The effects of fluvial and basal sediment properties on the morphodynamics of deltas undergoing sediment supply reduction

Deltas are home to hundreds of millions of people worldwide and form a key part of many coastal environments. Due to their low elevation, many deltas are threatened by sea level rise as well as direct human influences on flow and subsidence. Added to this, the volume of sediment exported by rivers to the coast has been reduced by around 1.4 billion tons per year, starving deltas of the building material needed to construct and maintain their valuable subaerial land in the face of these challenges. The calibre and cohesivity of sediment have both been shown to be important factors in determining the erosion, deposition and stability regimes within a delta system. However, it has not yet been shown how the qualities of river and substrate sediment affect how resilient deltas are to sediment reduction.

This study uses numerical modelling to investigate how the cohesivity of incoming river sediment and the erosion resistance of the delta’s substrate affect how deltas respond to a reduction in supplied sediment. Delft3D was used to create a series of stable deltas with varying fluvial and basal sediments, that where then exposed to sediment reduction. The loss of land area, change in channel geometry and other metrics where extracted from model output using Matlab to assess the effects of this sediment reduction, and how these effects varies between deltas.