Identifying the behavioural characteristics of clay cliffs using intensive monitoring and geotechnical numerical modelling

Coastal monitoring is routinely undertaken to provide an archival record of cliff-line movement that can be used in the development and validation of predictive coast retreat and evolution models. However, coastal monitoring is often purely quantitative in nature, and financial necessity requires deployment over extensive coastal sections. As a result, for local site conditions in particular, only limited geomorphological data are available or included during the development of such predictive models. This has resulted in many current models incorporating a simplistic or generalised representation of cliff behaviour, an approach that progressively loses local credibility when deployed over extensive heterogeneous coastlines. This study addresses this situation at a site of extreme coastline retreat, Holderness, UK, through the application of intensive monitoring of six representative cliff sections nested within a general geomorphological appraisal of the wider coastline as a whole. The data from these surveys have been used to validate a finite difference-based geotechnical modelling assessment of clay cliff stability. Once validated, the geotechnical model was used to simulate a range of scenarios that were sufficient to represent the range of topographic, hydrogeological, geological, and littoral conditions exhibited throughout the region. Our assessment identified that the cliff retreat occurs through the combined influence of direct marine erosion of the cliff, with shallow, structurally controlled failures or substantial mass failures. Critically, the predisposition to any one of these failure mechanisms arises principally as a result of initial cliff height. The results of the numerical modelling have been combined into an empirical slope model that derives the rate of landslide-induced retreat that would arise from mass failures under various future scenarios. Results of this study can be used in the selection and development of retreat models at coastlines of similar physiographic setting to that found at Holderness. The results represent a key step in linking material deformation properties to the processes of cliff change and the subsequent range of landforms found on clay cliffs. As such, the results could also be used more generally to illustrate the likely cliff behaviour of other soft rock coastlines. (C) 2010 Elsevier B.V. All rights reserved.