Flow structure over bed form with flexible vegetation patches

Understanding the effects of riparian vegetation under sediment-laden flow is becoming crucial due to the increase in frequency of extreme weather events. This study designed three densities and nine random distributions of bent flexible vegetation in flume experiments under sediment-laden flow. Sediments were continually added to the flume at a rate of 21 kg/h to simulate a natural river environment in a sediment-laden flow. The results showed that the evolutionary process of bed form under sediment-laden flow could be divided into four stages: scouring, development, recovery, and deposition stages, forming a dynamic cycle. Dunes were formed and backwater caused them to develop upstream, while structural resistance developed the dunes downstream. Contrary to clear water regime, sediments were deposited upstream of the vegetation area and scour occurred behind the vegetation. In addition, the vertical velocity profile showed to be dependent on the vegetation structure and four clear zones were identified: fixed, bent, canopy, and developed zones. The findings from this study provide crucial information towards river management through understanding the diverse vegetation effects under sediment-laden flows.

Download Full-text

To Investigate the Flow Structure of Discontinuous Vegetation Patches of Two Vertically Different Layers in an Open Channel

Water ◽

10.3390/w10010075 ◽

2018 ◽

Vol 10 (1) ◽

pp. 75 ◽

Cited By ~ 18

Author(s):

Naveed Anjum ◽

Usman Ghani ◽

Ghufran Ahmed Pasha ◽

Abid Latif ◽

Tahir Sultan ◽

...

Keyword(s):

Flow Structure ◽

Open Channel ◽

Vegetation Patches

Download Full-text

Study of frequency pattern of coherent turbulent flow over ripples using image processing with implication in river restoration

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-8-7845-2011 ◽

2011 ◽

Vol 8 (4) ◽

pp. 7845-7871

Author(s):

A. Keshavarzi ◽

J. Ball ◽

H. Nabavi

Keyword(s):

Image Processing ◽

Turbulent Flow ◽

Flow Structure ◽

Sampling Rate ◽

Three Dimensions ◽

Image Processing Technique ◽

Frequency Pattern ◽

Bed Form ◽

Bursting Events ◽

Sediment Particles

Abstract. River channel change and bed scourings are source of major environmental problem for fish and aquatic habitat. The bed form such as ripples and dunes is the result of an interaction between turbulent flow structure and sediment particles at the bed. The structure of turbulent flow over ripples is important to understand initiation of sediment entrainment and its transport. The focus of this study is the measurement and analysis of the dominant bursting events and the flow structure over ripples in the bed of a channel. Two types of ripples with sinusoidal and triangular forms were tested in this study. The velocities of flow over the ripples were measured in three dimensions using an Acoustic Doppler Velocimeter with a sampling rate of 50 Hz. These velocities were measured at different points within the flow depth from the bed and at different longitudinal positions along the flume. A CCD camera was used to capture 1500 sequential images from the bed and to monitor sediment movement at different positions along the bed. Application of image processing technique enabled us to compute the number of entrained and deposited particles over the ripples. From a quadrant decomposition of instantaneous velocity fluctuations close to the bed, it was found that bursting events downstream of the second ripple, in Quadrants 1 and 3, were dominant whereas upstream of the ripple, Quadrants 2 and 4 were dominant. More importantly consideration of these results indicates that the normalized occurrence probabilities of sweep events are in phase with the bed forms whereas those of ejection event are out of phase with the bed form. Therefore entrainment would be expected to occur upstream and deposition occurs downstream of the ripple. These expectations were confirmed by measurement of entrained and deposited sediment particles from the bed. These above information can be used in practical application for rivers where restoration is required.

Download Full-text

Frequency pattern of turbulent flow and sediment entrainment over ripples using image processing

Hydrology and Earth System Sciences ◽

10.5194/hess-16-147-2012 ◽

2012 ◽

Vol 16 (1) ◽

pp. 147-156 ◽

Cited By ~ 12

Author(s):

A. Keshavarzi ◽

J. Ball ◽

H. Nabavi

Keyword(s):

Image Processing ◽

Turbulent Flow ◽

Flow Structure ◽

Sampling Rate ◽

Three Dimensions ◽

Image Processing Technique ◽

Bed Form ◽

Bursting Events ◽

Sediment Particles ◽

Sediment Entrainment

Abstract. River channel change and bed scourings are source of major environmental problem for fish and aquatic habitat. The bed form such as ripples and dunes is the result of an interaction between turbulent flow structure and sediment particles at the bed. The structure of turbulent flow over ripples is important to understand initiation of sediment entrainment and its transport. The focus of this study is the measurement and analysis of the dominant bursting events and the flow structure over ripples in the bed of a channel. Two types of ripples with sinusoidal and triangular forms were tested in this study. The velocities of flow over the ripples were measured in three dimensions using an Acoustic Doppler Velocimeter with a sampling rate of 50 Hz. These velocities were measured at different points within the flow depth from the bed and at different longitudinal positions along the flume. A CCD camera was used to capture 1500 sequential images from the bed and to monitor sediment movement at different positions along the bed. Application of image processing technique enabled us to compute the number of entrained and deposited particles over the ripples. From a quadrant decomposition of instantaneous velocity fluctuations close to the bed, it was found that bursting events downstream of the second ripple, in Quadrants 1 and 3, were dominant whereas upstream of the ripple, Quadrants 2 and 4 were dominant. More importantly consideration of these results indicates that the normalized occurrence probabilities of sweep events along the channel are in phase with the bed forms whereas those of ejection events are out of phase with the bed form. Therefore entrainment would be expected to occur upstream and deposition occurs downstream of the ripple. These expectations were confirmed by measurement of entrained and deposited sediment particles from the bed. These above information can be used in practical application for rivers where restoration is required.

Download Full-text