Effect of Slope on the Stability of Riverbanks with a Mobile Bed: An Experimental Study

Document Type : Research Article

Authors

1 Department of Civil Engineering- Water Resources Engineering and Management, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran

2 Department of Water Engineering, Water Sciences and Environment Research Center, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran

3 Assistant Professor, Department of Civil Engineering- Water Resources Engineering and Management, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran

4 Assistant Professor, Department of Civil Engineering- Water Resources Engineering and Management, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran.

Abstract

Introduction
In general, bank protection methods are divided into two groups: indirect protection methods and direct protection methods. Changes in river banks are used in direct protection methods. In what follows, national and international research on the protection and stability of river banks using direct and indirect methods is presented. Depending on the channel bed, bank slopes and water surface slope, the gravity and buoyancy forces can either cause resistance to or provocation of overturning. In previous studies, the effect of the moving bed has not been considered, and most attention has been paid to the sloped bank wall and riprap particles
Methodology
In this research, the effect of the riverbank wall slope and river hydraulics on the stability of the river wall in moving bed conditions has been investigated. For this purpose, the influencing parameters in the study should be extracted. After determining these parameters, effective relationships are determined using the techniques used in dimensional analysis. In order to carry out the experiments, a flume measuring 15 meters long, 90 cm wide and 60 cm high was built at the Sediment Research Center of Khuzestan Water and Power Organization, Ahvaz, Iran, with a closed system with the possibility of providing a flow rate of up to 80 liters per second. In addition, the flow rate was measured by an electromagnetic flow meter with an accuracy of ±0.02 liters/second at the entrance of the flume. In the first and last 3 meters of the flume, the floor and walls of the flume were stabilized with concrete mortar, and in the middle 8 meters, the floor and walls of the riverbank (wall with three different slopes) were filled with river sediments with a grain size of D50 = 0.2 and D90 = 0.3 mm.
Results and Discussion
In this research, a total of 21 experiments were conducted in the laboratory of the Sediment Research Center of Khuzestan Water and Power Organization on a river model with a moving bed in order to investigate the effect of the bank slope and flow rate on the stability of riverbanks with a moving bed. Most previous reports have focused on solid beds; however, in this research, a solid bed was not considered, and the bed conditions were the same, similar to the riverbank conditions. In this research, the scouring and bank destruction processes were very diverse, as in real rivers. The conditions were stable up to a certain limit (Frd=0.21 and U/U*=1), and no changes or destruction occurred with time (up to 10 hours). After this limit, the foot of the slope started to erode, and the riverside sediments slowly entered the erosion hole, where the sediments were washed downstream. Finally, after a while, the lateral slope started to overturn and collapse. At more than U/U*=1.70, the lateral slope completely disappeared and became level with the bed. According to the charts and graphs, the riverbank with a slope of 20 degrees is destroyed later than the other two slopes (25 and 30 degrees). As the Froude number increases above 0.2, all the bank slopes become unstable, although this instability occurs later for the bank with a lower slope than for the bank with a higher slope in terms of the time parameter. On the other hand, the results showed that with the increase in the Froude number of Frd particles, the failure time Tf decreases due to the distance from the motion threshold of bed and wall particles. As the flow speed increases, the instability increases, and bank destruction occurs in a shorter time.
Conclusion
The results showed that the riverbank with a slope of 20 degrees is destroyed later than the other two slopes (25 and 30 degrees). As the Froude number increases to 0.2, all riverbank slopes reach instability. However, this instability occurs later for the bank with a lower slope than for the bank with a higher slope in terms of the time parameter. Besides, with the increase in the Froude number of particles Frd and the flow depth, the collapse time Tf decreased due to going farther from the incipient motion of the bed and wall particles.

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References

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History

  • Receive Date: 10 September 2023
  • Revise Date: 27 October 2023
  • Accept Date: 01 November 2023

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