HINDCAST OF A STORM-SURGE INDUCED BY LOCAL REAL WIND FIELDS IN THE VENICE LAGOON

This paper investigates the effects of the wind blowing over the Venic e Lagoon on both water level and circulation patterns. This has been c arried out using a finite element hydrodynamic model coupled, for the first time, with experimental wind stress fields obtained from measure ment of the horizontal and vertical wind components. A spatially varia ble wind field is used to compute the wind stress directly from the ve locity fluctuations. The hindcast concerns a case of strong northeaste rly wind (Bora) which occurred from the 22nd to the 24th December 1994 . Results are discussed both in terms of the spatial pattern of the st orm surge levels and of the water exchanges with the open sea through the three lagoon inlets. There is an area of set-down in the north of the lagoon and of one of set-up in the south. At the centre of the lag oon, the effects of the storm surge are small. Results show that the s et-down and set-up peaks occur in opposition of phase with the tide. I n the south, this is related to the bottom friction term which enhance s or reduces the magnitude of the storm surge elevation, whereas in th e north the mechanisms of nonlinear interaction between tide and storm surge are less clear. Also, a positive surge difference is found betw een the western and the eastern lagoon, and between the water levels c lose to the inlets and the open sea, the latter triggering the sea-lag oon water exchanges. Analysis of the discharges through inlets and ide al transects (which divide the lagoon into three parts) shows that the spatial and temporal behaviors of the storm surge in the south and in the north of the lagoon are almost independent. During the entire per iod under study, the sea water inflow through the northern inlet was c ontinuous and considerable compared with the outflow through the centr al and southern inlets. Interesting circulation patterns are found in the northern lagoon, whilst water dynamics in the southern lagoon are essentially driven by the outflow at the meridional inlet. Comparison of these results with the hindcast obtained using spatially constant w ind stress fields shows the importance of the wind spatial gradients i n determining the storm surge level patterns. (C) 1997 Elsevier Scienc e Ltd.