Modeling effects of tidal barrier closure of Venice Lagoon

As a combined result of land subsidence and sea level rise in the vicinity of the City of Venice there has been a net decrease in the difference between the mean elevations of land and sea of about 30 centimetres over the past century. This has resulted in an increased incidence of flooding in Venice with occasional disastrous consequences, as in November 1966 when storm and tide conditions combined to flood the Piazza San Marco to depths exceeding one metre. Among the solutions proposed to solve the flooding problem is a modular tidal barrier that is designed to close the three major entrances to the shallow lagoon in which Venice is situated. Because the lagoon is highly eutrophic and circulation is driven primarily by the tide in the Adriatic Sea, there is concern that operation of the barriers could exacerbate an already serious problem of pollution. The lagoon currently receives the untreated waste water from Venice with an organic loading equivalent to more than 400,000 persons during the tourist season, industrial discharges from the Port of Marghera, and non-point accretions of nutrients from adjacent agricultural areas. Preliminary investigation of the effects of the proposed barrier scheme using mathematical hydrodynamic and water quality models suggests that prolonged isolation of the lagoon from the sea, if this should become necessary for flood protection, may enhance primary production and induce unfavorable water quality conditions. This paper presents results of this investigation and offers suggestions for further refinement and application of the models to assist in the environmental impact assessment which must be performed prior to barrier construction.