Experimental Study on Uniform and Mixed Bed-Load Sediment Transport under Unsteady Flow

Zihao Duan,Bingbing Zhao,Jie Chen,Xiaojian Liu,Changbo Jiang

doi:10.3390/app10062002

Abstract

The scouring and deposition of sediment caused by unsteady flows (e.g., storm waves and floods) produces many secondary disasters. The resultant bed-load movement exhibits different transport laws compared with that by steady flow. In this study, the flume experiments were performed to study the bed-load movement under unsteady flow with different velocity skewness. The movement of uniform and non-uniform non-cohesive sediment under unsteady flow as well as the influence of the steady and unsteady flow on sediment transport rate are compared. Additionally, the non-uniform sediment transport formula of fine-to-coarse particle diameter ratio was investigated. The results showed that the sediment transport rate between uniform and non-uniform sand under the same median diameter is different. The non-uniform sediment transport rate is 1.27-, 3.19-, and 0.68-times as large as that in uniform sediment under d50 = 0.664, 1.333, and 2.639 mm under unsteady flow, respectively. For non-uniform sand, the transport rate of non-uniform sand with a larger adjacent particle size ratio (δ = 0.29) was 1.31-times greater than that of the non-uniform sand with a smaller adjacent particle size ratio (δ = 0.50). Moreover, theoretical deduction was carried out and the incipient sediment motion was analyzed from the force mechanism. A new unsteadiness parameter based on the acceleration concept was proposed. The relationship between the travel distance and velocity skewness of sediment particles was set up. The experimental results and theoretical analysis showed that sediment under unsteady flow were easier to start and transport than those under steady flow in the same flow effect. The travel distance of sediment particles was longer under unsteady flow than that under steady flow.

Highlights

The unsteady flow caused by storm waves and other disasters in nature often causes severe sediment movement and significant morphological changes, which exhibit extremely harmful effect to human activities and buildings [1,2]
The objective of this paper is to investigate the influence of unsteady flow on sediment incipient motion and transport, and compare bed-load transport rates with varying flow and sediment conditions
Whereas in large-sized particles (d = 1.333 mm), the transport capacity under the steady same non-uniform sand, there existed certain difference in the sediment transport rate of the same flow condition was weaker than that under the unsteady flow, and the transport rate difference is less than 12.1%

Summary

Introduction

The unsteady flow caused by storm waves and other disasters in nature often causes severe sediment movement and significant morphological changes, which exhibit extremely harmful effect to human activities and buildings [1,2]. These strong flows cause significant erosion and deposition in affected coastal areas, and results in severe economic losses [3]. Sci. 2020, 10, 2002 as a process of acceleration or deceleration and a strong unsteadiness effects on sediment particles during a short time [4,5,6,7], which affects the threshold motion of sediment and bed-load transport. Recent experimental studies provide some insight for better understanding mechanisms involved in sediment transport under different flow conditions

Objectives

Methods

Findings

Conclusion