CONTROLS ON FLUVIAL LONGITUDINAL PROFILE AND SEDIMENT CHARACTERISTICS: PREDICTIONS FROM A MORPHODYNAMIC MODEL

Open Access
Author:
Fambrough, Brooke Anderson
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 24, 2008
Committee Members:
  • Rudy L Slingerland, Thesis Advisor
Keywords:
  • comminution
  • subsidence
  • sediment transport
  • nonequilibrium transport
  • river
  • fluvial system
  • numerical model
  • variable discharge
Abstract:
Understanding the sources of variation in the longitudinal profiles and sediment characteristics of rivers is important for predicting hydrocarbon reservoir quality, and understanding river responses to climate and tectonic perturbations. Towards this end we have developed a numerical model named ZOE that simulates fluvial nonequilibrium transport and deposition of sand-sized heterogeneous mixtures. Using ZOE we conducted a series of numerical experiments to determine the dependency of longitudinal profiles, and the composition and texture of deposits on subsidence, degree of sediment comminution, sediment supply, and variable hydrograph. The experimental design consists of a standard steady-state control run for each sediment supply scenario to which are compared runs with subsidence, comminution, and simulated flood conditions with increased water discharge (Q) with a matching capacity sediment discharge (Qs) for each sediment/water relation. The water surface elevation at the downstream boundary and the feedstock distribution are held constant for each model run. Results show that climate change and sediment/water relations are the most important parameters determining the longitudinal profiles and sediment characteristics of fluvial and basin deposits. Climate change experiments achieve a dynamic equilibrium in which the bed alternates between aggradation and degradation with the frequency of the varying discharge. Increased Q lowers the bed slope and the fluvial system transports finer sediment relative to the feedstock. With a lower bed slope the system preferentially transports finer sediment to the basin, leaving behind a deposit enriched in feldspar. Climate change augments the effects of comminution and subsidence, which are otherwise negligible at the rates prescribed in this study. Three sediment/water relations were simulated 1) linear case (Qs~=Q), 2) upstream-weighted (Qs~=Q^a when a < 1), and 3) downstream-weighted case (Qs~=Q^a when a > 1). For a linear sediment feed, the longitudinal profile is linear; a slight concavity is achieved only when combined with the variable discharge of either a wet-dry climate (Period = 1 yr) or a stormy climate (Period = ¼ yr). These experiments and a simplified analytic model show that a necessary condition for a concave bed profile is a <1 and that a change to a wetter climate (all other factors being equal) causes a temporary fining of sediment delivered to a basin and in fluvial settings.