University of Twente Student Theses


Numerical modelling of the biophysical feedbacks of Salicornia at the constructed Marconi salt marsh

Broek, J.F. van den (2020) Numerical modelling of the biophysical feedbacks of Salicornia at the constructed Marconi salt marsh.

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Abstract:Salt marshes are valuable intertidal ecosystems because they can increase flood safety and ecological value of the coastal area simultaneously. An important part of the salt marsh ecosystem is its vegetation, which is known to trap and bind sediments and reduce the erosion of the soil due to dissipation of hydrodynamic energy. Recently, worldwide efforts have been made to restore and create these salt marshes. One of the recently constructed marshes is the Marconi salt marsh, which was constructed in the Ems-Dollard estuary at the end of 2018. The Marconi marsh is used to study the effects of different sediment compositions and sowing of seeds on salt marsh establishment and development. Despite the variability of characteristics between species, research on the development of salt marsh vegetation is limited to only a few genera. Therefore, the sowing of Salicornia Europaea (glasswort) at the Marconi marsh gives an interesting opportunity to gain more knowledge on the establishment and development in a salt marsh of an important pioneer species. By using the Marconi project as a case study, this thesis aims to determine the impact of hydrodynamics and morphodynamics on the development of the pioneer vegetation Salicornia Europaea at a (artificial) salt marsh by using a numerical modelling approach. To study the development of Salicornia, this study used a brand new hydrodynamic model (DFM) and combined it with a separate wave propagation model (D-Waves) as well as a vegetation growth model which comprises of the well-established Windows of Opportunity and population dynamics concepts for vegetation growth and simulates one growth season taking place between April-October. The Windows of Opportunity account for the relation of inundation and bed level dynamics with seedling establishment, which takes up the first few months of the vegetation development, while population dynamics govern the growth and decay of established salt marsh vegetation and addresses the rest of the plants life-cycle over the years. The characteristics of the Marconi site and Salicornia were determined by combining literature with elevation and stem density measurements of the site. The model results suggest that the morphodynamics are the most limiting factor for Salicornia’s development, since Salicornia was found to die due to bed level change, even when forced by calm hydrodynamic conditions. Furthermore, establishment of Salicornia seeds was found to be sensitive to hydrodynamics, with the model revealing a significant impact of the inundation frequency on the establishment of Salicornia and the resulting vegetation pattern found in the marsh. On the other hand, fully developed Salicornia clusters were found to be much more resilient to hydrodynamic factors. Due to this resilience, high-density groups were observed throughout the modelled site and appeared to form when two requirements were met; an early establishment to give Salicornia time to grow into fully-developed vegetation and a location that is protected well-enough from hydrodynamic energy to prevent excessive erosion or an excess of the bed shear stress threshold for plant mortality. The overall elevation of the marsh was found to affect the development of Salicornia in three stages. At elevations above the mean high water, Salicornia was well-established throughout the site, while, when lowered to below this water level, vegetation became sparse. Finally, at elevations of more than 1 metre below this mean high water level, all vegetation disappeared from the area. In addition, sediment compositions were found to have different effects on Salicornia as, according to field observations, layers with a high clay content promoted plant growth, while compositions with sand were found to increase the vegetation’s resistance to erosion in the model. This thesis has shown the sensitivity of Salicornia’s development to bed level change as well as the negative impact of inundation on the establishment of Salicornia. Furthermore, this study contributes to formulating requirements for high-density Salicornia vegetation at salt marshes and discovered different growth rates for Salicornia depending on its sediment composition.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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