Constructed Tidal Marshes

Abstract

Along river banks, on the transition zone between the river and see, tidal marshes can develop. The tidal marshes accommodate multiple plant and fish species. As a result of the stressful conditions in tidal marshes, unique intertidal ecology develops. However, due to the densifying of the river banks, tidal marshes are disappearing.
Restoration of tidal marshes can reintroduce the unique intertidal ecology on several locations in the estuary. To obtain restoration of tidal marshes, constructed tidal marshes come into play, which can be built at designated places along the river.
However, constructed tidal marshes are not necessarily built to restore the unique tidal nature, but can also have other functions as recreation and contribution in green city area.

Nevertheless, is it still unknown how constructed tidal marshes behave and what the optimal design is. Namely, a mismatch exists between the policymakers and designers on the one side and academic knowledge on the other side. To fill up this gap, accessible knowledge from the experts should be available for designers.
Therefore, this research provides guidelines for designers and gains more insight into the behaviour of constructed tidal marshes. This thesis focusses on extracting general knowledge from the results of a numerical model, applied on a case study.

Simulations of a numerical 1D Sobek model applied on the case study achieves the influence of model configurations on accretion. Therefore, tidal forcing, marsh design and system adjustments are divided into multiple components. The model simulations of the separate components give the influence on bed shear stress and potential sedimentation.
Furthermore, executed fieldwork calibrates the model on measured flow velocities. Besides, the Manning coefficient is estimated, and cross-sections are obtained by gps measurements. The parameters are subsequently used as model input.

From the simulations, it can be concluded that tidal asymmetry determines the duration of stagnant water and thus the settling of fine particles. A considerable increase in sedimentation is reached when flats are participating. However, an increasing flat area encourages ebb-dominance and can even lead to erosion.
Next, when the width of the cross-section is large compared to the depth, more sedimentation is predicted. With flood-dominant bed shear stresses, a broad cross-section leads to accretion.
In the case study, the presence of sand particles is not expected, as sand is deposited close to the inlet, where the bed shear stress is ebb-dominant. In contrast, silt settles throughout the system. Especially at low energetic conditions, such as bends, silt settles. As the occurring bed shear stress is mostly flood-directed, it is likely that sediment entering the system, does not leave the system anymore, and accretion of the bends is presumed.
The placement of gate culverts in flood direction leads to higher accretion rates due to the longer slack duration. However, at the exact locations of structures, higher bed shear stresses can develop, and erosion is expected.