Situazione altimetrica attuale del comprensorio veneziano e i suoi riflessi sull'ambiente lagunare

Abstract

Dopo aver inquadrato il processo della subsidenza che dagli inizi degli anni cinquanta ha interessato in modo particolare il territorio e la città di Venezia per le intensive estrazioni di acque sotterranee, viene effettuata un'analisi di dettaglio dell'attuale situazione altimetrica del comprensorio lagunare. Lo studio si basa sostanzialmente sulla interpretazione delle altimetrie 1993 confrontate a quelle del 1973 risultanti dalle due rispettive livellazioni di alta-altissima precisione condotte dal C. N. R., I.S.D.G.M. Questi rilievi, partendo da capisaldi stabili di terraferma (nei pressi della pedemontana) arrivano a Mestre, quotano la superficie della città storica con particolare dettaglio, i capisaldi lungo le linee circumlagunari e l'intero cordone litoraneo, chiudendo l'anello attorno alla laguna. L'analisi relativa all'arco di tempo ventennale ha permesso di confermare la stabilità del territorio accertata definitivamente nel 1975 nella zona industriale e nel centro storico dopo la diversificazione degli approvvigionamenti idrici, di definire con precisione l'andamento della subsidenza naturale, ove l'incidenza antropica è assente, e le aree più fragili del territorio quali quelle litoranee.

An overview of the land subsidence occurred in the city of Venice and its hinterland in the past decades is first presented. In particular the responsibility of groundwater withdrawals in the sinking process is briefly de-scribed. Man-induced subsidence averaged about 14 cm in the industrial area of Porto Marghera, the center of groundwater pumping, and 10 cm in the city of Venice, from 1952 to 1969. Starting in 1970 a general piezometric improvement began, because of the regulation and diversification of the water supply in the Venetian area and the shutdown 01 many artesian wells. In Marghera in 1975 only 170 l/sec were withdrawn against the 500 l/sec in 1969. Concurrently with the recovery 01 aquifers, the subsidence slowed down until it stopped and in 1975 a land rebound 01 about 2 cm was recorded in the city 01 Venice. In spite 01 this, one has to keep in mind that the subsidence is mostly irrecoverable and that its effects still remain. Again, since Venice lives on the water, its land elevation must be referred to the mean sea level. The overall loss in elevation with respect to water level can be synthesized as the result 01 three factors: subsidence due to groundwater exploitation, natural subsidence and eustatic rising 01 sea level. Their joint contribution in the «lowering» 01 the surface level 01 Venice with respect to the water level was a total of 22 cm from 1908 to 1975, a determining figure in the recurring phenomenon 01 «acqua alta» (flooding) . After reviewing the past altimetrical situation, an evaluation of the land elevation 01 the territory is then presented lor the last twenty years; a precise regional leveling was infact carried out in 1993 with the same path 01 the 1973 one. These surveys include the lines from Treviso (farther inland, the stable origin 01 the survey) to Mestre (Venice-mainland), from Mestre to the extreme part 01 the city 01 Venice and those all around the lagoon edges including the littoral strips. This 20-year period is quite significant to evaluate the present trend 01 land subsidence alter the cessation 01 the man-induced phenomenon measured first in 1973 and completely verified in 1975. In general the 1993 leveling, compared with that 01 1'973, has pointed out two different behaviors between the zones in the mainland and surroundings 01 Venice and in the city, which are decidedly stable, and the areas along the extreme edges 01 the lagoon and littoral where land sinking is still going on even il the rate 01 subsidence is much lesser than that measured during the previous critical 20- year period. In order to a have view 01 the whole area under examination, the map 01 the 1973-93 contour lines01 equal subsidence has been drawn, using the Kriging technique and comparing the height 01 the 208 bench marks distributed along the leveling lines. Reading these curves il appears even more evident the ground stability 01 the zones in the mainland, Venice and its surroundings, and the negative trend in soil elevation at the northern and southern extremities of the lagoon's edge and along the littoral. It is noted that the variations in elevation along the lines are progressively decreasing either southward or northward going towards the littorals. Concerning the littoral strips, one has to bear in mind that these areas are those 01 more recent deformation with respect to the others and that they are close to the rivers' outflows, so the natural compaction 01 recent deposits is more active here than elsewhere and the rate 01 natural subsidence is greater. An ideal line 01 demarcation between the two above mentioned stable and subsiding lagoon sectors can be outlined. Particular attention has been devoted to the altimetrical situation 01 Venice because 01 its importance in the lagoon environment. The city's area (about 8 km2) is covered by a network 01 123 benchmarks, 65 01 which constitute the base for comparison since 1961. A reliable map 01 subsidence contour lines was drawn by using again the Kriging technique. The pattern 01 these lines, that are positive almost everywhere, stresses the stability 01 the historical city's ground elevation. Some 01 the curves 01 equal subsidence are closed outlining small negative «bowls». Examination 01 data indicates that very localized sinkings occur in the more recently lilied areas (Marittima-Tronchetto and S. Elena-Giardini) where one can suppose that the process 01 consolidation is still going on (even though at a very small rate), and in certain areas bordering the principal canals; the latter could be partly linked to an erosion action induced by the speed 01 water currents along the canals. By analyzing the temporal piezometric surface variations 01 the once exploited aquifers, one can confirms that presently the contribution 01 groundwater withdrawals to land subsidence doesn't exist as a general cause. Only very localized influences can be found.