Cross-shore parallel tidal channel systems formed by alongshore currents

Abstract

Abstract Tidal channel systems arising from morphodynamic interactions exhibit a suite of diverse morphological configurations. A prevalent type is represented by linear dendritic channels formed by single-thread streams aligned roughly parallel or subparallel to each other in the cross-shore direction. Despite their ubiquity, the processes driving the formation of these parallel channel systems remain elusive. We conducted a morphological analysis of 275 parallel tidal channels from 20 different locations worldwide and found that the angle between individual parallel branches and the parent stream, from which they develop, consistently falls within the range of 80-100º. We employed numerical modeling to shed light on the underlying mechanisms governing their formation, revealing that alongshore uniformity in bed topography and the strength of tidal currents condition the alignment of parallel channels. Cross-shore parallel channels can be formed by alongshore tidal currents, and the channel orientation is largely governed by the shape of the bed profile. Straight and shore-normal parallel branches tend to form where the bed profile changes sharply around the mean sea level, while linearly sloping profiles lead to oblique parallel branches. By unraveling the physics underlying the formation of these striking but poorly understood geomorphic features, our results bear significant ramifications for the understanding and management of valuable tidal ecosystems.