Abstract
As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.
Highlights
Seldom can rivers in nature be observed to be following a straight path, and they tend to flow in meandrous or braided patterns
As a result of the interaction between the longitudinal and secondary flows in bent paths and the generation of helical flows [4], the flow’s force becomes more complex. This leads to more sediment transport than that occurring in upstream straight paths
The downstream bend’s presence influences bed topography variations and changes in incipient motion conditions along the upstream bend; In the downstream bend, in addition to the downstream straight path’s influencing the incipient motion conditions, the upstream bend geometry affects bed topography variations in the downstream bend; There are more bed topography variations created in the downstream bend than in the upstream bend; The maximum sedimentation height occurs near the inner bank, and the maximum scour depth occurs near the outer bank
Summary
Seldom can rivers in nature be observed to be following a straight path, and they tend to flow in meandrous or braided patterns. Hydrodynamic forces are applied onto sediment particles on the river bed through the river path. If the flow force can overcome bed sediments’ weight force, this process will result in sediment transport [3]. As a result of the interaction between the longitudinal and secondary flows in bent paths and the generation of helical flows [4], the flow’s force becomes more complex. This leads to more sediment transport than that occurring in upstream straight paths. Studying and understanding sediment transport and sediment incipient motion mechanisms in bends require more investigation than studying those in straight channels
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