TXT-tool 3.044-1.1: The Runout of Chalk Cliff Collapses—Case Studies and Physical Model Experiments

Chalk exposures in coastal cliff faces extend from Yorkshire to Devon in England and from Boulonnais to Normandy in France. The failure of chalk cliffs is a long-recognised hazard along parts of these coastlines, however variation in the chalk’s lithology, stratigraphy and physical properties coupled with changing topographic relief, means that the nature of the hazard is variable. This tool is concerned with cliff failures that may lead to chalk flow resulting in catastrophic flow slide or rock avalanche-like behaviour. We discuss the nature of chalk flows and how they relate to other types of flow-like avalanches. Recently published and unpublished work is used to unify simple mechanical index properties between the French and English coastal chalks. Some well-documented studies are then re-examined, using these results to show how runout is controlled by chalk porosity. Finally, a series of physical model tests are described in which some of the controls on runout of the chalk are examined.