Simulation of High Suspended Sediment Concentrations and Options for a Reduction in the Lower Ems

摘要

Estuaries with hyper-concentrated suspended sediment concentration (SSC) provide critical conditions for water management, channel shipping and ecological affairs. Due to the extreme situation of the Ems concerning the SSC, ecologists and water management authorities try to find solutions by different restoration measures, which aim to damp the hydrodynamic asymmetry and to reduce the suspended sediment concentration. Indicators especially for mobilization, resuspension and settling of fine cohesive sediments provide important benchmarks to evaluate restoration potential. Therefore hydro- and morphodynamic indicators like changes in flood and ebb currents, in flood- and ebb current gradients, net sediment flux and the shift of the turbidity zone were assessed. Especially indicators for fine sediments were selected to analyze the potential of meso-scale suspended sediment reduction. Numerical simulations were performed to increase the Understanding of this estuary. But it shows limits to the existing physical approaches for cohesive sediment transport. For a performance of high suspended sediment concentrations two approaches, one with a simplified flocculation and typical hindered settling, and one with a new flocculation approach taking turbulence and salinity into account, were compared for the Ems Estuary. The hydrodynamic processes, salinity and suspended sediment transport were resolved by a numerical three dimensional finite volume model for the Outer and Lower Ems. Considering the uncertainties already inherent in meso-scale simulations the long-term effects are difficult to anticipate. Based on an existing long-term approach for a tidal marsh river with cohesive sediments, first conclusions for a promising long-term concept in an estuarine environment are presented.