87 Figure 5.1 Velocity profiles plotted against flow depth at three different measurement sites for infiltration, 10% infiltration and 15% infiltration (a) vegetation spacing of 15 cm (b) vegetation spacing of 10 cm (dashed line shows height of deflected vegetation top). 123 Figure 6.11 Vertical turbulent intensity profiles at different measurement locations for no seepage, 10% seepage and 15% seepage.
Overview
Therefore, a good understanding of the interaction between flow, sediment and vegetation is required to understand the problem. Based on the flow conditions and vegetation characteristics, aquatic vegetation can be grouped into two classes: (i) submerged and emergent vegetations and (ii) rigid and flexible vegetations.
Types of vegetation
Recently, efforts are being made to restore rivers, re-naturalize and rehabilitate watersheds and watercourses in which revegetation is the first and foremost step (Kothyari et al, 2009a). Flexible elements can assume different configurations (Kouwen et al, 1969; Gourlay, 1970; Kowobari et al, 1972) due to the hydrodynamic action of the flow and the bending stiffness EI, where E is the modulus of elasticity of the flow.
State of the art
The velocity profile shows an inflection point near the top of the vegetation and the flow becomes unstable, resulting in the formation of three-dimensional vortices. The Reynolds stress increases linearly from the free surface to the top of the vegetation.
Need for research
Erosion of channel banks contributes to sheet flow due to increased channel bed shear stress. An empirical relation for the thickness of the sheet flow layer has been developed, which includes suction as an independent parameter along with others.
Objectives
Flow characteristics in a channel covered with uniformly distributed vegetation
Effect of mixed vegetation densities on flow structure
Hydrodynamics of seepage affected channel with vegetation bundles
Experimental study of flow through natural vegetation
Thesis Organization
The change in various flow characteristics due to the use of downward seepage is studied. The chapter highlights the flow characteristics for two different cases: (i) when vegetation is fully covered over the entire test section and (ii) when vegetation is partially covered for half the width of the test section.
Overview
Apparatus and Methods
The Flume
Test section
Bed material
Flow discharge in the main channel
Seepage discharge
The flow meters had a control valve and a digital display that was used to apply the desired percentage of seepage discharge (Figure 2.6).
Flow velocity
The acceleration threshold method (Goring and Nikora, 2002; Dey et al, 2012) was used to remove the spikes in the velocity data (Figure 2.8). It can be seen from Figure 2.9 that power spectra for filtered velocity pulses were in good agreement with Kolmogorov's −5/3 law in the inertial subrange.
Flow depth
Water surface elevation
Temperature and Kinematic Viscosity
Fixing of Flow depth
Theoretically, the value of Y should be zero, but for practical purposes Miller (1977) recommended a value of 10-6. During the experiment, an attempt was made to keep the Y value as close as possible to the recommended value.
Vegetation characteristics
Carollo et al (2005) and Okatomo and Nezu (2009) classified the different flow patterns in flexible vegetated channels as upright or rigid, gently swaying or without organized movements, monami or with organized movements and inclined.
Experimental Program
The vegetation zone was located in the middle of the ditch and covered an area of 5m long and 1m wide (Figure 2.17). The height of the vegetation (hv) was 4 cm and the deflected vegetation height is in the range 3-3.5 cm (average deflected vegetation height, hd = 3.25 cm).
Introduction
This chapter presents the results of velocity profiles and turbulence characteristics in uniformly distributed vegetation stems. It discusses the effect of vegetation height, vegetation spacing and placement pattern of the vegetation element on flow characteristics.
Velocity
The velocity in the free stream region is higher than the velocity measured along the vegetation stems (Liu et al, 2008). This indicates that the application of downward seepage results in an increase in velocity in the vegetated zone.
Reynolds Stress
The presence of a shear layer leads to the production of oscillations near the top of the vegetation. For both staggered and uniform patterns, a vegetation spacing of 10 cm has a higher value of Reynolds stress than 15 cm (Figure 3.4).
Turbulence Intensities
Additionally, it is observed that the 15 cm center-to-center spacing has slightly lower turbulence intensity than the 10 cm center-to-center spacing, regardless of the vegetation pattern. For the 10% and 15% drainage also, the turbulence intensity as for the vegetation height is greater, reaching an average increase value of 15% for the 10% and 25% drainage.
Moment Analysis
As the percentage of seepage increases, the positive values of M30 near the riverbed increase. M12 started to have positive values near the bottom and the positive nature increases as the percentage of seepage down increases.
