PhD defence: Biogeomorphology shaping coastal and estuarine systems the long-term development of landscapes under fluvial and coastal agents

to

Coastal, fluvial and estuarine landscapes develop through interaction of water, sediments and biota. These landscapes are an ever evolving product of hydrodynamic forces, such as river discharges, tides and waves, sediment transport and interactions with biota on the pre-existing morphology.

The objective of this thesis is to systematically determine the long-term and large-scale development of coasts, estuaries and tidal basins under combinations of tides, waves, river discharges, sea level rise, sediment supply and vegetation. Hypotheses posed by paleogeographical reconstructions were tested with biomorphodynamic numerical models.

First, model developments were necessary to perform the desired long-term simulations. Here we focused on the parameterisation of both wave-driven sediment transport and transverse bed slope effects. These parameterisations showed large effects on the morphodynamic simulations and the model improvements and knowledge gain were further applied in the later models.

Then, scenarios of landscape evolution with various boundary conditions and sediment availability, with and without vegetation were explored in two contrasting idealised settings: a fluvial-tidal estuary, and a barrier coast (wave-tidal) system. The fluvial-tidal estuarine setup allowed levees to form and evolve under combinations of fluvial and tidal discharges, sediment supply and different eco-engineering species, namely reeds and trees. The wave-tidal setup of a comprehensive sand-mud barrier coast system tested combined effects of wave climate, tides, sea level rise and formation of marshes.

The tidal-fluvial basin model, inspired by paleoreconstructions of the Old Rhine, showed that vegetation played a major role in transforming the coastal landscape from subtidal to intertidal and supratidal. The dense reed reduced the levee width and inhibited the formation of crevasses. Conversely, trees (sparse vegetation) enhanced the formation of crevasses. Furthermore, vegetation alone was able to reduce the tidal effect and connected the levees to the coastal barrier, which transformed the tidal basin into an estuary. Essentially, this shows that the transition of the Old Rhine from a tidal basin to an estuary can have been entirely driven by the settling of reeds (followed by later peat formation) with all other boundary conditions (offshore tides and fluvial discharge) being equal.

The back-barrier basin model responded strongly to the presence of marsh vegetation in a similar way. The vegetation in the tidal basin changed the local configuration of channels and shoals. This triggered a cascade of effects beyond the vegetation patches: it changed the sediment balance of the basin. The unvegetated basins, regardless the magnitude of the offshore supply of mud, imported sediments and kept up with sea level rise. On the other hand, the vegetated basins showed a net export of sediments after the marsh establishment. Without sea level rise, the basin reached a steady-state (equilibrium). However, with sea level rise the growth of accommodation space and tidal prism combined with the net sediment export trend led to basin drowning and extensive marsh mortality. This suggests that, in contrast with most literature, vegetation may not invariably contribute to the infilling that potentially counters sea level rise.

Start date and time
End date and time
Location
Academiegebouw, Domplein 29, Utrecht
PhD candidate
Marcio Boechat Albernaz
Dissertation
Biogeomorphology shaping coastal and estuarine systems the long-term development of landscapes under fluvial and coastal agents
PhD supervisor(s)
prof. dr. M.G. Kleinhans
prof. dr. B.G. Ruessink
Co-supervisor(s)
dr. H.J. Pierik
dr. A. van der Spek