ABSTRACT

Dredging operations are an important source of sediment resuspension at sea. The finest fraction of the resuspended sediment is often transported by the local regime of currents, and the extension of the resulting plumes may affect large portions of sensitive marine environment and possibly induce its alteration.

In this context, a specific modeling study has been conducted with a particular focus on the environmental impacts due to sediment dispersion during post-trenching operations for a submerged pipeline (sealine) offshore Sicily (Italy). The sealine has a length of about 70 km, and it crosses a very steep bathymetry, varying between 20m and 650m.

The study is based on 3D modelling with an unstructured mesh approach. This allows coarse spatial resolution far from the dredging area and very high spatial resolution for the detailed representation of the sediment release, both in terms of magnitude and position, which can vary in time according to the planned schedule of dredging operations. The hydrodynamic component, which is the main forcing for the transport of fine material, has been modelled using realistic marine conditions as a combination of a local high resolution model and a general circulation model at basin scale, during several time windows possibly planned for the operations.

As a result, the project showed how relevant suspended sediment concentrations (SSC) are confined to an area few hundred meters wide along the sealine, only involving the deepest layers of the water column. The deposition at seabed is negligible, in the order of millimeters, progressively decreasing with the distance from the sealine.

The present study provides an example of realistic and physically based approach for the assessment of the effects of post-trenching operations at sea, thus providing quantitative answers to the requirements of the Environmental Authorities.

INTRODUCTION

Dredging operations are an important source of sediment in marine water. This happens, for example, during excavation works related to submerged pipeline trenching: the sediment released along the water column during the operations is advected and dispersed by the local regime of currents, and the resulting plume may affect large portions of sensitive marine environment and possibly induce its alteration.

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