Abrasion of a series of tracer materials on a gravel beach, Slapton Sands, Devon, UK

Abstract

Abstract While increased attention is being paid to the use of quarried rock in groynes, revetments and breakwaters, and to beach recharge, the large volumes of materials and the associated environmental costs of obtaining these resources has begun to focus minds. Interest in the possible future use of a wider range of material sizes and qualities for beach recharge and in the use of smaller armour block sizes which move during profile adjustment has been growing. Deployment of these dynamic materials for coastal protection structures has focussed research on the long-term performance of these materials and the implications for design life. This reflects a growing concern about the more intelligent use of finite material resources. The vast range in intrinsic resistance of different rock types to laboratory mill abrasion is well reported and these results, when presented as the QMW mill abrasion resistance index, can give relative lifetimes of differing shingle types. However, field studies and/or degradation models of the type discussed in this paper are needed to convert these relative laboratory lifetimes to prototype environmental lifetimes in years so that they are appropriate for predicting the long-term changes in beach material volumes. The author's 1991 rock degradation model, which gives an absolute prediction of weight loss rates, was proposed for armourstone placed within structures designed for static stability. It encorporates the mill abrasion test result for the rock type in question. However, it also attempts to embrace dynamically stable rock protection structures from gravel beaches up to berm breakwaters, depending on the mobility parameter. Before this laboratory and field study, no prototype data was available to check its applicability to gravel-sized material. This paper presents the field results for pebbles consisting of three rock types. Their rates of weight loss with respect to interactions with indigenous beach materials in both laboratory and field trials are given. Whereas weight losses from flint or gneiss pebbles in a field experiment will be negligible, their long-term losses may be predicted from experiments and observations on beaches such as Slapton Sands, where pebbles of rapidly abrading rock types have been measured.