Sediment Resuspension in the Lagoon of Venice: Short-term Observations of Natural and Anthropogenic Processes

摘要

To monitor sediment resuspension in the Lagoon of Venice, a small benthic lander was deployed next to the main shipping channel, used by cargo ships and oil tankers sailing to the port of Mar-ghera.The site has asiltysand bed and the deployment lasted for about 16 hours, with water depths ranging between 1.1 to 2.0 m. During the first part of the experiment there were no wind waves but the ship traffic was consistent, generating waves with H,mx of the order of 0.23 m. Due to the presence of ship wakes, resuspension reached peaks of 30 mg 1 but the suspended material settled very rapidly. The vessel traffic decreased during the night of the deployment, but a night breeze occurred, generating small amplitude waves with Hs = 0.08 m. Suspended matter concentration during the flood tide (afternoon/evening of 19 August 1998) averaged at about 30 mg 1 . At high water the suspended sediment concentration dropped to very low values, but as the tide turned, picking up speed, resuspension took place: constant concentrations of about 60 mg 1 were observed for a period of about four hours. During this period short crested wind waves were present, therefore it is believed that the concomitant action of wave resuspension and wind driven currents was preventing sediment settling. Calculation of current only bed shear stresses confirmed that wind-induced currents were able to resuspend sediment of the coarse silt-fine sand fraction.The correlation between wave orbital velocities and resuspension events indicates that vessel generated waves are able to resuspend the sandy fraction of the bed sediment. Wind waves have comparable resuspension capacity, but the sediment fluxes generated under choppy seas are larger than under vessel waves, because of the additional importance of wind-driven currents.