[S-CG64] [EE] Morphodynamics and Genetic Stratigraphy for Understanding Landforms and Strata

[SCG64-09] The roles of hydrodynamic backwater and relative sea-level rise in setting deltaic avulsion frequency

*Austin John Chadwick¹, Michael P Lamb¹ (1.California Institute of Technology)

Keywords:Deltas, Avulsions, Sea-level rise

Many of the world’s deltas are built through periods of construction of depositional lobes punctuated by lobe-scale avulsions. Deltaic rivers avulse when backwater effects create a locus of deposition in the river that reaches a critical thickness that scales with the channel flow depth. Recent work suggests that relative sea-level rise can play an important role in setting the pace of aggradation and frequency of avulsions within the backwater zone, but the fundamental relationship between avulsion frequency and sea-level change remains unexplored. We address this knowledge gap using an analytical model and a quasi-2D morphodynamic model. In our preliminary simulations, avulsion frequency increases under faster rates of normalized relative sea-level rise, because more sediment is deposited on the topset relative to the foreset and thus less time is required before the channel avulses. This behavior is well-predicted by our analytical solution, at least for low rates of relative sea level rise. At higher rates of relative sea level rise, avulsion frequency becomes more variable because lobes partially drown between avulsions. Results have implications for the sustainable management of modern deltas undergoing relative sea-level rise and for interpreting avulsions in stratigraphic sequences.