Three-Dimensional Numerical Modeling of Sediment Transport For Coastal Engineering Projects in British Columbia, Canada

Quantitative understandings of sediment transport for coastal engineering projects, such as removing and installing underwater cables, installing and operating underwater turbines and disposal of dredged marine sediment (or terrestrial overburden) are one of the key requirements in planning these projects, assessing potential environmental impact and obtaining regulatory approvals from government agencies. In support of the environmental assessment and approval, the highly-integrated, three-dimensional finite difference COastal CIRculation Model COCIRM-SED was recently adapted and optimized to predict the sediment transport processes associated with a number of coastal engineering projects in Roberts Bank, Canoe Pass and Brown Passage, British Columbia, Canada. In these applications, the circulation module was validated using historical ocean current data located in the study areas. For the Roberts Bank application, the model was used to predict the sediment plumes and deposition resulting from the removal and installation activities of existing and replacement underwater transmission cables across the Strait of Georgia. In the model, a total of six sediment categories from fine silt to medium sand were classified and simulated together in terms of sampled sediment characteristics. The model results were obtained with a trenching rate of 300 m/h and two trench sizes, a wide trench of 1.0 m wide x 1.0 m deep and a narrow trench of 0.2 m wide x 1.0 m deep. The amount of sediment that is suspended above the trench was taken to be 30% of the total volume for the wide trench and 25% for the narrow trench. In the application of Canoe Pass, the model was adapted to predict the sediment transport resulting from installing and operating the underwater turbines in Canoe Pass, where the causeway dam, being in place since the 1940's, is planned to be completely removed and replaced by two underwater turbine systems for electricity generation in this area of large tidal currents. In the model, a total of 10 sediment categories from medium silt to coarse gravel were classified and simulated together in terms of sampled sediment characteristics. For a worst case scenario regarding sediment transport, the numerical modeling was conducted with the two turbine systems removed, leaving only the duct openings in the barrage. In this case the installation flow resistance is minimal resulting in the highest level of flows between the two basins on either side of the barrage, as in the scenario of having both systems removed for service. The detailed model results of sediment transport were used to examine such regimes and potential issues associated with the underwater turbines as (1) effects on the HMCS Columbia dive site due to silt transport and altered current flows; (2) effects on the Yellow Island Aquaculture Facility due to silt transport and altered current flows and current jets; (3) sediment transport characteristics on either side of Canoe Pass in both Seymour Narrows and the bay to the east of Canoe Pass including the dive site and the Yellow Island Aquaculture Facility. In the application of Brown Passage, the model was adapted to predict the bottom accumulation and TSS plume resulting from marine dredgate and terrestrial overburden disposal from the Prince Rupert Harbor area, via a barge at a designated disposal site with a water depth of about 200 m in Brown Passage. The distribution of the disposal sediment was initially simulated with the short-term fate model of sediment disposal STFATE, which ran over the initial 45 minutes of sediment disposal under average flood and ebb currents. The STFATE model results of the suspended sediment concentration and initial accumulation on the seabed were then input to the COCIRM-SED model, which simulated the quantity and pattern of the short-term and long-term deposition of disposal sediment and TSS plume during and after the disposal operations.