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

Abstract

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.

Effects of estuarine mudflat formation on tidal prism and large‐scale morphology in experiments

“…In addition, when the results are compared to data from real-world estuaries, we note that for many real-world estuaries the relative extent of mudflats is larger upstream, similar to our experiments: Western Scheldt (McLaren, 1993(McLaren, , 1994, Ems-Dollard (Van Heuvel, 1991), Dovey (Baas et al, 2008), Severn (Allen, 1987) and the Salmon River estuary (Dalrymple and Choi, 2007). This trend was also observed in numerical models (Braat et al, 2017;Lokhorst et al, 2018). Since the field data support the experimental results, the experiment can help us understand how the mudflats in the system are formed.…”

“…These areas could potentially be a starting point where pioneer marsh species can find their window of opportunity (Cao et al, 2017;De Haas et al, 2018). This was recently also concluded in a numerical modelling study of estuaries with mud and vegetation (Lokhorst et al, 2018). An important question related to vegetation and mud settling is whether the vegetation supports mud settling, or the other way around, or both.…”

Growing Forced Bars Determine Nonideal Estuary Planform

“…High elevated parts developed on the oldest parts of bars that accreted over time and lack flooding and morphodynamic activity in later phases. The relative scarcity of high elevated areas is caused by the lack of cohesive material and vegetation, which would otherwise accrete tidal bars and estuary banks (Braat et al, , ; Lokhorst et al, ). Low elevated parts are previous channels or scours on bars for which time was too short to fill in.…”

Salt Marsh Establishment and Eco‐Engineering Effects in Dynamic Estuaries Determined by Species Growth and Mortality

“…The interacting processes cause a temporal‐physical plant variation by growth and temporal‐spatial dynamics induced by mortality. The dynamic vegetation model is based on the riparian vegetation model of Van Oorschot et al (), but the intraannual salt marsh growth and treatment of the periodic tides is novel, as previous studies simplified the hydromorphodynamic stresses by the periodic tides (Kleinhans et al, ; Lokhorst et al, ).…”

“…e Poppema et al (2017). (2017), but the intraannual salt marsh growth and treatment of the periodic tides is novel, as previous studies simplified the hydromorphodynamic stresses by the periodic tides Lokhorst et al, 2018).…”