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

Tobias Müller,Marwan A Hassan

doi:10.5194/esurf-6-1041-2018

Abstract

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 1-D model “BESMo” (Bedload Scenario Model). The model performs well in replicating flume experiments of a similar scope (where sediment was fed constantly, in one, two, or four pulses) and allowed the exploration of alternative event sequences. We show that in these experiments, the order 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 the 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 decrease 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.

Highlights

Mass movements in mountainous regions often deliver sediment directly to the stream network, resulting in coupled conditions that can trigger immediate channel responses during relatively large delivery events
All runs ended at about the same slope after the 280 h simulation time, which implies that while the low frequency, large magnitude events strongly alter the channel in the short term, the sequencing of events does not play an important role in the long run
To test the effect that different episodic sediment supply regimes can have on the morphology of a mountain stream, we developed a numerical model to recreate and extend simulations from flume experiments

Summary

Introduction

Mass movements in mountainous regions often deliver sediment directly to the stream network, resulting in coupled conditions that can trigger immediate channel responses during relatively large delivery events. The local response rate and trajectory following a delivery event is a function of the prevailing watershed flow regime, the magnitude of the delivery event, gradients in channel width (Ferrer-Boix et al, 2016), and in some instances the concentrated activity of aquatic species such as salmon (Hassan et al, 2008a). Lisle et al (1997) conducted flume experiments and numerical modelling that showed that sediment pulses are mainly reworked in situ, in contrast to a downstream translation in the form of a sediment wave. This finding is supported by Lisle et al (2001), where little evidence of sediment waves was found

Methods

Results

Discussion

Conclusion