VERBI VEnice Research BIbliography

This paper outlines progress made within the CORILA Research Lines 3.15 and 3.16, regarding the origins and transport pathways of sand within the inlets of Venice lagoon. The approach taken is multi-facetted and involves long-term direct measurements within the inlets, inferences based on morphological changes of the bed, direct measurements of sand transport, and geophysical monitoring of key features. Results to date show a clear transport of sand in the inlets of Venice lagoon throughout the water column. We note that there is a strong import of fresh water, fine-grained sediment and plant matter through Lido entrance about 2 days after a high river discharge event. This (freshwater) plant matter moves close to the bed, as seen at the mouth of the Po river (Tesi et al., in review), and is most abundant during flooding tides whilst the fine sediment moves inwards near the surface. Bathymetric and geophysical surveys of the ebb tidal deltas of Lido and Chioggia have been completed and show clear progradational foresets diagnostic of rapid accumulation (up to 20 cm/annum). The Lido ebb tidal delta has accreted up to 3 x 106 m3 sand/annum.
The sand appears to come in part from by-passing the northern breakwater and in part from Treporti canal (Umgiesser et al., 2006). Chioggia ebb tidal delta also shows accretion of the 50,000 m3 sand/annum. The ebb deltas are a new phenomenon of the shore face (about 70 years old) and are the product of construction of the jetties (in the early C20th). Sand transport in the inlets has been measured using Helley-Smith sand traps (equipped with 63 micron nets) in conjunction with a variety of other techniques. The traps have an efficiency of about 4% only (in agreement with Rey et al., 1987), but were well suited to the conditions in Venice lagoon and provided enough material for detailed analysis.
The results of this work show that Chioggia inlet is dominated by the export of macrophytes and is sediment limited, whereas Lido inlet is more complex showing strong buoyancy- driven stratification and resulting vertical gradients in salinity, temperature, sediment load, and organic content. We found considerable evidence of the transport of very fine sand in Lido inlet. The sand transport is greatest in the bottom 10% of the water column and follows closely the Rouse profile. Sand concentration in suspension is simulated well by the Rouse exponent (* U Ws ß? ) = -0.98 (Lido inlet) and -0.58 (Chioggia inlet) (setting ß=1 and ? = 0.4). Fine sand extends to a height of about 4 m above the bed during peak flows corresponding to the estimated thickness of the boundary layer (Amos et al., 2006) and observed in ADCP profiles. The measured mass transport rate of sand at the bed agreed well with the sand transport predictions of Engelund and Hansen (1967) and from the model simulations of SHYFEM + Sedtrans96. This transport comprised largely inorganic sediment (>95%) whereas the organic content of suspended material varied over a wide range and was greatest (> 90%) near the surface. The survey of the bassi fondi yielded strong evidence for continued bed erosion in the southern lagoon. A comparison of our bathymetry and that of 1970 showed approximately 0.5 m of erosion. By contrast, a paleo-cordone (paleo-barrier) evident in the survey region did not erode and hence is being exhumed by ongoing erosion elsewhere. This sandy feature, therefore, does not appear to be contributing sand to the export of sediment in the southern lagoon. Sidescan sonar backscatter was used to map the distribution of macrophytes, and the position of the paleo-cordone (evident as intermediate backscatter). Finally, the Benthic Roughness Acoustic Device (BRAD) (deployed at 6 sites) was effective in mapping bed roughness beneath a plant canopy, as well as the distribution of macrophytes within the plant canopy, and the presence of epiphytes and other shelly material within the field of view. It appears that epiphytes are strong acoustic scatterers and may well be the reason for poor results obtained with shallow sub-bottom profilers in Venice Lagoon.