Abstract
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.
Highlights
Triggered events, with above and below average precipitation during El Niño Southern Oscillation (ENSO) warm (El Niño) and cold (La Niña) events [50,51,52,53]. The dynamics of this system are driven by the river discharge (Q) when freshwater discharge Q > 2000 m3 /s [54], while the remote and local wind effects that act on the northeastern (NE)–southwestern (SW) directions become more important during lower river flows (Q < 2000 m3 /s)
In order to answer the question of whether the discharged dredged sediment remains in the oceanic disposal site or is transported towards Cassino Beach, IBAMA demanded an investigation on alternative disposal sites for the Port of Rio Grande, which originated this study
During the entire simulated period, Patos Lagoon exported an average of 895.4 t/h, totaling 642,000 t of material in suspension exported to the coastal zone during the simulation period
Summary
In [43], the authors applied the IH-Dredge model to simulate the seabed evolution and the transport pathways of suspended sediment and toxic substances during dredging operations at the. [45] studied the seasonal variability of the depth of the pycnocline during the plume dispersion generated by the bottom mining operation on the South Korean coast using the ROMS model and remote sensing, and [46] used an integrated hydrodynamic model with a suspended sediment transport module (Delft3D). The fate of the dredged sediment once it is disposed of raises an important question as it involves several environmental and economic aspects for the region. Port of Rio Grande, located inside the Patos Lagoon estuary (Figure 1), disposed of in four different open ocean disposal sites using numerical modeling techniques. Current bathymetric data were provided by the Brazilian Navy and Rio Grande Port Authority and interpolated for the domain
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