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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 May 2018

Research article | 04 May 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).

Fluvial response to changes in the magnitude and frequency of sediment supply in a 1D model

Tobias Müller and Marwan Hassan Tobias Müller and Marwan Hassan
  • Department of Geography, The University of British Columbia, Vancouver, BC, Canada

Abstract. In steep headwater reaches, episodic mass movements can deliver large volumes of sediment to fluvial channels. If these inputs of sediment occur with a high frequency and magnitude, the capacity of the stream to rework the supplied material can be exceeded for a significant amount of time. To study the equilibrium conditions in a channel following different episodic sediment supply regimes (defined by grain size distribution, frequency, and magnitude of events), we simulate sediment transport through an idealized reach with our numerical 1D model BESMo (Bedload Scenario Model), which was configured using flume experiments with a similar scope. The model performs well in replicating the flume experiments (where sediment was fed constantly, in 1, 2 or 4 pulses) and allowed the exploration of alternative event sequences. We show that in these experiments, the ordering of events is not important in the long term, as the channel quickly recovers even from high magnitude events. In longer equilibrium simulations, we imposed different supply regimes on a channel, which after some time leads to an adjustment of slope, grain size, and sediment transport that is in equilibrium with the respective forcing conditions. We observe two modes of channel adjustment to episodic sediment supply. 1) High-frequency supply regimes lead to equilibrium slopes and armouring ratios that are like conditions in constant feed simulations. In these cases, the period between pulses is shorter than a fluvial evacuation time, which we approximate as the time it takes to export a pulse of sediment under average transport conditions. 2) In low-frequency regimes the pulse period (i.e. recurrence interval) exceeds the fluvial evacuation time, leading to higher armouring ratios due to longer exposure of the bed surface to flow. If the grain size distribution of the bed is fine and armouring weak, the model predicts a lowering in the average channel slope. The ratio between the fluvial evacuation time and the pulse period constitutes a threshold that can help to quantify how a system responds to episodic disturbances.

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Tobias Müller and Marwan Hassan
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Tobias Müller and Marwan Hassan
Tobias Müller and Marwan Hassan
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Latest update: 18 Sep 2018
Publications Copernicus
Short summary
We used a numerical model to study how mountain streams adjust to series of sediment input events, e.g. landslides. We first recreated laboratory stream experiments with similar scope and then expanded their parameter space, allowing us to find when the stream becomes overloaded with sediment. This occurs when the stream cannot evacuate a sediment input in the time between pulses. Our results can help to better understand the long term adjustment of mountain streams to episodic sediment supply.
We used a numerical model to study how mountain streams adjust to series of sediment input...