Cliff stability evaluation using geomorphological maps based on oblique aerial photographs

Abstract

Abstract Geomorphological mapping is a well established tool in terrain evaluation. A case record is presented in which high-oblique aerial photography from a light aircraft has been used for basic mapping of a 3 km length of coastline with the purpose of assessing relative landslide hazard. The cliffs at Budleigh Salterton, East Devon, are formed in a succession of Permo-Triassic mudstone, conglomerate and sandstone which dip gently eastwards along the coast. The coastline is divided into five units based on changes in the stratigraphic succession exposed in the cliff face; each unit correlating with distinctive geomorphological features and cliff profiles. Groundwater discharges, wave attack and weathering ensure that there is continuing geomorphological activity. This necessitates frequent updating of local hazard assessment. Overlapping aerial photographs of the cliff face have been taken at intervals during the past six years. With limited ground inspection these photographs provide adequate data for basic geomorphological mapping of the principal forms represented in the cliff. Successive series of photographs are used to follow the development of individual features and to identify the hazards present in each section of the cliff. Data from early Ordnance Survey maps, old photographs and personal reminiscences extend parts of the history of the coast back into the last century. The positions of landslides are controlled largely by geological and hydrological factors whereas the timing of events is often a function of external environmental triggering mechanisms such as weather, tide or waves. A classification of the relative hazard of cliff-top instability has been used to map hazard zones for the coastline. Together with the system of photographic data acquisition advocated, this provides a rapid and economic method to assist planning authorities in determining development strategy.