Sand transport in Chioggia Inlet, Venice Lagoon and resulting morphodynamic evolution

Abstract

The magnitudes and pathways of sand transport in Chioggia Inlet, southern Venice Lagoon, and the resulting morphological evolution have been investigated. Bathymetric analysis has established that there is a net export of sediment from the southern Lagoon Basin, and that significant morphodynamic changes in the barrier-inlet system have taken place. The total loss of sediment from the lagoon was evaluated as 125,770 m3/a, 10% of which is estimated to be sand. In addition, the presence of an ebb-tidal delta, extending from the mouth of Chioggia Inlet, as well as two subaqueous spits, were identified. The ebb-tidal delta annual growth was estimated as 50,000 m3/a, suggesting the existence of an additional source of sand for the delta, in addition to the Lagoon. The pathways and provenance of sand in transport were determined through seabed sampling and mineralogical analysis. Three predominant sources were identified: sand eroded from the tidal flats in the Southern Basin; a northelrly littoral drift of quartz-rich sand supplied by the Adige and Brenta rivers, to the south of the Chioggia Inlet; and a southerly littoral drift of carbonate-rich renourishment sand from Pellestrina, to the north of the Inlet. The nature and magnitude of transport through the inlet was investigated through direct measurements and modelling. It was found that the export of sand through the inlet is bed grain size dependant, ebb dominant and mostly in the bottom 1 m of the water column. The shape of the profile of concentration is well reproduced by the Rouse parameter, and thresholds for motion are between those derived by Van Rijn (1993) and by Amos et al. (2010b) for the Venetian inlets. Modelling results suggest that present export rates are 40% lower than those evaluated from longterm bathymetry changes. Volumetric and modelling estimates of drift along the shores adjacent to the Inlet indicate that the main contribution to the inlet features is littoral drift, with a longshore transport rate of 118,000 m3/a, from the south, and 91,000 m3/a, from the north. A conceptual sand budget for Chioggia Inlet was proposed. Littoral drift was found to be dominant over transport through the Inlet; thus, it is suggested that although the overall budget of sand in Chioggia Inlet is still negative, a small import of sand is starting to take place. Furthermore, the contribution of Chioggia Inlet to the net export of sand from the lagoon, based on present net budget estimates was calculated as 15%.

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ORCID for Robert J. Nicholls: orcid.org/0000-0002-9715-1109

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