Quadrant Analysis
In the present observations for staggered pattern, it is observed that in the flow region above the vegetation, the flow is dominated by ejection events for cases without seepage, but equal contribution of ejection and sweep for lower flow region. One of the reasons may be that the flow is not very shear-wise in the unobstructed region, A, and is therefore only dominated by the ejection event.
Turbulent Kinetic Energy (TKE) Budget
Tt has negative values in the upper flow region and positive values in the lower vegetation zone (Figure 3.14). The energy transported by the turbulent diffusion is partially balanced by the pressure term shown by positive values above the vegetation zone and negative values in the vegetation zone (Figure 3.14).
Conclusions
The presence of downward seepage changes the velocity distribution, leading to an increase in velocity in the vegetation zone. The turbulent diffusion or transport is compensated by the pressure transport, which is reflected in positive values in the upper catchment area and negative values in the vegetation zone.
Introduction
74 the flow pressure, promotes the stirring movement of the flow body and washes away the sediment around the plant. The various factors influencing the resistance to flow in an open channel are size, shape and irregularity of the channel, curvature of the channel, types of roughness and vegetation including the length, density and stiffness (Chow, 1959).
Velocity Profiles
Stem/vegetation resistance is one of the important flow resistance parameters in vegetated open channels. When evaluating flow resistance in a straight vegetated channel, both the effects of the hydraulic cross-sectional reduction and the dissipative effects due to the presence of the roughness elements (shape, size, arrangement and concentration of the elements) must be taken into account. In fact, both the geometry of the vegetation elements and the turbulence characteristics of the flow affect the hydrodynamic resistance and the size of the wakes generated downstream of the elements themselves (Shen 1973; Ferro and Giordano 1992).
Reynolds stress
This in turn leads to more impulse exchange near the top of the vegetation and ultimately results in more Reynolds stress at that location. As the current goes downstream, the Reynolds voltage increases again by a value in the range of 3-9%.
Turbulence Intensities
Turbulence generated by shear occurring near the top of vegetation increases the intensity of vertical as well as downstream turbulence. It is observed in all profiles that the application of downward drainage increases the turbulence intensities compared to no drainage in the range of 6-12% and 3-7% from 10% drainage to 15% drainage cases.
Moment Analysis
82 The negative values of M03 and M21 in the vicinity of the bed suggest that the flow current takes place in the area and the flow coming towards the bed is again carried away by the flow in the direction of flow, which is observed from positive values of M30 and M12. However, for the case of 5 mm upstream-10 mm downstream (Figure 4.5a, 4.5b and 4.5c), the positive values of M30 and M12 and the negative values of M03 and M21 increase in the range of 4–8% when the flow occurs from upstream 5 mm part towards the middle part and 5-10% when the flow goes from the upstream part towards the downstream 10 mm diameter part.
Quadrant Analysis
While for a diameter of 10 mm up and a diameter of 5 mm down, the difference between sweep events and ejection events increases by 2-5%. An interesting feature is observed for this case, where a decrease in the sweep-to-ejection difference in the range of 4–7% is observed as the flow goes from upstream to downstream.
Drag Coefficient
85 The difference between sweeping and ejection increases in the range of 3-5% for 10% seepage compared to no seepage and 2-4% for 15% seepage compared to 15% seepage. For the case of upstream with a diameter of 5 mm - downstream with a diameter of 10 mm (Figure 4.6a, 4.6b and 4.6c), the difference in the sweep and ejection events increases by 3-6% as the flow of upstream with a diameter of 5 mm towards the middle part and 4-8% from the middle towards downstream.
Conclusions
In the presence of downward drainage, the sweep action dominates over the entire region and the difference in contribution between sweep and extraction increases as the percentage of drainage increases. The change in average drag coefficient with drainage was also studied and it was found that the value of CD decreases as the percentage of drainage increases.
Introduction
Nepf (1999) developed a model to describe the drag, turbulence and diffusion for flow through emergent vegetation and covered the natural range of vegetation density and stem Reynolds numbers to extend the cylinder-based model for vegetative resistance by the to include dependence on the drag coefficient. , stem density and highlights the importance of mechanical diffusion in vegetated flows. Experimental measurements were assessed to characterize the flow phenomenon in a natural channel with vegetation patches.
Velocity
However, the speed on the upstream 8.5 m is always higher than the downstream 5.5 m track regardless of the application of downstream seepage.
Reynolds stress
It is also observed that the maximum Reynolds stress for a 15 cm gap is lower (2-7%) compared to a 10 cm gap. The maximum Reynolds stress near the top of the vegetation field is increased in the range of 4-9% from no seepage to 10% seepage and 7-18% from no seepage to 15% seepage.
