This report considers the recent history of debris flow in Scotland as it relates to the trunk (strategic) road network and the rainfall climate of Scotland. The development of a deterministic rainfall threshold for Scotland is described in the context of a brief review of international rainfall trigger thresholds. This highlights the potential for a probabilistic approach to be taken to the development of such thresholds; and for rainfall event data that does not lead to a landslide, as well as rainfall event data that does lead to a landslide, to be incorporated into the analysis. The probabilistic analyses presented here use Bayes’ theorem to undertake both one-dimensional analyses (for rainfall duration and intensity, and total rainfall) and two-dimensional analyses (for paired values of rainfall duration and intensity) to determine conditional landslide probability. Probabilistic thresholds for the conditional probability of landslide occurrence dependent on rainfall duration and intensity P(D|I) are derived in the range 0.05 (5%) to 0.3 (30%). It is proposed that a formal trial be undertaken of a probabilistic landslide forecast system.

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