Sediment resuspension during vessel manoeuvres in port areas: evidence from field observations

Purpose: The increase in maritime traffic in the Bay of Koper (Gulf of Trieste, northern Adriatic Sea) has been made possible given the increase in the local port capacity and logistics, which, in turn, means an increase in the number of arrivals of larger vessels (which were the original motive for port expansion); this poses a potential risk for coastal environments due to the impact of the resuspension of bottom sediment which affects the physical and chemical characteristics of the water column. The aim of this work was to assess the magnitude of these perturbative events.Materials and methodsTurbidity (NTU) measurements were made using a CTD multiprobe during a vessel manoeuvre in the port navigational canal and in the entire Bay of Koper. In the highest turbidity zone, samples were collected from the surface water layer (0.5 m), at a depth of 6.0 m and 12.0 m. Total suspended solids (TSS) and suspended organic matter (SOM) were measured gravimetrically. TSS grain-size distribution was determined using a laser granulometer.Results and discussionThe effect of the vessel manoeuvre was evident on turbidity with a maximum value of 137 NTU (TSS = similar to 139 mg l(-1)) and a sampled concentration of TSS of 37 mg l(-1) (bottom layer) in the water column immediately after the ship manoeuvre. Grain-size analysis shows a spectrum of particles with a mode size between 22 and 88 mu m (medium silt and very fine sand, respectively). The estimated resuspension mass of total suspended solids (TSS) was similar to 109 t in the restricted manoeuvre area extending about similar to 736 x 493 m and with an average depth of 15 m.ConclusionsThe results demonstrated the significant impact of vessel manoeuvres on the measured parameters, the impact of which cannot be underestimated in terms of marine environmental protection and maritime traffic safety. Moreover, this impact is expected to increase in the near future.