Experimental Analysis of Incipient Motion for Uniform and Graded Sediments

Khabat Khosravi,Amir H N Chegini,Patricia M Saco,Andrew D Binns,Luca Mao,Jose F Rodriguez

doi:10.3390/w13131874

Abstract

So far, few studies have focused on the concept of critical flow velocity rather than bed shear stress for incipient sediment motion. Moreover, few studies have focused on sediment mixtures (graded sediment) and shape rather than uniform sediment for incipient motion condition. Different experiments were conducted at a hydraulic laboratory at the University of Guilan in 2015 to determine hydraulic parameters on the incipient motion condition. The aim of this study is to conduct a comparison between uniform and graded sediments, and a comparison between round and angular sediments. Experiments included rounded uniform bed sediments of 5.17, 10.35, 14 and 20.7 mm, angular uniform sediment of 10.35 mm, and graded sediment. Results demonstrated that angular sediment has a higher critical shear velocity than rounded sediment for incipient motion. Results also showed that for a given bed sediment, although critical shield stress and relative roughness increased with the bed slope, the particle Froude number (based on critical velocity) decreased. In terms of the sediment mixture, the critical shear stress (Vc*) was higher for the graded sediment than for the three finer uniform sediment sizes. The finer fractions of the mixture have a higher particle Froude number than their corresponding uniform sediment value, while the coarser fractions of the mixture showed a lower stability than their corresponding uniform sediment value. Results demonstrated that the reduction in the particle Froude number was more evident in lower relative roughness conditions. The current study provides a clearer insight into the interaction between initial sediment transport and flow characteristic, especially particle Froude number for incipient motion in natural rivers where stream beds have different gravel size distribution.

Highlights

We used the statistical package for social sciences (SPSS) to compute the Pearson correlation coefficient, a parametric method measuring the linear correlation between two variables X and Y as follows: N ∑ XY − ∑ X ∑ Y
The Pearson correlation coefficient shows that the highest correlation coefficient for sediment transport was found with flow velocity (0.757), followed by shear stress (0.717), bed slope (0.504), sediment diameter (0.315), and flow depth (0.269), with p-values of 0, 0.007, 0.02, 0, and 0, respectively
The results of Pearson correlation coefficient showed that the impact of flow velocity on sediment transport and incipient motion is as important the impact of flow velocity on sediment transport and incipient motion is as important as as shear stress

Summary

Introduction

Coarse bedload transport and especially incipient motion of stream-bed materials is one of the most relevant and practical processes to investigate for hydraulic engineers, fluvial geomorphologists and ecologists. The incipient motion of sediment is defined as the condition at which a sediment particle is entrained. The hydrodynamic condition for determining the movement of sediments is generally defined in terms of critical shear stress [6]. Shields [6] used zero or small observable transport rate, whereas other researchers defined threshold visually as the first displacement of single particle [14], scattered particle movement [15], weak movement [16], and general movement [17]

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Conclusion