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Lookup NU author(s): Dr Stuart Dunning
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Landslides cause potentially costly damage and disruption to Scottish road infrastructure. Debris-flows, a fast-moving torrent of solids mixed with water, are by far the most frequent cause of such disruption and are capable of causing significant impacts, due to their high mobility. Many debris flow events have been recorded across Scotland, particularly at the highly active A83 Rest and be Thankful (RabT). The influence of gullies in amplifying the hazard from these failures is well understood, and much work has been carried out to characterise the way that rainfall intensity-duration influences slope activity. Until recently, smaller sediment movements and failures have proven difficult to quantify and thus questions have remained over their potential significance. However, small changes on the slope may also have the potential to highlight wider geomorphological controls of activity that are not so well understood. Observations alongside accurate volume estimations may aid the long-term understanding of hazards as well as potentially proving helpful in the consideration and design of management and mitigation measures.Advances in long range Terrestrial Laser Scanning (TLS) have opened up opportunities to record such changes. This report presents the primary findings of a two-year monitoring project at three sites on the Scottish trunk road network. Seasonal variations in vegetation make comparisons of surveys undertaken at different times of the year difficult. This problem arises from the cyclic growth and decline of widespread bracken cover, which introduces false change signals that obscure real geomorphic changes. A tailored monitoring workflow has been developed for sites that undergo such significant seasonal vegetation changes, enabling the detection of slope material losses smaller than 1 m³. Monitoring has been carried out at two to three month intervals at the A83 RabT, and a slightly lower frequency at the A82 Glen Coe and A85 Glen Ogle.In total, six shallow failures have been detected and, where possible, quantified at RabT during the monitoring period. A number of secondary mobilisations and other changes have also been detected; these other changes potentially represent long-term precursors and should be re-analysed in the context of future events. A limited number of changes have been detected at the A82 Glen Coe and A85 Glen Ogle sites, although a large rockfall was detected some distance from the A82 carriageway prompting an inspection by the road authority.A key finding of the monitoring has been the segregation of slope failure processes into either the modification of existing hillslope gully systems (producing net deposition or scour), or the development of new channels (< 0.5m depth) or gullies (≥ 0.5m depth). Both effects are significant, as they may result in the release of otherwise relatively immobile sediment stores. Sub-annual monitoring at RModelling of difficult to observe abT (monthly where weather permits) has further highlighted secondary mobilisations from landslide scars, up to one year after failure. For instance, a 224 m³ secondary mobilisation, days after the initial failure associated with Storm Desmond (labelled SDLS2 – Storm Desmond Landslide 2), may have reduced the retention capacity of a debris flow catch net. The same net was later subject to a flow associated with the Storm Frank (SFLS1 – Storm Frank Landslide 1) flow, from which material reached the A83 carriageway. Quantification of the SDLS2 event, particularly the loss of 85 % of the failed mass to a nearby gully, has also demonstrated the potential for smaller events to recharge gullies with significant quantities of sediment for later entrainment. This process has previously resulted in debris flow hazard amplification; the largest recorded debris flow recorded at RabT (October 2014) almost doubled in magnitude as a result of entrainment, resulting in closure of the A83.To further develop the hazard assessment of the most active of the monitored sites, the A83 RabT, two modelling approaches are presented; these are numerical modelling of debris flow propagation and simulation of likely hydrological flow lines over the slope. Back-analysis and calibration of the former has been found to yield realistic deposition distributions, demonstrating the credibility of the model to simulate events at A83 RabT and thus perform hazard-analysis. The initial experimental hazard modelling results suggest that such an approach could be very powerful, enabling both zones of relatively high mobility and potential gaps in current mitigation efforts to be identified. This approach does however require further development, with particular focus required around whole-slope coverage and increased resolution to account for sources on inter-channel ridges; these may be split and flow into multiple channels and do not give a true indication of mobility potential.Modelling of difficult to observe hydrological pathways shows some promise in demonstrating the potential role of water flow in triggering some shallow failures. Model results appear to corroborate on-slope evidence of small ephemeral drainage channels, which may be of significance to shallow failure initiation and gully development. A larger collection of TLS derived change may be required to fully substantiate this theory. Continued TLS monitoring would be beneficial for this purpose and has already proven beneficial through the identification of at least six additional small failures at A83 RabT following the original two-year monitoring period, the results of which are presented in this report.Multiple surveys around the winter months are advised for RabT, whilst annual change might be of benefit at A82 Glen Coe. Lower frequency surveys, every few years, may be appropriate for the A85 Glen Ogle site in order to assess steady-state sediment dynamics.
Author(s): Sparkes B, Dunning SA, Lim M, Winter MG
Publication type: Report
Publication status: Published
Online publication date: 15/03/2018
Acceptance date: 14/03/2018
Report Number: PPR852
Institution: Transport Research Laboratories
Place Published: Wokingham
Notes: ISSN: 2514-9652