Analysis of the rainfall pattern triggering the Lemešná debris flow, Javorníky Range, the Czech Republic

Two significant rainfall episodes affected the eastern part of the Czech Republic in May 2010 causing dozens of landslides, including a potentially damaging debris flow on Lemesna Mt. in the Javorniky Range on the 2 June 2010. The rainfall data from the rainfall gauges managed by the Czech Hydrometeorological Institute situated 7, 12 and 20 km from the debris flow were analysed and a new rainfall gauge was installed in the immediate vicinity of the debris flow. The following rainfall parameters were calculated as moving values for each day within the period from 1983 to 2018: cumulative rainfall of 2, 3, 5, 10, 20, 30, 60 days and an antecedent precipitation index of 5, 10, 20, 30, 60 days. The rainfall totals, which exceeded the debris flow triggering precipitation by many times, but no slope deformation was recorded during them, were also analysed. The debris flow triggering rainfall values were assessed and they showed a single concordance of all of the tested rainfall parameters on the day of the debris flow. We found that the combination of cumulative rainfall for 30 days together with 1-day and 3-day amounts, overall rainfall pattern and the development of the rainfall situation were more important for triggering the Lemesna debris flow than the individual rainfall extremes. This provides a new perspective to the rainfall thresholds issue. The importance of choosing the calculating method between the cumulative rainfalls and the antecedent precipitation index is illustrated by the significant differences between the values. The significance of the rainfall gauge selection is also emphasised, since the orographic position together with the distance between gauges can significantly influence the differences between on-site and measured rainfall amounts.