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.
How plausible are high-frequency sediment supply-driven cycles in the stratigraphic record?
