Bed Morphology around Various Solid and Flexible Grade Control Structures in an Unstable Gravel-Bed River

Jia-Mei Wang,Xing-Guo Yang,En-Quan Lv,Hong-Wei Zhou,Xing Lin,Rui Jiang

doi:10.3390/w10070822

Abstract

Flexible grade control structures (GCSs) are increasingly popular alternative countermeasures consisting of conventional solid structures for protecting riverbeds from erosion. Among this structure typology, tetrahedron framed permeable weirs (TFPW) were proposed to stabilize riverbeds and protect cross-river structures. The bed morphology as well as the interactions among the structures, water flow and bed features have not been systematically analyzed. Therefore, based on the field data collected in the Jianjiang River, a series of physical model experiments were carried out in this study to investigate the river morphology in unstable gravel-bed rivers under five different structural conditions, including no GCSs, solid bed sills and flexible TFPW with three different layout types. Data on bedform features, topography changes and the volume of erosion or deposition were collected and analyzed. The results indicated that dramatic general erosion and bed armoring would occur without any protection. The bed would be protected from upstream erosion with bed sills, but, at the same time, in addition to general erosion and bed armoring downstream of the structures, deep local scour might pose a serious threat to structural safety. TFPW had good protective effects on riverbeds by preventing the bed from erosion and even inducing sediment deposition both upstream and downstream from the structures. Because the flexible structures have advantages in ecology, structural stability and construction, TFPW were suitable and optimal for riverbed stabilization in unstable gravel-bed rivers.

Highlights

Both natural processes such as landslide damming [1,2] and anthropogenic causes such as dam construction [3,4], bed mining [5], land-use changes [6,7] and channelization [8] could destroy the relatively steady equilibrium states of rivers
To optimize the hydraulic structures for river sediment control and bed stabilization in unstable gravel bed rivers, based on the prototype data of the plain reach of the Jianjiang River, a series of flume experiments were carried out to study the effects of solid bed sills and flexible tetrahedron framed permeable weirs (TFPW) on the bed morphology
This paper presents the results of a field investigation and experimental study of bed morphology under different structural conditions in an unstable gravel-bed river

Summary

Introduction

Both natural processes such as landslide damming [1,2] and anthropogenic causes such as dam construction [3,4], bed mining [5], land-use changes [6,7] and channelization [8] could destroy the relatively steady equilibrium states of rivers. Gravel-bed rivers that flow from large-gradient mountain regions to low-gradient plains regions would become extremely unstable [9,10]. The instability of streambeds causes damage in riverbanks and other infrastructure and failure of cross-river structures, such as bridges and underwater pipelines. Such mountain flood disasters occur in many unstable gravel-bed rivers from large-gradient mountain regions to low-gradient plains regions, and they urgently need to be prevented and controlled. To stabilize riverbeds and riverbanks and to protect river crossing structures, grade control structures (GCSs) are usually constructed across the channel, and they are called drop structures, Water 2018, 10, 822; doi:10.3390/w10070822 www.mdpi.com/journal/water. GCSs can protect fish habitat by preventing excessive riverbed degradation [15], they constitute barriers for fish migration if they are high [8,18,19]

Methods

Results

Discussion

Conclusion