Simulation of River Bluffs and Slip-Off Slopes With a Discrete Particle-Based Model
Abstract
A discrete particle-based model simulates evolution of two-dimensional valley cross sections similar to those produced by bedrock meandering rivers and thereby suggests that characteristic features such as overhanging cliffs and talus slopes are dependent on specific relationships among process rates. Discrete coordinates on a gridded cross-section define locations of particles of intact bedrock, sediment (loose material with half the bulk density of bedrock), water, or air on that grid, and each particle of rock or sediment has a unique (or zero) concentration of terrestrial cosmogenic nuclides (TCNs). Stochastic processes determine both the possible locations of process actions and the results of those actions. Stochastic discharges generate boundary shear stresses, calculated by an approximation to the ray-isovel model, that determine removal probabilities for candidate particles of bedrock or sediment from the boundary of a self-formed channel. An asymmetric probability distribution governs the selection of candidate particles on the wetted perimeter and drives asymmetric fluvial erosion and transport that can undermine adjacent slopes, so that the channel migrates laterally. Sediment is produced from intact bedrock by weathering and rock fall. The latter acts only on candidate bedrock particles that are undermined and exposed at the surface. Weathering produces two sediment particles from one of bedrock, and thereby inflates the surface, when slope-normal random walks from candidate sites on the surface end at bedrock particles, so that the sediment-bedrock interface is irregular and discontinuous. Diffusive transport moves candidate particles on random walks in random directions along the surface, where transition probabilities depend on local topography. TCNs are produced when the randomly situated and oriented random walks of cosmic rays end at bedrock or sediment, and not water, particles. The model produces asymmetric channels and valley cross sections, where the two slopes have contrasting bedrock lowering rates, regolith thicknesses, TCN concentrations, and gradients. In simulated valleys, talus-mantled slopes grade smoothly into steep outer channel banks at the bases of overhanging cliffs, all with small TCN concentrations, and thickly mantled slip-off slopes grade smoothly into shallow inner banks similar to point bars, all with large TCN concentrations. Dimensional analysis suggests, and simulations confirm, relationships that can be tested in the field: cliffs form when bedrock lowering due to weathering is small relative to fluvial bedrock and weathering rate is small relative to fluvial transport capacity; for a given slope length, smaller rock-fall rates produce greater cliff heights; and greater fluvial transport capacity relative to sediment production by bedrock lowering produces thinner talus.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP43F..07L
- Keywords:
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- 1825 HYDROLOGY Geomorphology: fluvial;
- 1826 HYDROLOGY Geomorphology: hillslope;
- 1862 HYDROLOGY Sediment transport;
- 1886 HYDROLOGY Weathering