Dispersion Plumes in Open Ocean Disposal Sites of Dredged Sediment

Management of estuarine systems under anthropogenic pressures related to port settlement and development requires thorough understanding about the long-term sediment dynamics in the area. In an era of growing shipping traffic and of ever larger ships; millions of tons of bottom sediments are dredged annually all over the world and the major question concerning dredging operations is not whether they should be done, because it is obvious that they are extremely important and necessary, but where the dredged sediments can be disposed of with the least possible ecological impact. The present study involves the evaluation of transport trends of dredged material from a turbid estuary disposed of in four different open ocean disposal sites using numerical model techniques, aiming to contribute to minimizing potential environmental impacts and maximizing efficiency of the dredging operation. The study is carried out in southern Brazil, investigating the fate of dredged material from the Port of Rio Grande, located inside the Patos Lagoon estuary. Simulations were carried with the TELEMAC-3D model coupled with the suspended sediment (SEDI-3D) module and incorporating results from the wave module (TOMAWAC) to evaluate the dispersion of the suspended sediment plume and its interaction with coastal currents. This modeling structure proved to be a valuable tool to study the hydrodynamics and sediment transport pathways in estuarine and coastal areas. Results indicate that the natural Patos Lagoon coastal plume was observed under the predominant ebb flows and NE winds, promoting fine sediment entrapment south of the mouth of the lagoon (in front of Cassino Beach). The dispersion plumes in the disposal sites responded to the wind intensity and direction and did not present any transport tendency towards Cassino Beach. Part of the dredged sediment disposed of in the proposed alternative sites located in deeper areas (Sites B and C) left the site and was transported parallel to the coast (SW–NE direction) according to the wind direction (NE–SW). The area where the disposal sites were located took around 4 days to recover from the dredging operation and reach the usual suspended sediment concentrations and the actual Port of Rio Grande Licensed Site for dredged material proved to be the best alternative among the investigated options

Determining the maximum sediment dispersion in an ocean disposal site for dredged material in Buenaventura, Colombia

This paper presents an analysis of the affected area at an ocean disposal site, in terms of sediment dispersion, as a consequence of the dis-posal of dredged material from the approach channel for Buenaventura´s Port. The analysis was carried out at three points within Buenaven-tura´s Bay, one located at the authorized disposal site, a second point at a proposed future disposal site and a third point at the Bocana Sector, where the highest tidal velocities are identified. The PTM (Particle Tracking Model) mathematical model was used for this investigation, showing that due to the hydrodynamic conditions, a sediment particle can reach a longer dispersion distance from its original disposal point. Finally, sediment dispersion at the three analysis points were compared in terms of current velocity and maximum displacement.