Experimental Investigation on Flow Configuration in Flexible and Rigid Vegetated Streams

Riparian vegetation could be an appropriate solution for the flood control and sustainable river management technique as it is useful for the energy dissipation of the stream flows. The present experimental investigation is conducted to understand the flow configuration and energy dissipation of stream flows considering the flexible and rigid vegetation. The laboratory-based physical models are tested in a rectangular flume to observe the flow field in the upstream and downstream of both the vegetations. In this study, rigid vegetation is considered of wooden dowels of equal height and almost uniform diameter whereas for flexible vegetation paddy plants were used. Acoustic Doppler Velocimeter is utilized to observe the velocity profiles at different sections and then it is compared with and without the vegetation. It is noticed that around 24% reduction in stream velocity occurs due to introduction of rigid vegetation whereas 90% reduction happens due to flexible vegetation in the channel. Additionally, energy dissipation at all the sections of both vegetation types was found to give a more comprehensive understanding of the flow field. Overall, the present study provides an insight into the fact that with the help of flexible and rigid vegetation one can restore the ecological balance in the river. The vegetation with high density, height and flexibility will be useful for the dissipation of energy in effective manner. This study also suggests that flexible vegetation will be an effective tool in river management by decreasing scouring in the channel, thereby reducing erosion and sediment discontinuity.


