Comprehensive post‐event survey of a flash flood in Western Slovenia: observation strategy and lessons learned

The limited extent of the areas affected by flash floods and strong spatial and temporal gradients of rainfall cause conventional measurement networks of rain and discharges to be inadequate for an effective observation of these events. The documentation of flash floods urges post-event survey strategies encompassing accurate radar rainfall estimation, field observations of the geomorphic processes associated to the flood, indirect reconstruction of peak discharges and interviews of eyewitnesses. This paper describes the methods applied and the results achieved in the survey of a flash flood that occurred on 18th September 2007 in the Selka Sora watershed (Western Slovenia). The documentation of this flash flood reveals high peak flood discharges and unit peak discharges and a complex flood response. Observations on geomorphic activity show widespread erosion and debris flows, although generally involving relatively small debris volumes. Relevant amounts of large wood were mobilised, with a large variability of the intensity of supply processes. The field study of the Selka Sora flash flood outlines the importance of geomorphological surveys as a prerequisite for flood discharge reconstruction in mountainous watersheds with active sediment dynamics, the basic role of the accounts of eyewitnesses of the flood, and the need of qualitycontrolled weather radar, which permit coupling field observations with rainfall-runoff modelling.