Mechanics of River Avulsions on Lowland River Deltas

Citation

Chadwick, Austin John (2020) Mechanics of River Avulsions on Lowland River Deltas. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/97C1-JG94. https://resolver.caltech.edu/CaltechTHESIS:02032020-134346875

Abstract

Lowland deltas are home to over 0.5 billion people and some of the most biodiverse ecosystems on Earth. Deltas are highly dynamic landscapes, and at the largest scale grow through repeated construction of depositional lobes punctuated by river avulsions – abrupt shifts in river course to the shoreline. River avulsions have been responsible for dangerous floods and civil unrest over human history, but also counter land loss due to sea-level rise and coastal subsidence by nourishing wetlands with sediment. Despite the central role avulsions play on lowland deltas, the processes controlling their location and frequency remain poorly understood compared to steeper environments such as alluvial fans. This thesis is focused on the mechanics of river avulsions on lowland deltas, and the factors controlling their location and frequency. Chapter 1 addresses the origin of a preferential avulsion site on river deltas, using a novel modeling framework that unites previous work to incorporate backwater hydrodynamics, river-mouth progradation, relative sea-level rise, variable flood regimes, and cycles of lobe construction, abandonment, and reoccupation. Chapter 2 focuses on changes to avulsion frequency caused by relative sea-level rise, incorporating a combination of theory, field data, and numerical modeling. Chapter 3 explores general model predictions for avulsion location and timing during climate change, including rising and falling sea level, imbalances in upstream water and sediment supply, and the magnitude and frequency of storm events. Finally, Chapter 4 presents a scaled laboratory experiment where models and theory for lowland delta avulsion mechanics were put to the test. The work presented in this thesis offers new tools to predict river avulsions on densely populated lowland deltas, and allows for comparison with existing models of coastal restoration that fail to account for river avulsion mechanics and the hydrodynamics of lowland rivers.

Thesis Files