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Abstract: This model is based on the concept of transport concentration, defined as the time-averaged concentration in a given location of a lagoon, which determines the long-term net transport of sediments as the sum of a dispersive and an advective flux. Dispersive net flux of sediments is due to the alternate components of the tidal flow, while the advective net flux of sediments is due to the residual (Eulerian) component of the tidal, fluvial and littoral flow and possibly to the asymmetry between flow and ebb tide. The long-term deposition (or erosion) rate in a given location is expressed as proportional to the difference between the local transport concentration and the local equilibrium concentration, depending in turn on the local stirring action by waves and currents. The two-dimensional model is able to reproduce the ontogeny of a tidal lagoon as a consequence of a breaching of the littoral dune line. During the ontogenetic phase, the system tends rapidly toward a quasi-equilibrium condition apparently characterized by a stable planimetric configuration of the channel network. The structure of the channels and their watersheds is qualitatively similar to the Venice Lagoon''s ones. After the relatively fast ontogenetic phase, the subsequent morphological evolution of the bottom continues at a slower pace until a stable bathymetric configuration of the lagoon is reached. The model is able to reproduce the morphological effects of subsequent perturbations of natural or anthropogenic origin. [Copyright &y& Elsevier]

Abstract: A comparison of 1927, 1970 and 2002 bathymetric surveys in the Lagoon of Venice was used to reconstruct historical changes in sedimentation. A detailed GIS-based analysis of the charts revealed the timing and pattern of geomorphic changes and allowed calculation of sediment deposition and erosion for the entire lagoon and each of its four sub-basins: Treporti, Lido, Malamocco and Chioggia. Two main developments are discernible from comparative observation of the areal distribution of the main elevation ranges: the diminution in area of the saltmarshes, which decreased by more than 50%, from 68km2 in 1927 to 32km2 in 2002, and the progressive deepening of the lagoon, with a huge increase in the area of subtidal flats (between −0.75 and −2.00m depth), from 88 to 206km2 during the same period. Generally, the lagoon showed a clear-cut change in the most frequent depths (modal depth) from a value of −0.62m in 1927 to −0.88m in 2002. The deepening of the lagoon affected mostly the lagoonal sub-basins south of the town of Venice, where modal depth increased from −0.65 to −1.12m in Lido, from −0.64 to −1.75m in Malamocco and from −0.39 to −0.88m in Chioggia. Large changes in lagoonal morphology were caused by human-induced subsidence, the dredging of navigation channels between 1927 and 1970, and intense natural erosion enhanced by sediment re-suspension due to Manila clam fishing between 1970 and 2002. There was a net loss of about 110Mm3 of sediment from the lagoon, most of which (73Mm3, ca.70%) was in the earlier period. A significant amount was lost by dredging and direct disposal outside the system, either on land or at sea, and there was a net loss of 39Mm3 from the lagoon to the sea through the inlets, at an annual rate of 0.5Mm3. Comparison of erosion rates in the two periods revealed an alarming acceleration, from a net sediment loss of 0.3Mm3 yr−1 in the period 1927–1970 to 0.8Mm3 yr−1 in 1970–2002. Deterioration caused a shift from a highly differentiated lagoon morphology in the 1930s to a sediment-starved and subsidence-dominated structure in the 1970s, and from there to the high-energy and more open (bay-like) lagoon of today. The results demonstrate the potential application of GIS to reconstruct the recent chronology of sediment distribution and to improve the understanding of the geomorphic processes shaping the seafloor, whilst providing an insight into the possible impacts of environmental changes induced by natural and anthropogenic forcing. [Copyright &y& Elsevier]

Abstract: This paper presents the results of a 3-year-long (November 2004–November 2007) study based on the use of acoustic Doppler current profilers (ADCPs) to estimate the solid transport through the three inlets of Venice lagoon. In each of the three inlets instruments were mounted both on survey boats and deployed on the channel bed. The three bottom-mounted ADCPs were positioned in the central part of the inlets, continuously monitoring vertical profiles in the water column. Periodic transects along the investigated sections were collected by the boat-mounted ADCP. Both installations measured current speed and acoustic backscatter intensity. The latter expresses the attenuation of acoustic energy due to material in the water column. The conversion of acoustic backscatter into suspended solids concentration (SSC) was carried out by means of direct measurements of concentration; also an indirect method was used. Boat-mounted ADCP acquisitions were used to calibrate and to validate the bottom-mounted ADCP data. Hourly time series of water discharge and SSC were obtained by calculation from the current speed and acoustic backscatter data recorded by the fixed ADCPs. Hourly solid flux time series were computed. The solid flux and SSC time series at the three inlets were analyzed in relation to the hydrodynamic and atmospheric conditions, highlighting the impact of intense meteorological events on the resuspension process. The lagoon sediment budget is estimated to be about 0.5×106 t/yr and shows a tendency for sediment loss. [Copyright &y& Elsevier]