T.J. Grandjean1,2*, R. Weenink1,2; D. van der Wal1,3, E.A. Addink2; Z. Hu4,5,6; S. Liu4; Z.B. Wang7,8; Y. Lin9,10; T.J. Bouma1,2
1 NIOZ, NL; 2 Utrecht University, NL; 3 University of Twente, NL; 4 Sun Yat-Sen University, CN; 5 Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, CN; 6 Pearl River Estuary Marine Ecosystem Research Station, CN; 7 Delft University of Technology, NL; 8 Deltares, NL; 9 SKLEC, CN;
10 Yangtze Delta Estuarine Wetland Ecosystem Observation and Research Station, CN
* Corresponding author: tim.grandjean@nioz.nl
Introduction
The balance of sediment supply and loss is critical for tidal flats to maintain, especially under future conditions with rising sea levels. Estuarine sediments may come from several sources, mainly rivers, the sea, primary production and shore erosion. Conversely, sediment loss primarily occurs through erosion by waves and tidal currents, which is often redistributed within the estuary. Changes in the balance may occur either through natural changes and global change-induced changes (e.g., increased storm events and droughts) or human activities (e.g., dam construction, dredging, and land use changes) (Giosan et al., 2014; Syvitski et al., 2009). In the end, insufficient sediment supply can lead to erosion and loss of essential ecological habitats such as tidal flats, salt marshes and mangroves (Mariotti & Fagherazzi, 2013). Further, this potential loss of intertidal areas poses a significant risk to the natural flood defence systems (Bouma et al., 2014), as their diminished capacity against storm surges and rising sea levels compromise coastal protection. While the critical role of sediment balance in maintaining tidal flats is acknowledged (Gao, 2019), empirical evidence is lacking to identify the minimum amount of sediment required to maintain under different conditions intertidal areas globally.
Objective and Methods
To identify the sediment thresholds essential to maintain intertidal areas, we quantified the morphological evolution of tidal flats over time and assessed the available sediment within estuarine systems globally. Recognising the absence of temporal datasets of sediment load and morphological development at global scales, we used Landsat 5 imagery to retrieve this information. Utilising 4,939 images, we constructed Digital Elevation Models (DEMs) for 40 estuaries, covering two distinct periods: 1986-1988 and 2009-2011. The DEMs allowed us to discern morphological changes within estuaries, offering insights into the trajectories in two dimensions: lateral (expanding or retreating) and vertical (accreting or eroding). Furthermore, we used satellite-derived turbidity (Dogliotti et al., 2015) as an accessible proxy for sediment availability worldwide. Based on these results, in combination with the tidal range data from tidal gauges, we identified a critical turbidity threshold crucial to maintaining estuarine tidal flats.
Results
While previous research has concentrated on the lateral changes of intertidal areas (Murray et al., 2019, 2022), our findings show that lateral changes do not invariably correspond to vertical alterations and vice versa. This discrepancy underscores the need to monitor morphological changes in both dimensions to understand estuarine dynamics. Moreover, the interconnectivity of intertidal habitats - such as salt marshes, mangroves, and tidal flats - plays a crucial role in morphological development; our study highlights the importance of including all intertidal habitats in analyses to determine a critical turbidity threshold. This resulted in our research identifying for the first time a tidal-amplitude-dependent turbidity threshold crucial for the existence of intertidal areas, providing new insights about the importance of managing turbidity levels in intertidal areas.
References
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Dogliotti, A.I., Ruddick, K.G., ... Knaeps, E. (2015). A single algorithm to retrieve turbidity from remotely-sensed data in all coastal and estuarine waters. Remote Sensing of Environment, 156, 157–168.
Gao, S. (2019). Geomorphology and Sedimentology of Tidal Flats. In Coastal Wetlands (pp. 359–381). Elsevier.
Giosan, L., Syvitski, J., Constantinescu, S., & Day, J. (2014). Climate change: Protect the world’s deltas. Nature, 516, 31–33.
Mariotti, G., & Fagherazzi, S. (2013). Critical width of tidal flats triggers marsh collapse in the absence of sea-level rise. Proceedings of the National Academy of Sciences, 110(14), 5353–5356.
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