SEDIMENTATION ASSESSMENT OF A SMALL DRINKING WATER RESERVOIR: A COMBINED FIELD MEASUREMENT AND NUMERICAL MODELLING APPROACH

The research activity, here described, refers to a combined approach of numerical modelling of the erosion phenomena and sediment field measurement campaigns. On a small water supply reservoir, in Tuscany (Italy), phenomena involved in soil erosion as well as in sediment transport and deposition, of the upstream drainage river basin, have been analyzed and the water storage capacity reduction has been estimated. The small artificial lake, of about 0.18⋅106 m3 of volume, has allowed the experimentation, on a reduced scale, of different monitoring methodologies related to reservoirs sedimentation. Particularly two bathymetry surveys, at an 18-year interval, have provided the calibration data of the numerical modelling. The modelling has included the implementation of some watershed scale models for an estimation of the mean annual soil erosion rates. A simple conceptual sediment transport model, based on linear reservoir cascades, coupled with sediment deposition and degradation, has also been developed to evaluate the routing of the eroded sediment to the river channels network and to the reservoir. The model has been applied on the catchment, at event scale on a period of nine years, to estimate the amount of sediment transport and the long term reservoir sedimentation. The combined approach of measurements and modelling has given useful indications for reservoir sedimentation monitoring and control, and for defining a policy of reservoir management. Moreover the comparison between different monitoring techniques has provided the definition of optimization criteria of the measurements procedures for reservoir water storage capacity evaluation as well as of the analysis tools for soil erosion and sediment budget estimation.