Groundwater numerical modeling as a complementary tool for designing hydraulic structures

The city of Parma (Italy) is characterized by a junction of two small rivers: Parma and Baganza. The city, since 2004, is served by a flood control reservoir on the Parma River with the aim at mitigating the flood risk in urban areas. Recently, in order to increase the safety of the city, a new flood control reservoir on Baganza River has been planned. The first carried out study has allowed to define the reservoir location and geometry, the storage area (1200 m x 700 m) and volume, and the maximum acceptable head stage inside the basin. The reservoir consists of a main structure that limits the flow rate downstream, 1700 m of levees and three check dam upstream. These allow to lower the river bed and the storage area with the aim at increasing the storage volume and simultaneously limiting the elevation of levees. Moreover, in order to avoid the piping, grout walls below the main structure and the levees have been planned. During the last year, the aquifer beneath and surrounding the study area has been investigated by means of 18 boreholes, 14 monitoring wells, geoelectrical and geophysical surveys and several pumping tests. The head levels inside the wells have been monitored in order to evaluate the seasonal fluctuations and the influence of the river on groundwater. The local stratigraphy could be simplified in: 0 – 28 m gravel-sand with a succession of thin clay lens, 28 – 35 m clay and 35 – 50 m gravel-sand. The monitoring wells have allowed to identify two different water tables that demonstrate the existence of two aquifers: a phreatic one (0- 28 m) connected to the river stage and a confined one (35-50 m). The phreatic aquifer extends in a wide region that covers not only the reservoir location but also a residential and an agricultural area; for this reason a great attention has been paid on the wells used for human activities and especially for irrigation. A numerical model of the aquifer has been developed by means of MODFLOW 2000. All available data have been analyzed with the goal of representing with high reliability the observed head levels and to forecast the two most soliciting cases: maximum stage reached during a flood event and minimum stage (empty basin) considering two different grout walls depth (15 m, 28 m). The numerical model represents only the phreatic aquifer and consists of 200 x 300 cells (each one of 10x10 m2) and three layers in order to describe also a thin clay deposit that is located below the main structure area at about 15 m. Constant head upstream and downstream, an impervious bottom and the river have been considered as boundary conditions. The model has been set up on observed values collected during spring 2013. The first results have estimated that the grout walls should fully penetrate the phreatic aquifer in order to prevent piping and an excessive variation of the aquifer levels. This condition will preserve the area surrounding the basin to too high head levels during flood events; but on the downside, during empty periods, an excessive withdrawal could cause a dewatering of the aquifer. In fact the river, in the basin area, will no longer drain or recharge the beneath aquifer.