Il Fiume Tevere: produzione di sedimenti, trasporto solido, centrali idroelettriche e deficit sedimentario alla foce. Il “Nodo” di Corbara-Alviano

The River Tiber: sediment supply, solid transport, hydroelectric power stations and sedimentary deficit at mouth. The “node” of Corbara-Alviano. – The Corbara-Alviano “node”, on the Tiber River, corresponds to the reach that extends from the upstream of Corbara Lake to downstream of the Alviano hydroelectric power stations. The node is made up of three main components: 1) the lake and the related Corbara hydroelectric power station; 2) the reach of riverbed between the two lakes, where the River Paglia flows into the Tiber River; 3) the lake and the relative hydroelectric plants of Alviano. As for the sections upstream and downstream of the node, the characteristics and processes that affect each of the three components have been defined, following the methods of the morphological-sedimentary approach. The level of definition, considering the available budget, is that of setting and first approximation, but sufficient to formulate the problems, to evaluate their effects and to identify possible solutions through suitable design hypotheses. In particular, the following were defined: 1) the volumes and grain size of sediments and organic matter currently trapped in the two lakes; 2) the nature, quantity, methods and gran size of the solid transport upstream and downstream of the node, before the construction of the power stations, in the current situation and in the presence of the bypass; 3) the contribution of the node to the state of sedimentary deficit that affects the mouth of the Tiber. On the basis of the data acquired, the terms of the problem have been redefined: 1) the spatial limits of the bypass, which can affect only the Corbara lake; 2) it does not seem realistic to recover the sediments currently trapped, due to the environmental problems that would cause in the riverbed downstream. Finally, some indications are provided for a design hypothesis that allows to bypass the sandy component of the current and future solid transport, according to methods very similar to those of the natural phenomenon, without decreasing the production of hydroelectric energy and without disastrous consequences for the riverbed downstream of the node. The note relates to the results of the morphological-sedimentary approach, but the activity was carried out in synergy with the working group that developed the problem, following the hydrological-hydraulic approach and reaching similar and share results.