USING RAINFALL AND RUNOFF PEAKS OVER THRESHOLD IN THE ANALYSIS OF FLOOD GENERATION MECHANISMS

We investigate some properties of the rainfall and runoff processes at the daily scale looking at their combination related to the flood generation mechanisms and we show how these relationships can improve the prediction of flood risk. In alpine regions, the effects on floods of snow accumulation and melting can be recognised by comparison of rainfall and runoff peaks at a seasonal basis. Similarly, evaluation of the timing and magnitude of rainfall and runoff events can provide information about the homogeneity of flood regimes both at a site and within morphologically homogeneous regions. In this paper we evaluate the within-year distribution of daily precipitation and runoff considered as peaks over threshold (POT), which provides meaningful additional information to traditional annual maximum series finalised to flood frequency analysis (FFA). Application to some 30 time series in Piemonte (Italy) shows the characters of different regimes of flood generation (snow-ice dominated and/or rainfall dominated), where joint action of both the climatic dominant factors produce non-homogeneous flood processes. Climatic implications in the selection of homogenous regions in FFA are discussed.