A monitoring and modelling led investigation into the debris flow geohazard in the west of Scotland

Sparkes, Bradley Scott (2019) A monitoring and modelling led investigation into the debris flow geohazard in the west of Scotland. Doctoral thesis, Northumbria University.

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Abstract

Debris flows are a particularly disruptive form of mass movement due to their ability to propagate at high velocities over large distances, to increase in magnitude through erosion and entrainment and to inflict large impact pressures. A number of recent events in Scotland have resulted in widespread disruption and damage, including road closures that require expensive repairs and mitigation installations. Climatic forecasts suggest that such events could become more commonplace.
Scotland’s hillslopes afford a rare opportunity to observe interacting processes from source to sink, over a relatively short elevation range. This study uses a combination of monitoring, modelling and mapping to investigate the factors driving debris flows, to characterise the geohazard and to gain insight into debris flows within a wider geomorphological context
Monitoring data encompasses multiple changes over three study sites, particularly a period of intense storminess (winter 2015), during which three major slope failures each in excess of 300 m³ occurred at the primary study site the Rest and be Thankful. A lack of high frequency and low-magnitude changes at all sites suggests that precursory changes, such as progressive gully loading, may be less significant than first considered. Instead, larger magnitude, low frequency slope failures appear critical for loading channels for future entrainment. Using a combination of primary and secondary data, this study also demonstrates the ability for debris flows to directly scour and develop new gully systems and increase in magnitude via this mechanism.
The runout model RAMMS-DF was found to effectively model the effect of convergent topography on debris flow runout magnitude and direction, proving it to be a useful tool for the appraisal of mitigation efforts, such as catch net distributions. The continuum model however struggled to model channelization, not accounting for rheological changes occurring when flows enter areas containing hydrological flow. Nonetheless, a combination of monitoring and modelling demonstrates that source location relative to topographic confinement, channelisation and entrainment form major components of the debris flow geohazards.
Modelling of observed ephemeral drainage has highlighted the significant interconnectivity of shallow failures with slope drainage. It is hypothesised that such interconnectivity may be significant in triggering slope failures, as well as the subsequent incision of gullies.
Furthermore, periodic drainage switching may explain an observed propensity for spatial clustering of slope failures.
Synthesis of these findings highlight the role of debris flows within a geomorphic continuum and presents a conceptual model which hypothesises that successive scouring debris flows connect to rapidly form long gully systems, a mechanism of paraglacial sediment evacuation. At the Rest and be Thankful, recent debris flow activity towards the centre of the slope, and resultant gully development, may be acting to increase the gully density and match that of other slope areas where the gullies are already well developed. Low gully maturity may therefore be indicative of a future hazard. These findings call for hazard management and mitigation approaches to better account for drainage pathways and sediment cascade mass balances.

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