Building and Raising Land: Mud and Vegetation Effects in Infilling Estuaries

Abstract

Many Holocene estuaries were infilled to form convergent, single‐channel systems, while others remained partially or wholly unfilled. This difference in the degree of infilling depends partly on the balance between fluvial and coastal sediment input and the hydrodynamics that can export sediment. However, it remains unclear to what degree this balance is tipped by mud supply and eco‐engineering vegetation, and by what planform patterns the infilling proceeds. This study aims to explore experimentally how mud and vegetation change the degree and process of infilling, elevate and merge bars above intertidal levels and affect the planform of estuaries. To this end, three experiments were conducted in the Metronome, a flume that tilts periodically to create tidal currents, wherein forced tidal asymmetry resulted in net importing estuaries. In the second and third experiments, mud was supplied and in the third experiment seedlings were released of three vegetation species with eco‐engineering traits at a laboratory scale. With only sand, the estuary fills sufficiently to form a multi‐channel pattern with intertidal bars. Both mud and vegetation settle on intertidal bars and on the fluvial bay‐head delta, thereby contributing to bar stabilization and further estuary infilling, pointing at effective strategies to keep up with future sea‐level rise. This reduces channel mobility and effectively narrows the summed subtidal channel width toward an ideally converging funnel shape. This seems especially effective where vegetation stabilizes the mud. The experiments suggest that a range of steady states exists between the end‐members of an unfilled and a completely infilled, ideal estuary.

Effects of Sea‐Level Rise on Dredging and Dredged Estuary Morphology

“…Scale experiments have proven useful in identifying and confirming empirical estuary processes and relations, including the identification of channel and bar patterns (Leuven, Braat, et al., 2018), the influence of mud (Braat et al., 2019) and of vegetation (Kleinhans et al., 2022) on morphological development, the influence of dredging on multi‐channel tidal systems (van Dijk et al., 2021) and the long term development of infilling estuaries (Weisscher et al., 2022). These experiments have been proven to be robust in recreating meaningful and accurate estuarine hydrodynamics (Weisscher et al., 2020) and morphodynamics (Leuven, Braat, et al., 2018) at a small scale and will therefore be used to assess the effects of SLR on dredged and non‐dredged alluvial estuaries.…”

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Estuaries are bodies of water with one or more open connections to the sea (Leuven et al, 2016) which develop at the land-sea interface due to delivery of sediment from both rivers and the coast (Nicholls et al, 2020). Estuaries that have developed naturally tend to have a converging planform shape, often with mutually evasive ebb and flood channels which create a multi-channel system (Jeuken & Wang, 2010;van Dijk et al, 2021;Weisscher et al, 2022). They have several intertidal shoals and bars, particularly at their widest points (Leuven et al, 2016), and extensive floodplains (van Veen et al, 2005).

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Representing the impact of Rhizophora mangroves on flow and sediment transport in a hydrodynamic model (COAWST_rh v1.0): the importance of three-dimensional root system structures

“…considerably vary among genera, such as pneumatophores or "pencil roots" of Avicennia and Sonneratia genera, and prop roots of Rhizophora genus (Krauss et al, 2014). Especially due to the presence of aboveground root systems, mangroves exert drag against water flow that slows down flow velocity and creates conditions preferable for the deposition and retention of tidally and fluvially transported sediments (Furukawa et al, 1997;Krauss et al, 2003;Horstman et al, 2015;Chen et al, 2016Chen et al, , 2018Willemsen et al, 2016;Best et al, 2022) similar to other wetland vegetation habitats such as salt marshes (Temmerman et al, 2005;Bouma et al, 2007;Mudd et al, 2010;Weisscher et al, 2022). The vegetation-flow-sediment interaction is considered a major driving factor of the long-term geomorphic evolution of wetland vegetation habitats (Mariotti and Fagherazzi, 2010;Mariotti and Canestrelli, 2017;Brückner et al, 2019;Kalra et al, 2022;Willemsen et al, 2022) that enabled them to counteract the threats due to sea-level rise (Fagherazzi et al, 2012(Fagherazzi et al, , 2020Lovelock et al, 2015;Kirwan et al, 2016).…”

Saved or starved? The importance of sediment management in determining the future response of estuaries and deltas.

“…Scale experiments have proven useful in identifying and confirming empirical estuary processes and relations, including the identification of channel and bar patterns (Leuven et al, 2018a), the influence of mud (Braat et al, 2019) and of vegetation (Kleinhans et al, 2022) on morphological development, the influence of dredging on multi-channel tidal systems and the long-term development of infilling estuaries (Weisscher et al, 2022b). These experiments have been proven to be robust in recreating meaningful and accurate estuarine hydrodynamics (Weisscher et al, 2020) and (Leuven et al, 2018a) at a small scale, and will therefore be used to assess the effects of SLR on dredged and non-dredged alluvial estuaries.…”

“…Estuaries are bodies of water with one or more open connections to the sea (Leuven et al, 2016) which develop at the land-sea interface due to delivery of sediment from both rivers and the coast (Nicholls et al, 2020a). Estuaries that have developed naturally tend to have a converging planform shape, often with mutually evasive ebb and flood channels which create a multi-channel system (Jeuken and Wang, 2010;Weisscher et al, 2022b). They have several intertidal shoals and bars, particularly at their widest points (Leuven et al, 2016), and extensive floodplains (van Veen et al, 2005).…”