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Longshore size grading on a boulder beach

Green, Andrew, Cooper, Andrew and Saltzmann, Leslee (2016) Longshore size grading on a boulder beach. Journal of Sedimentary Research, 86 (10). pp. 1123-1128. [Journal article]

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URL: http://jsedres.sepmonline.org/content/86/10/1123

DOI: 10.2110/jsr.2016.71

Abstract

Longshore size sorting on boulder beaches has not previously been reported. In a boulder beach comprising beachrock slabs, we report systematic longshore clast size grading. The boulder beach at Mission Rocks, South Africa is deposited on an elevated (þ3 m MSL) shore platform and comprises imbricated clasts up to 5 m in A axis dimension (9 tonnes). The clasts are derived from adjacent intertidal beachrock and eolianite outcrops and are emplaced during high-magnitude wave events. Four distinct downdrift-fining cells are present. Each is 40–50 m long.The sorting is attributed to post-emplacement clast redistribution in which the smallest clasts are transported most frequently (by lower-magnitude storms) and therefore travel farthest downdrift. The updrift cell boundary is marked by boulders (5 m in length) that exceed the transport threshold for all but the most energetic of storms while the downdrift limit contains clasts 0.3 m in length. This mechanism of longshore size sorting does not rely on variations in longshore wave power as does cell development on sand and gravel beaches. The alongshore sorting via multiple high- (and variable-) magnitude events over a period of time, distinguishes these coarse clast deposits from those of single extreme events (whether storms or tsunamis).

Item Type:Journal article
Keywords:Boulder beach, sorting, longshore transport, storm, tsunami
Faculties and Schools:Faculty of Life and Health Sciences > School of Geography and Environmental Sciences
Faculty of Life and Health Sciences
Research Institutes and Groups:Environmental Sciences Research Institute > Coastal Systems
Environmental Sciences Research Institute
ID Code:36989
Deposited By: Professor Andrew Cooper
Deposited On:17 Feb 2017 11:02
Last Modified:14 Dec 2017 11:35

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