Marchi L., Amponsah W., Borga M., Cavalli M., 2014, Stream power of recent flash floods in Europe_ data collection and analysis, International Geographical Union Regional Conference - Changes Challenges Responsability, Krakow (Poland), 18-22 August 2014,
URL: http://www.cnr.it/prodotto/i/283149

Stream power is a key variable for the evolution of landforms associated to the fluvial systems. Several studies on stream power have related to bankfull or near-bankfull discharge, which is regarded as morphologically significant. The analysis of stream power for bankfull discharges permits homogeneous comparisons of stream power between different parts of a channel network, whereas the stream power assessment for large floods provides important insights on river energy expenditure that caused major, abrupt morphological changes in channels and on floodplains. Because of the strong spatial gradients of rainfall inputs, flash floods often hit different patterns of a river basin with different intensity. Resulting values of stream power show spatial variations that depend on geological controls on channel geometry and sediment characteristics, as well as on variations of flood intensity_ this urges a field approach that takes into account such variations of the factors that influence stream power. Post-flood assessment of peak discharge after major floods makes it possible to analyse stream power in fluvial systems stressed by high-intensity floods. This study analyses the stream power of some extreme (return period > 100 years) flash floods that occurred in mountainous basins of various European regions in the last decade. The spatial variability of stream power is examined taking into account topographic variables, such as upstream contributing area, channel slope and cross-sectional width, and with regard to differences of flood response intensity in various sectors of the river basin. In most of the analysed cross-sections, high values of specific stream power were observed_ this is consistent with the high severity of the studied floods. Field observations have shown that, depending on flood intensity, channel geometry and material, and sediment supply due to landslides and debris flows in the tributaries, the studied flash floods produced geomorphic changes of different intensity in alluvial channels and on floodplains. Not surprisingly, lined channels, usually characterised by narrow width, display higher values of specific stream power than natural cross-sections draining catchments of similar size. Channel erosion in lined channels, as well as in bedrock channels, was usually negligible. Continuing post-flood surveys by means of standardised procedures is of utmost importance for extending and updating quantitative information on flash floods in Europe. In this context, data on stream power are of great importance for linking hydrological and hydraulic characteristics of the floods to their geomorphic consequences.

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