Comparing threshold definition techniques for rainfall induced landslides: a national assessment using radar rainfall

Translational landslides and debris flows are often initiated during intense or prolonged rainfall. Empirical thresholds aim to classify the rain conditions that are commonly associated to landslide occurrence and therefore improve understating of these hazards and predictive ability. Objective techniques which are used to determine these thresholds are likely to be affected by the length of the rain record used, yet this is not routinely considered. Moreover, remotely sensed spatially continuous rainfall observations are under-exploited. This study compares and evaluates the effect of rain record length on two objective threshold selection techniques in a national assessment of Scotland using weather radar data. Thresholds selected by ‘Threat Score’ are sensitive to rain record length whereas, in a first application to landslides, ‘Optimal Point’ (OP) thresholds prove relatively consistent. OP thresholds increase landslide detection and may therefore be applicable in early warning systems. Thresholds combining 1 and 12-day antecedence variables best distinguish landslide initiation conditions and indicate that Scottish landslides may be initiated by lower rain accumulation and intensities than previously thought.