Temporal variability of sediment transport in steep channels

During my PhD, I studied the interaction between sediment transport and channel morphology in step-pool channels. I analysed experimental data collected at UBC in the context of the PhD thesis of André Zimmermann. André conducted experiments in a steep flume to study the conditions leading to the formation and collapse of steps, using sediment of 12 different grain-sizes.

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Steep flume at UBC (source: PhD thesis of A. Zimmermann)

High-resolution (1 s) time series of sediment transport were measured for individual grain-size classes at the outlet of the flume for different combinations of sediment input rates and flow discharges. I analyzed these data to quantify the relation between discharge and sediment transport and the nature and strength of memory in grain-size-dependent transport in steep channels.

The results show that no simple statistical descriptor of sediment transport (mean, extreme values, and quantiles) display a clear relation with water discharge. Actually a large variability between discharge and sediment transport was observed.

Relation between discharge and sediment transport in two repeated experiments.

I also quantified the strength and nature of memory in sediment transport rates by estimating the Hurst exponent and the autocorrelation coefficient of the time series for different grain sizes. The results show the presence of the Hurst phenomenon in transport rates, with a tendency towards long-term memory which is grain-size dependent. se found that Short-term memory characterizing coarse-grain transport increases with temporal aggregation and this reveals the importance of the sampling duration of bed load transport rates in natural streams, especially for large fractions.

Hurst exponent for sediment transport rates and its dependence on grain size.

Saletti, M., P. Molnar, A. Zimmermann, M. A. Hassan, and M. Church (2015), Temporal variability and memory in sediment transport in an experimental step-pool channel, Water Resour. Res., 51, 93259337, doi:10.1002/2015WR016929.



Modelling step formation with a reduced-complexity model

The core of my PhD has been the development of a new model for step-pool morphology. I developed  a particle-based reduced-complexity model, CAST (Cellular Automaton Sediment Transport) which 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.

CAST: spatial domain and processes.

The model reproduces a realistic bed morphology and typical fluctuations in transport rates observed in steep channels. Notably, particle hop distances from entrainment to deposition,are well fitted by exponential distributions, in agreement with field data.

Main outputs of CAST.

CAST also 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).

Step-pool structures due to jamming generated with CAST.

Saletti, M., Molnar, P., Hassan, M. A., and Burlando, P.: A reduced-complexity model for sediment transport and step-pool morphology, Earth Surf. Dynam., 4, 549-566, doi:10.5194/esurf-4-549-2016, 2016.