A New 3-D Non-linear Model of the Subsidence of Venice

A new quasi 3-D nonlinear finite element model is developed for the simulation and prediction of land subsidence due to groundwater withdrawal in the Venice Lagoon. The nonlinearity is accounted for by the use of aquitard hydraulic conductivity and specific storage related to the effective intergranular stress and solved by a block successive overrelaxation (SOR) procedure with the optimal block SOR factor. The hydrologie response is then fed into a nonlinear 1-D vertical compaction model. The overall modelling approach is applied to the regional multiaquifer system underlying the Venetian Lagoon where extensive groundwater pumping from the upper aquifers has caused an alarming land subsidence at Venice over the period 1950-1970 and a more modest settlement in the southern and northern lagoon areas thereafter. The 3-D nonlinear flow and 1-D compaction models have been calibrated against the past available piezometric and settlement records and reproduce very satisfactorily the event as was observed from the inception of pumping until the early seventy when the cone of depression achieved the largest extent. The simulations for later years up to the present time are currently under way.