Variability of Bed Load Transport During Six Summers of Continuous Measurements in Two Austrian Mountain Streams (Fischbach and Ruetz)

Abstract

Previous measurements of bed load transport in gravel bed streams revealed a large temporal and spatial variability of bed load transport rates. Using an impact plate geophone system, continuous bed load transport measurements were made during 6 years in two mountain streams in Austria. The two streams have a snow‐melt and glacier‐melt dominated hydrologic regime resulting in frequent transport activity during the summer half year. Periods of days to weeks were identified which are associated with approximately constant Shields values that indicate quasi‐stable bed conditions. Between these stable periods, the position of the bed load transport function varied while its steepness remained approximately constant. For integration time scales of several hours to 1 day, the fluctuations in bed load transport decreased and the correlation between bed load transport and water discharge increased. For integration times of about 70–100 days, bed load transport is determined by discharge or shear stress to within a factor of about 2, relative to the 6 year mean level. Bed load texture increased with increasing mean flow strength and mean transport intensity. Weak and predominantly clockwise daily hysteresis of bed load transport was found for the first half of the summer period.

Bedload transport: a walk between randomness and determinism. Part 2. Challenges and prospects

“…2, one cannot but be amazed by the deviation between the trend shown by the field data Q s ∝ Q 3 w and an MPM-like bedload transport equation Q s ∝ Q w . Field surveys in other gravel-bed rivers have shown similar deviations between field data and empirical bedload equations (Barry et al, 2004;Recking, 2010;Recking et al, 2012;Rickenmann, 2018) of up to as large as one order of magnitude. Various explanations have been suggested, including the poor performance of empirical equations at capturing low sediment transport rates (near the threshold of incipient motion), the effect of grain sorting, surface processes such as bed armouring, the bedload transport rate's nonlinear dependence on flow rate and bed geometry, variations in sediment supply, and the effects of migrating bedforms (Recking, 2012;Yager, Venditti, Smith, & Schmeeckle, 2019).…”

“…A power-law function fitted to the dataQ s = cQ 3 w shows that despite large fluctuations, the mean transport rate is well correlated with water discharge Q w . As found in a number of field investigations (Barry, Buffington, & King, 2004;Rickenmann, 2018), the dependence of Q s on Q w is more pronounced than that given by classic bedload equations, such as the MPM Eq. (6).…”

History‐Dependent Threshold for Motion Revealed by Continuous Bedload Transport Measurements in a Steep Mountain Stream

“…This implies that history dependence in τ c is likely to arise in gravel channels of many forms. We expect that similar patterns of stabilization under low and intermediate flows and disruption of that stabilization under high flows should persist, though the details of this dependence may vary (Rickenmann, ). For example, previous results from Turowski et al () indicate that interevent flows are less effective in modifying channel stability in proglacial streams.…”

Impact of Unsteady Flow Events on Bedload Transport: A Review of Laboratory Experiments

“…One reason for this is that the violent character of unsteady flow is a serious constraint preventing field measurements of sediment transport [15]. Nonetheless, some monitoring of bedload in rivers has been conducted and has provided valuable field data [3,7,16]. However, both flow and transport processes are highly variable in time and space, and observations and measurements of detailed processes, such as dynamics of bed morphology during unsteady flow events, still pose a technical challenge.…”