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

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 inmountain 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 andwater discharge for three different slopes (S= 6.7%, 9.9%, and 13%). Sediment transport, bulkmean flowvelocities, and variables describing the channel-bed morphologywere measured regularly during the experiments. Periodic bedload pulses were clearly visible in all of the experiments, alongwith flowvelocity and channel-bed morphology fluctuations. Correlation analysis showed that the durations of these cycleswere similar, although theywere 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 showthat for higher streampower 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.