Regional frequency analysis of extreme precipitation in the eastern Italian Alps and the August 29, 2003 flash flood

The main objective of the study is to characterise the severity of a flash flood-generating storm occurred on 29 August 2003 on the upper Tagliamento river basin, in the eastern Italian Alps. This storm was characterised by extraordinary rainfall amounts and large spatial variability. Regional frequency analysis based on the index variable method and L-moments is utilized to analyse short duration annual maximum precipitation for the Friuli-Venezia Giulia region, in north-eastern Italy, which includes the storm location. It is shown that the regional growth curves based on the Kappa distribution may be useful for the subregions specified. This analysis provides a framework to investigate the frequency characteristics of the 29 August 2003 flash-flood-generating storm for various rainfall durations. Radar rainfall estimates, adjusted by using a physically-based methodology and data from a raingauge network, are used to characterise the return period of the storm rainfall amounts, highlighting the importance of considering its spatial dimension. Severity graphs are developed to visualise the return periods and their variability for different rainfall durations within the storm. It is shown that adjusted radar rainfall estimates may suffer for considerable uncertainty and that the uncertainty magnifies in the evaluation of the relevant return periods. The analysis shows also that i) attributing a single return period to a storm event is not realistic, and ii) the severity of flash flood generating storms is poorly captured by using conventional raingauge networks. The reported results show that estimates obtained by using careful adjustment of radar observations may be useful to evaluate the severity of the storm for ungauged basins and to evaluate the spatial dimension of the frequency characterisation.