T.J. Grandjean1*, J. van Belzen1,2, D. van der Wal1,3, G.S. Fivash4, S. Temmerman4, M.G. Kleinhans5, T.J. Bouma1,5
1 NIOZ, NL; 2 Wageningen Marine Research, NL; 3 University of Twente, NL; 4 University of Antwerp, BE; 5 Utrecht University, NL
* Corresponding author: tim.grandjean@nioz.nl
Introduction
Estuaries have long attracted dense human settlement and economic activity (Day et al., 2007), as their semi-enclosed waters form natural gateways between the open sea and inland transport networks. Major ports such as Antwerp, Hamburg and Shanghai are situated within large estuarine systems and depend on reliable navigational access. Beyond their logistical importance, estuaries provide essential ecosystem services, including flood attenuation and highly productive, biodiverse habitats (Barbier et al., 2011). To maintain shipping access, many estuaries undergo continuous maintenance dredging and, in numerous cases, progressive channel deepening to accommodate increasing vessel size and trade volumes. Although individual case studies show that such interventions alter intertidal morphology (de Vet et al., 2020; Grandjean et al., 2026; van Dijk et al., 2021), it remains unclear whether this reflects a general and consistent response across disturbed estuarine systems. Resolving this question is crucial for understanding long-term resilience and for guiding estuarine management under sustained human pressure and accelerating sea-level rise.
Objective and Methods
Building on local evidence from the Western Scheldt, where long-term dredging and sediment disposal are associated with pronounced intertidal change (Grandjean et al., 2026), we expanded the analysis to a broader spatial scale. We compiled a dataset of estuaries documenting dredging activities and sediment disposal practices over recent decades. For both dredged and non-dredged systems, intertidal elevation models were reconstructed using remote sensing. Digital Elevation Models of the intertidal zone were derived using the waterline method (Grandjean et al., 2024), combining time series from Landsat 8 and 9 and Sentinel-2 imagery. These elevation datasets were analysed using morphological metrics that characterise the vertical distribution and spatial configuration of intertidal areas. This allowed us to systematically assess whether dredged estuaries exhibit distinct morphological signatures compared to relatively unmodified systems.
Results
Our results show that dredging-dominated estuaries exhibit a systematically different vertical distribution of intertidal area compared to non-dredged systems. Specifically, these systems are characterised by a reduced proportion of low-elevation tidal flats and a greater proportion of higher-elevation flats. In several cases, intertidal area extends above Mean High Water level, increasing the likelihood of marsh establishment (Fivash et al., 2023).
In the short term, an increase in higher-elevation flats may appear favourable, for instance by providing accessible foraging habitat for waders. However, over longer timescales, these elevation patterns are associated with progressive marsh expansion and a decline in low-elevation habitats. Moreover, several dredging-dominated systems show indications of reduced multi-channel complexity, suggesting that vertical shifts coincide with broader changes in morphodynamic structure and potentially reduced ecological resilience.
Taken together, our findings suggest that sustained dredging does not necessarily cause abrupt collapse, but instead drives a gradual and persistent loss of geomorphic diversity—capturing the idea of dredging as a slow-motion degradation of estuarine systems.
References
Barbier, E. B., .. & Silliman, B. R. (2011). The value of estuarine and coastal ecosystem services. Ecological Monographs, 81(2), 169–193.
Day, J. W., .. & Horton, B. P. (2007). Emergence of complex societies after sea level stabilized. Eos, Transactions American Geophysical Union, 88(15), 169–170. https://doi.org/10.1029/2007EO150001
de Vet, P. L. M.,.. & Wang, Z. B. (2020). Sediment Disposals in Estuarine Channels Alter the Eco-Morphology of Intertidal Flats. Journal of Geophysical Research: Earth Surface, 125(2). https://doi.org/10.1029/2019JF005432
Fivash, G. S., .. & Bouma, T. J. (2023). Early indicators of tidal ecosystem shifts in estuaries. Nature Communications, 14(1), 1911. https://doi.org/10.1038/s41467-023-37444-6
Grandjean, T. J., .. & Bouma, T. J. (2026). De Westerscheldenatuur: Een mooie toekomst vraagt keuzes nu! In 2026-01 (pp. 1–38). NIOZ. https://doi.org/10.25850/nioz/7b.b.fk
Grandjean, T. J., .. & Bouma, T. J. (2024). Critical turbidity thresholds for maintenance of estuarine tidal flats worldwide. Nature Geoscience, Accepted.
van Dijk, W. M., .. & Kleinhans, M. G. (2021). The vulnerability of tidal flats and multi-channel estuaries to dredging and disposal. Anthropocene Coasts, 4(1), 36–60. https://doi.org/10.1139/anc-2020-0006
