Pietro Teatini - Articles in Int. Conf. Proceedings

Contribute of remote sensing, ground radiometry and in situ investigations to recognize outcropping peat soil

V. Nicoletti, S. Silvestri
Consorzio Venezia Nuova . Servizio Informativo, Venezia, Italy

F. Rizzetto, L. Tosi
Istituto per lo Studio della Dinamica delle Grandi Masse, CNR, Venezia, Italy

M. Putti, P. Teatini
Dept. Mathematical Methods and Models for Scientific Applications, University of Padova, Padova, Italy

EXTENDED ABSTRACT
The present study is part of a more comprehensive research project called VOSS (Venice Organic Soil Subsidence), which addresses the problem of the peat subsidence in the Zennare Basin, one of the most critical catchment located just south of the Venice Lagoon between the Adige and Bacchiglione Rivers. Because of the low ground elevation of the basin, future climatic changes may have a negative impact on its sustainable development. The various lithologies and geological features are related to the different paleo-depositional environments. Sandy and silty soils characterize the remnants of ancient fluvial and beach ridges, whereas clayey silts, often rich in organic matter, fill the inter-distributary lowlands; bogs with peat layers occur in the reclaimed marshy areas. With the purpose of creating new agricultural areas, reclamation works started in the second half of the XIX century and were completed before the Second World War. Soil drainage and ploughing caused the oxidation of the peat soil with consequent formation and emission of carbon dioxide, mass loss which lead the subsidence process. Rate of 2-3 cm/year have been estimated by the in situ evidences and through the analysis of historical data where the sinking due to the natural sediment consolidation seams to be very small (2-3 mm/year). The comparison between levelling survey performed by the local authority in 1965 and the Regional Topographic Map of the 1983 shown that the overall land sinking totalled up to 2 m, leading surface elevations between 3 and 4 m below the mean sea level. The peat soil placed in the lowest south-western area are characterized by the highest subsidence rate, whereas sandy-silty deposits, located in the north-eastern partion of the basin, representative of ancient fluvial ridges, are most stable.
The main objective of the VOSS project is the implementation of a mathematical model able to describe and predict the subsidence processes: the large amount of data collected in-situ (hydrogeological parameters, CO2 fluxes into the atmosphere, etc.) are of key importance to calibrate the model and to validate its results. Moreover several field campaigns and the accurate analyses of aerial photographs allowed to produce a new detailed geomorphological map of the Zennare Basin. Several traces of paleo-river beds with their main flow direction towards the southern Venice Lagoon margin were found. Two of them, probably related to an ancient water course of the Adige River, cross the Zennare Basin with a SWNE direction, whereas the ancient southernmost course of the Brenta and Bacchiglione River system can be observed in the north-eastern sector. The old course of the Canale dei Cuori is recognizable by the traces of its meanders, intersecting the new canal (Canale Nuovo dei Cuori) built at the end of the 19th century. Finally, the meanders located close to the north-eastern boundary represent the northernmost ancient branch of the Po River.
The field activities already in place gave the opportunity to collect a large dataset, which has been used for ground control point identification to validate and calibrate the satellite data analyses. Moreover during the field campaigns spectral signatures of peat, sand and silt soils have been collected using a GER 1500 portable radiometer. The spectra have been compared to satellite data collected over the Zennare Basin in different seasons. In particular, one Landsat 5 TM scenes has been acquired in Dicember 2001, two Landsat 7 ETM+ in April 2001 and May 2002; one IKONOS scene in November 2001, and, finally, two ASTER scene in September and December 2001. Spectra comparisons and the application of different classification algorithms allowed the production of accurate soil maps.
An example of classification results on IKONOS data is presented, which is in good agreement with the geomorphological map: peat areas are mainly distributed in the south of the basin, while a mixture of sandy and silt soils occupy the northern part of the basin. In dependence on the type of data, the most feasible classifications have been obtained applying two methods:

1. the Principal Components Analysis;
2. the Soil Brightness Index calculated applying a Tasseled Cup rotation.
The major difficulties encountered during the analyses were related to the presence of vegetation that cover the soil surface. Besides, the fields are often covered by the cut vegetation that rests on the ground after threshing from October to febrary; the IKONOS scene presents many fields in this condition. At the end all data have been organized in a Geographic Information System (GIS) which is a fundamental instrument not just for data retrieval during the project activities, but also for the future possible development of the research.