Paulina vegetation model: current attenuation by salt marsh vegetation
Kusters, L.E.M. (2003). Paulina vegetation model: current attenuation by salt marsh vegetation. MSc Thesis. TU Delft, Civil Engineering and Geosciences, Hydraulic Engineering: Delft. [diff. pag.] pp.
The present report represents the second phase of my M.Sc. dissertation. The first phase concentrated on thorough data processing of field measurements in the Paulina Polder, see Kusters et al (2003) in Appendix A. For the present phase of my end thesis I have investigated the influence of vegetation on currents in a mudflat salt marsh ecosystem in the estuary of the Westerschelde. This has been done within the framework of the Delft Cluster research program 'ecomorphology' and is part of the project Biogeomorphology of estuaries and coasts. The main objective of this thesis is to explore and evaluate the application of Delft3D, a physically-based hydrodynamic model linked with sedimentation transport in an environment where vegetation is important, e.g. a salt marsh, against field data. Once validated this Delft3D model can be used to simulate flooding and sedimentation patterns in other tidal marshes in the estuary of the Westerschelde. In a next stage this modelling can be extended to long time geomorphologic evolution simulations. The study site is the Paulina Polder, situated in the estuary of the Westerschelde in the southwest of the Netherlands. The Paulina Polder is a typically Westerschelde mudflat-salt marsh ecosystem, characterised by an extended mudflat with rich benthic community and salt marsh vegetation that contains all gradual stages from pioneer to late successively. The extended and viable zone of pioneer vegetation mainly consists of common cord grass. The sandbanks in the middle of the Schelde protect the salt marsh from extreme hydrodynamic conditions. To study the impact of vegetation on the estuarine morphodynamics, on a scale of a salt marsh the 'Paulina vegetation model' was designed. This is a three dimensional Delft3D model with an implemented routine to include vegetation. This routine incorporates an adaptation of the vertical flow distribution, turbulence generation and dissipation based on a 1DV-model. The mudflat and salt marsh system is modelled by an almost rectangular grid using a semi diurnal tide on the open sea boundary. The vegetation input is dependant on the location of the different plant types, the vertical plant structure and the number of plants per vegetation area. An important conclusion of this study is that the designed Paulina vegetation model can be used to study the effects of vegetation on flow in an estuarine environment. To obtain the most realistic simulation results detailed information about the bathymetry and vegetation is needed.The most important conclusion from the sensitivity analysis is that because of the presence of vegetation the flow area reduces, forcing the water to be pushed to the outer layers. The area is even more reduced when the vegetation is denser and the vegetation is modelled with leafs. In the upper layers, where there is no vegetation anymore, the water can overflow the plants, the velocity as a function of time increases. To al lesser degree there is an increase in velocity in the bottom layer. The changes in flow area have considerable effects on the velocities in the water column. Another finding is that because of the vegetation and the flow resistance of the marsh the water chooses a different route, the vegetation free gully. In this research the effects of waves, wind, leafs and the bending of stems are left out of the computations. In future research these effects should be investigated. Careful thought is needed about the influence of the hydraulic roughness of the vegetation on the morphological development of the salt marsh, concerning the small flow velocities.
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