Process Inference from Coastal-Protection Structures to Boulder Beaches

Abstract

Because boulder beaches and rubble coastal-protection structures have environmental and compositional similarities, beach-boulder movement is examined in the light of engineering studies of protection-structure stability. On a studied boulder beach, sediment transport occurred during 11 storms over a period of 2 years. Wave conditions from these 11 storms are used in two no-damage design formulae in an attempt to mathematically predict boulder movement. The design formulae over-predict the movement threshold of smaller sized beach sediment and under-predict the movement threshold of the larger sediment, however the Hedar formula (developed for low-angle protection structures) yielded the more realistic prediction. Introduction Five boulder beaches located along the central New South Wales, Australia, coast were selected in order to study their sedimentary properties, form, and sediment transport. These beaches were found to differ fundamentally from cobble beaches (Oak, 1984). Each studied beach is aligned obliquely to the approaching waves and is composed of local sediment, the mean particle size being -84) or less (>256 mm). One boulder beach, Kiama, was chosen for the monitoring of boulder movement because of all the studied beaches it is the least accessible and it is not adjacent to a sand beach suitable for recreation. For these reasons there was little chance of human interference with the marked boulders. Kiama Boulder Beach Kiama boulder beach is located approximately 100 km south of Sydney and has a north-northeasterly exposure to the open ocean. This beach behaves as a sediment trap for material liberated from disintegrating cliffs extending approximately 1 km south. These cliffs are composed of Westley Park Sandstone (Branagan and Packham, 1970), the lowest member of the Geringong Volcanics. There are no massive joint patterns or distinct cleavage planes that might influence the shape and size of particles supplied to Kiama beach. A shore platform provides a base and protection from waves for one-third of this boulder beach, while the remainder is underlain by pebbles, cobbles, and boulders; no sand is present. The length of the beach is 150 m and the width averages 23 m, with the minimum being 17 m. Mean foreshore slope ranges between 6° and 12°, a cliff backs only the headland zone, and a berm extends along the whole beach. Mean sediment size is -8.646. Monitoring Boulder Movement On Kiama beach 300 boulders at the intersection points of a grid surveyed from a fixed bedrock position were marked with marine paint and their a, b, and c axes measured so that any sediment movement could be documented. Once a month and after every major storm during a two year period, the beach was examined for evidence of boulder movement. Because the position of each marked boulder was known, the number of those transported could be readily ascertained. When a moved boulder was found, the distance and direction of the movement were measured. When a boulder was removed from its grid-point position, the boulder in its place at the grid point was marked and measured, so at all times a grid of 300 boulders of known size and position was maintained. Coastal-Protection Structures and Boulder Beaches Although little is known about the processes that form and maintain boulder beaches, there has Geografiska Annaler · 68 A (1986) · 1-2 25 This content downloaded from 157.55.39.213 on Sat, 02 Jul 2016 04:19:04 UTC All use subject to http://about.jstor.org/terms