Capturing the Role of Soil Moisture in Driving Soft Cliff Erosion Through Terrestrial Laser Scan (TLS) Return Intensity Data

Abstract:

Characterisation of cliff retreat, especially in soft ‘rocks’ is considered highly problematic and complex where rates of lateral retreat can vary between 1 ma^-1 and > 10 ma^-1. Limited historical observations, and temporally and spatially variable retreat often mean that the fundamental relationships driving erosion e.g. rainfall intensity, rainfall frequency and localised storm events, are overlooked. Providing the basis for empirical modelling under projected climate change, measurement has traditionally relied upon the use of ground survey, historical maps and aerial photographs. Although these techniques offer insight into cliff top position and shoreline change over extended timescales, they are inappropriately placed to capture short-term, focused rates of change.

Here, we quantify failure across the cliff face with localised volumetric change and spatially distributed surface moisture patterns determined from the return intensity measured by a terrestrial laser scanner (TLS). We conducted repeat TLS surveys at a 200 m site of cliff over a 12 month survey period along the Holderness coastline, UK. We demonstrate how TLS can be used to quantity variations in material grain-size and surface moisture (where TLS return intensity is calibrated to in-situ soil moisture measurements), which in turn, can be used to associate active vs stable areas on the cliff face.

This work is aimed at exploring the relationship between surface moisture content and grain size as indicators of stability in coastal cliffs. It is expected that the outcome of this study will have important implications for the understanding, and future modelling of coastal erosion under predicted climate variability.