Articles | Volume 4, issue 3
Earth Surf. Dynam., 4, 549–566, 2016
Earth Surf. Dynam., 4, 549–566, 2016

Research article 20 Jul 2016

Research article | 20 Jul 2016

A reduced-complexity model for sediment transport and step-pool morphology

Matteo Saletti1, Peter Molnar1, Marwan A. Hassan2, and Paolo Burlando1 Matteo Saletti et al.
  • 1Institute of Environmental Engineering, ETH Zurich, Zurich, Switzerland
  • 2Department of Geography, The University of British Columbia, Vancouver, BC, Canada

Abstract. A new particle-based reduced-complexity model to simulate sediment transport and channel morphology in steep streams in presented. The model CAST (Cellular Automaton Sediment Transport) contains phenomenological parameterizations, deterministic or stochastic, of sediment supply, bed load transport, and particle entrainment and deposition in a cellular-automaton space with uniform grain size. The model reproduces a realistic bed morphology and typical fluctuations in transport rates observed in steep channels. Particle hop distances, from entrainment to deposition, are well fitted by exponential distributions, in agreement with field data. The effect of stochasticity in both the entrainment and the input rate is shown. A stochastic parameterization of the entrainment is essential to create and maintain a realistic channel morphology, while the intermittent transport of grains in CAST shreds the input signal and its stochastic variability. A jamming routine has been added to CAST to simulate the grain–grain and grain–bed interactions that lead to particle jamming and step formation in a step-pool stream. The results show that jamming is effective in generating steps in unsteady conditions. Steps are created during high-flow periods and they survive during low flows only in sediment-starved conditions, in agreement with the jammed-state hypothesis of Church and Zimmermann (2007). Reduced-complexity models like CAST give new insights into the dynamics of complex phenomena such as sediment transport and bedform stability and are a useful complement to fully physically based models to test research hypotheses.

Short summary
This study presents a new reduced-complexity model with few parameters linked to basic physical processes, which aims to reproduce the transport of sediment as bed load and the formation and stability of channel morphology in steep mountain streams. The model is able to simulate the formation and stability of steps, bed structures commonly encountered in steep channels, by assuming that their formation is due to intense sediment transport during high flows causing jamming of particles.