AN OVERVIEW OF THE SUBSIDENCE OF VENICE. L. Cabognin and P. Gatto Istituto Studio Dinamica Gandi Masse - CNR, Venice, Italy. Abstact A geneal view of the aspects of the Venice subsidence situation up to the pesent time, as studied by the CNR in Venice since 1969, is supplied. A look at the cause and effect elationship is given, along with an illustation of the thee factos making-up the land elevation loss duing this centuy (namely, natual subsidence, man-induced subsidence and eustacy). Finally it is demonstated that the ecent incease in ecuence of flooding is mainly due to the 22 cm of land elevation loss ecoded with e spect to mean sea level. The subsidence of Venice is a poblem which has come to light in the last decades since the city has been eve moe fequently subjected to "acqua al ta" (flooding). Both natual and man-induced esponsabilities wee hypothesized at the beginning of the '60s fo the pocess of land sinking. Howeve, on the basis of woldwide expeiences it appeaed cetain that the intensive goundwate withdawals played a fundamental ole in the phenomenon (Lofgen, 1969; Poland, 1970). Fom 1969 the new "Istituto pe lo Studio della Dinamica délie Gandi Masse" (ISDGM), CNR - Venice, included the analysis of subsidence among its eseach themes, and so a long-tem theoetical and expeimental, oganic an ational eseach began, specifically oiented to the analysis of this poc ess (Fig. 1). Subsidence induced by ash exploitation of goundwate has been studied in depth, the cause and effect elationship has been quantified and a mathematical model was developed to pedict the evolutionay pocess which was late veified by time (see efeences and main bibliogaphy) Only a bief eview of this occuence is given hee. The exploited Venetian aquife system included 6 stata fom gound level to about 350 metes deep (Fig. 2). The exploitation of undegound wate pefomed mainly fo industial use, began aound 1930 with modest quantities and inceased until it became intensive afte the second wold wa, with the industial boom. Consequently the hydaulic heads went down with espect to the oiginal condition at the beginning of the centuy, and the aveage piezometic decline in the industial zone was moe than 20 metes, 80% of which happened in the citical phase between 1950 and 1970 (Fig. 3) Consequently, the induced land loweing was most intense in this peiod. In fact, due almost exclusively to natual causes in the fist decades of the centuy, it eached the highest values between 1952 and 1969. With the 1970 closue of atesian wells and the divesification of wate supply in the Venetian aea, a geneal impovement occued quite quickly. 321
SUBSIDENCE STUDY HISTORICAL DATA COLLECTION RATIGRAPHY HYDROLOGY GÉOTECNICS [» GEOLOGICAL METHODS I HISTORICAL DATA SELECTION AND RELIABILITY ANALYSIS l-h STATISTICAL METHODS Hi HISTORICAL DATA STORAGE HISTORICAL DATA SYNTESIS AND GEOLOGICAL INTERPRETATION FIELD MEASUREMENTS AND DATA CALIBRATION COMPACTION RATE OF THE SOIL LABORATORY ANALYSIS WATER ANALYSIS PIEZOMETRIC MEASUREMENTS MEASUREMENTS GEODETICAL SURVEYS C" MEASUREMENTS D- FIELO DATA STORAGE ELABORATIONS HYDROGEOLOGICAL PATTERNS GEOLOGICAL SUBSIDENCE EVALUATION MATHEMATICAL SIMULATIONS HYDROLOGICAL MODEL SUBSIDENCE MODEL Î PREDICTIVE SIMULATION OF SUBSIDENCE FIG. 1 - Blok diagam of the investigation of subsidence of Venice (afte Cabognin and Gatto, 1976). The hydaulic heads ecoveed and late ose to gound level, e-establishing the condition which existed befoe the intensive goundwate pumpage (Fig. 4). At the same time, the gound, which had been in a quiescent peiod, showed a ebound in 1975, which, in the histoic cente, was about 2 cm (Fig. 5); an insignificant figue, but sufficient to show the cessation of induced subsidence. This positive behaviou of the gound had peviously been foseen, unde the hypothesis of the closue of the atesian wells,by the mathematical simulation of the subsidence of Venice (Fig. 6); futhe it was, and is still today, confimed by the bahaviou of sea level diffeences (Fig. 7). Compaing the tidal ecods in Rovigno and Buccai, sites on the Yugoslavian coast which is taken to be stable, in Tieste and those of Venice, until 1969 - that is when subsidence was occuing - the aveage annual sea level ecoded in Venice was appaently inceasing with espect to that of the othe stations. In ecent yeas this has not occued any moe, and even the ebound appeas. 322
