Modelling Suspended Sediment Concentrations in the Firth of Forth

A two-dimensional depth-integrated sediment transport model has been applied to the Firth of Forth to study the resuspension and movement of sediment in the lower estuary. The dominant particle size in the region is 10 μm with about 70% of the near-bed particles and over 90% of the surface particles being in the size range 0–15 μm. The tidal velocity was assumed to be logarithmic with height above the bed, and the vertical structure of the suspended particulate matter (SPM) concentration was represented by a modified Rouse profile. Erosion and deposition threshold parameters were extracted from two sets of observed SPM concentrations. The most realistic simulations were obtained when the erosion and deposition parameters were made functions of the spring/neap cycle. On the approach to neap tides the erosion threshold increased due to consolidation of the bed sediment, while after mean tide the increased bottom stress caused the erosion threshold to be exceeded for longer during each tidal cycle which resulted in more sediment being removed from the bed. The importance of the time-varying nature of these parameters has been demonstrated using a single grid-point model that calculated SPM concentration fromin situerosion and deposition processes alone. The inclusion of the shallow-water constituents in the tidal currents was required for realistic simulation of SPM concentrations.