The use of a combined monitoring system for following a turbid plume generated by dredging activities in a port

Abstract

The potential negative effects of dredging on a sensitive marine environment (e.g. phanerogamic meadows, beaches, benthic populations) can be justification for restrictions on the dredging project or the creation of a dredging monitoring plan. The dredging of the Port of Genoa (Italy) provided the opportunity to study the concentration of total suspended solids, the physical characteristics of the water column, and the winds and currents determining the hydrodynamic characteristics of the port, and to test a double monitoring system for turbidity control. In the dredging operation of the Port of Genoa, we positioned a couple of fixed monitoring systems operating 24/7, consisting each of a conductivity–temperature–depth probe and two acoustic Doppler current profilers (ADCP), near the two port entrances to monitor turbidity and currents. To make the monitoring strategy more efficient, to periodically control the data transmitted by the fixed stations and to ensure coverage of those areas not covered by the fixed monitoring system, a vessel equipped with a vertical ADCP and a conductivity–temperature–depth probe with a turbidimeter periodically followed the dredger during its daily operations. Using the data acquired during the pre-dredging and dredging phases, we considered turbidity and suspended solids variations caused by the dredging operation to study the evolution of the plume. The trailing suction hopper dredge (TSHD) plume extended from the surface throughout the entire water column at a distance of 50 m, with higher turbidity close to the bottom. At a distance of 200 m, the plume was much reduced. Instead, at a distance of 50 m, the turbidity produced by the backhoe was lower than that measured around the TSHD, while at a distance of 100 m the plume was reduced with only noticeable values near the bottom. Finally, we compared the turbidity data of the dredging with the background conditions near the Posidonia oceanica meadow present near the port. The data presented in this paper indicate that the choice of a combined monitoring system can be a good practical solution for reaching two different objectives: (a) to follow the evolution and movement of the turbid plume, and ensure that it does not flow out of the port, contaminating the surrounding area or damaging nearby coastal habitats or the Posidonia oceanica meadows; and (b) to study the differences between the turbid plumes created by two dredging tools (backhoe and TSHD) under different wind–wave conditions.