MAFIOHERA l:<5 y..»i :':;-. :,-.;:': MII<I I idiy-iiit t^si feg'..nj l=-l Q_) iquifsi FIG. 2 - Schematic section of the Venetian aquifeaquitad system.the six exploited aquife ae numbeed (afte Cabognin and Gatto, 1976). Land subsidence due to goundwate withdawal is hence no longe a poblem today. As long as intensive exploitation will be avoided, the steady pesent condition will pesist. But a geat concen a'ose afte the sevee 1976 eathquake, when all notheasten Italy was hit. The geodetic suvey caied out immediately afte, showed in the focal aea notable upheavals which attenuate towad South, in the Venetian Plain (Talamo et al., 1978). Venice itself seemed 1970 yeas FIG. 3 Piezometic decline obseved fom 1910 to 1970 all ove the Venetian teitoy (afte Cabognin and Gatto, 1976). 323
-Ah.»H", INDUSTRIAL ZONE*-. VENICE-^ o a. gound suface 5 O oiginal condition stating dawdown maximu m dawdown ecovey 1920 1930 1940 1950 Y E A R S 1960 1970 FIG. 4 - Aveage piezometie level in the industial zone compaed with that of Venice. The A h values efe to the industial zone (afte Gatto and Cabognin, 1981). not be involved in the effect: the above mentioned tidal compaison stesses this statement. Next levelling, which is going to be pefomed in 1984 1985, will futhe detect possible land elevation desplacements in the Venetian mainland. Notwithstanding the eassuing pesent situation and the ebound, the 80% of the occued subsidence is ievesible and its effect still emains. We must keep in mind, howeve, that Venice is built in a lagoon envionment and it is with espect to wate that it must come to tems. Fo this eason we must emembe that natual causes contibute to loweing the height of the city with espect to sea level, even if they ae only slight: natual subsidence and eustacy. Apat fom natual subsidence which has been quantified as 0.4 mm/y fo the pesent centuy, and theefoe has a elatively modest incidence on the phenomenon (3 cm fom 1908 to today), the pocess of positive eustacy must not be neglected. The sea level is ising all ove the wold and also involves the Adiatic and theefoe the Venice Lagoon. This pocess, always assuming a linea tend, has been evaluated as 1.27 mm/y and so it has totalled 9 cm duing this centuy. The combined effect of the thee factos (natual subsidence, man-induced subsidence and eustacy) has been quantified as a land elevation loss of about 22 cm fom the beginning of the centuy (Fig. 8). This loss in gound suface elevation, even if only of vey modest popotions with espect to subsidence measued elsewhee both in 324
MAINLAND INDUSTRIAL ZONE VENEZIA I 2-4 - 6-8 - 10-1952(BASE) 1975 / 1969 FIG. 5 - Aveage piezometio level and land subsidence between 1952 and 1975 along a levelling line fom the mainland (A) to Venice (B). The close coelation between the two phenomena is evident eithe in the citical peiod (1952-1969) o in the positive stage (1969-1975) (afte Cabognin et al., 1976). 12-14 - Km 0 2 6 FIG. 6 - Pedicted subsidence unde the effect of the twe ex teme head decline pattens: a) countinuation of ash exploitation b) closue of atesian wells (afte Gambolati et al., 1974). 90 2000 325
