Cross-shore parallel tidal channel systems formed by alongshore currents.

Parallel tidal channel systems, characterized by commonly cross-shore orientation and regular spacing, represent a distinct class of tidal channel networks in coastal environments worldwide. Intriguingly, these cross-shore oriented channel systems can develop in environments dominated by alongshore tidal currents, for which the mechanisms remain elusive. Here, we combine remote sensing imagery analysis and morphodynamic simulations to demonstrate that the deflection of alongshore tidal currents at transitions in bed elevation determines the characteristic orientation of the parallel tidal channels. Numerical results reveal that sharp changes in bed elevation lead to nearly 90-degree intersection angles, while smoother transitions in bed profiles result in less perpendicular channel alignments. These findings shed light on the potential manipulation of tidal channel patterns in coastal wetlands, thus equipping coastal managers with a broader range of strategies for the sustainable management of these vital ecosystems in the face of climate change and sea level rise.

Vegetation enhances curvature-driven dynamics in meandering rivers.

Stabilization of riverbanks by vegetation has long been considered necessary to sustain single-thread meandering rivers. However, observation of active meandering in modern barren landscapes challenges this assumption. Here, we investigate a globally distributed set of modern meandering rivers with varying riparian vegetation densities, using satellite imagery and statistical analyses of meander-form descriptors and migration rates. We show that vegetation enhances the coefficient of proportionality between channel curvature and migration rates at low curvatures, and that this effect wanes in curvier channels irrespective of vegetation density. By stabilizing low-curvature reaches and allowing meanders to gain sinuosity as channels migrate laterally, vegetation quantifiably affects river morphodynamics. Any causality between denser vegetation and higher meander sinuosity, however, cannot be inferred owing to more frequent avulsions in modern non-vegetated environments. By illustrating how vegetation affects channel mobility and floodplain reworking, our findings have implications for assessing carbon stocks and fluxes in river floodplains.

Loss of geomorphic diversity in shallow tidal embayments promoted by storm-surge barriers.

Coastal flooding prevention measures, such as storm-surge barriers, are being widely adopted globally because of the accelerating rise in sea levels. However, their impacts on the morphodynamics of shallow tidal embayments remain poorly understood. Here, we combine field data and modeling results from the microtidal Venice Lagoon (Italy) to identify short- and long-term consequences of flood regulation on lagoonal landforms. Artificial reduction of water levels enhances wave-induced sediment resuspension from tidal flats, promoting in-channel deposition, at the expense of salt marsh vertical accretion. In Venice, we estimate that the first 15 closures of the recently installed mobile floodgates operated between October 2020 and January 2021 contributed to a 12% reduction in marsh deposition, simultaneously promoting a generalized channel infilling. Therefore, suitable countermeasures need to be taken to offset these processes and prevent significant losses of geomorphic diversity due to repeated floodgate closures, whose frequency will increase as sea levels rise further.

Remotely-sensed planform morphologies reveal fluvial and tidal nature of meandering channels.

Meandering channels extensively dissect fluvial and tidal landscapes, critically controlling their morphodynamic evolution and sedimentary architecture. In spite of an apparently striking dissimilarity of the governing processes, planform dimensions of tidal and fluvial meanders consistently scale to local channel width, and previous studies were unable to identify quantitative planimetric differences between these landforms. Here we use satellite imagery, measurements of meandering patterns, and different statistical analyses applied to about 10,000 tidal and fluvial meanders worldwide to objectively disclose fingerprints of the different physical processes they are shaped by. We find that fluvial and tidal meanders can be distinguished on the exclusive basis of their remotely-sensed planforms. Moreover, we show that tidal meanders are less morphologically complex and display more spatially homogeneous characteristics compared to fluvial meanders. Based on existing theoretical, numerical, and field studies, we suggest that our empirical observations can be explained by the more regular processes carving tidal meanders, as well as by the higher lithological homogeneity of the substrates they typically cut through. Allowing one to effectively infer processes from landforms, a fundamental inverse problem in geomorphology, our results have relevant implications for the conservation and restoration of tidal environments, as well as from planetary exploration perspectives.

Field migration rates of tidal meanders recapitulate fluvial morphodynamics.

The majority of tidal channels display marked meandering features. Despite their importance in oil-reservoir formation and tidal landscape morphology, questions remain on whether tidal-meander dynamics could be understood in terms of fluvial processes and theory. Key differences suggest otherwise, like the periodic reversal of landscape-forming tidal flows and the widely accepted empirical notion that tidal meanders are stable landscape features, in stark contrast with their migrating fluvial counterparts. On the contrary, here we show that, once properly normalized, observed migration rates of tidal and fluvial meanders are remarkably similar. Key to normalization is the role of tidal channel width that responds to the strong spatial gradients of landscape-forming flow rates and tidal prisms. We find that migration dynamics of tidal meanders agree with nonlinear theories for river meander evolution. Our results challenge the conventional view of tidal channels as stable landscape features and suggest that meandering tidal channels recapitulate many fluvial counterparts owing to large gradients of tidal prisms across meander wavelengths.