Mass movement hazard & global climate change: Physical-deterministic modelling study of the rest and be thankful pass, Scotland UK

Mass movements frequently affect infrastructure corridors, particularly so in mountainous areas. The A83 at the Rest and be Thankful pass (Scotland, UK) is a prime example of a road experiencing frequent closure due to precipitation-triggered landslides and debris flows. The closures result in long (>90 km) diversions and severe economic losses (>£60 million). To ensure persistence of transport services, future engineered interventions and maintenance schedules must anticipate the potential consequences of climate change that affects the frequency and magnitude threshold precipitation events that are capable of triggering mass movement. In turn this requires a better understanding of the hydrological implications of climate change on precipitation-triggered landslides and debris flows. The area is prone to mass movements due to steep topography, the glacial history, reworked deposits of mud and siltstone, and landslide-triggering precipitation. The objective here is to assess the severity of future events, i.e. the coming decades, was assessed by means of an event-based runout model for selected events that were identified using a present-day precipitation threshold applied to hydro-meteorological forecasts for this part of the UK. A convex hull method was applied to these forecasts to create a comprehensive selection of modelled events and a much reduced computational load. This selection is considered to be representative for the largest variation within the forecasts. Results show that 61 mm of daily precipitation will likely trigger mass movement. This threshold reduces by 0.15 mm for every millimetre accounted for in an antecedent precipitation index calculated based on 8 antecedent days. On the basis of the hydro-meteorological forecasts it can be concluded that the frequency of these trigger conditions gradually declines, as seven out of the eleven RCM (Regional Climate Model) configurations show longer recurrence intervals. This corresponds with the decline in the modelled future precipitation. However, it is observed that, for scenarios with lower average daily precipitation and antecedent precipitation indices, event severity is likely to increase as intense daily precipitation follows drier antecedent conditions. For scenarios with higher average daily precipitation and antecedent precipitation indices, persistent wetting appears to expand source areas and increases material entrainment. The simulations show that the main source areas, situated between 300 and 500 metre elevation, extend upslope when the antecedent precipitation regime has peak precipitation values in the latter stages of the 8 antecedent days.