A model of sand transport in Treporti channel: northern Venice lagoon

Abstract

The origin of the sands in the Venice lagoon has been the subject of an extensive field survey in parallel with numerical modelling. Four transects along Treporti and Burano canals were conducted from which 33 bottom sediment samples were collected. These samples were analysed for grain size and sorting to examine any trends in the granulometry of these sediments that might shed light on transport paths. The modelling study consists of three parts: the sediment transport model SEDTRANS96 was used with a finite-element hydrodynamic model (SHYFEM) and an empirical wave model (US Army Corps of Engineering) to simulate sand transport in the Treporti canal. A type of link box model was created where finite elements of the hydrodynamic model have been combined to macroboxes on which the water and sediment flux over the sections, and a mass balance has been computed. Several grain size classes were simulated; the distributions before and after the simulation were examined. Idealised wind and tidal values were initially used to force 12 h simulations to test the sediment transport sensitivity. Finally, a full-year simulation (1987) has been carried out using measured tidal and wind data. Only a part of Venice lagoon was covered by the simulation: a major channel (Treporti) running from Lido inlet towards the northern lagoon. The total sand transport through all of the sections was computed for 1 year. Sediment mass balance was determined, and the resulting trends of erosion and deposition were computed. There were no trends in the median grain diameter and sorting of bottom samples from the Treporti canal; all sands were fine (120 μm, one outlier of 300 μm was removed). The absence of a trend in grain size suggests that there is no significant import of sand to the lagoon through the Lido inlet. The results from the simulations seem therefore to confirm the hypothesis of reworking of sand within the lagoon. The computed erosion is some centimeters per year diagnostic of channel scouring and enlargement with time. The Treporti canal is subject to strong current velocities of around 1 m/s, which hold fine sand in suspension and thus prevent sedimentation.

Links between hydrology and sedimentology in the Lagoon of Venice, Italy

“…This issue has also confirmed by the results of Umgiesser et al (2006), in which a sediment transport model was applied to the Lido inlet. Comparing results from three idealised simulations (tide only, tide with scirocco, tide with bora winds) with a simulation using real forcings over one year, it was found that the long-term results were closest to the simulation when only tidal forcing was taken into account.…”

Modelling fluxes of water and sediment between Venice Lagoon and the sea

“…As a result of their 1-D sediment transport model they concluded that the yearly loss of sand though the Venice inlets is in the order of 0.1 × 10 6 m 3 yr −1 . As well, modelling by Umgiesser et al (2006) indicates an export of sand from the lagoon through Lido inlet through erosion of the major tidal channels.…”

Tidal inlet seafloor changes induced by recently built hard structures

“…Lido inlet, in particular, was considered depositional. Defendi et al [88] measured the total transport in the Lido inlet based on calibrated ADCP measurements in 2005-2006 and by coupling the transport model SEDTRANS96 ([89], [90]) to the SHYFEM hydrodynamic model ([91]; [92]), showing a sediment loss of 381 ⋅ 10 3 m 3 /year for the lagoon and of 256 ⋅ 10 3 m 3 /year for the Lido inlet, with a bed-load transport of 29 ⋅ 10 3 m 3 /year. These studies referred to the inlet before the most recent modifications.…”