Establishing a relationship between rock control and rates of coastal erosion using high precision sequential UAV photogrammetry data and Leeb rebound impact hardness testing

In Northern Europe, chalk is an important material affecting many coastal communities. This has led to a great deal of research into coastline recession to quantify rates of coastal erosion and to understand the processes and mechanisms that control coastal rock mass instability of chalk cliffs. Along the UK and French chalk coastlines, estimates for rates of coastal retreat vary, but are generally understood to be between approximately 0.2 my^-1 and 0.6 my^-1.  Along the Sussex coastline rockfalls from the coastal chalk cliffs tend to be small in nature, typically producing less than 1,000 m³ of material. Larger rockfalls, however, do occasionally occur and can exceed 20,000 m³. Understanding the relationship between rock strength and wave energy is of great importance in predicting cliff recession. Although the importance of understanding the contribution of rock control on coastal erosion is widely recognised, establishing a direct link between rock material properties and rates of erosion has proven to be difficult.                                                               

This study presents the findings from five years of coastal monitoring at a site in Sussex (United Kingdom) using UAV photogrammetry. Rates of coastal erosion have been calculated and are found to be in general agreement with those in the published literature. Sequential monitoring undertaken at the site enabled the identification of many rockfalls with calculated volumes ranging between <100 m³ and >4,000 m³. Sampling of rock materials in the field for laboratory testing was undertaken. Extracted dried core sub-samples were tested with a Leeb hardness tester (Equotip) to derive a surface hardness at regular intervals along the cliff-line. Previous research (Thrower et al, 2022) has already demonstrated that the Leeb hardness of chalk has a good relationship with intact dry density and can therefore be a useful tool for estimating rock strength. The results of this study show a good correlation between Leeb hardness and back-wear along the cliff-line at the study site, indicating a relationship between rock strength and rates of coastal erosion. The data show that sections of the study area characterised by softer relatively low-density chalk have been eroded at faster rate than areas characterised by harder relatively high-density chalk. This work demonstrates the potential that Leeb hardness testers have in the characterisation of rock mass properties for quantifying rock control in coastal studies. Furthermore, such testing could provide useful data in predicting future cliff recession behaviour at coastal sites.