Influence of alluvial slope on avulsion in river deltas

Abstract. Changed hydrological regimes, sea-level rise, and accelerated subsidence are all putting river deltas at risk across the globe. One mechanism by which deltas may respond to these stressors is that of avulsion. Decades of delta avulsion studies have resulted in conflicting hypotheses that avulsion frequency and location are primarily controlled by upstream (water and sediment discharge) or downstream (backwater and sea-level rise) drivers. Here we use Delft3D morphodynamic simulations to test the upstream-influence hypothesis by varying the initial alluvial slopes upstream of a self-formed delta plain within a range (1.13 × 10⁻⁴ to 3.04 × 10⁻³) that is representative of global deltas and recording avulsion, while leaving all other parameters constant. Avulsion timing and location were recorded in six scenarios modelled over a 400-year period. We measured independent morphometric variables including avulsion length, delta lobe width, bankfull depth, channel width at avulsion, delta topset slope and sediment load and compare these to natural and laboratory deltas. We find that larger deltas take more time to avulse as avulsion timing scales with avulsion length, delta lobe width and bankfull depth. More importantly, we also find a strong (p < 0.05) negative correlation between delta topset slope and avulsion timescale. We argue that topset slope is directly dependent on the varying upstream alluvial slope which determines sediment supply to the delta. Increases in upstream alluvial slope raise transport capacity so bringing more sediment into a delta plain, leading to higher aggradation rates and, consequently, more frequent avulsions. These results induce further debate over the role of downstream controls on delta avulsion.