A degradation model specifically for rock armour in coastal structures is proposed. The objective is to give the coastal engineer a new design tool. The model provides a prediction of the armourstone weight loss with time on the structure. This information can help in the assessment of alternative design costings while taking account of the quality of rock from different possible sources.

Factors affecting the degradation rate are the intrinsic material properties of the rock source, the production-influenced geometric properties of the armourstone, the environmental boundary conditions at the coastal site and the armour layer design concepts used. To apply the model to a given rock source, a sample of the material is tested in an abrasion mill simulation of the wear process. This gives a weight versus laboratory time plot. Laboratory time is converted to years on site using an equivalent wear time factor which is derived from a combination of nine weighted parameters. The effects of the other principal degradation mechanisms, fracturing and spalling, are included as well as abrasion. For example by using parameter estimates which couple meteorological climatic conditions with particular rock type susceptibilities, the influence of physico-chemical degradation can be assessed.

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