The 29 August 2003 storm on the upper Tagliamento River basin in the eastern Italian Alps is examined as a prototype for organized convective systems that dominate the upper tail of the precipitation frequency distribution and are likely responsible for the majority of flash flood peaks in this area. The availability of high-resolution rainfall estimates from radar observations and rain gauge networks, together with flood response observations derived from stream gauge data and post-event surveys, provides the opportunity to study the hydrometeorological and hydrological mechanisms associated with this extreme storm and the associated flood. The flood occurred at the end of a climatic anomaly of prolonged drought and warm conditions over Europe and the Mediterranean region. A characteristic of the event is its organization in well-defined banded structures, some of which persisted in the same locations for the duration of the event. The steadiness of these rainbands led to highly variable precipitation accumulations and, associated with orographic enhancement, played a central role in the space-time organization of the storm. Two dominant controls on extreme flood response are recognized and analyzed: steadiness of convective bands and dry antecedent soil moisture conditions.

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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 Selška 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 km 2 . Among the lessons learned from the field study of the Selška 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.

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Abstract. High resolution radar rainfall fields and a distributed hydrologic model are used to evaluate the sensitivity of flash flood simulations to spatial aggregation of rainfall at catchment scales ranging from 10.5 km 2 to 623 km 2 . The case study focuses on the extreme flash flood occurred on 29 August 2003 on the eastern Italian Alps. Four rainfall spatial resolutions are considered, with grid size equal to 1-, 4-, 8-and 16-km. The influence of rainfall spatial aggregation is examined by using the flow distance as a spatial coordinate, hence emphasising the role of river network in the averaging of space-time rainfall. Effects of rainfall spatial aggregation are quantified by using a dimensionless parameter, represented by the ratio of rainfall resolution (L r ) to the characteristic basin length (L w ), taken as the square root of the watershed area. Increasing the L r /L w parameter induces large errors on the simulated peak discharge, with values of the peak discharge error up to 0.33 for L r /L w equal to 1.0. An important error source related to spatial rainfall aggregation is the rainfall volume error caused by incorrectly smoothing the rainfall volume either inside or outside of of the watershed. It is found that for L r /L w <1.0, around 50% of the peak discharge error is due to the rainfall volume error. Remaining errors are due to both the distortion of the rainfall spatial distribution, measured with respect to the river network, and to the reduced spatial variability of the rainfield. Further investigations are required to isolate and examine the effect of river network geometry on the averaging of spacetime rainfall at various aggregation lengths and on simulated peak discharges.