A 3-DIMENSIONAL NUMERICAL-MODEL OF SUSPENDED SEDIMENT TRANSPORT IN HOWE SOUND, BRITISH-COLUMBIA

Numerical models of the hydrodynamics of Howe Sound and of the transpo rt and deposition of suspended sediment from the Squamish River enteri ng Howe Sound are described Two different versions of a three-dimensio nal 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 t he velocity field as well as the time-varying density field. The model s are forced al their open boundaries by a larger three-dimensional mo del of the Strait of Georgia, by surface wind stress and by the Squami sh 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-dimensiona l hydrodynamic model. Sediment transport and deposition are simulated using a Monte Carlo procedure, in which the transport, diffusion and s ettling 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 addi tion to the velocity derived from the three-dimensional model and the intrinsic settling rate. Temporal variations in supply are simulated b y corresponding variations in the rate of installation of pseudo-parti cles. The grain size distribution of the source sediment is simulated by randomly selecting, for each pseudo-particle, a sinking velocity fr om a range of sinking velocities whose distribution reproduces the dis tribution 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 report ed by Hicken (1989).