The evaluation of rating curve in bed rivers with sediment transport

The measure of the water discharge in rivers is important within the cognitive actions addressed to identify a management policy concerning the water resources. In particular, it is essential for a planning of the interventions in a middle and long term in relation to the control activity and environmental protection respectly to the forecasting and prevention of natural hazard. The measure methods in natural channels are generally based on the use of hydrometers combined with rating curves in steady flows. The relation between water level and discharge is determined by measures of velocities in conditions of low and mean flows, and more rarely during high flows. In the latter case, practical difficulties in measurement lead to extrapolate discharge values on statistical basis which could involve significant errors in rating curves at high levels. Furthermore the steady rating curve cannot to estimate in rigorous way the discharges in presence of sediment transport, which often happens with the high flows modifying the geometrical and morphological river characteristics. In this context, the present work describes a numerical-experimental approach to estimate the water discharges on the basis of numerical models at fixed and mobile bed, calibrated by simultaneous measure series of water level and velocities in gauged stations. In the first step, the application of fixed bed model allowed a more accurate extrapolation of rating curves for elevated discharges through the roughness coefficient correction. Successively the use of the mobile bed model, considering simplified hypothesis, allowed to analyze the sediment transport effects on rating curve shape. The reliability of such approach has been evaluated on a gauged section located upstream a single thread alluvial reach in the middle valley of the Noce River in Basilicata region (Italy). This reach is characterized by a narrow and confined bed, which was interested by a progressive morpho-hydrodynamic change caused by a landslide in the right side slope of the basin in July 2007.