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

Spatial and temporal scales of occurrence of flash floods, combined with the space and time scales of conventional measurement networks of rain and discharge, make these events particularly difficult to observe. The effective documentation of flash floods requires post-flood survey strategies encompassing accurate radar rainfall estimation, field observations of the geomorphic processes associated with 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 Selska Sora watershed (Western Slovenia). Hydrometeorological analyses of the storm are based on radar reflectivity observations. The documentation of the flash flood reveals high peak flood discharges and a complex flood response. Peak discharges were estimated at 22 cross sections, with drainage areas ranging from 0·2 to 147 km2. Among the lessons learned from the field study of the Selska Sora flash flood, there are three key conclusions that can inform similar studies. Firstly, geomorphological surveys are an important prerequisite for flood discharge reconstruction in mountainous watersheds affected by debris flow and intense sediment transport. Secondly, the accounts of eyewitnesses of the flood provide a unique contribution to event reconstruction. Finally, it is necessary to have quality controlled weather radar data, which may permit coupling field observations with rainfall-runoff modelling.