<p>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<sup>-1</sup> and 0.6 my<sup>-1</sup>.  Along the Sussex coastline rockfalls from the coastal chalk cliffs tend to be small in nature, typically producing less than 1,000 m<sup>3</sup> of material. Larger rockfalls, however, do occasionally occur and can exceed 20,000 m<sup>3</sup>. 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.                                                               </p> <p>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 m<sup>3</sup> and >4,000 m<sup>3</sup>. 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.</p>

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Intact dry density (IDD) is a key parameter for characterising the rock mass behaviour of chalk and is often the main classification property reported for engineering design. The role of IDD in coastal chalk settings has been discussed in literature as an important rock control for coastal cliff recession behaviour. This study demonstrates a tool that can reliably estimate the IDD of chalk samples. Originally developed for testing sheet steel materials, the Equotip has been shown to be adaptable for geological applications and the results of this study show that the Equotip can be used to derive a surface hardness value of dry chalk samples that has a positive correlation with IDD. Furthermore, the results show that the device is sensitive to the anisotropy of chalk and that testing undertaken in the diametral orientation give more accurate correlations. The results of this study also show that the Equotip is sensitive to environmental setting and the effects of weathering, giving a separate correlation for samples collected along the Sussex coastline to those obtained inland. A statistical assessment of the results shows that a characteristic hardness within an acceptable margin of error can be achieved from an optimal number of single impacts.