A process typology of regional floods -: art. no. 1340

We propose a framework for identifying types of causative mechanisms of floods. The types are long-rain floods, short-rain floods, flash floods, rain-on-snow floods, and snowmelt floods. We adopt a catchment perspective, i.e., the focus is on the catchment state and the atmospheric inputs rather than on atmospheric circulation patterns. We use a combination of a number of process indicators, including the timing of the floods, storm duration, rainfall depths, snowmelt, catchment state, runoff response dynamics, and spatial coherence. On the basis of these indicators and diagnostic regional plots we identify the process types of 11,518 maximum annual flood peaks in 490 Austrian catchments. Forty-three percent of the flood peaks are long-rain floods, only 3% are snowmelt floods, and the relative contribution of the types changes with the flood magnitude. There are pronounced spatial patterns in the frequency of flood type occurrence. For example, rain-on-snow floods most commonly occur in northern Austria. Runoff coefficients tend to increase with rainfall depth for long-rain floods but are less dependent of rainfall depth and exhibit much larger scatter for flash floods. All types exhibit seasonal patterns, both in terms of flood magnitudes and catchment altitudes of flood occurrence. The coefficient of variation (CV) of the flood samples stratified by process type decreases with catchment area for most process types with the exception of flash floods for which CV increases with catchment area.