Author(s): Tamara Ghilardi; Mario J. Franca; Anton J. Schleiss
Keywords: Bedload fluctuations; Macro-roughness elements; Boulders; Steep channel; Wide grain size distribution
Abstract: Bedload fluctuations over time in steep rivers with wide grain size distributions have been observed, even under constant sediment feeding and water discharge. Observed bedload pulses are periodical and a consequence of grain sorting. Along with bedload, flow velocity and bed morphology fluctuate in time as well. The presence of large relatively immobile boulders, such as erratic stones often present in mountain streams, have an impact on flow conditions. Their influence on these bedload fluctuations has not been studied yet, namely in what concerns the frequency and amplitude of bedload fluctuations. The influence of boulders on sediment transport and flow conditions is herein investigated by means of laboratory experiments carried out in a 8 m long and 0. 25 m wide tilting flume. The detailed analysis of one set of measurements obtained during a 12. 8 hours (774 minutes) test is presented in this paper. For this experiment 12 boulders of diameter D=0. 075 m were randomly placed in the flume, corresponding to 3% of the flume surface initially occupied by boulders. The flume slope was set to 6. 7% . Input sediment discharge was constant and equal to qs, in=0. 0563 x10-3 m2/s and the liquid discharge was fixed at q=1. 68 x10-2 m2/s. Sediment transport and boulder protrusion are measured regularly during the experiment. Furthermore, bulk mean flow velocities are measured every 15 minutes. Periodical bedload pulses are clearly visible on this long duration experiment. Bulk velocity and boulder protrusion are linked with the fluctuations of the bedload transport as demonstrated by correlational analysis. The period and the phase of the fluctuations are similar for all the measured variables. Observations indicate that the detected periodical fluctuations correspond to different bed states. Grain size distribution through the channel, varying in time and space, is clearly influencing these bedload pulses. During low sediment discharges, coarse stable riffles are formed. On the other hand, in high sediment fluxes the destruction of riffles followed by bed fining is observed. A detailed description of the experimental observations and an analysis of amplitude and frequency of fluctuations for this long duration test are presented in this article.