The stream power variation in a GIS environment as an index to evaluate the most 'sensitive' points of a river

The note deals with the problem of measurement and variability, along the fluvial reaches of a hydrographic network, of the stream power, understood according to the definition originally provided by Bagnold (1966) “The available power supply, or time rate of energy supply, to unit lenght of a stream is clearly the time rate of liberation in kinetic form of the liquid’s potential energy as it descends the gravity slope S”. In formula: Ω = ρ g Q S where: Ω = stream power per unit of flow length (W/m) ρ g = density x gravity acceleration = specific weight of the fluid (kg/m3) Q = liquid discharge (m3/s) S = slope of the considered reach (m/m). Recent digital elevation models allow the calculation of channel gradient and consequently stream power with a finer spatial resolution, opening promising and novel opportunities to investigate river geomorphological processes and forms. The work carried out consisted in defining and implementing a methodological approach that could be automated within a geographic information system and that meets two requirements: 1) use input data that is easy to find as DEM at a low resolution; 2) estimate, in the best possible way and on the basis of the available data, the stream power and its variability along the considered stream. In particular, the methodological approach has been implemented in the GIS environment (GRASS Gis, Qgis), and it has been applied to a sample basin, highlighting its variability along the streams of a higher order (in practice the main stream and its most important tributaries). The sudden and more substantial variations of stream power were then related to the processes acting in the fluvial system. This approach has made it possible to highlight how the erosion, solid transport and sedimentation phenomena occurring along the fluvial reaches (and the geomorphological and geological-applicative problems that these involve, especially in anthropized areas), are correlated precisely to abrupt variations (increase/decrease) of the ”power” available. Hence the idea that automated and standardized screening of stream power variability along a stream can be used as a preliminary diagnostic element to identify the most ”sensitive” points of the same, on which to concentrate subsequent investigations (field checks, to verify the causes), aimed at mitigating the risk due to the dynamics of the riverbed.