Environmental Engineering - Sable Offshore Energy Project - Sediment dispersion modeling

A monitoring program was carried out to assess the environmental effect of pipeline trenching operations on areas near the landfall site in Stormont Bay. 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 for the sediment dispersion simulations.

A finite element model was set up using the coastlines and bathymetry of Country Harbour, Isaacs Harbour and Stormont Bay, extending offshore to 45° N latitude. Modeling was carried out separately for trenching by a bucket dredge in the nearshore region and a subsea trencher in the deep region. Characteristic sediment sizes for nearshore and deep regions were obtained from geotechnical survey data. The sediment distribution for each region was divided into five class ranges with representative grain sizes and settling velocities. In order to examine a ‘worst case’ scenario, the assumption is made that all the material escapes from the trenching operation. The vertically averaged nonlinear finite element model solves the convection-diffusion equation to determine suspended sediment concentrations and total bed change.

The computed suspended sediment concentrations, for both nearshore and deep regions, are similar to background turbidity levels that have been measured during ‘non-storm’ conditions (5 mg/l). These values are relatively low, compared to turbidity levels that have been generated by storms (100 mg/l). Changes in the seabed are minimal, with deposition occurring primarily in close proximity to the trenching operations.