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

Spatial and temporal patterns of sediment storage over 45 years in Carnation Creek, BC, a previously glaciated mountain catchment

“…These results collectively suggest that the timescale of adjustment to a major pulse of sediment can be quite long (>30 years in this system) in close proximity to the input location, but will vary substantially over short distances. The specific timescale of adjustment will depend on the magnitude of the sediment delivery event, grain size distribution of the delivered material, frequency of slope failures and transport capacity of the system (e.g., Mueller and Hassan, ).…”