Sediment dynamics of a tidal freshwater wetland system: A case study in the Biesbosch in the Rhine-Meuse delta

Abstract

Many deltas are threatened by land subsidence, sea level rise, and sediment starvation. The diversion of water and sediment into drowning delta wetlands aims at renewed sediment accumulation for ecosystem regeneration. This thesis investigates the Brabantse Biesbosch freshwater microtidal river system where new wetlands have been developed. The hydro-morphologic processes and their controls, and resulting sediment accumulation are investigated to determine critical conditions for sedimentation, and how these may be influenced or optimised. The major part of this project comprised collection and analysis of measurements of water and sediment flow, and sediment deposition. Furthermore, using the 1D SOBEK3 hydrodynamic and sediment transport model, a scenario analysis was exerted to explore future sediment accumulation. The two rivers are the major source of sediment for the area. The sediment load into the wetland system decreases with distance from the rivers. The new wetlands have improved river connectivity of the middle and northeast of the area, which results in less sediment depletion and thus more sediment accumulation. However, in channel sections close to the feeding rivers shear stresses are high, causing sediment resuspension during periods of increased river discharge. The newly developed wetland with continuous supply of river sediment effectively traps sediment, leading to positive sediment budget. In contrast, a new wetland at the interior of the Biesbosch functions as a source of material instead of a sediment trap, likely due to primary production within the wetland. Erosion occurs in inlet and outlet channels, whereas sediment is deposited in the central part of the wetlands. Both erosion and sedimentation rates have decreased over time, since channels reach morphodynamic equilibrium. Sand is deposited at the river inlets, and within and close to channels. Mud is more uniformly distributed over intertidal flats due to micro-topographic flow paths and shear stresses by wind waves that hamper sediment settling or cause resuspension at this bare and shallow area. Sediment fluxes vary at daily to weekly time scales due to river discharge, tide, and wind. Sediment deposition increases with upstream river discharge and sediment influx. However, trapping efficiencies decrease for increasing wind strengths and raised river discharges, due to the associated increased shear stress in the water, preventing settling or even causing resuspension of previously deposited material. An efficient freshwater tidal wetland system would comprise multiple proximal flow through or dead-ending tidal channels, with limited length, so not all sediment is progressively lost at short distance from the river. The connectivity between channels and intertidal area can be improved by a reduction of the total gradient of the intertidal zone, so large areas will submerge during high water. Furthermore, resuspension of sediment by wind or flow related shear stresses should be reduced.