|Effects of intertidal ecosystems on estuarine hydrodynamics and flood wave attenuation: a multi-scale study|
Stark, J. (2016). Effects of intertidal ecosystems on estuarine hydrodynamics and flood wave attenuation: a multi-scale study. PhD Thesis. University of Antwerp. Ecosystem Management Research Group (ECOBE): Antwerp. xii, 166 pp.
Marien; Brak water; Zoet water
Estuaries and their intertidal ecosystems, such as marshes and tidal flats, provide for numerous socio-economic and ecological functions such as biogeochemical nutrient cycling, navigation or flood protection. While historical land reclamations significantly reduced intertidal habitat, intertidal areas are nowadays being restored again. The results of this thesis improve insight in the impact of changes in the size and geometry of intertidal areas on estuary scale and marsh scale tidal hydrodynamics, with a special focus on their potential to attenuate storm surges.Firstly, water level measurements and a hydrodynamic model are used to study tidal and storm surge propagation through the Saeftinghe marsh along the Scheldt estuary. Results indicate that intertidal ecosystems can indeed reduce storm surges locally. In particular, tides with peak water levels above the marsh platform are mainly attenuated. Conversely, tides with lower peak water levels are slightly amplified along the converging channels. Model simulations also show that attenuation can be limited by marsh size, due to blockage of the tidal wave against levees surrounding a wetland. Ultimately, modelled attenuation rates are related to the channel size relative to the storage volume on the marsh platform. Storm surge reduction is thus dependent on wetland geometry and on the flood wave height itself.This thesis also addresses tidal asymmetry in intertidal channels during the eco-geomorphological development of marshes. In particular, vegetation establishment leads to marsh-scale flow concentration to the channels, causing a less flood-dominant asymmetry associated with reduced import of sediments and other materials. This development continues as the platform grows vertically and the sediment-demand of the platform decreases. However, as the marsh elevation gets higher and tidal discharge through the marsh channels decreases, tidal asymmetry becomes more flood-dominant again.Furthermore, this thesis shows that the elevation of intertidal areas within the estuary’s flow-carrying section determines their influence on tidal asymmetry on estuary scale. Ebb-dominance is most strongly enhanced for tidal flats slightly above mean sea level, while lower tidal flats of this type may even enhance flood-dominance. Intertidal areas that only provide for storage volume enhance ebb-dominance throughout the estuary, except directly downstream where flood dominance is enhanced due to a velocity surge effect. Finally, model results indicate that, in addition to known effects of tidal prism decrease upstream and tidal prism increase downstream of additional storage areas, tidal prism may decrease far downstream of intertidal storage areas due to a decreasing tidal range.