The Subsidence of Como: Human Impact or Natural Tendency?
by
Valerio Comerci
APAT (Italian Agency for Environment Protection and Technical Services), via Vitaliano Brancati 48, 00144 Roma
Coauthors: L. Bonci, A.M. Michetti, E. Vittori

The town of Como (Lombardia, Northern Italy) is seated at the southern edge of the western branch of Lake Como at the foot of the Alpine Chain. It is an important cultural and industrial settlement since Roman times, because located along a key commercial route, easily connecting Italy with northern Europe across the Alps. Lake Como (or Lario) is the third largest and the deepest lake in Italy, and lies in a 60 km long, downstream-bifurcated river-glacial trough cutting across the Central Alps, intensely modelled by an ice tongue up to ca. 2 km thick during the Last Glacial Maximum.

The urbanized area is affected by subsidence phenomena. In particular, remarkable rates of ground sinking have been recorded immediately after World War II.

To effectively understand the origin of this phenomenon, we have taken into account the role of all the different natural (Climate, Geology, Tectonics) and human (e.g., hydrological works, water withdrawal from wells) components in the geologic and environmental evolution of the Como sedimentary basin, from Late-Pleistocene to present.

Our work is based on A) field survey, B) air photo interpretation, C) literature stratigraphic, archaeological, topographic, and subsurface data compilation, D) drilling of exploratory shallow boreholes, E) radiocarbon dating, F) pollen analysis, G) 3D geological model reconstruction, H) a new precision levelling survey; I) SAR interferometry (PSInSAR).

Literature data (Castelletti and Orombelli, 1986; Bini, 1993) and our new analyses show that during the early late glacial the Como area hosted a proglacial lake at 270 m a.s.l. with a S-ward drainage. With the progressive ice thawing, the lake level in the Como area dropped to an elevation of 205 to 200 m a.s.l., very close to the present-day level (198 m a.s.l.), as proven by the St. Abbondio lacustrine terrace. The formation of this terrace implies a dramatic change in the Lario landscape and the birth of the modern Lake Como. We examined this site in detail by analyzing 17 boreholes drilled for the St. Abbondio church restoration, calibrated through three new shallow boreholes. The stratigraphy of the St. Abbondio recent deposits (as the one under the Como center) shows pro-glacial lake sediments poor in organic matter in the lower part, and lake shore deposits rich of wood horizons in the upper part. A wood sample from this site was first dated at 11730 ± 180 yr 14C B.P.\ (Castelletti & Orombelli, 1986). A wood sample we collected in 2003 from the same deposits has yielded 14C age of 13230 ± 120 yr B.P. (Centre for Isotope Research, University of Gröningen). Therefore, at this time (ca. 13-11 ka B.P.), while along the border of the basin (St. Abbondio site) a lacustrine shore developed, a marsh environment occupied the middle part of the basin, as testified by the tens of meters of loam and sand rich in organic matter found in many of the boreholes analyzed (more than 70).

The maximum depth at which this stratigraphic layer was drilled has been utilized to estimate the rate of natural subsidence from ca. 13, 000 yr B.P. to present. The velocity of subsidence is given by: V=(S-Dz)/T where V is the velocity, S is the sediment thickness, Dz is the sums of depth variations due to level water oscillations, T the sediments age. Supposing Dz = 0, the maximum subsidence rate during the last ca. 13, 000 yr is bracketed between 1 to 4 mm/yr, with the higher value distributed in the lakeshore area. Moreover three archaeological sites of Roman age, now buried under 2.5-3 meters of sediments, have provided the historical long-term average subsidence rate.

In order to assess the ground movements, from 1928 to 1997, several institutions (Italian Military Geographic Institute, Politecnico di Milano, regional and local authorities) carried out precision leveling surveys. Values of ground lowering higher than 20 mm/yr have been measured in the districts close to the lakeshore from 1955 to 1975, presumably induced by the indiscriminate ground water exploitation. With the drastic reduction of water pumping in the Como plain, the subsidence rate slowed down to mean values of 1-2 mm/yr in town, with some benchmarks still or even reversing their trend (data refer to 1981, 1983, 1990, 1997 leveling campaigns). However, along the lakeshore the ground continued to rapidly sink, for example 6 mm/yr in Piazza Cavour.

In May 2004 a new precision leveling survey was carried out by APAT to assess the present-day ground vertical movements. Except for a few benchmarks that since 1997 have continued to lower with a rate higher than 10 mm/yr, all the benchmarks in the Como plain showed rates comparable to that of the natural subsidence, with velocities of 2-3 mm/yr along the lakeshore and even smaller in the inner part (e.g. 1 mm/yr in the duomo area).

Furthermore, TRE (Tele Rilevamento Europa) has measured the ground deformation by means of the Permanent Scatterer (PSInSAR) technique during the years 1992-2003. The velocities obtained in the Como plain by the two different techniques are in very good agreement, proving the reliability of the data. On the reliefs that border the plain the InSar measures show positive movements (with rates higher than 2 mm/yr on Brunate Mount), confirming the uplift of the Como pre-Alpine chain known in literature (e.g. Arca and Beretta, 1985).

Thus, the Como subsidence seems to be a localized phenomenon, affecting only the geomorphologic depression on which seats the town, filled by unconsolidated Late-glacial and Holocene sediments.

The ban of water exploitation in the Como plain (since 1989 the lake feeds the local aqueduct) has slowed down the subsidence but it is not enough: the shoreline area is still sinking and the risk of lake floods progressively increases.

Further investigations and monitoring already scheduled will allow a more precise constraining of the present-day trend of the subsidence in order to predict its near-future evolution and to lay down the basis for a possible prevention and mitigation planning.

Date received: July 18, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-40.