HR: 17:45h
AN: H34C-08    [Abstracts]
TI: Long term net exchange of sand through Venice inlets
AU: * Tambroni, N
EM: nicotam@diam.unige.it
AF: Universita' di Genova DIAM, Via Montallegro 1, Genova, ITA 16145 Italy
AU: Seminara, G
AF: Universita' di Genova DIAM, Via Montallegro 1, Genova, ITA 16145 Italy
AB: Venice lagoon has undergone through the centuries significant morphological changes due to several natural events as well as antropic actions: the diversions of rivers discharging into the lagoon, the construction of long jetties bounding the inlets, sea level rise and subsidence. As a result, the lagoon has progressively deepened and it is claimed nowadays that a loss of roughly one million cubic meters of sediments is experienced by the lagoon each year! Relatively sophisticated models of the inlet hydrodynamics and morphodynamics may be used in order to estimate the net exchange of sediments between lagoon and sea, however they are still computationally demanding to allow for "long term" predictions. In the present work, we formulate a simple model of the inlet hydrodynamics which allows us to estimate the net exchange of sediments associated with the sequence of tidal events recorded for several years. The exercise proves instructive. Firstly, it turns out that the present configuration of each of the inlets is far from the static-equilibrium limit, defined as the condition such that the maximum speed is equal to the critical speed for sediment motion. Secondly, we obtain an estimate of an upper bound for the loss of sediments experienced by each inlet through the years assuming that the amount of sediments carried by both flood and ebb currents is determined by their transport capacity: results suggest that the yearly loss of sand experienced by Venice Lagoon is an order of magnitude smaller than usually claimed. Thirdly, we formulate a simple framework to account for the effect of the sediment overload induced by resuspension due to breaking waves during storm events in the far field: assuming the flood current to be modelled as plane and irrotational, we are able to determine the flow pattern driven by the tidal oscillations and littoral currents in a neighbourhood of the lagoon inlets. Having determined the flow field, the convection-diffusion equation for sediment concentration has been solved numerically and allows us to explain the tendency to inlet siltation associated with sediment resuspension during storm events. Finally, the implications of the above results for the interpretation of the recent morphodynamic evolution of Venice lagoon are discussed.
DE: 4235 Estuarine processes
DE: 4558 Sediment transport
DE: 4560 Surface waves and tides (1255)
DE: 3020 Littoral processes
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting