Effects of Vessel Traffic on Erosion and Sedimentation in Ports: A New Tool Supporting Port Maintenance Plans and Practices
Type:
Presented during:
WODCON XXIII - Dredging is changing - The Practice. The Science. The Business.
Authors:
A. Pedroncini, A. Guarnieri, S. Saremi, S. Torretta, J.H. Jensen, M. Vaccari, C. Vincenzi
Abstract
"The action of ship propellers in ports is often the main process that influences the sediment erosion and deposition patterns inside the basin. This can cause both a reduction of vessels clearance - in case of bed accretion - and it can become a threat for the stability of the berthing structures - in case of bed erosion, with the consequent rise of issues related to operational management of the port areas. Propellers induced jets generate shear stresses at the sea bottom, that are often responsible for the resuspension of sediment. The resuspended sediment is then transported within the basin by the combined effect of the propellers induced jet and the tidal, wind and wave generated currents. When the sedimentation dynamics are particularly intense and fast (such as bottom accretion of the order of tens of centimeters/year, or even higher), Port Authorities are often forced to mitigate the issues with maintenance dredging of the seabed, to fully recover the required conditions for safe maneuvering, evolution and berthing of the vessels. To date, Port Authorities in Italy still do not implement any tool to provide a robust and scientifically based forecast of sedimentation and deposition that might support the maintenance plan of the basins; the limited knowledge of the vessels induced erosional and depositional processes prevent the Authorities to achieve an accurate quantification of the phenomenon, with limited capacity of planning maintenance operations. The integrated numerical modelling of hydrodynamics and sediment transport may represent an important aid to Port Authorities and more broadly to port managers and operators, through a proper simulation of the sediment dynamics induced by ship propellers on the medium and long-time scales. This could potentially become an important instrument for planning dredging activities in the perspective of an operational maintenance of the bathymetry. The present work, commissioned to DHI by Ports of Genoa, shows a pilot study of sediment transport induced by ship propellers in the passenger port of Genoa, where the naval traffic involves mainly ferries and cruises (generally self-propelled) and where the sediment dynamics previously introduced are particularly relevant. Methodological approach The study was based on the most advanced modelling tools developed by DHI, combined with several pre and post processing tools specifically developed. Two modules of the flexible mesh version of the code MIKE 3 were used: HD (Hydrodynamic module) was used in the non-hydrostatic version for a correct and realistic representation of the fluxes induced by the ship propellers; MT (Mud Transport module) was used for the simulation of the erosion, resuspension, transport, and consequent deposition of fine sediment. A specific analysis of the natural currents in the basin demonstrated that tidal, wind and wave generated currents can be assumed as negligible in this very sheltered area of the port, if compared to the strong fluxes induced by the direct action of the propellers. Therefore, natural currents have been excluded from the analysis. This assumption can be adopted in harbours (or basins) where the traffic is primarily associated to self-propelled vessels (mainly cruise and ro-ro) and the typical range of natural currents is in the order of cm/s. A thorough investigation of the historic vessel traffic data provided by Stazioni Marittime SpA allowed to identify the typology of the most representative ships for all the berths of the passenger port in terms of average length, beam, draft, and propellers dimensions. Then, a three-dimensional hydrodynamic, non-hydrostatic model has been implemented at ..."
Keywords: Port maintenance, Sediment transport in ports, Decision Support Systems