Numerical Analysis of Land Subsidence due to Natural Compaction of the Upper Adriatic Sea Basin

Abstract

We have simulated the evolution of the accreting Quaternary column at three strategic locations (Venice, Po river delta, Ravenna) along the Upper Adriatic coastline. The analysis is performed by a I-D nonlin-ear finite element model of soil compaction driven by groundwater flow in an isothermal sedimentary basin subject to a continuous vertical sedimentation process. The constitutive relationship of the most important parameter controlling the event, i. e. the soil compressibility vs the effective intergranular stress, has been derived from a number of oedometer tests carried out on samples taken from shallow and deep exploratory boreholes scattered through the study area. Dating of the sediments has allowed for the estimate of an average depositional rate at selected time intervals spanning the Middle-Upper Pleis-tocene, the Holocene and the historical times. The model has been run so as to reproduce the present day sediment thickness with a small excess hydrostatic pressure since the Adriatic Sea basin in known to be normally pressured. It is assumed that sedimentation in the last 0.5 My was substantially in equilibrium with subsidence so that the deposition surface remained at a constant elevation. The results of our analysis indicate a natural land settlement in the next century between 0.5 mmjy at Venice and 4+5 mmjy in the Po delta, with an intermediate value of 2+2.5 mmjy at Ravenna.