THE GROWTH AND EVOLUTION OF RIVER-DOMINATED DELTAS AND THEIR DISTRIBUTARY NETWORKS

Open Access
Author:
Edmonds, Douglas Arthur
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 17, 2009
Committee Members:
  • Dr Rudy Slingerland, Dissertation Advisor
  • Rudy Slingerland, Committee Chair
  • Eric Kirby, Committee Member
  • Richard B Alley, Committee Member
  • David N Hill, Committee Member
Keywords:
  • bifurcation
  • avulsion
  • delta
  • delta stability
  • delta growth
Abstract:
River-dominated deltas are dynamic environments and because they are home to a significant fraction of the world’s population we need to understand their growth and evolution so their behavior can be predicted and hazards can be mitigated. Here, using a combination of numerical modeling, physical experiments, and field data, I investigate the processes that participate in the growth and evolution of river-dominated delta channel networks. Using physical experiments I document that deltaic avulsions are caused by an upstream migrating wave of sedimentation that is triggered by a stagnated river mouth bar. An avulsion occurs at the levee location where the greatest average shear stress has been exerted for the longest time. The subsequent evolution of the delta network is a function of the configuration and stability of the individual bifurcations that divide water and sediment. Numerical modeling experiments show that the discharge ratio in the downstream bifurcate channels is a function of the Shields number in the upstream channel. There are two equilibrium functions where one defines symmetrical configurations (equal partitioning of discharge), while the other two define asymmetrical configurations (unequal partitioning of discharge). A network of equilibrium bifurcations is stable to perturbations. Using numerical experiments and field data I show that delta networks are generally stable to perturbations in the form of a closure of a bifurcate channel. Interestingly though, the effect of that perturbation redistributes the water and sediment fluxes throughout the delta, which has the potential to change the long term evolution of a delta. Most notably, these results have implications for engineering fluvial systems. For example, in the 1960s it was thought that all the water from the Mississippi River would flow down the Atchafalaya River. To stabilize this bifurcation a control structure was built to regulate the discharge distribution. The results in this thesis suggest that perhaps a stable bifurcation could have been designed without the aide of control structures.