Influence of alluvial slope on avulsion in river deltas

Abstract. Changing 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 as to whether avulsion timing 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 m m−1) that is representative of global deltas, 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 find strong negative correlations between sediment load avulsion timescale and sediment load initial alluvial slope. Sediment load is directly dependent on the upstream alluvial slope, and increases in this slope raise transport capacity and introduce more sediment into a delta plain, leading to higher aggradation rates and, consequently, more frequent avulsions. These results introduce further debate over the role of downstream controls on delta avulsion.