A methodology for the study of the relation between coastal cliff erosion and the mechanical strength of soils and rock masses

A methodology is proposed to make approximate estimates of the destructive force of waves at the bottoms of cliffs where no accurate bathymetric data are available, by means of correlation between the erosion rate of rocky coasts and the mechanical strength of soils and rock masses. The correlation used makes it possible to obtain a linear semi-logarithmic expression between the long-term average erosion rate (from an engineering standpoint) of coasts consisting of different lithologies, wave energy, and the uniaxial compressive strength of soils and rock masses forming the cliffs. The uniaxial compressive strength of soils consisting of landslide debris was measured using a pocket penetrometer and the Rock Mass index (RMi) method was used for rock masses. These methods were chosen after critical analysis of the methodologies currently available. The proposed approach was tested in the Cilento area of southern Italy, and in particular along a 6 km stretch of coast typified by rock masses and landslide debris that are particularly susceptible to erosion. This coastline is exposed to south-westerly waves which in deep water reach significant heights (between 1 and 3 m) and significant wave periods (between 4 and 7 s). Estimates of the coastal erosion rates were made by studying the variations, especially in restricted headland areas, occurring over two time spans, 30 and 45 years, respectively by comparing topographic maps and aerial photographs taken at different times. These comparisons give approximate indications of retreats of 15 m over the time span of 30 years (average erosion rate of 0.5 m year−1) and of 35 m over the time span of 45 years (average erosion rate of ca 0.8 m year−1). These data do have a local significance but clearly show how quick these processes sometimes are. Lastly, the study of a notch incised by wave action on a concrete sea wall built to protect a terrace made it possible to quantify the impact of erosion on concrete. It was possible to obtain RMi-values through field observations. Furthermore, it was possible to obtain the constants of the Hoek and Brown failure criterion using RMi. On the basis of RMi-values, the rock masses studied were defined as ‘moderately strong’ to ‘strong’ and, according to the Hoek and Brown criterion, as ‘poor’ to ‘fair’. A semilogarithmic linear relation was found between the strength of landslide debris, rock masses and concrete, as well as their respective long-term erosion. Consequently, a rough estimate of the destructive force of waves (ca 24 MPa) was calculated. This value is given by the product of the mean value of their hydraulic pressure for a non-dimensional constant which, in turn, takes into account the sandy-pebbly beach sediment at the foot of the cliffs that acts as an abrasive on the rock mass.