|Tidal channel development and the role of vegetation: fundamental insights and application for tidal marsh restoration|
Vandenbruwaene, W. (2011). Tidal channel development and the role of vegetation: fundamental insights and application for tidal marsh restoration. PhD Thesis. Universiteit Antwerpen. Onderzoeksgroep Polaire Ecologie, Limnologie en Geomorfologie: Antwerpen. 140 pp.
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Tidal channel networks play an essential role in tidal ecosystem functioning since they are the major flow paths for water, sediments, nutrients and biota between the intertidal zone and the subtidal estuarine or coastal area. Understanding their morphogenesis and evolution is crucial for (1) the evolution of natural tidal flats and marshes under the influence of environmental changes such as sea-level rise, and (2) the restoration of tidal marshes on formerly embanked land.During the evolution of an unvegetated tidal flat towards a vegetated tidal marsh, the intertidal landscape becomes colonized by dynamic vegetation patches. Chapter 2 explores the flow acceleration around these dynamic vegetation patches (Spartina anglica) in a large-scale flow facility, and discusses the implications for the evolution of the intertidal landscape. Results demonstrate that the amount of flow acceleration next to vegetation patches, and the distance from the patch where maximum flow acceleration occurs, increase with increasing patch size. In between the patches, the accelerated flow pattern starts to interact as soon as the ratio patch size (D) / inter-patch distance (d) = 0.43-0.67. As the patches grow further, the flow acceleration increases until D/d = 6.67-10, from which the flow acceleration between the patches becomes suppressed, and the two patches start to act as one.
Chapter 3 relates the long term evolution of tidal channels to the observed changes on the intertidal platform, i.e., establishment of vegetation and the reduction of tidal prism. Tidal channels properties were hereby determined by use of aerial photographs. This chapter demonstrates that there is a strong impact of intertidal vegetation on the evolution of channel network dimensions, while the role of tidal prism changes is of minor importance.
Chapter 4 compares the flow hydrodynamics of an unvegetated tidal flat with the flow hydrodynamics of a vegetated tidal marsh, and assesses the effect of vegetation on the observed tidal channel properties. Flow patterns were determined by measuring water levels, flow velocities 8 and flow directions. Results show that during the flood and ebb phase flow patterns in a vegetated tidal marsh are clearly routed through the tidal channel network, whereas on an unvegetated tidal flat the platform floods and drains more like a sheet flow. This results in larger channel widths and channel depths in the tidal marsh compared to the tidal flat (for comparable watershed areas), and the presence of a levee-basin topography on the vegetated marsh platform, which is absent on bare tidal flats.
Chapters 5 and 6 focus on the morphological evolution of a de-embankment site after introducing a daily tidal regime and compare the results with a nearby natural tidal marsh. Chapter 5 hereby handles the morphological evolution of the platform of the de-embankment site by measuring platform elevation changes at locations with different inundation heights. Based on these measurements a model was built which predicts long-term (75 years) elevation changes under different scenarios of mean high water level (MHWL) rise. In the de-embankment site (CRT) the MHWL follows the increase in CRT surface elevation (for details see chap