Environmental Engineering - Sable Offshore Energy Project - Sediment modeling of river

Construction of a pipeline crossing at Milford Haven River required a study to determine the optimal time for dredging operations to be carried out and to predict the impact of dredging activities with respect to sediment dispersion. Sediment dispersion modeling was conducted to predict whether suspended sediment would be transported by currents toward sensitive biological areas. A hydrodynamic model with tidal forcing provided current velocity fields in Milford Haven River for the sediment dispersion simulations.

A finite element model was set up using the coastlines and bathymetry of Milford Haven River and Chedabucto Bay, extending offshore to 61? 15’ W longitude. Characteristic sediment sizes at the pipeline crossing were based on an analysis of 3 samples from a borehole. The sediment distribution for each sample was divided into five class ranges with representative grain sizes and settling velocities. The vertically averaged nonlinear finite element model solves the convection-diffusion equation to determine suspended sediment concentrations and total bed change.

Continuous trenching over a 2-day period by a traditional bucket dredge was simulated using a smaller mesh covering only the upper region of Milford Haven estuary for an intermediate tidal range and a low tidal range. The bucket dredging operations were assumed to have a 15% loss of sediment into the water column. Computer simulations were carried out to predict sediment concentration for two cases. A ‘Best Estimate’ scenario based on estimated volumes of coarse and fine sediment in the pipeline trench and a ‘Worst Case’ scenario based on the assumption of only fine sediment in the pipeline trench.