Peatland subsidence in the Venice watershed

Large agricultural areas reclaimed from 1892 to 1967 south of the Venice Lagoon, Italy, are characterized by soils with high organic content. At present, these areas lie almost entirely below the mean sea level mainly because of land subsidence due to peat oxidation. The farmland is artificially kept drained by a complex network of ditches and pumping stations that discharge the drainage water into the Venice Lagoon or the Adriatic Sea. Flooding from the sea and the lagoon is prevented in normal conditions by levees. The maintenance of a given water table depth, as is dictated by agricultural requirements, has caused the lowering of the inlet shaft of the drainage pumping stations and the simultaneous increase of the pumping head. As a major consequence, the efficiency of the pumps has decreased with corresponding increase of the drainage costs. Moreover, the risk of flooding during severe storms cannot be ruled out as well as the occurrence of adverse events such as saltwater contamination from the nearby rivers, the lagoon, and the sea (Rizzetto et al., 2003). Hence, the development of the area as a cereal farmland is becoming increasingly expensive, and is clearly unsustainable over the long term. In view of the above, the research project VOSS (Venice Organic Soil Subsidence) was undertaken with the aim at characterizing the composition of the histosols, defining the extent of the subsiding area, understanding the basic processes affecting the occurrence, quantifying the past and present land subsidence rate, and finally developing a predictive tool for helping to plan the most appropriate management strategies in relation to the dominant agricultural practices and the maintenance of an efficient drainage network, which would be able to safely protect the farmland from exceptional floods. A field site was instrumented at the end of 2001 to investigate the occurrence of land subsidence in the Zennare Basin (45° 10' E and 12° 9' N), a reclaimed agricultural area in the south catchment of the Venice Lagoon. Drainage of outcropping peat soils has resulted in an overall settlement of 1.5 - 2 m since the 1930's. Continuous measurement of the hydrological regime and peat surface displacements by an ad hoc tool proposed by Deverel and Rojstaczer (1996) has allowed for an accurate estimate of both the peat reversible movement and irreversible subsidence. This chapter presents a review of the VOSS project, that was conducted in close collaboration with the Land Reclamation Authority (Consorzio di Bonifica Adige-Bacchiglione) and the farmland owners, and funded by Co.Ri.La. (Research Program 2001-2003) and Sistema Informativo MAV-CVN. After a short description of the area of interest and the experimental site, the collected data are shown and discussed. The recoverable and unrecoverable components of the measured displacements are evaluated on the basis of a direct analysis of the recorded time series and by use of mathematical models relating peat porosity to moisture content, and peat oxidation to soil temperature and depth to the water table, respectively. Finally, some remarks are provided by way of conclusion.