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Peat Mass Movements and Geomorphological Impacts of Extreme Rainfall
A research project of the Department of Geography.
Peat mass movements are relatively common geomorphological events in UK and Irish uplands but the causes and movement mechanisms are poorly understood. Most peat failures are triggered by high magnitude rainfall events, and impacts include damage to stream ecosystems, loss of high quality, protected blanket bog, and damage to property and infrastructure. On 19 September 2003, an intense rainfall event lasting less than 3 hours caused around 35 landslides within an area of 3km2 on Dooncarton Mountain, Co. Mayo, Ireland. Most of the landslides were peat slides, and unusual in that the majority slid along a failure surface within the peat, evident from the shiny, black and discontinuous thin veneer of peat left on the failure scars; even some of the steeper slopes that failed (e.g. 20-25º) involved peat up to 0.5m deep. Impacts on the local community included inundation of farmland, loss of livestock and farm machinery, damage to bridges and devastation of the Pollathomas graveyard. One house was destroyed and up to 50 families were affected by the landslides: at the time of writing, 14 families were still not rehoused. Geomorphologically the events are of enormous significance because they represent the largest cluster of failures yet recorded (globally) in both spatial and temporal terms (occurring within two hours of one another, and over an area of only 3km2).
The aim of this research is to determine the hydrogeomorphic conditions which produced these failures and assess the geomorphological impact in terms of local sediment budgets. This will be coupled with an appraisal of the impacts on the local community in order to build hazard scenarios for similar conditions arising in the future.
Methods & Objectives
- To determine how and why so many peat slides and other shallow landslides were triggered on certain slopes of Dooncarton Mountain, Co. Mayo, by the storm of 19 September 2003
- To quantitatively assess the geomorphological impacts of the storm event in terms of sediment production, transport and deposition, in the context of local topography, land use/management and community infrastructure
- To incorporate previously ignored factors such as tensile strength into stability modelling of peat slopes, as a step towards development of a method for routine assessment of potential instability of peat slopes
- To provide a hazard map and a quantified assessment of future risk which would be of direct benefit to the inhabitants of the affected villages
Results & Outputs
The principal outcomes of the work have been:
- The first quantitative analysis of a multiple failure event involving blanket peat that establishes failure conditions and geomorphological impacts of the entire event
- Demonstration of spatial patterns of subsurface structures (e.g. pipes, iron pan) associated with a slope catena sequence, and the influence of the catena-topography relationship on the occurrence of failure
- Establishment of a complete spatially-distributed sediment budget for the event
- Quantitative assessment of the role of ditches and peat-cutting activities in the occurrence of failure
- Development of hazard mapping approaches based on sediment budget analysis
The significance of this work lies in the assessment of impacts of future climate change, in the form of more frequent high magnitude and intensity rainfall events, with respect to upland blanket bog environments. This work has demonstrated a previously unknown density of sub-peat pipe networks (with unclear relationships to hydrological pathways through the peat), and the presence of slickenside surfaces within basal peat layers. Both of these factors indicate that blanket peat elsewhere may be much more susceptible to failure during intense rainfall than previously thought. This has significant implications for future hazard assessment, particularly in areas where the peat cover has been disrupted by peat-cutting or ditch excavation. The sediment runout data comprise the first such data from a single storm producing multiple landslides and will therefore be of significance for future hazard assessments. Coupling between the slope and channel sediment systems and the downstream impacts that ensue is an important area for further research.
Detailed results are given in the following publications:
Warburton, J. and Dykes, A.P. (In preparation) Application of sediment budget methods in the quantification of peat mass movement hazards. Natural Hazards and Earth Systems Science
Warburton, J. and Dykes, A.P. (In preparation) Rapid mass wasting of peat hillslopes under an extreme rainfall event – a sediment budget appraisal.Geomorphology
Dykes, A.P. and Warburton, J. (2008) Failure of peat-covered hillslopes at Dooncarton Mountain, Co. Mayo, Ireland: analysis of topographic and geotechnical factors. Catena 72, 129-145
Dykes, A.P. and Warburton, J. (2007) Significance of geomorphological and subsurface drainage controls on failures of peat-covered hillslopes triggered by extreme rainfall. Earth Surface Processes and Landforms 32, 1841-1862
Dykes, A.P. and Warburton, J. (2007) Mass movements in peat: a formal classification. Geomorphology, 86, 1-2, 73-93