Introduction
Riparian vegetation such as submerged, emergent and floating are found to be major source of ecological restoration by maintaining the sediment continuity in the channel by reducing the scouring.Generally, riparian vegetation shows a complex hydrodynamic phenomenon because of different morphology of plant species and flexibility.In the river course, it is found that vegetation consists of deciduous woody trees and shrubs, distributed with branches and leaves throughout the height (Schnitzler 2007).The presence of emergent vegetation in the rivers alters the flow configuration, water level and the conveyance capacity of the channel (Nepf 2012;Rowiński et al. 2018).In the past, several researches have been conducted for observing the flow resistance and water conveying capacity in order to determine the stage and discharge characteristics of artificial and natural streams.Several empirical equations have been suggested for estimating the river-bed roughness and flow resistance in the channels.Some of them have derived empirical flow resistance equations from velocity measurements in gravel and boulder bed streams (Ferguson 2007;Hey 1979;Rickenmann and Recking 2011).In most of these methods the friction factor may be defined as a function of the given relative submergence, y/D, where y stands for an average flow depth and D is considered as the grain size.Although these empirical relationships have been investigated by restricted range of laboratory and field measurements data, none of them predict an acceptable flow resistance in different hydraulic and morphological situations.Hence adopting and applying a suitable flow resistance is difficult and requires adequate skill and practice with special consideration to the hydraulic condition of the river.
Numerous literatures show that the well established flow resistance formulas such as Darcy-Weisbach, Chezy and Manning equations have been widely used to observe the river flows.Sustainable river management could be done by the restoration of the aquatic and riparian vegetation.Both experimental and numerical studies were attempted to understand the hydrodynamics of the bed morphology in vegetated streams.Flow resistance developed by vegetated streams can be predicted considering the cylindrical roughness rods (Li and Shen 1973).The flow characteristics of aquatic vegetation have been studied by various investigators in the past and it was seen that turbulence is significantly associated with the vegetation (Nepf and Vivoni 2000;Finnigan et al. 2009;Nepf 2012).Wilson et al. (2003) conducted experimental tests on two different kind of submerged vegetation and noticed that the extra thinnest zone affected the momentum transfer between the vegetative and upper zone of the aquatic vegetation.Chen et al. (2011) studied turbulent characteristics on the different patterns of submerged flexible vegetation arranged longitudinally and transverse with various spacing of plants.Interestingly, it was found that fully developed flow is divided into three zones such as top non-vegetated zone, middle vegetated zone and the sheath zone (separated zone).The flow profile along the stream direction of the submerged vegetation was greatly influenced due to the drag of the aquatic vegetation which resulted into the complicated velocity profile (Huai et al. 2009;Wilson 2007;Zhang and Nepf 2009;Cheng 2007;Klopstra et al. 1997;Neary 2003;Pietri et al. 2009;Righetti and Armanini 2002).However, the velocity profile was different for flexible vegetation when compared with the rigid type aquatic vegetation (Ghisalberti and Nepf 2006).The existence of vegetation plays a significant role in the shear layer dynamics by considering the drag force acting in the lateral vegetated area and the total settled velocity difference.Also, it was seen that large scale vortices are liable for the exchange process between the main channel and the vegetation portion (Ghisalberti and Nepf 2002;White and Nepf 2007).This exchange process plays a vital role in river system by regulating the movement of sediments, nutrients and pollutant (Jirka 2001;Montakhab et al. 2012;Box et al. 2019).Kasiteropoulou et al. (2017) have carried the numerical study of turbulent flow in an open channel considering bed covered with vegetation.It was found that the wakes were produced behind the rigid cylinders upright to the channel bed.The effect of the discharge on the velocity pattern was also investigated to understand the turbulence characteristics.Mavrommatis et al. (2022) have performed the laboratory based test considering the different types of artificial simulated vegetation.The flow characteristics were mainly reported in the study and the effect of vegetation on the flow downstream of the channel was studied.
However, uncontrollable growth of the vegetation throughout the channel can also reduce the performance of the channel.For instance, emergent vegetation provides large hindrance to the channel flow and if not controlled, affects the conveyance capacity of the channel (Montakhab et al. 2015).Therefore, controlled vegetation growth is supposed to be the sustainable method for river restoration and erosion prevention works.A brief insight into the advantages and disadvantages is shown below in Fig. 1 for a better understanding of the effects of vegetation on a channel.
Although many studies have investigated flow characteristics in vegetated open-channel flows, but the flow and turbulence characteristics in the vicinity of natural vegetative models have not been explored extensively (Chen et al. 2011).Moreover, limited comparative studies are found to understand the effects of vegetation type (i.e., flexible and rigid) on the flow field considering the natural roughness of the stems and plants in the channel.The main objective of this study is to investigate the effects on the flow structure within emergent flexible and rigid vegetation.Therefore, the flow velocity profile across different water depths and sections were observed and compared so as to gain an insight into this complex vegetation-flow interaction.The results of this study would help the readers to get a comprehensive idea about the flow in a vegetated channel and the results may be further extended to understand the process of sedimentation and ecological restoration of polluted rivers.The results of this study on the vegetations with natural flow condition of roughness will be useful for maintaining the ecological health of the river.was placed on the top of the channel for measuring the head at different sections.Acoustic-Doppler velocimeter (ADV) of Nortek-AS made with 10 megahertz (MHz) was used to study the three dimensional.The ADV applied in the study had an accuracy of ± 0.5% of observed value ± 1 mm/s as shown in Fig. 2(b).The velocity was measured at different local points as given in 1(b).The values of the minimum measured depth of water (at 0.5 m upstream of the vegetation) and flow velocity (at 0.120 m upstream of the vegetation) were 0.260 m and 0.192 m/s, respectively.Therefore, the maximum uncertainty may be 1+ (0.0001/0.260)×100+ (0.0001/0.192)×100% = 1.09% (say 1.1%), which is very nominal.
A wooden box consisting of two kinds of vegetation was provided in the mid section of the flume in order to measure the velocity at different sections of the vegetation field.The dimensions of the boxes were 0.5 m × 0.38 m × 0.1 m and were constructed with the waterproof ply board.Flexible and rigid vegetation chosen for the present study were represented by using natural rice plants grown in the laboratory and wooden dowels respectively.Several wooden dowels were arranged in the columnar pattern for the investigation of flow field and the same pattern was also adopted for the flexible rice plants as shown in Fig. 3. Discharge was maintained at 20 × 10 − 3 m 3 /s.For comparison purpose two similar boxes (flexible vegetation) were placed in order to measure the velocity across the vegetation for emergent case.The flow depth (H) was maintained at 0.26 m during the experiment.For collection of data, points were marked on the top of the flume.A strip of gap was provided in between the vegetation so that ADV probe could move easily without disturbing the flow around the vegetation.The movement of probe in the vicinity of vegetation was done in such a manner that it didn't disturb the flow configuration within the channel.The present investigation is carried out for the uniform and steady flow.1998).These filtered data were finally processed to calculate the mean velocities along XY plane for constructing vector fields near vegetative models using MATLAB software.The velocity profile for both the vegetation was pared at the different sections.The same procedure was adopted to compare the effect of increased vegetative density (flexible vegetation only) on the flow field.Finally, the flow field with and without the vegetation were found in order to understand the hydrodynamics and bed morphology of vegetative streams.
The time-averaged stream wise velocity (u), transverse velocity (v) and vertical velocity (w) for all the sections were determined by the following equations: The energy dissipation for the respective vegetation type was found by using the concept of Specific Energy (E) as shown below: Where Y is the water depth at the respective section and U is the mean flow velocity at the respective section.

