Characterising soil moisture in transport corridors using remote sensing

Abstract

This thesis assesses the ability of remote sensing techniques to characterise soil moisture in a transport corridor environment. Much of the world’s transport networks are built on earthwork embankments or in cuttings. In the UK, many of these earthworks were constructed in the mid-19th Century and are susceptible to slope instability. Instability in transport corridors is often triggered by an increase in pore pressure, which is directly influenced by an increase in soil moisture. Although a number of studies have investigated the use of remote sensing techniques for estimating soil moisture, they have tended to be conducted under controlled conditions and few have considered their capacity for being operational. This study addresses this point by exploring the use of high spatial resolution digital elevation models (DEMs) and airborne hyperspectral imagery for characterising soil moisture in transport corridors. A number of terrain (topographic wetness index (TWI), potential solar radiation, aspect) and spectral analysis (red edge position estimation, derivative stress ratios, continuum removal analysis, partial least squares (PLS) regression modelling, mapping biological indicator values) techniques were assessed using terrestrial systems over a test embankment, and airborne data for a transport corridor. The terrain analysis metrics TWI and potential solar radiation were found to be highly sensitive to the DEM spatial interpolation routine used, with a thin plate spline routine performing best in this study. This work also demonstrated that Ellenberg indicator values extended for the UK can be mapped successfully for transport corridor environments, providing potential for a number of different applications. Individually, the techniques were shown to be generally poor predictors of soil moisture. However, an integrated statistical model provided an improved characterisation of soil moisture with a coefficient of determination (R2) of 0.67. Analysis of the model results along with field observations revealed that soil moisture is highly variable over the transport corridor investigated. Soil moisture was shown to increase in a non linear fashion towards the toe of earthwork slopes, while contribution from surrounding fields often led to concentrations of moisture in cutting earthworks. Critically, while these patterns could be captured using the data investigated in this study, such spatial variability is rarely taken into account using analytical slope stability models, potentially raising important challenges in this respect.