Soil survey and monitoring of a reclaimed basin
south of the Venice Lagoon (Italy)
M. Bonardi, F. Rizzetto, L. Tosi, F. Furlanetto
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 Zennare Basin (45° 10' E and 12° 9' N) is an area of about 23 km2 located
south of the Venice Lagoon, between the Brenta and Adige rivers. Since the XIX century,
land reclamation activities progressively improved the territory, and marshes and swamps
have been completely replaced by agricultural land. Nowadays the ground surface of the
Zennare Basin lies down to about 4 m below mean sea level. Flooding is prevented by
water-pumping stations and the critical ground elevation appears to be related to land
subsidence. The average subsiding rate is between 2 and 3 cm/year, primarily caused by
organic soil oxidation whereas natural compaction gives only a secondary contribute
(2-3 mm/year). The oxidation of the outcropping peat soil causes the CO2
emission in the atmosphere with consequent loss of organic matter and ground surface
lowering; this process is induced by the intensive agricultural activity and by the
water pumping needed to maintain the water table at 50-100 cm below the ground surface.
An overall settlement of about 1.5 m during the last fifty years matches pretty well
with the protrusion of old hydraulic structures located within the basin and funded on
the mineral soils underlying the outcropping peat layer.
In the framework of the VOSS Project (Venice Organic Soil Subsidence), aimed at studying
land subsidence processes by means of field experiments and mathematical models,
the present work is concerned with the determination of the morphological and
sedimentological characteristics of the Zennare Basin. Landforms and features of
the superficial deposits are reported in a geomorphological map, which constitutes the
geological input for the mathematical model simulating the land subsidence process.
The inner coastline position reached during the Flandrian transgression,
5-6,000 years B.P., was identified by Favero and Serandrei Barbero, just in the
north-western part of the Zennare Basin. Even if the transgression crossed the basin,
no traces of marine-lagoon environments were found in the outcropping sediments,
whereas they clearly appear from cores at a depth of 6-7 m below mean sea level.
These deposits were buried by intensive fluvial sediment supplies, which caused the
rapid eastward progradation of the coastline. Aerial photographs and field surveys
evidence the presence of beach ridges at approximately 1-2 km east of the Zennare Basin.
The coastline reached this area about 4,500 years B.P. and stationed here longer
than in the inner position.
Subsequent fluvial depositional events filled up the back barrier lagoon and the
surrounding swamps and caused a new eastward migration of the coastline.
The analysis of several historical land use maps has indicated the environmental
transformations of this territory during the last two centuries. An 1833 map shows the
predominance of swamps, marshlands, and only close to the fluvial ridges located in
the north-eastern sector small drained sites were used for agricultural activities
or urban settlements. The first water pumping station and a few canals are
indicated in an 1862 map, whereas the first well developed network of drainage
canals appears in an 1896 map.
Presently, fluvial and deltaic sedimentation constitutes the outcropping deposits:
in particular sandy and silty soils characterize remnants of ancient fluvial ridges,
whereas clayey silts, often rich in organic matter, fill the interdistributary lowland;
bogs with peat layers, up to 2 m thick, occur in the reclaimed marsh areas.
Two paleo-river systems, probably related with the ancient Adige River, cross the
central part of the Zennare Basin with SW-NE direction. While the northern trace
disappears next to Sista Alta, the southern one follows the direction going from
Motta Molara to Ca Bianca, along the Cordenazzo, Sista Bassa and Ca Zennare alignment.
In the north-eastern part of the Zennare Basin, the evidence of the Brentone Vecchio,
i.e. the ancient southernmost path of the Brenta and Bacchiglione river systems,
is also recognizable.
The old course of the Canale del Cuori is evidenced by the traces of its meanders,
which intersect the new canal (Canale Nuovo dei Cuori) built at the end of the XIX century.
The northernmost ancient branch of the Po River, which dates back to the Bronze Age,
passed through Agna, Cona and Conca d'Albero leading towards Chioggia, as evidenced
by the meanders located WNW of Ca Bianca. Next to Conca d'Albero a paleo-course
of the Adige River flowed into this branch of the Po River. This information
indicates that complex relationships among the ancient flow directions of the Po,
Adige, Brenta and Bacchiglione rivers existed in this coastal plain sector.
Further analyses, such as mineralogical and radiocarbon dating, are needed for a
complete reconstruction of their paleo-courses.
Finally, detailed investigations on several narrow channels which were frequently
branched, indicated that most of them are related to the earliest reclamation works.
