Morphological effects of vegetation on the tidal–fluvial transition in Holocene estuaries
<p>Vegetation enhances bank stability and sedimentation to such an extent that it can modify river patterns, but how these processes manifest themselves in full-scale estuarine settings is poorly understood. On the one hand, tidal flats accrete faster in the presence of vegetation, reducing...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-10-01
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| Series: | Earth Surface Dynamics |
| Online Access: | https://www.earth-surf-dynam.net/6/883/2018/esurf-6-883-2018.pdf |
| Summary: | <p>Vegetation enhances bank stability and sedimentation to such an extent that it
can modify river patterns, but how these processes manifest themselves in full-scale
estuarine settings is poorly understood. On the one hand, tidal flats accrete
faster in the presence of vegetation, reducing the flood storage and
ebb dominance over time. On the other hand flow-focusing effects of a tidal
floodplain elevated by mud and vegetation could lead to channel concentration
and incision. Here we study isolated and combined effects of mud and tidal
marsh vegetation on estuary dimensions. A 2-D hydromorphodynamic estuary model
was developed, which was coupled to a vegetation model and used to simulate
100 years of morphological development. Vegetation settlement, growth and
mortality were determined by the hydromorphodynamics. Eco-engineering effects
of vegetation on the physical system are here limited to hydraulic
resistance, which affects erosion and sedimentation pattern through the flow
field. We investigated how vegetation, combined with mud, affects the average
elevation of tidal flats and controls the system-scale planform. Modelling
with vegetation only results in a pattern with the largest vegetation extent
in the mixed-energy zone of the estuary, which is generally shallower. Here
vegetation can cover more than 50 % of the estuary width while it remains
below 10 %–20 % in the outer, tide-dominated zone. This modelled distribution
of vegetation along the estuary shows general agreement with trends in
natural estuaries observed by aerial image analysis. Without mud, the
modelled vegetation has a limited effect on morphology, again peaking in the
mixed-energy zone. Numerical modelling with mud only shows that the presence of
mud leads to stabilisation and accretion of the intertidal area and a slight
infill of the mixed-energy zone. Combined modelling of mud and vegetation
leads to mutual enhancement with mud causing new colonisation areas and
vegetation stabilising the mud. This occurs in particular in a zone
previously described as the bedload convergence zone. While vegetation
focusses the flow into the channels such that mud sedimentation in intertidal
side channels is prevented on a timescale of decades, the filling of
intertidal area and the resulting reduction in tidal prism may cause the infilling of
estuaries over centuries.</p> |
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| ISSN: | 2196-6311 2196-632X |