Investigating and quantifying tidal effects on the formation of bifurcated channel networks in modern river deltas

Deltas are unique landforms that develop where rivers debouch into standing bodies of water such as oceans or lakes. Characterized by intricate networks of interconnected channels, they evolve in response to dynamic environmental factors, including sediment particle size, sediment supply, vegetation growth, waves, tides, and climate change. Among these factors, tidal currents play a significant role by continually modifying delta morpholodynamics. However, quantitative measures for assessing tidal influence on delta morphology remain challenging and are poorly understood. Here, we conducted hydro-morphodynamic modeling using Delft3D, varying tidal amplitude and the ratio of mud and sand supply to capture changes over a broad range of timescales. Furthermore, we measured bifurcation lengths, the distances between two adjacent bifurcation points along the channel centerlines in deltaic channel networks, and analyzed the spatial pattern of these lengths. The results indicate that higher tidal amplitude leads to a spatial increase in bifurcation length with bifurcation orders and that a higher proportion of muddy composition responds more sensitively to the tidal effects. Channel geometry, governed by fluid flow properties and sediment compositions, and the evolution of mouth bars collectively explain the observations in this study. We propose that stronger tidal currents and cohesive sediment composition facilitate channel deepening and narrowing, ultimately increasing the advection length and thus bifurcation length. Our study aims to elucidate the spatial pattern of branching channel networks, providing a quantitative measure compared to conventional methods for predicting delta morphology. Building on these findings, we can further enhance our understanding of how channel networks evolve across global scales under a variety of coastal processes.