Predominant landward skewing of tidal meanders

Highly sinuous, high-amplitude meander bends shaped by unidirectional, downstream-oriented flows in alluvial rivers are predominantly upstream-skewed, offering an opportunity to infer flow direction from meander planforms. In contrast, it remains unclear whether tidal meander bends formed by bidirectional, potentially asymmetric flows display similar skewing preferences, despite being characterized by planform dynamics similar to their alluvial counterparts. Geomorphological characteristics of meandering channels in different tidal environments thus merit further research. Here we used satellite images to measure meandering patterns for 232 purely tidal channels located at 25 study sites worldwide, to disclose imprints of tidal flows embedded in meander planforms. Our study sites span micro- to macro-tidal settings characterized by different tidal flow asymmetries, and include both vegetated mangrove swamps and salt marshes, as well as unvegetated intertidal flats. We found that tidal meanders exhibit morphological features consistently scaled to local channel width regardless of tidal regimes, and that they are typically less sinuous than fluvial bends. Exponential landward reduction of channel widths (i.e. funnelling) is counterbalanced by a linear increase in width-adjusted bend amplitude that is more pronounced in ebb-dominated environments. More importantly, the majority of tidal bends appear to be preferentially landward-skewed, and the degree of skewing increases as bend sinuosity and amplitude increase as well. Although the mechanism controlling such skewing preference remains elusive and further fieldwork is needed to elucidate this point, we hypothesize that it might be related to the local dominance of ebb flows characterizing most tidal environments. Overall, our results demonstrate a statistical preference for landward skewing in high-amplitude, highly sinuous tidal meander bends that does not depend on the site-specific characteristics of the considered tidal environment and can be potentially employed to infer the direction of tidal flows based exclusively on remotely sensed meander planforms.