Experimental Study on Backflow Patterns Induced by a Bilateral Groin Pair with Different Spacing

Cuiping Kuang,Xuejian Han,Qingping Zou,Yuhua Zheng,Jie Gu

doi:10.3390/app11041486

Cuiping Kuang, Xuejian Han + Show 3 more

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https://doi.org/10.3390/app11041486

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Abstract

Groins are one of the popular manmade structures to modify the hydraulic flow and sediment response in river training. The spacing between groins is a critical consideration to balance the channel-depth and the cost of construction, which is generally determined by the backflow formed downstream from groins. A series of experiments were conducted using Particle Image Velocimetry (PIV) to observe the influence of groin spacing on the backflow pattern of two bilateral groins. The spacing between groins has significant effect on the behavior of the large-scale recirculation cell behind groins. The magnitude of the wake flow induced by a groin was similar to that induced by another groin on the other side, but the flow direction is opposite. The spanwise velocity near the groin tip dictates the recirculation zone width behind the groins due to the strong links between the spanwise velocity and the contraction ratio of channel cross-sections between groins. Based on previous studies and present experimental results, quantitative empirical relationships are proposed to calculate the recirculation zone length behind groins alternately placed at different spacing along riverbanks. This study provides better understanding and a robust formula to assess the backflow extent of alternate groins and identify the optimum groins array configuration.

Highlights

IntroductionTermed spur dikes, have been used widely to protect coastlines and riverbanks from scour and erosion for centuries
The downstream recirculation zone formed behind a groin was subdivided into three main zones, i.e., shear layer, backflow, and reattachment zone
The mainstream velocity significantly increases from the groin tip to the opposite bank due to the decreased cross-section caused by protrusion of the groin, with a velocity gradient rapidly increasing near the groin tip, resulting in the shear layer zone

Summary

Introduction

Termed spur dikes, have been used widely to protect coastlines and riverbanks from scour and erosion for centuries. They are typically built perpendicularly or at an oblique angle to the bank so that the flow is kept away from the bank [1]. Installing the groins, the laterally confined flow attains a sufficiently high velocity to rectify a navigable river [2]. The flow field in vicinity of groins is a visual indicator of the turbulent flow characteristic and plays a key role in the understanding of hydrodynamics induced by the groin [4,5]. The flow field around such a kind of groin can be divided into two subzones due to flow separation from the groin tip: the recirculation zone with a large-scale vortex downstream of the groin and the main stream with contraction and diffusion by the protrusion of the groin (see Figure 1 for detail)

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