Laboratory Study of the Effects of grain size and Boundary Reynolds Number on Displacement Number and Stability Number Against Relative Roughness on Movement Threshold in River Bed Loads

Subject Areas : Water resources management

Alireza Zamani Nouri ¹ , Jalal Soltani parishan ² , Hossein Hassanpour Darvishi ³

1 - Associate Professor, Department of civil engineering, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran
2 - Department of Civil Engineering, Shahr-e- Qods Branch, Islamic Azad University, Tehran, Iran.
3 - Department of Civil Engineering, Shahr-e- Qods Branch, Islamic Azad University, Tehran, Iran.

Received: 2024-04-20 Accepted : 2024-11-02 Published : 2024-12-11

Keywords: Incipient Motion, Movability Number, Diameter without grain dimension, particle stability number, Relative roughness parameter,

Abstract :

Background and Aim: The presence of suspended sediments in water crossings of water transfer structures leads to a reduction in the flow cross-section and an increase in roughness, which consequently results in decreased discharge of the structure. The rivers in our country carry a higher sediment load compared to rivers around the world. This indicates the severity of erosion, the poor condition of natural resources, excessive pressure on pastures, destruction of forest lands, and inappropriate exploitation of agricultural lands.

Method: One of the important issues that play a fundamental role in river hydraulics is the prediction of threshold conditions and the initial movement of riverbed particles. In general, the moment when particles start to move is referred to as the threshold of movement, which is usually observed visually. Given that clear water conditions are more suitable for observing particle movement, most researchers have studied the threshold of movement under clear water conditions. However, considering that in nature, river water always transports sediments as well, there has not yet been any laboratory research on the effect of suspended sediment concentration on the threshold of movement due to the difficulties in observing particle movement and the challenges in conducting experiments. So, in this article the experiments conducted in the laboratory on five different grain size samples are initially described. The focus became on the threshold of particle movement in non-cohesive sediments in riverbeds under two conditions: clear water and suspended load. A comparison of these two conditions bacame made in a laboratory setting. Subsequently, calculations are performed based on the obtained results, including the dimensionless particle diameter, mobility number, critical shear velocity, particle stability number, boundary Reynolds number, relative roughness parameter, and settling velocity and compared to the comparison of the diagram with the relationships and results of other researchers in the diagram, as well as comparing the data on the provided threshold curves (alternative methods).

  Results: Meanwhile, the results of particles in the range of 0.5 to 15 mm show that the stability number changes in the range of 1.7 to 2.8. The results of the experiments have shown that the conditions of the movement threshold in bed sediments are not only dependent on the size of particles and sediments, but also related to the water. Using the mobility number   as a function of ، has certain advantages over Shields mobility function ( ) and with the increase of relative roughness, a decreasing trend in particle stability is obtained.

Conclusion: As much as the amount of suspended load increases, the critical shear stress of the flow at the threshold of particle movement  increases. Therefore, the greater shear stress in the state of suspended load compared to the shear stress of clear water will lead to earlier observation of the movement of bed particles in the state of suspended load. With the increase of the suspended load in each section of the river, the destructive effects and sedimentation increases in the upstream.

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