Time-Averaged Velocity
The variation of time-averaged velocities gives an idea about the effects of vegetation on the flow.Flow measurements were taken along different vertical points (Z) as well as along the transverse direction, i.e., along the y direction.The collected data were then used to plot graphs, which were non-dimensionalized as Z/B along the Y-axis and u/u 0 along the X-axis.Here, u o denotes the mean flow velocity of fully developed flow.It was observed that there was a considerable decrease in streamwise time-averaged velocity (u) near-bed as flow approaches vegetation.At Sect. 2, there was a decrease of about 24% in u for rigid vegetation whereas the presence of flexible vegetation reduces u by about 90%.Further reduction in u was observed as flow moves through vegetation, i.e.Section 3 for flexible vegetation.However, the reduction of velocity near the bed resulted in an increased velocity near the water surface for Sect. 2 (rigid and flexible) and Sect. 3 (flexible) as shown in Fig. 5 (a) to (f) and Table 1.
As the flow moves downstream, there is an increase in the near-bed velocity (Sects.4 and 5 for flexible vegetation and Sects.3 and 4 for rigid vegetation), but the magnitude is much lesser than upstream Sects. 1 and 2. Thus, it can be seen that the flow gets deviated from the vegetated zone to the unvegetated zone, thereby increasing the stream-wise near-bed velocity u.This will ultimately result in more bed shear stress and bed material transport in the unvegetated region as compared to the vegetated region.This is similar to what was observed by Devi et al. (2017) and Devi and Kumar (2016) in their study.
The three-dimensional velocity profiles for both flexible and rigid vegetation were plotted using MATLAB for a better understanding of the flow field in the presence of vegetation as shown in Fig. 6.It is seen that for flexible vegetation, as the flow moves towards the vegetation, the vertical velocity component (w) starts acting downwards having comparatively less magnitude than the upstream Sect. 1.This is also observed within the vegetative region, i.e., Sect.3. Also at Sect. 4, 'w' starts acting upward with a large magnitude.The same is observed in the case of rigid vegetative model.This observation supports the fact that vegetation helps in stabilizing the channel bed and hence reducing erosion.
However, the magnitude of 'w' at Sect. 4 (flexible vegetation) was found to be less than that of Sect. 3 (rigid vegetation).This may be due to the larger wakes being produced by Further calculations were carried out to compare the stream-wise velocity (u) in the presence of both flexible and rigid vegetation.It was observed that flexible vegetation retards the velocity profile more efficiently and hence would prove to be more efficient in stabilizing the channel bedand reduce erosion.This comparison is depicted in the graphs shown in   1 for a comprehensive understanding of the effect of vegetation type on the flow.
From Fig. 7, it is observed that there are certain points where the velocity profile of flexible vegetation intersects the velocity profile of rigid vegetation.This may be due to the flexibility of the vegetation, which induces swaying motion in them.
In Table 1, the negative sign implies that the flow velocities near the surface for both the vegetative models are higher than the flow velocity near the surface observed in the flume without vegetation.This may be due to the energy transfer by the vegetation from the channel bed towards the surface, thereby making the bed more stable.

Energy Dissipation
The mean flow velocity (u o ) for the fully developed flow without vegetation was found to be 9.9219 cm/s and this value was further used to calculate the energy dissipation using Eq. 4 for each vegetation type.

Rigid Vegetation
The mean-flow velocities (U) and the corresponding water depths (Y) at different sections for rigid vegetation are listed in Table 2.
Using Eq. 4, it was found that there is a small gain of 0.0120 cm of energy at Sect. 1 with respect to the fully developed un vegetated flow.Similarly, there is a loss of 0.0190 cm of energy as flow moved from Sect. 1 to Sect. 2. Further comparisons have been tabulated in Table 3 below.

