Author(s): Terunori Ohmoto; Kanji Adachi

Linked Author(s): Terunori Ohmoto

Keywords: Hyper-concentrated sediment; Flow resistance; Particle image velocimetry; Large-scale organized vortices; The Q criterion

Abstract: Flow resistance in an open channel is considerably affected by not only bedform and the size, shape and relative positions of roughness elements, but also by the presence of sediment being transported and the hydraulic conditions. Flow resistance in a river channel occurs as a macroscopic consequence of interacting elements, namely, the river channel, the sediment being transported and the flow, and the elucidation of their mechanisms and the establishment of methods for predicting them constitute one of the most important subjects of research in the field of flood control. Those objectives may contribute to improved accuracy in the prediction of the relationship between the level of the water, the flow rate in a river, and the amount of sediment transported in a river. However, as existing methods depend mostly on empirical assumptions, universal mechanisms remain mostly unknown. This study deals with the influence of hyper-concentrated sediment-laden flow on flow resistance and momentum transport in open channel flow over two-dimensional prismatic roughness elements. Momentum transport was examined in detail by using kaolin and a poly sodium acrylate (PSA) solution resembling hyper-concentrated sediment-laden flow in viscosity characteristics, measuring flow velocity by particle image velocimetry (PIV) and comparing the artificial flow field thus produced with the clear water flow field. In this study, experiments using poly sodium acrylate (PSA) solutions, which resemble hyper-concentrated sediment-laden flow in viscosity characteristics, were conducted by applying particle image velocimetry to investigate the resistance characteristics and mechanism of hyper-concentrated sediment-laden flow over two-dimensional prismatic roughness elements by comparison with clear water flow. The results obtained from this study can be summarized as follows: 1) In the case where the roughness element height k is 10 mm (λ/k = 10, λ: the longitudinal spacing of roughness elements), the sediment-laden flow shows lower resistance than the clear water flow when the volumetric sediment concentration CV is lower than 8.5% and higher resistance when CV is higher than 8.5%. 2) In cases where the volumetric sediment concentration CV is lower than 8.5%, resistance at k = 5 mm shows a minimum value when CV is 4% to 6%. At k = 10 mm, a minimum value is shown when CV is 6%, showing a significant decrease in resistance. 3) Stream lines of the time-averaged flow of the PSA solution at the concentrations CW of 300 mg/l (equivalent to the kaolin concentration CV of 6%) and 800 mg/l (equivalent to the kaolin concentration CV of 10%) showed that significant flow separation or circulation did not occur. This shows that form resistance is smaller in the PSA solution (i.e. high viscosity) flow than in the clear water flow.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022616

Year: 2022