Turbulence Intensities
Furthermore, the reduced nature of the Reynolds stress is attributed to the fact that the further the location is from the top of the vegetation, the lower the turbulence caused by the vegetation stems. This reiterates that the use of downward seepage increases the Reynolds stress and thereby increases bed material transport.
Third order moments
The flow is slightly more sheared for 10 cm center-to-center than for 15 cm center-to-center, evidenced by a higher value of the Reynolds stress. As observed, downward seepage increases the shear stress and thus with an increase in the downward seepage percentage, the flow becomes more sheared leading to more turbulence intensities.
Quadrant analysis
M12 being positive near the bed and M21 negative near the bed indicates that w w diffusion propagates in the upstream direction and u u diffusion occurs in the downstream direction respectively. The negative values of M03 and M21 near the bed infer that flow flooding is occurring in the region and the flow coming towards the bed is carried back by the stream in the direction of the flow which is observed by the positive values of M30 and M12.
Drag Coefficient
Downward seepage increases the maximum Reynolds stress near the top of the vegetation field by 4-9% from no seepage to 10% seepage and 7-18% from no seepage to 15% seepage. The watershed above the vegetation field has an almost equal contribution from ejection and sweep.
Introduction
The hydraulic resistance caused by vegetation depends on many factors, including the size of the vegetation stem, plant height, vegetation density, and depth of flow. James et al (2006) developed a hypothetical vegetation-influenced flow model to identify the most important variable in determining flow resistance.
Fully submerged
- Velocity
- Reynolds Stresses
- Turbulence Intensity
- Moment Analysis
- Integral Scales of flow
- Drag coefficient
- Conclusions
Thus, the maximum value of Reynolds stresses in the vegetation zone occurs near the top of the vegetation. However, an increase in the maximum Reynolds stress and turbulent intensities is obtained at the downstream end of the ungrown region.
Partially covered
Velocity
This shows that the reduced velocity in the vegetated region is deflected to the unvegetated area. This shows that for seepage cases the velocity that is reduced in the vegetated area is also redirected in the unvegetated region.
Reynolds stress
Higher value of maximum Reynolds stress is achieved for vegetated region compared to the non-vegetated region due to the local effect of erosion and deposition around the vegetation. In all profiles, with the increase in seepage percentage, an increase in the value of the maximum Reynolds stress is achieved.
Turbulent Intensities
From the contour profiles, σu and σw have maximum turbulent intensities at the center of the vegetated test section (A section). The turbulent intensities increase for unvegetated section when the flow goes downstream, while it is opposite for vegetated section, which implies that the presence of vegetation impairs the fluctuation accompanying the flow, thus protecting the downstream section from erosion.
Moments
This means that the transport of flux in the flow direction occurs more when seepage is applied. This means that the transport of flux in the direction of flow in the nearby bed area is more with the application of seepage, which causes more sediment movement.
Quadrant analysis
Drag coefficient
Integral Scales of flow
Vegetation is known to help protect erosion, but the main point lies in investigating the flow and sediment transport conditions in the unvegetated area of a partially vegetated channel. The high flow velocity in the unvegetated region can cause soil erosion and sediment transport.
Conclusions
The maximum Reynolds stress and turbulence intensities in the vegetation zone are higher than in the bare area, leading to local sediment movement around the vegetation stems. For the vegetated part, the dominance of sweeping event in the entire catchment area is observed at 15% seepage, which is related to more positive values of M30 and more negative values of M03. The application of downward seepage reduces the vegetation roughness shown by the reduction in CD.
Flow characteristics in a channel covered with uniformly distributed vegetation
The present study is carried out to study the flow characteristics in a vegetated channel using artificial vegetation as well as natural vegetation. The main objective of this study is to incorporate the effect of downward seepage and explore the flow characteristics in a vegetated channel.
Effect of mixed vegetation densities on flow structure
Hydrodynamics of seepage affected channel with vegetation bundles
Experimental study of flow through natural vegetation
Maximum value of length and time scales near the top of the vegetation, inferring the occurrence of more momentum exchange as observed from the Reynolds stress. Presence of vegetation forces the flow from the vegetated zone towards the unvegetated region, leading to the occurrence of erosion in the unvegetated region.
Recommendations for future work
Prediction of the contributions to Reynolds stress from bursting events in open channel flows. Higher Reynolds stress is achieved at the onset of the vegetation zone, where contraction in the flow occurs due to the presence of vegetation elements.
