Sediment trapping, turbidity maxima, and bathymetric stability in macrotidal estuaries

Abstract

This paper examines how fine marine sediments are trapped in macrotidal estuaries and what processes coexist to maintain bathymetric stability. An analytical emulator is constructed integrating explicit formulations for tidal and residual current structures together with sediment erosion, suspension, and deposition. The emulator estimates suspended concentrations and net sediment fluxes and indicates the nature of their functional dependencies and corresponding sensitivities to tidal elevation amplitude ̂ζ (or tidal current Û), channel depth D, bed friction coefficient f, sediment size (specifically, settling velocity WS), and river flow velocity UR. Associated scaling analyses reveal the relative impacts of terms representing tidal nonlinearities, gravitational circulation, and " delayed" settling. Using the emulator to determine conditions consistent with zero net flux of sediments or bathymetric stability, we deduce WS ≈ fÛ. Overall, finer sediments are imported and coarser ones are exported, with more imports (extending to a coarser fraction) on spring tides than on neaps. The emulator indicates that the spring-neap shifting of this coarse-fine boundary corresponds to particle diameters between 30 and 50 μm. This constitutes selective sorting, maintaining a widely distributed source of continuously resuspended material in a size range commonly observed. By illustrating the acute sensitivity of sediment fluxes to the bed friction coefficient, we anticipate feedback adjustments between suspended and surficial sediments. Factors governing the generation, location, and characteristics of turbidity maxima are indicated, including dependence on ̂ζ/D. A direct link is shown between parameters that control sediment flux at a specific cross section and net upstream tidal energy dissipation.