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Tidal marshes: bio-geomorphic self-organization and its implications for resilience to sea level rise and changing sediment supply

Periodic Reporting for period 1 - TIGER (Tidal marshes: bio-geomorphic self-organization and its implications for resilience to sea level rise and changing sediment supply)

Reporting period: 2019-09-01 to 2021-08-31

Intertidal landscapes are complex environments located between land and sea, which are regularly flooded by tides. They provide highly valuable ecosystem services that are threatened by increasing sea level rise and decreasing sediment supply.

Previous studies showed that the small-scale (order of square meters) interactions between vegetation dynamics, water flow and sediment transport (so-called biogeomorphic feedbacks) have a great impact on channel network formation and evolution at the landscape-scale (order of square kilometers). This process is called biogeomorphic self-organization.

Our objective is to investigate the impact of plant species traits on biogeomorphic self-organization of intertidal landscapes. More specifically, we hypothesize that (1) different plant species traits lead to the self-organization of different channel network patterns, and (2) the resulting self-organized landscape structures determine the efficiency to distribute and trap sediments on the intertidal floodplains, and hence the resilience of the landscape to increasing sea level rise and decreasing sediment supply.

By using a combination of remote sensing analyses and numerical simulations, we aim at producing new fundamental knowledge on landscape self-organization by biogeomorphic feedbacks, and its implications for the resilience of intertidal landscapes against environmental changes.
We have developed the software program TidalGeoPro to extract tidal channel network characteristics from aerial pictures and numerical model results. This allows us to assess how efficient specific tidal channel networks are to transport and deliver sediments to the tidal floodplains, which contributes to the resilience of tidal wetlands to increasing sea level rise and decreasing sediment supply.

By analyzing aerial pictures from 14 tidal marshes along the coasts of the USA and China, we found out that physical factors (such as marsh elevation, tidal range and inundation time) are more likely than biological factors (such as biomass, stem density, stem diameter and stem height) to contribute to the high diversity encountered in tidal channel network characteristics (Liu et al., Limnology and Oceanography, accepted for publication).

We are currently analyzing aerial pictures from 11 tidal marsh and mangrove systems to investigate the impact of plant morphologies (here, grass vs. trees) on the characteristics of the tidal channel networks that dissect their vegetated floodplains.

We have developed the biogeomorphic model Demeter to simulate explicitly, in tidal wetlands, the feedbacks between (i) the tidal hydrodynamics, (ii) the sediment erosion, transport, deposition, and resulting bed level changes, and (iii) the vegetation establishment, expansion and die-off, following different colonization strategies (e.g. patchy vs. homogeneous). We have adopted a multiscale approach in which the hydro-morphodynamics is simulated at coarser resolution than the vegetation dynamics. As a results, Demeter is able to account for relevant fine-scale flow-vegetation interactions (less than a square meter) together with their impact on vegetation and landform developments at the landscape scale (several square kilometers) and on the long term (decades), which was crucial for the simulations planed in this project.

Our multiscale approach required the development of novel multiscale coupling techniques. For instance, we have developed a convolution method to spatially refine coarse-resolution hydrodynamic simulations of flow velocities around fine-resolution patchy vegetation patterns. and we have provided evidence that it can substantially improve the representation of important biogeomorphic processes in Demeter (Gourgue et al., Journal of Advances in Modeling Earth Systems, 2021).

The first application of Demeter is a tidal marsh restoration project at the Belgian/Dutch border along the Scheldt Estuary, for which we had important datasets for model calibration and validation. It is also an important milestone for this project because it serves as proof of concept that Demeter can be used for assessing tidal marsh resilience to sea level rise and sediment supply (Gourgue et al., Earth Surface Dynamics, in review).

We are currently analyzing simulation results to answer two fundamental questions: (1) How much of tidal marsh resilience is inherited from self-organization of tidal channel networks at the plant colonization stage? (2) What is the range of physical conditions that allow for biological characteristics to play a significant role in tidal channel self-organization?
The main objective of this project is to understand the role of vegetation in shaping channel networks in tidal wetlands, and the resulting impact on tidal wetland resilience to increasing sea level rise and decreasing sediment supply. The socio-economic impact of this research is very high, because of the important ecosystem services provided by tidal wetlands (e.g. carbon sequestration and coastal protection).

To reach our objectives, we have developed two software programs at the state of the art in the field of (bio-)geomorphology. TidalGeoPro is the first toolbox able to extract channel network characteristics from aerial pictures and numerical model results that is specifically designed for intertidal systems. Demeter is the first biogeomorphic model able to account for relevant fine-scale flow-vegetation interactions (less than a square meter) together with their impact on vegetation and landform developments at the landscape scale (several square kilometers) and on the long term (decades). It makes use of state-of-the-art biogeomorphic coupling techniques (Gourgue et al., Journal of Advances in Modeling Earth Systems, 2021). TidalGeopro and Demeter are both published with open-source licenses and are expected to support state-of-the art research by other groups internationally.

Our remote sensing analyses have revealed that the high diversity encountered in tidal marsh channel network characteristics throughout the world is better explained by physical factors (marsh elevation, tidal range and inundation time) than biological ones (biomass, stem density, stem diameter and stem height) (Liu et al., Limnology and Oceanography, accepted for publication). Further analyses will determine if tidal channel networks are significantly different in salt marshes and mangroves, and if those differences are especially due to the contrasting morphologies of their dominant species (grass vs. trees).

We have demonstrated that the resilience of restored tidal marshes (and the preservation of the ecosystem services they provide) can be steered by restoration design (Gourgue et al., Earth Surface Dynamics, in review) and that Demeter is a useful resource in that perspective.

In the final year of this project, we aim at (1) evaluate how much of tidal marsh resilience is inherited from self-organization of tidal channel networks at the plant colonization stage, and (2) determine the range of physical conditions that allow for biological characteristics to play a significant role in tidal channel self-organization. This will allow to better assess the future socio-ecological value of newly developed tidal marshes, whether as a natural response to climate change (marsh migration) or by planned restoration.
Bed level and vegetation cover evolution for two species with contrasting colonization strategies.