Fluvial-tidal Interplay Captures the Loopy Morphology of Real Coastal Channel Networks

Abstract:

Many of Earth's river deltas and marshes present loops in the form of islands surrounded entirely by river or tidal channels. Recent studies have made advances in characterizing delta topology and flow dynamics, yet no works have sought to provide an explanation for loops' emergence and long-term stability. We have adapted a network model for vascular development, previously used in the context of biological systems, to reproduce the loopiness found in coastal channel networks by varying a dimensionless parameter T* that represents the ratio between tidal and fluvial fluxes. Balance between these influences produces maximally loopy systems in simulations, between T*=1-10. At either limit of overwhelming river current (T*<1) or tidal flux (T*>100), our model analytically recovers the well-explored optimal channel network model of treelike, branching networks with steady state flow. We test our model by analyzing 16 real-world deltaic channel networks and find a similar pattern wherein maximum loopiness occurs where riverine and tidal flux inputs are approximately balanced (T*=1-10). As a proof of principle that fluctuations in hydrodynamic control are capable of resulting in loops, our simple model provides insight into the effects of changing, dynamic flows on landscape evolution. With better understanding of this phenomenon, we build towards the ability to attribute or predict change within these complex systems from hydroclimatic or anthropogenic forcing.