Peatland subsidence in the Venice watershed
M. Camporese
Department of Hydraulic, Maritime, Environmental and Geotechnical
Engineering, University of Padova, Padova, Italy
G. Gambolati, M. Putti, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
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.
