2018 CSDMS meeting-053
Predicting the location of avulsion hazards on deltas in the face of changing discharge regimes and relative sea-level rise
On densely populated deltas, the tendency for river channels to catastrophically avulse poses a hazard to human life and property. Previous work has shown that river avulsions preferentially occur around a spatial node with a distance from the shoreline that is controlled by backwater hydrodynamics, the interplay of dynamic river discharge and standing water near the shoreline. Our ability to forecast the location of future avulsion hazards is limited, however, because avulsions are relatively rare and many deltas are experiencing drastic changes in river discharge and sea level due to land-use and climate change. Building upon previous work, we present a predictive model of delta-lobe morphodynamics and repeated avulsion that is applicable to deltas over a range of spatial scales, sediment supplies, flood regimes, and relative-sea-level-rise conditions. In our model, delta lobes build on top of one another, demonstrating a distribution of avulsion lengths that is sensitive to flow regime and relative sea-level change. Variable flood regimes lead to a consistent avulsion length when low flows (less than bankfull) and high flows (greater than bankfull) compete to intermittently fill and scour portions of the backwater reach. The avulsion node is a spatial maximum in channel superelevation set by the downstream extent of low-flow deposition between erosive high-flows, and in general channels avulse farther upstream when high-flow events are more extreme and more frequent. Relative sea-level rise leads to a more variable avulsion node, driven by intermittent retreat and advance of the delta shoreline as the river periodically shifts the distribution of sediment. If rise rates are sufficiently high to sequester all sediment upstream of the river mouth, avulsions occur progressively farther upstream or not at all. These results have implications for the forecasting of avulsion hazards on modern deltas undergoing relative sea-level rise and human management.