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

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.

Co‐evolution of coarse grain structuring and bed roughness in response to episodic sediment supply in an experimental aggrading channel

“…Figure shows the time‐series of sediment transport rate at the outlet for the seven runs (for more details see Elgueta‐Astaburuaga and Hassan, ; Elgueta‐Astaburuaga et al ., ; Müller and Hassan, ). Run R1 was conducted under zero feed conditions: at the beginning of the experiment large amounts of mostly fine gravel were evacuated from the bed surface or perhaps infiltrated between larger bed surface grains into the subsurface.…”

“…Although the range of experimental conditions was limited, numerical simulations conducted by Müller and Hassan () suggest that our results should be broadly applicable to similar aggrading fluvial systems. Müller and Hassan () conducted a set of numerical simulations based on the same flume experiments presented here. They extrapolated the results by systematically varying the sediment feed characteristics to gain insight on the generalization of the results and to find thresholds of the response of the bed to different pulse frequencies and magnitudes.…”

“…In order to compare the sediment supply regimes, we fed the same cumulative amount of sediment (300 kg) in each run. Although the range of experimental conditions was limited, numerical simulations conducted by Müller and Hassan () suggest that our results should be broadly applicable to similar aggrading fluvial systems. Müller and Hassan () conducted a set of numerical simulations based on the same flume experiments presented here.…”

“…Additional experimental details are provided in Elgueta‐Astaburuaga and Hassan (), Elgueta‐Astaburuaga et al . () and Müller and Hassan ().…”

“…Based on these experiments Elgueta-Astaburuaga and Hassan (2017Hassan ( , 2019 and Elgueta-Astaburuaga et al (2018) reported on the impact of episodic sediment supply on sediment transport rate, sediment transport variability, sediment mobility, sediment storage and channel adjustment. Müller and Hassan (2018) developed a numerical model to study the impact of magnitude and frequency of episodic sediment supply on channel adjustment. They used the same experiments to calibrate their numerical model.…”