Abstract
Tidal wetlands provide a habitat for a wide range of vegetation communities and animal species that are found nowhere else, and also provide a wide range of social, economic and environmental services to human well-being. Tidal wetlands are under threat of drowning by sea level rise (SLR) as a result
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of climate change (CC). Enhancing sedimentation inside tidal wetlands is currently one of the main mitigating measures to prevent wetlands from drowning due to SLR. Tidal freshwater wetlands (TFWs) in the transition zone between tidally-dominated and fluvially-dominated sections of a river delta have received little attention in scientific literature. This study focused on which factors control sedimentation and erosion in tidal freshwater wetlands and how climate change affects these controls and thus sedimentation and erosion. To study how sedimentation rates and patterns in TFWs are affected by the interplay of river discharge, wind waves and tide, a hydro-morphodynamic model was constructed of a small de-embanked polder located in the Brabantse Biesbosch TFW in the Rhine-Meuse delta in the Netherlands. This model was used to carry out multiple scenario analyses to examine how different combinations of hydro-meteorological boundary conditions control sedimentation and erosion under current climate conditions. Next, transient scenario runs were carried out for the period 2015-2050 with gradually changing boundary conditions in order to assess current and future net sedimentation rates, patterns and trapping efficiencies, and compare predicted rates of bed level change to predicted rates of SLR. After that, the effect of climate change as well as river diversions on sedimentation patterns and trapping efficiencies of the entire Brabantse Biesbosch wetland was analyses. TFWs depend on riverine discharges for inflow of water & sediment, but more discharge does not automatically mean more sedimentation: in the Brabantse Biesbosch, larger discharge events generally lead to a net loss of sediment due to resuspension and outflow of previously settled material. Wind as a boundary condition has a major effect on sedimentation rates and trapping efficiencies, and only negative. The effect of tidal range on sedimentation rates is more limited than the effect of wind and discharges; however this strongly depends on the location of the wetland in the delta. The de-embanked polder study area is likely to drown due to CC. The effect of CC on sedimentation rates and morphological developments is limited: morphological stabilization of this newly developed area is a far more important driver of bed level changes than CC. TFWs are dynamic, complex systems in which the sedimentation rates are controlled by the interplay between river discharges, tides, wind and wetland characteristics. There is not a single ‘perfect measure’ that can be applied everywhere to enhance sedimentation rates. For example, the construction of river diversions towards wetlands is only effective in case the SSC of the feeding river is sufficiently high to counteract the eroding effect of the incoming current.
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