Analysis of Flash-Flood Runoff Response, with Examples from Major European Events

Flash floods are typically associated with short, high-intensity rainstorms. As such, they are characterized by short response time and have the potential to severely impact and damage communities in different climatic settings all over the world. Despite their scientific and social importance, the fundamental processes triggering a flash-flood response are poorly understood. This contribution aims to provide a review of the hydrological mechanisms driving hillslope runoff response to intense rainfall and to characterize runoff response from selected extreme flash floods in Europe. The first part of the chapter provides general concepts regarding the hydrological mechanisms controlling catchment and hillslope runoff response to intense precipitation. Furthermore, we present an overview of scientific investigations carried out in different hydro-climatic settings to characterize the runoff-generation processes occurring under intense rain rates. In the second part of the chapter, we provide an analysis of the runoff properties of a number of extreme flash floods that occurred in Europe since 1994. More specifically, we examine the climatic settings of the flash floods considered, analyzing the distribution of event runoff coefficients and assessing the role of antecedent saturation conditions in controlling the magnitude of extreme flash floods.