Comparing Aggradation, Superelevation, and Avulsion Frequency of Submarine and Fluvial Channels

IN REVIEW IN "FRONTIERS IN EARTH SCIENCE" (30 Aug 2019). Constraining the avulsion dynamics of rivers and submarine channels is essential for predicting the distribution and architecture of sediment, organic matter and pollutants in alluvial, deltaic, and submarine settings. Submarine channels are well known to be more aggradational than rivers, and aggradation of the channel, levee, and floodplain are key forcing mechanisms for avulsion. We create a geometric channel-belt framework relating channel, levee, and floodplain stratigraphy that allows comparative analysis of avulsion dynamics for rivers and submarine channels. We utilize 52 channe...  more

IN REVIEW IN "FRONTIERS IN EARTH SCIENCE" (30 Aug 2019). Constraining the avulsion dynamics of rivers and submarine channels is essential for predicting the distribution and architecture of sediment, organic matter and pollutants in alluvial, deltaic, and submarine settings. Submarine channels are well known to be more aggradational than rivers, and aggradation of the channel, levee, and floodplain are key forcing mechanisms for avulsion. We create a geometric channel-belt framework relating channel, levee, and floodplain stratigraphy that allows comparative analysis of avulsion dynamics for rivers and submarine channels. We utilize 52 channel cross sections within this framework to provide avulsion criteria for submarine channels and how they differ from rivers.
Superelevation and a new channel-floodplain coupling metric are the two key parameters that control channel-belt thickness in both rivers and submarine channels. While rivers can only superelevate an amount equivalent to 1 channel depth above the floodplain prior to avulsion, submarine channels are more stable during aggradation, with superelevation values commonly >3 channel depths. Channel-floodplain coupling in rivers is weak, with floodplain aggradation being negligible compared to channel aggradation. However, floodplain aggradation is more significant for submarine channels, resulting in stronger channel-floodplain coupling.
The combination of enhanced superelevation and strong channel-floodplain coupling results in channel-belts for submarine channels that can be as thick as ~ 10 channel depths, while fluvial channel belts are limited to 2 channel depths. We interpret that levee aggradation and thus superelevation is promoted by turbidity current overspill. Floodplain aggradation is also influenced by overspill and hemipelagic sedimentation. As a submarine channel reaches aggradation that would cause a river to avulse, the submarine channel is stable because the flow has far less potential energy to create an avulsion because turbidity currents have ~50x less density contrast between flow and ambient fluid as compared to rivers. The Amazon channel showcases this stability, with a channel belt that is ~ 5 channel-depths thick for more than 400 streamwise km, more than twice the aggradation that a river is capable of.