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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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Volume 6, issue 4
Earth Surf. Dynam., 6, 883-901, 2018
https://doi.org/10.5194/esurf-6-883-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Earth Surf. Dynam., 6, 883-901, 2018
https://doi.org/10.5194/esurf-6-883-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Oct 2018

Research article | 09 Oct 2018

Morphological effects of vegetation on the tidal–fluvial transition in Holocene estuaries

Ivar R. Lokhorst1, Lisanne Braat1, Jasper R. F. W. Leuven1, Anne W. Baar1, Mijke van Oorschot2, Sanja Selaković1, and Maarten G. Kleinhans1 Ivar R. Lokhorst et al.
  • 1Faculty of Geosciences, Utrecht University, P.O. Box 80115, 3508 TC Utrecht, the Netherlands
  • 2Department of Freshwater Ecology & Water Quality, Deltares, P.O. Box 177, 2600 MH Delft, the Netherlands

Abstract. 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.

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In estuaries, mud sedimentation enhances salt marsh accretion. Here we explore system-scale effects of plants and mud on planform shape and size of estuaries. We coupled Delft3D for hydromorphodynamics with our vegetation model and ran controls for comparison. Effects are greatest at the fluvial–tidal transition, where for the first time in a model, a bedload convergence zone formed. Regardless of local vegetation effects, mud and vegetation cause gradual filling of estuaries over time.
In estuaries, mud sedimentation enhances salt marsh accretion. Here we explore system-scale...
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