s* / / f- / VENEZIA-ROVIGNO VENEZIA- TRIESTE VENEZ1A- BUCCARI ANNI FIG. 7 - Yealy mean sea levels in Venice and Tieste, Rovigno, Buccai: tend of the diffeences (afte Cabognin et al., 1981). Italy and aboad, has been vital fo the Venice's existence, in as much as Venice lives in the wate. In fact, floodings in Venice have undegone a stiking incease in fequency, and those tide levels which 80 yeas ago would not have flooded the city ae now included in "acque alte" because of the 22 cm land elevation loss (Fig. 9). < 9 - n T ~ o i «DC y STAÏ D NCE (cm) en ~i c/> < O «b 4 3 2. -0 1 2 3 4 5 6 l 8 ; 9 10 " 12 13 14 *15 J j i I IREND VALUE OF NA TURAL COMPONEN1S 11896-196/1 eustiilic ising 1 27 mm/y 04 mm/y befoe 192 SI subsidence! ^ ^ ^ ^ ^ \» * ^ ^- ^"nvi.l. in 1908 1 1 *» - NAT URAL SUBSIDENCE NATURAL TREND ""OUCEO By [ 1 i MAN PERIOD INFLUENCED BY HUMAN ACTIVITY REBOUND i " * ' WS 1 i 2 i & i R i' 2 M,8 I J20 1,27 s «o =; a~ > u. ii 2 s o, Si Z ï î FIG. 8 - The oveall land elevation loss of the Venetian suface level with espect to mean sea level: the contibution of the thee esponsible factos - eustacy, natual subsidence, subsidence due to goundwate withdawal - is indicated fom 1908 to 1980 (afte Gatto and Cabognin, 1981). 326
1910 1920 1930 1940 1950 1960 1970 1980 Y E A R S FIG. 9 - The "aoqua alta" occuences fom 1908 to 1980. The incease with time is clealy dependent on the loss in gound suface elevation (afte Gatto and Cabognin, 1981). Refeences and main bibliogaphy Albeotanza L., Faveo V., Gatto P., Masutti M., Mozzi G., Pianetti F., & Seandei Babeo R., 1972, Catasto pozzi del Comune di Venezia: vols. I,II,III,IV,V, CNR - 1st. Studio Dinamica Gandi Masse, Venice, TR 23; Caputo M., Folloni G., Gubellini A., Piei L., Unghendoli M., 1972, Suvey and geometic analysis of subsidence in the egion of Venice and its hinteland: CNR - 1st. Studio Dinamica Gandi Masse, Venice, TR 9- Cabognin L., Gatto P., 1976, A methodology fo hydogeological data collection in the Venetian plain: Poceeding IBM Semina on Regional Goundwate Hydology and Modelling, Venice, Italy, p. 267-291- Cabognin L., Gatto P., Mozzi G., Gambolati G., Riccei G., 1976, New tend in the subsidence of Venice: Second Intenational Symposium on Land Subsidence, Anaheim, CA, USA, I.A.H.S., no. 121, p. 65-81. Cabognin L., Gatto P., Mozzi G., Gambolati G., Riccei G., 1978, Lo sfuttamento delle isose idiche sotteanee e 1'abbassamento del suolo a Venezia: Atti del Convegno "I poblem! della subsidenza nella politica del teitoio e della difesa del suolo" (9-10 novembe), Pisa. Cabognin L., Gatto P., Mozzi G., 1981, La lduzione altimetica del teitoio veneziano e le sue cause: 1st. Veneto Sc. Lettee ed Ati, Rappoti e Studi, vol. VIII, p. 55-83. C.N.R., 1971, Livellazione geometica di pecisione della città di Venezia - 1970: CNR - 1st. Studio Dinamica Gandi Masse, Venice, TR 11. Gambolati G., Feeze R.A., 1973, Mathematical simulation of the subsidence of Venice. 1: Theoy, Wate Resouces Reseach, vol. 9, no. 3- Gambolati G., Gatto P., Feeze R.A., 1974a, Mathematical simulation of the subsidence of Venice. 2: Results, Wate Resouces Reseach, vol. 10, no. 3, P- 563-577. Gambolati G., Gatto P., Feeze R.A., 1974b, Pedictive simulation of the subsidence of Venice: Science, vol. 183, p. 849-851. Gatto P., Cabognin L., 1981, The Lagoon of Venice - natual envionmental tend and man-induced modification: Hydological Sciences Bulletin, 26, 4, p. 379-391. 327
Lofgen B.E., 1969, Land subsidence due to the application of wate: epinted fom Reviews in Engineeing Geology II. Poland J.F., 1970, Memoandum summaizing esults of discussions at Venice, Italy, with the committee fo wells of CNR Ve test and obsevation wells fo subsidence studies in the Venetian aea: UNESCO Repot. Talamo R., Pampaloni M., Gassi S., 1978, Risultati delle misue di livellazione di alta pecisione eseguite dall'istituto Geogafico Militae nelle zone del Fiuli inteessate dalle ecenti attività sismiche: Boll. di Geodesia e Sc. Aff., anno XXXVII, no. 1, p. 62-75, Fienze. 328