A three‐dimensional numerical model of suspended sediment transport in Howe sound, British Columbia

Abstract

Numerical models of the hydrodynamics of Howe Sound and of the transport and deposition of suspended sediment from the Squamish River entering Howe Sound are described Two different versions of a three-dimensional hydrodynamic model of Howe Sound have been developed One is a fine-grid model employing a 390-m grid size, the other a coarse-grid model with a 1.95-km grid size. In the present implementation, both models have 9 layers in the vertical, and calculate the three components of the velocity field as well as the time-varying density field. The models are forced al their open boundaries by a larger three-dimensional model of the Strait of Georgia, by surface wind stress and by the Squamish River flow. The coarse-grid model is computationally less demanding and is used for long-term simulations, whereas the fine-grid model is used for detailed simulations for shorter time periods. The sediment model derives its velocity field from the coarse grid three-dimensional hydrodynamic model. Sediment transport and deposition are simulated using a Monte Carlo procedure, in which the transport, diffusion and settling of suspended sediment are reproduced by tracking the fate of a large number of pseudo-particles, each having dynamics corresponding to the sediment grain size it represents. The effects of diffusion are simulated by prescribing a random velocity for each particle, in addition to the velocity derived from the three-dimensional model and the intrinsic settling rate. Temporal variations in supply are simulated by corresponding variations in the rate of installation of pseudo-particles. The grain size distribution of the source sediment is simulated by randomly selecting, for each pseudo-particle, a sinking velocity from a range of sinking velocities whose distribution reproduces the distribution of sinking velocities in the supply. The model was run for a 69-day period, sufficient time for virtually all the sediment in the summer Squamish River freshet to settle to the floor of Howe Sound The resulting annual rate of deposition is similar to calculations reported by Hicken (1989).