Gambolati, Giuseppe

Abstract. The Venice Lagoon is characterized by a fast morphodynamics appreciable not only over the geological scale but also in historical and modern times. The lagoon environment proves very sensitive to even minor modifications of the natural and anthropogenic controlling factors. An important human endeavor accomplished in the past century is the reclamation of the southernmost lagoon area that has been turned into a fertile farmland. The reclaimed soil is reach in organic matter (peat) that may oxidize with release of carbon dioxide to the atmosphere. The continuous loss of carbon is causing a pronounced settlement of the farmland that lies below the present sea/lagoon level. This enhances the flood hazard and impacts noticeably on the maintenance and operational costs of the drainage system. Total peatland subsidence is estimated at 1.5 m over the last 70 years with a current rate of 1.5-2 cm/year. The geochemical reaction is primarily controlled by soil water content and temperature, and is much influenced by agricultural practices, crop rotation, and depth to the water table. A small (24 km2) controlled catchment located in the area has been instrumented for accurately monitoring the basic parameters and recording the ground motion. The in situ measurements have been integrated with the combined use of remote sensing data to help cast light on the process and identify the mitigation strategies.

ABSTRACT - This paper describes the studies performed and the results obtained under the framework of the Project "Sistema Lagunare Veneziano", Research Line 2. 7. A short review of the land subsidence evolution is first presented, including the more recent period (1950 - 1970) of human induced land sinking. The altimetrica! situation of the entire venetian territory is then analysed in detail for the fast twenty years through the comparison of two precise regional levelling carried out in 1973 and 1993. This twenty-year period is quite significant to evaluate the present trend of land subsidence after the cessation of human-induced process measured in 1973 and completely verified in 1975. Different behaviours in altimetrica! evolution have clearly come out. A ground stability belongs to the zones in mainland, Venice and its surroundings, while a certain subsidence stili occurs at the northern and southern extremities of the lagoon's edge and along the littoral where the natural compaction of recent deposi ts is more active than elsewhere and the rate of natural subsidence is greater. The degenerative state of the venetian littoral strips has worsen in the recent past by land sinking. In the fast twenty years the relatively slight subsidence has helped in permanently establishing the de-stabilisation of the submerged littoral zone that is evidenced by the deepening of the near shore bottom slope. In order to analyse in-depth the subsidence process, more theoretical studi es have been performed.New appropriate and complex statistica/ methodologies use.fùl in formulating a judgement on the linearity of the dependence between groundwater withdrawal and land subsidence were elaborated. A new 3-D non linear .finite element model was developed for the simulation and prediction of land subsidence due to intensive groundwater exploitation. The model is based on a quasi 3-D approach for flow and makes use of a 1-D vertical compaction model. The overall modelling approach, solved with an origina! block SOR iterative procedure, is applied to the regional aquifer-aquitard system underlying the venetian lagoon. The flow . and consolidation models, suitably calibrated, have allowed to well reproduce thephenomenon. SOMMARIO Questo lavoro presenta la sintesi degli studi effettuati in tema di subsidenza nell'ambito della linea di ricerca 2. 7 del Progetto Sistema Lagunare Veneziano, prima fase. I risultati qui descritti si basano sostanzialmente sull'analisi comparata delle altimetrie 1973 e 1993 risultanti dalle livellazioni di alta precisione (effettuate con gli stessi criteri dal CNR, ISDGM Venezia) che partendo dai capisaldi base di Rua di Feletto e Treviso hanno quotato 208 capisaldi stabiliti lungo le direttrici Conegliano-Treviso-Venezia e circumlagunare. L'analisi relativa all'arco di tempo ventennale ha pennesso di confennare la stabilità delle zone di terraferma, nonché del centro storico di Venezia. Le area più subsidenti sono risultate essere quelle alle estremità lagunari nord e sud e lungo i litorali. L'esame di alcuni profili batimetrici ha evidenziato che in corrispondenza delle zone di maggior perdita altimetrica del litorale emerso si è verificata w1a certa destabilizzazione del litorale sommerso con un incremento della pendenza media. Ai rilievi di campagna sono stati affiancati studi più teorici. L'interdipendenza esistente tra depressurizzazione del sistema acquifero e subsidenza è stato analizzato con i più recenti criteri statistici evidenziando una relazione lineare tra la causa e l'effetto. Il fenomeno è stato simulato con un nuovo modello matematico tridimensionale non lineare agli elementi finiti fonnato da un modulo idrologico "quasi 3-D" ed un modulo di consolidazione monodimensionale verticale in cui le caratteristiche idrogeologiche degli aquitard dipendono dalla tensione intergranulare efficace. La non-linearità è stata risolta con uno schema iterativo che, per la particolare stmttura del modello, è equivalente ad uno schema di sovrarilassamento a blocchi. La calibrazione del modello ha consentito di riprodurre il fenomeno in modo soddisfacente dal 1930 al 1973, durante il periodo più critico per il sistema lagunare