Hydrological controls of surficial mass movements in peat

Peat deposits occur in areas where water logging is common. Large areas of blanket mire occur throughout the temperate and cold regions of the Northern and Southern Hemispheres and are particularly extensive in the UK Uplands. Mass movements of peat, reported usually as peat slides and bog bursts, have been well documented for over 150 years. However, the fundamental controls of this form of shallow instability are still poorly understood. The aim of this paper is to review critically the available evidence linking peat hydrology and slope instability. Then, using data from the North Pennine uplands in Northern England, we examine the importance of rainfall, macroscale drainage conditions and peat hydrological processes in initiating slope instability. Shear failure by loading, buoyancy effects, basal liquefaction and surface or marginal rupturing have all been proposed as potential failure mechanisms. Data from Northern England demonstrate that hydrological processes are fundamental in determining the spatial and temporal occurrence of peat slides. Most instabilities are associated with convective summer thunderstorms, although instances of rapid snowmelt and intense winter rainfall have also triggered these events. All instabilities occur in association with distinct drainage features. Slides are initiated along natural drainage lines or in association with artificial drainage often brought about by mining activity or agricultural practices. At the scale of the soil profile the special hydrological properties of peat, in particular shallow water tables and low soil hydraulic conductivity, offer important clues to failure mechanisms. Pore water pressures are generally low and vary little in the peat profile and throughout the year. It is clear from observations of subsurface flow that significant volumes of runoff reach considerable depths in the soil. Macropores (flow occurring in pores greater than 1 mm in diameter) are important in delivering surface runoff to deeper parts of the profile and in some instances high pipe water pressures may develop. These findings define several important hydrological controls for surficial mass movements in peat but the prospect of predicting the location and timing of such events is still a long way off.