Peat slides : morphology, mechanisms and recovery

This thesis investigates the morphology, mechanisms and recovery of peat slide failures using a combined field and laboratory approach. Detailed field investigations are based in the North Pennines, Northern England. The North Pennine regional sample is compared with a global database of peat mass movements, compiled from literature sources and field survey in the UK and Ireland. The database identifies two major types of peat mass movement, peat slides and bog bursts. Both types have been reported in documentary sources during the last 300 years, although peat slides appear to be increasing in frequency. Peat slides occur at higher altitudes and on steeper slopes than bog bursts. Previous studies of peat slides have suggested a translational failure mechanism, with a failure plane at or below the peat-substrate interface. Investigations in the North Pennines identified 14 failures spanning the last 68 years, of which four are previously undocumented. All show similar characteristic features including: excavated crescenticlinear failure scars, rafted and/or blocky debris and widespread deposition of slurry. Sedimentation patterns suggest a rapid and progressive transition from slide to flow, with localised extensive and compressive movement. peat slurry comprises the largest volume of transported material. Laboratory investigation of materials at each site suggest that the peat-substrate zone was the most likely plane of failure in all cases. The irregular nature of this contact zone promotes pockets of weak material, prone to sub-horizontal fracture in the peat mass and low shear strengths in the substrate. A chronosequence approach is applied to investigate the rate of recovery of the slide scars after failure, by comparing the nature of vegetation assemblages on six North Pennine peat slides of various ages. Initial regrowth of vegetation is slow (1-5 years), with reworking of the exposed substrate by fluvial processes. Thereafter, revegetation progresses at a broadly linear rate (5-50 years). Physical rather than chemical changes in substrate character aid revegetation. The field evidence suggests the process of recovery is completed within 70 years. This partly explains the difficulty of identifying older documented sites. Sediment budget analysis applied to the slide events and to their post-failure development demonstrates that the event phase yields the greatest volume of sediment (between 48 and 96% of the overall budget). Overall, peat slides represent a minor component in the geomorphological activity of the North Pennine region (0.02 — 1.00%). Because of the bias toward peat slides, a case study of the Glendun (Ireland) bog burst was undertaken to justify the treatment of peat slides and bog bursts as separate event types. This indicated that bog bursts show differing patterns of sedimentation to peat slides, with less vertical erosion, and more elongated peat raft forms. Vertical variations in material properties show wetter, less dense and slightly more humified peat at depth, with a failure zone within the peat mass. The greater quantities of peat that remain allow more rapid revegetation, and impede the development of surface drainage. Consequently, recovery of bog burst scars may be more rapid than for peat slide scars.