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Preliminary evidence on laboratory experiments to detect the impact of transient flow on bedload transport

Przyborowski Łukasz, Nones Michael, Mrokowska Magdalena, Książek Leszek, Phan Cong Ngoc, Strużyński Andrzej, Wyrębek Maciej, Mitka Bartosz, Wojak Szymon

Acta Geophysica

2022

Vol. 70, no 5

2311--2324

Trapezoidal-shaped hydrographs are typical of anthropized rivers, as this form is generally associated with the release of water from hydropower dams. To investigate how such unnatural waves can affect bedload rate, preliminary fume experiments were performed in Krakow, Poland, looking at bedload transport rate, bed shear stress and bed morphology. In addition, close-range bed surface photogrammetry was used to investigate bed changes due to the passage of the food wave. Three scenarios, having the same water volume but different wave magnitudes, were tested. The lowest wave showed almost no sediment transport and no visible changes in bed morphology, while higher waves changed the bed morphology, creating erosion and accumulation zones. The highest wave was characterized by an 8-shaped hysteresis of the bedload rate with a peak during the wave maximum. The lag time between the maximum bedload rate and the wave plateau was longer than expected due to zero-slope conditions.

Effects of sampling configurations of acoustic Doppler velocimeter (ADV) on turbulence measurements around large roughness elements

Golpira Amir, Baki Abul B. M., Azimi Amir H.

Acta Geophysica

2022

Vol. 70, no 5

2251--2267

In this experimental study, the effects of sampling strategies on the acoustic Doppler velocimeter (ADV) measurements in clear water flow around a boulder, including the near-bed and boulder’s wake regions, were investigated. This study found that adjusting sampling volume heights and frequencies noticeably influenced the mean and turbulent flow characteristics, especially measurements of time-averaged velocity and the Reynolds shear stress near the bed. Increasing sampling frequency and reducing sampling volume size improve ADV quality parameters while it increased the noise variances. Generally, the recorded mean and turbulent flow parameters with the smallest sampling volume and the highest frequency showed the least sensitivity to applying a signal correlation filter. Assuming the normality of true velocity data, this study found that by keeping at least 70% of data, average signal correlation thresholds as low as 57% and 40% can be applied for filtering of measurements in the near-bed and boulder’s wake regions, respectively. These findings indicated that applying common strict signal correlation thresholds is not necessarily the best practice to filter velocity data in the near-bed and turbulent regions.

Experimental Study on Velocity Profiles with Different Roughness Elements in a Flume

Wang X.-K., Ye Ch., Wang B.-J., Yan X.-F.

Acta Geophysica

2015

Vol. 63, no. 6

1685--1705

The classical log law for velocity profile is applied to engineering practice. Field observations indicate that the composition of the bed materials obviously influences the shape of vertical velocity distribution. To clearly understand the roughness effect, six types of materials were laid separately at various depths for the investigation of the effects of roughness elements on the vertical velocity distribution. A down-looking 3D acoustic Doppler velocimeter was used to measure the velocity profiles. The experimental results showed that the curve characteristics of velocity profiles are strongly dependent on the roughness scale and related flow parameters. If d/R, Fr, and Re are larger than 0.15, 0.47, and 60 000, respectively, the velocity distribution may resemble an S-shape profile. The inflexion position Z*/H for a given S-shape profile was empirically deduced as Z*/H = -0.4481d/R + 0.3225. Otherwise, the velocity profile agrees well with the logarithmic law. The findings of this study are useful in engineering practice (i.e., depth-averaged velocity and flow rate estimate).