HR: 1340h
AN: H43D-0524    [Abstracts]
TI: Normal and Anomalous Dispersion in Fluvial Sediment Transport
AU: * Bradley, D N
EM: nate.bradley@colorado.edu
AF: CIRES and Department of Geological Sciences, University of Colorado, Campus Box 399, Boulder, CO 80309
AU: Tucker, G E
EM: gtucker@cires.colorado.edu
AF: CIRES and Department of Geological Sciences, University of Colorado, Campus Box 399, Boulder, CO 80309
AB: Understanding the rate of motion and pattern of dispersion in fluvial sediment transport is essential for a variety of applications, including predicting the fate and transport of solid-phase contaminants and modeling the cosmogenic-nuclide inheritance of water-borne sediment. In order to create a probabilistic model of sediment particle motion, it is necessary to characterize the statistical properties of fluvial sediment dispersion. In general, two modes of behavior have been observed in advective-diffusive transport systems: normal and anomalous dispersion. Normal dispersion is characterized by a well-defined mean position and spatial variance and the time evolution of particle concentration is described by a simple advection-diffusion equation. In contrast, a transport system that exhibits anomalous dispersion will tend to have a heavy-tailed spatial distribution, a mean position that is different from the peak concentration, and a large variance. The fundamental difference lies in the probability distribution of individual particle velocities. When the distribution is sufficiently heavy-tailed, the resulting dispersion pattern will be anomalous. Anomalous dispersion has been observed in geophysical systems ranging from turbulent flow to transport in heterogeneous porous media. Several lines of evidence from the sediment transport literature suggest that fluvial sediment may undergo anomalous dispersion. Tracer experiments show a preference for right-skewed travel distance distributions, a characteristic of anomalous diffusion. Studies suggest that large inputs of sediment to rivers (such as a landslide) tend to disperse in place rather than translate downstream. In addition, the fact that sediment grains can become trapped in flood plains and bars for long periods of time and then move long distances in rare, short duration events such as floods suggests a potential for anomalous dispersion due to a broad distribution of particle residence times. We develop a random-walk model for bedload and suspended-load motion. Particles alternate between entrainment and deposition and the transition between these states is governed by a probability distribution of instantaneous shear stress and an entrainment threshold. Once a particle is entrained, its motion is governed by probability distributions describing turbulent velocity fluctuations. This model provides a framework to explore the conditions under which rivers are likely to exhibit anomalous sediment dispersion.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1862 Sediment transport (4558)
DE: 3245 Probabilistic forecasting (3238)
DE: 3265 Stochastic processes (3235, 4468, 4475, 7857)
SC: Hydrology [H]
MN: Fall Meeting 2005