Use of optical and acoustic instruments to study the turbid plumes generated by three different types of dredges during dredging activities inside and outside of a port

The turbid plumes generated by different types of dredges have specific behaviours and diffusion patterns. The dredging of the Oil Port of Genoa Multedo (Italy) with three different dredges (grab, backhoe and trailing suction hopper) enabled us to study: the physical and hydrodynamic characteristics of the water column of the port and the area just outside it; the characteristics of the turbid plumes; and the different responses of our optical and acoustic instruments. In the dredging operations, a vessel equipped with a vertical acoustic Doppler current profiler and a conductivity-temperature-depth probe with a turbidimeter followed the different dredges during their daily operations. Using the acoustic backscatter and turbidity data acquired during the dredging phase, we investigated the turbidity variations caused by the dredging operations to study the evolution of the plume generated by the three different dredges. The grab plume was present only on the bottom up to a distance of 50 m from the grab, while at greater distances the turbidity fell to background values. The backhoe plume was present close to the bottom at a distance of 50 m from the dredging and, in the entire water column, at distances of 50-400 and 100 m inside and outside of the port, respectively. The trailing suction hopper dredge plume extended from the surface through the entire water column at distances from the dredging of 400 and 100 m inside and outside of the port, respectively. The highest values were found close to the bottom at distances from the dredging at 50 and 30 m inside and outside of the port, respectively. The choice of a combined monitoring system is a good practical solution for studying the differences between the turbid plumes created by the three dredging tools (grab, backhoe and trailing suction hopper dredge) under different hydrodynamic conditions. Furthermore, two different types of instrument should give complementary information on different aspects of sediment resuspension during a dredging operation.