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Discussion papers | Copyright
https://doi.org/10.5194/esurf-2018-29
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Apr 2018

Research article | 10 Apr 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth Surface Dynamics (ESurf).

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

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

Abstract. Vegetation enhances bank stability and sedimentation to such extent that it can modify river patterns, but whether similar strong biogeomorphological feedbacks exist in estuarine environments 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, while flow-focusing effects of tidal floodplain elevated by mud and vegetation could lead to channel concentration and incision. Here we study isolated and combined effects of mud and saltmarsh vegetation on estuary dimensions. A 2D hydromorphodynamic sandy 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 herelimited 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. Results show that the vegetation reaches its largest extent in the mixed energy zone of the estuary. Here vegetation can cover more than 50% of the estuary width while it remains below 10–20% in the outer, tide dominated zone. Aerial image analysis shows general agreement with trends in natural estuaries. The presence of mud leads to stabilization and accretion of the intertidal area and a slight infill of the mixed energy zone, which acts as a bed load convergence zone at the fluvial-tidal transition. Without mud, the modelled vegetation has a limited effect, again peaking in the mixed energy zone. Combined modelling of mud and vegetation leads to mutual enhancement with mud causing new colonization areas and vegetation stabilizing the mud. 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 resulting reduction of tidal prism may cause infilling of estuaries over centuries.

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Short summary
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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