Sand transport in northern Venice Lagoon through the tidal inlet of Lido

Abstract

The provenance and transport of sand has been investigated around Lido Inlet, the largest of
three tidal inlets in Venice Lagoon, Italy. Morphological analysis has established the presence
of an ebb-tidal delta extending from the mouth of Lido Inlet as well as other features typical
of tidal inlets. The stability of the inlet, as well as the canals of Treporti and Burano,
was determined through the application of the tidal prism/cross-sectional areas relationship
theorized by O’Brien (1969). Whilst Lido Inlet and Treporti Canal have both remained
in equilibrium in terms of this relationship, Treporti Canal has suffered erosion due to a
fluctuating tidal prism. Lido Inlet is slightly flood dominant although grain trend analysis
of bottom sediment reveals net export of sand. Treporti Canal is ebb dominant and is the
source of this sand, but it is becoming increasingly flood dominant as average current speeds
have reduced and ebb currents are weakening at a faster pace than flood currents. This is
proposed as a reason to why the northern lagoon is accreting (0.44 cm yr?1), contrary to
trends in the southern (-0.37 cm yr ?1) and central lagoon (-0.23 cm yr?1). A sediment
budget formulated for the whole lagoon has revealed that whilst the overall rate of erosion
is reducing, the area subject to erosion is increasing and the rate of accretion is decreasing,
resulting in no net change in the net sediment loss rate between 1930-1970 to 1970-2000 (-0.05
cm yr?1). Mineralogical analysis on bottom samples, beach and riverine samples confirmed
that longshore transport is from north to south along the northern lagoon; carbonate grains
are dominant in the north with proportions gradually replaced by quartz to the south. Lido
Inlet proved to have similar mineralogy to the River Piave with a higher calcite/dolomite ratio
than inner lagoon samples suggesting a less mature sediment and thus import of sediment.
Conversion of ADCP backscatter into bedload transport rate, suspended sand, and suspended
fines concentrations has shown that no sand is transported at velocities below ?0.4 m s?1.
55% of sediment transported during an ebb flow was sand in suspension (peak: 527,000
kg hr?1), 37% was fines in suspension (peak: 283,000 kg hr?1), and the remaining 8% of
sediment was transported as bedload (peak 68,700 kg hr?1).

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