Period and amplitude of bedload pulses in a macro-rough channel

• A set of 38 steep flume experiments with different macro-roughness configurations
• Fluctuations are observed for parameters bedload, flow velocity and bed morphology.
• All the parameters fluctuate with the same period, but a time delay is observed.
• A link between the period and the amplitude of bedload fluctuations is observed.
• The period and amplitude of the fluctuations are linked to the stream power.

It is known that bedload fluctuates over time in steep rivers with wide grain size distributions, even under conditions of constant sediment feed and water discharge. Bedload fluctuations, which are a consequence of grain sorting, are periodic and are related to fluctuations in the flow velocity and channel-bed morphology. The presence of large relatively immobile boulders, such as erratic blocks that are often present in mountain streams, has a strong impact on flow conditions and sediment transport. However, their influence on bedload fluctuations has not been studied. Sediment transport fluctuations were investigated in this study in a set of 38 laboratory experiments carried out on a steep tilting flume under several conditions of constant sediment and water discharge for three different slopes (S = 6.7%, 9.9%, and 13%). Sediment transport, bulk mean flow velocities, and variables describing the channel-bed morphology were measured regularly during the experiments. Periodic bedload pulses were clearly visible in all of the experiments, along with flow velocity and channel-bed morphology fluctuations. Correlation analysis showed that the durations of these cycles were similar, although they were not necessarily in phase. The pulses were characterized by their amplitude and period as a function of various boulder spatial densities and diameters. We could show that for higher stream power the fluctuations decrease in cycle duration and in amplitude. The presence of boulders increases the stream power needed to transport a given amount of sediment, thus decreasing the bedload fluctuations.