Collecting a multi-disciplinarity field dataset to model the interactions between a flood control reservoir and the underlying porous aquifer

During the last decades, a large number of flood control reservoirs were developed in Northern Italy, in order to mitigate flood risk in urban areas. The city of Parma, located on the large alluvial fan of the Parma River, is served by a flood control reservoir (i.e., dry dam), completed in 2004. The reservoir can store a volume of 12·106 m3 over an area of 1.2 km2 surrounded by about 4 km of artificial levees and closed downstream by a concrete dam 15 m high, equipped with 3 movable floodgates. The structure has the purpose to store the excess water in the case of high return period flood events, releasing it downstream at a controlled rate. A stilling basin is located downstream the dam in order to dissipate the kinetic energy of the discharged flow. The stilling basin is made up of 2 m thick concrete slabs, on which 3 dissipating blocks are located. The deposits below the stilling basin are surrounded by a grout wall (20 m deep) with the aim of realizing a confined "box". Groundwater levels inside the box are controlled by a 110 m long drainage trench located upstream the stilling basin, 3 m below its floor. In the perspective of a long-term management of the reservoir, after the completion of the works, a phase of investigation, control and monitoring of the efficiency of the entire system has been carried out, mainly to highlight the interactions between the reservoir and the underlying aquifer. This task was accomplished filling the reservoir at the maximum retaining level by means of capturing the tails of spring 2008 flood events. The aquifer beneath and surrounding the structure has been investigated by means of several tests, such as: stratigraphical, lithological and hydrogeological characterization, in situ permeability tests and lab analysis. Moreover, a groundwater monitoring system made up by 44 piezometers with dataloggers and real- time data transmission to a dedicated website has been set up. Monitoring data before, during, and after the infilling of the reservoir show that the aquifer below the structure is multilayered, with prevailing silty gravels and relatively thin silty and clayey strata. The aquifer can be simplified in three layers: a phreatic aquifer (from 0 to 20 m depth), a thin clayey layer (20 to 25 m) and a regional semi-confined aquifer (beneath 25 m), whose level tend to respond to the reservoir levels. The multidisciplinary database collected so far is the basis of a numerical model that is going to be developed to understand the interactions between the reservoir and the aquifer, in different scenarios.