Author(s): Fiona Maager; Benjamin Hohermuth; Volker Weitbrecht; Robert Boes

Linked Author(s): Fiona Maager, Benjamin Hohermuth, Volker Weitbrecht, Robert Boes

Keywords: Flow history; Hydrographs; Sediment supply; Bed stability; Scour depth

Abstract: Built step-pool systems are promising to stabilize the bed in steep streams. Flume experiments have already been conducted to assess scour development and stability of such step-pool sequences. However, mainly stationary conditions were tested so far, which hardly represent the processes in steep streams. In this study, three distinct flow loading conditions (LC) were applied to the same step-pool sequence to determine potential differences in scour development and step stability. The 1:20 scaled physical experiments were conducted in a flume with a bed slope of 8%. The base material was selected according to typical Swiss mountain streams with dm = 0.005 m and d₉₀ = 0.0126 m (model scale). Six equally spaced steps were artificially placed into the channel bed. The steps consisted of two rows of blocks with a weight between 1.1 and 1.4 kg and a height of ~0.065 m each. First, a stationary hydraulic load was applied to the step-pool sequence for 80 min (LC A). The unit discharge was stepwise increased by Δq = 0.014 m²/s until the steps collapsed. Second, a series of hydrographs was applied to represent the processes occurring in steep streams more realistically (LC B). Similarly to LC A, the peak discharge of each hydrograph was increased by Δq = 0.014 m²/s until the steps collapsed. The total duration of the hydrographs was set to 54 min, the peak arrived after 13.5 min and lasted for 4.5 min. Due to the shorter peak duration and the descending limb of the hydrographs in LC B, differences in scour depth and stability were expected. LC C was identical to LC B, but a small stationary discharge was applied in between each hydrograph to simulate low flow periods occurring between major flood events. An increase in scour depth was expected in LC C, because the jet of low flows impinges more vertically on the channel bed. The step-pool system sustained a discharge of q = 0.110 m²/s for LC A and C, and q = 0.124 m²/s for LC B. Considering the uncertainties, the step-pool system failed at similar hydraulic loads independently of the LC applied. Major changes in channel bed appeared to occur within 5 to 10 min after the peak discharge was reached. Furthermore, all LC led to a similar scour depth. Even for periods of low flow in LC C, scour depths did not further increase at the toe of the steps. Consequently, applying either one of the LC appears to be reasonable to test step stability. However, all three LC were conducted under clear water conditions and the effect of sediment feed on the stability was neglected. Scour development of step-pool systems with sediment feed will be investigated in future experiments.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022473

Year: 2022