5750, United States.

Abstract

Burley Beach (southeastern Lake Huron) exhibits a multi-barred shoreface, the long-term equilibrium morphology characteristic of many low angle, sandy beaches in the Canadian Great Lakes. During a single major storm, a new bar emerged 50-60 m offshore as an irregular trough-crest form, through differential erosion of an existing shore terrace. Emergence, bar growth and offshore migration were associated with: (a) an overall negative sediment balance in the inner surf zone initially (‑2.30 m3>/m beach width), but with a large positive sediment balance (+5.10 m3/m) subsequent to the storm peak and during the storm decay; (b) progradation of the beach step to produce a new shore terrace; and (c) offshore migration of the two outer bars to provide the accommodation space necessary for the new bar. The primary transport mechanisms accounting for emergence of the new bar, its growth and migration were: (a) the mean cross-shore currents (undertow), which always transported suspended sediment offshore; and (b) the onshore transport of suspended sediment by incident gravity wave frequencies early in the storm and subsequently by infragravity waves (at the storm peak and the decay period). The longshore transport of sediment was significant in terms of the gross transport, although the net result was only a small transport to the south-west (historic littoral transport direction). It did not cause bar initiation, but it may have supplied some of the sediment for bar growth. The primary mechanism for bar initiation and growth was the cross-shore displacement of sediment by wave-driven (oscillatory) transport and cross-shore mean currents (undertow).

Publisher

Consortium Erudit

Subject

Paleontology,Geology

Reference60 articles.

1. Aagaard, T., Greenwood, B. and Nielsen, J., 2001. Bed level changes and megaripple migration on a barred beach. Journal of Coastal Research, Special Issue 34, p. 110-116.

2. Aagaard, T. and Greenwood, B., 1994. Suspended sediment transport and the role of infragravity waves in a barred surf zone. Marine Geology, 118: 23-48.

3. Aagaard, T. and Greenwood, B., 1995a. Suspended sediment transport and morphological response on a dissipative beach. Continental Shelf Research, 15: 1061-1086.

4. Aagaard, T. and Greenwood, B., 1995b. Longshore and cross-shore suspended sediment transport at far infragravity frequencies in a barred environment. Continental Shelf Research, 15: 1235-1249.

5. Aagaard, T. and Greenwood, B., 1999. Directionality of cross-shore sediment transport in the surf zone under high-energy conditions. Coastal Sediments ’99, American Society of Civil Engineers, New York, 16 p.

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