Collaborative Research: Investigating the coupling between superelevation and gradient advantage in river avulsions

Rivers occasionally experience a process called avulsion when they jump out of their banks and carve a new path across the landscape. The resulting floods are more extreme than typical floods caused by rainfall; avulsions can devastate entire communities. River avulsions occur infrequently, so we have very few observations of them. As a result, the scientific understanding of this important natural hazard lags behind other comparable hazards like earthquakes. For example, the number of people in the United States living in the potential path of river avulsions is not known because we do not even know for sure what conditions prime rivers to avulse. In this project, the team will develop a new unified theory for the processes that prime river avulsion, and test the theory using experiments and observations from satellite images. Two main conditions are thought to destabilize rivers and set them up to avulse. The first is called superelevation (β), where sediment accumulates on the levees and the riverbed, lifting the river above the surrounding floodplain. The second is called gradient advantage (γ), where a steeper alternative path is available to the river. These two conditions have long been considered either mutually exclusive or unrelated. Recent observations have revealed that avulsions occur when the combined values of superelevation and gradient advantage reach a joint threshold; specifically, where β • γ ≈ 2. Based on this, the team will test three hypotheses: 1) the joint threshold for river avulsion arises because alluvial ridge superelevation grows faster on fans and gradient advantage grows faster on deltas; 2) avulsions occur in isolated river reaches where β • γ is locally elevated, and do not occur elsewhere; and 3) the threshold of β • γ for avulsion will decrease with increasing trigger size. To build on this discovery, the project team will develop a new theory for river avulsion setup into a physics-based modeling framework that seeks to model how β and γ evolve over time on a given river. That model will be tested against new remote-sensing observations of how β and γ vary along river reaches, and new lab experiments aimed at testing the controls on the threshold value of β • γ. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.