Bed load fluctuations in a steep channel

Abstract

Bed load transport rate fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions even under constant sediment feeding and water discharge. The observed bed load transport rate pulses are periodic and a consequence of grain sorting. Moreover, the presence of large, relatively immobile boulders, such as erratic stones, which are often present in mountain streams, has an impact on flow conditions. The detailed analysis of a 13 h laboratory experiment is presented in this paper. Boulders were randomly placed in a flume with a steep slope (6.7%), and water and sediment were constantly supplied to the flume. Along with the sediment transport and bulk mean flow velocity, the boulder protrusion, boulder surface, and number of hydraulic jumps, which are indicators of the channel morphology, were measured regularly during the experiment. Periodic bed load transport rate pulses are clearly visible in the data collected during this long-duration experiment, along with correlated fluctuations in the flow velocity and bed morphology. The links among the bulk velocity, the time evolution of the morphology variables, and the bed load transport rate are analyzed via correlational analysis, showing that the fluctuations are strongly related. A phase analysis of all observed variables is performed, and the average shapes of the time cycles of the fluctuations are shown. Observations indicate that the detected periodic fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, which varies in time and space, clearly influences these bed load transport rate pulses. Finally, known bed load transport rate formulae are tested, showing that only the application of a drag shear stress allows a correct estimation of the time fluctuations.

Analysis of mechanical-hydraulic bedload deposition control measures

“…2A). The rough surface of the channel consisted of grains cast in mortar with the size of the D 84 of the supply mixture or larger, as a representative grain size for roughness (Ferguson, 2007;Ghilardi et al, 2014;Rickenmann and Recking, 2011;Zimmermann, 2010). The water-sediment mixture was separated at the downstream model end by a filter basket.…”

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

“…The absence of vertical sorting does not permit the creation of coarse plane bed structures, the subsequent destruction of which causes bedload fluctuations (cf. Section 3.1; Ghilardi, 2014;Ghilardi et al, 2014b). The attenuation of fluctuations is rapid at first (increase in low values of stream power), but quickly reaches a lower limit for a stream power of~15-20 W m −2 .…”

“…Both phenomena mentioned by Iseya and Ikeda (1987), grain sorting and bedform migration, were observed (cf. Ghilardi, 2014;Ghilardi et al, 2014b).…”

“…At the bedload transport corresponding to average values (before and after a peak), the surface grain size distribution corresponds to that of the supplied sediments. This process is described in detail in Ghilardi et al (2013Ghilardi et al ( , 2014b and Ghilardi (2014).…”

“…The definition of the period of a bedload pulse is based on a correlation analysis similar to the one performed in Ghilardi et al (2014b) and Ghilardi (2014). Fig.…”