Valutazione dell'interrimento di alcuni invasi collinari nel territorio umbro

Thousands of small reservoirs have been built in Italy since the 1960s, which are defined as “hilly dams” because they are generally located at moderate high elervation and created by artificially damming of creeks, to accumulate water upstream, to be used mainly for irrigation purposes and, more generally, as a water reserve. Despite the fact that the realization of new reservoirs has slowed down markedly in recent decades, the number of lakes in the area remains very high. The current situation of these reservoirs is very often of poor conservation; in fact, in their natural evolution, the hill lakes undergo a more or less rapid silting. In fact, the construction of a dam along the creek significantly alters the distribution of sediments along the course, blocking solid transport upstream and creating a sedimentary deficit downstream. The silting phenomenon, which constitutes the very reflection of normal erosion phenomena, has as its main effect a decrease in the useful volume for water storage, as well as negative consequences also on the blocked creek, including an elevation of the riverbed upstream of the reservoir, erosion phenomena downstream of the dam, and a reduction of solids input towards the hydrographic network. The main goal of the note is the estimate of the annual soil loss, using the RUSLE model (Universal Soil Loss Equation), of the hydrographic basins relating to some hilly lakes of the Umbrian territory in the Upper Valley of the River Tiber, in Umbria. This study constitutes the preliminary phase of a larger project, which also includes direct bathymetric measurements of the reservoirs taken into consideration. From the comparison between the data coming from the analysis of the initial projects of the dams and the bathymetric measurements carried out with an echo sounder, it will in fact be possible to evaluate the degree of elevation of the bottom with respect to the original project and therefore the volume and the silting rate can be estimated. A further comparison between the soil loss calculated on the basis of the modeling using RUSLE and the real quantity of sediment present on the bottom will also allow to test the model itself for the purpose of evaluating the soil loss and the actual silting of the reservoirs. By quantifying the silting of the reservoir, the impact of the dam on the solid transport of the riverbed downstream of the reservoir itself can also be estimated, with the consequent verification of the possible triggering of erosion phenomena and deepening of the riverbed.