Tidal channel development and the role of vegetation: fundamental insights and application for tidal marsh restoration

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 cha

Formation and evolution of a tidal channel network within a constructed tidal marsh

The morphogenesis of tidal channel networks that dissect intertidal flats and marshes has been studied especially by morphodynamic modeling, while relatively few empirical data exist on high-resolution field observations. Here we measured the spontaneous formation and evolution of a tidal channel network in a newly constructed tidal marsh (Scheldt estuary, Belgium) over a period of 4 years, by high-accuracy topographic surveying with a temporal resolution of 1 year at high spatial resolution considering all channels deeper than 0.1 m. As a reference, topographic measurements with a similar high resolution were performed in a nearby, mature natural tidal marsh network. Based on the field surveying and additional GIS processing, we derived several geometric and hydraulic parameters (channel width, depth, eroded volumes, crosssectional area, length profiles, drainage density, mainstream length, tidal discharge and watershed area), and compared the evolution of geometric relationships in the constructed marsh with the natural tidal marsh. In this way we have evaluated how fast an equilibrium state was attained. We found that after 2 to 3 years of tidal working the cross-sectional areas of former ditches in the constructed marsh were in equilibrium with the corresponding tidal discharge. Furthermore we observed that the mainstream lengths and the drainage densities for the smaller watershed areas were comparable with the natural tidal marsh, demonstrating the rapid headward growth of newly forming channels and tributary channel formation near the channel heads. Newly formed channels preferentially developed in the low elevation zones of the constructed marsh and channel extension was not significantly influenced by the presence or absence of vegetation. However, the overall channel drainage density and channel cross-sectional areas of the newly formed channels were still lower compared to the natural tidal marsh after 4 years. This indicates that further channel network extension and continued channel deepening can be expected in the coming years

Bio-geomorphic effects on tidal channel evolution: impact of vegetation establishment and tidal prism change

The long-term (10–100 years) evolution of tidal channels is generally considered to interact with the bio-geomorphic evolution of the surrounding intertidal platform. Here we studied how the geometric properties of tidal channels (channel drainage density and channel width) change as (1) vegetation establishes on an initially bare intertidal platform and (2) sediment accretion on the intertidal platform leads to a reduction in the tidal prism (i.e. water volume that during a tidal cycle floods to and drains back from the intertidal platform). Based on a time series of aerial photographs and digital elevation models, we derived the channel geometric properties at different time steps during the evolution from an initially low-elevated bare tidal flat towards a high-elevated vegetated marsh. We found that vegetation establishment causes a marked increase in channel drainage density. This is explained as the friction exerted by patches of pioneer vegetation concentrates the flow in between the vegetation patches and promotes there the erosion of channels. Once vegetation has established, continued sediment accretion and tidal prism reduction do not result in significant further changes in channel drainage density and in channel widths. We hypothesize that this is explained by a partitioning of the tidal flow between concentrated channel flow, as long as the vegetation is not submerged, and more homogeneous sheet flow as the vegetation is deeply submerged. Hence, a reduction of the tidal prism due to sediment accretion on the intertidal platform, reduces especially the volume of sheet flow (which does not affect channel geometry), while the concentrated channel flow (i.e. the landscape forming volume of water) is not much affected by the tidal prism reduction