Print Email Facebook Twitter Fine Sediment Infill Reduction in Offshore Dredged Approach Channel Title Fine Sediment Infill Reduction in Offshore Dredged Approach Channel Author Van Velzen, B.J.M. Contributor Winterwerp, J.C. (mentor) Luijendijk, A.P. (mentor) Van Overeem, J. (mentor) Jacobs, W. (mentor) Rijks, D.C. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Date 2014-07-04 Abstract An offshore terminal is proposed to be constructed adjacent to the entrance of an Estuary in the Niger Delta. A dredged approach channel of around 10 km long, 250 m wide and to a depth of -14 m LAT is required to allow ships to reach the terminal. From earlier research it is found that high transport rates of fine sediment, in the order of 10 million m3/year, are expected to cause significant sedimentation in the proposed channel. This research aims to describe the complex processes associated with the expected cohesive sediment infill, and find mitigation measures to reduce the scale of required maintenance dredging effort for this project specifically and for similar projects in general. The local environment is subject to strong seasonal variations in wave heights, wind speeds, residual current velocities as well as varying discharges of water and sediment by the Estuary. Significant wave heights during the wet season are typically 1.5 m with a peak period of 13 s. The capacity of the waves to erode sediment in the coastal zone is larger than the capacity of the flow to keep the sediment suspended. This causes sediment induced buoyancy effects to take place. Two main processes were identified that determine the infill of the proposed channel. Episodic discharge of sediment from the Estuary and interception of sediment transported by a residual current in the coastal zone. Fluid mud events were observed in the field, which could result in significant infill rates if the mud layer is able to reach the proposed channel. This process is highly uncertain and requires further research in order to estimate the relative effects. By the use of a Delft3D sediment transport model, sediment transports in the project area were simulated. It was concluded that the expected sediment infill is predominantly caused by the residual transport in the coastal zone and to a lesser extent from sediment originated from the Estuary. A list of potential mitigation measures to reduce sediment infill for similar projects as this specific case is proposed. From an analysis of these measures it is concluded that no single solution is able to reduce sediment transport towards a dredged channel on a long term. The complexity of such muddy deltaic systems make it difficult to change the sediment transport processes significantly, therefore the focus should be on the channel configuration to minimize trapping of sediment and to optimize maintenance dredging efficiency. A downward sloping channel bed was proposed to achieve a self-cleansing channel by transporting the mud infill in offshore direction under the influence of gravity. Due to the gentle surrounding bed slope and resulting maximum slope of the channel bed (1:2000), the effectiveness of this solution is expected to be low. A reduction of the stratification induced residual current was proposed to be achieved by redirecting the fresh water discharge from the Estuary away from the project area. By the use of the Delft3D model it is found that this measure is not effective for the considered dimensions of the channel. The effect of a sediment trap was analyzed using a 2-DV trapping model, set up with the Delft3D software package. The reduction in sediment transport towards the proposed channel by a sediment trap of the considered dimensions is estimated at 20 - 55%. During down–time for maintenance dredging due to shipping traffic in the approach channel, the sediment trap can be maintained. This results in more time available to perform maintenance dredging activity. A limited amount of data was available for this study, therefore additional data is required to calibrate the Delft3D model and obtain more accurate results. Further research on the optimum use of a nautical depth is recommended, as a low expected consolidation rate of the mud infill might reduce the amount of required maintenance dredging. Subject Delft3Dcohesive sedimentNiger deltatrapping efficiencysediment infill To reference this document use: http://resolver.tudelft.nl/uuid:d0485d1f-4778-4fd9-bda5-03b50825ebe7 Embargo date 2016-07-04 Part of collection Student theses Document type master thesis Rights (c) 2014 Van Velzen, B.J.M. Files PDF BJM_van_Velzen_2014_Thesis.pdf 25.78 MB Close viewer /islandora/object/uuid:d0485d1f-4778-4fd9-bda5-03b50825ebe7/datastream/OBJ/view