Flexible Vegetation
The mean-flow velocities (U) and the corresponding water depths (Y) at different sections for flexible vegetation are listed in Table 4.
Using Eq. 4, it was found that there is a negligible gain of 0.00077 cm of energy at Sect. 1 with respect to the fully developed unvegetated flow.Similarly, there is a loss of 0.0184 cm of energy as flow moved from Sect. 1 to Sect. 2. Further comparisons have been given in Table 5.
On the other hand, there is a loss of energy of 0.0029 cm at Sect. 1 as compared to the unvegetated flow as the vegetation density is increased.Further loss of energy of 0.0200 cm is observed as flow moves from Sects. 1 to 2. Thus, it is seen that increasing the vegetation density results in further retardation of velocity profile and subsequently, the energy dissipation is also increased.Further study will be carried out to understand the turbulence characteristics of the flow taking place through the vegetative models.Turbulence in flow generally occurs either because of any obstructions/sharp corners or because of the high speed which induces drag between the adjacent fluid layers and between the fluid and its surrounding, creating swirls and eddies.The previous study shows that the presence of vegetation significantly attenuates the production of turbulence within the vegetative region.The study of Reynolds Shear Stress (RSS) will give an insight into the momentum diffusion mechanism in a vegetated channel.

Conclusions
Flow field of emergent flexible and rigid vegetation with aligned columnar arrangements are analyzed in a flume using an Acoustic Doppler Velocimeter (ADV).The discharge is kept constant for all the experimental runs.It was noticed during the experimental analysis that the velocity reduces as the flow approaches and moves towards the vegetative models.The near-bed stream-wise velocities (u) at upstream sections for each vegetation type decreases as flow moves towards vegetation.However, the percentage decrease in velocity is around 90% in case of flexible vegetation whereas in the case of rigid vegetation it is around 24%.This decrease in the near-bed stream-wise velocity is accompanied by an increase in u near the water surface, signifying the transfer of energy from the channel bed towards the water surface.On the other hand, the velocity keeps on increasing downstream in order to achieve the velocity distribution of the fully-developed unvegetated flow.Moreover, it is noticed that vegetation diverts the flow from vegetated region towards the unvegetated region, thereby increasing the risk of bed instability and erosion in the downstream unvegetated regions.Generally, in fluids, shear stress is a function of the rate of shear strain, which is related to the velocity gradient of the fluid flow.From the observation it is noticed that the velocity of the streams downstream the rigid vegetation is more as compared to the flexible vegetation.Therefore, from the present study it is found that the sediment erosion/scour downstream will not occur due to the vegetation fixation and thus the sediment continuity will be maintained.Moreover, the present study is indicative that the flexible vegetation should be adopted in the channels/rivers in order to restore the ecological disturbances and this method would be quite effective for river management.In future, further studies are needed to understand the importance of buffer width of the vegetation on the sides of bank and growth of the flexible vegetation in the rivers.These variations in flow field significantly affect the process of sedimentation and hence the bed stability in rivers and streams with the presence of vegetation.The present study aims at providing the readers an idea about these flow variations in vegetated streams.The work can further be extended to understand the turbulence characteristics in such streams which will help engineers and researchers in carrying out various river restoration works including the ecological restoration of rivers.The several types of vegetation with different density, height and flexibility should be studied.The flow velocity in the vicinity of the vegetation is not known.The ADV could not recognize the velocity profile around the canopy.This study was carried out for uniform and steady condition but during flood it is difficult to have such conditions so further study should be considered for understanding the flow configuration.The free flow condition is observed in the present study.In future, submerged case can be considered for further study to understand the flow behavior around the canopy.

Fig. 1
Fig. 1 Effect of vegetation in a channel

Fig. 2
Fig. 2 (a) Experimental setup (b) ADV setup for the experimental runs

Fig. 3
Fig. 3 Vegetative models used for the experiment (a) Rigid wooden dowels (b) Flexible rice plants

Fig. 6
Fig. 6 Three-dimensional velocity profiles in MATLAB for (a) Flexible vegetation (b) Rigid vegetation

Fig. 7 .
Fig. 7.Moreover, the quantification of the velocity data (without vegetation versus Sect. 1 of each vegetation type) is provided in Table1for a comprehensive understanding of the effect of vegetation type on the flow.From Fig.7, it is observed that there are certain points where the velocity profile of flexible vegetation intersects the velocity profile of rigid vegetation.This may be due to the flexibility of the vegetation, which induces swaying motion in them.In Table1, the negative sign implies that the flow velocities near the surface for both the vegetative models are higher than the flow velocity near the surface observed in the flume without vegetation.This may be due to the energy transfer by the vegetation from the channel bed towards the surface, thereby making the bed more stable.

Fig. 7
Fig. 7 Stream-wise velocity distributions at different measurement loactions for comparative study