Fluvial Hydraulics in the Presence of Vegetation in Channels

1. Introduction

In many semi- and arid regions, water does not flow from middle spring to early fall, leading to the development of vegetation patches with irregular distribution on channel beds and banks. These vegetation patches influence the characteristics of flow in rivers and streams. The presence of vegetation patches on channel beds and banks can be either submerged or emergent. The height of an emergent vegetation patch exceeds water depth, while the height of a submerged vegetation patch is less than the water depth. Vegetation patches on both channel beds and the banks of natural rivers play a significant role in many aspects of the environment, including water quality, sediment transport and bank stability.

In the presence of a vegetation patch in a channel, the estimation of key hydraulic parameters such velocity, Reynolds stress and turbulence intensities depends on the features of the vegetation patch, such as submergence ratio, vegetation density and configuration. There are numerous key issues for researchers to apply turbulent flow characteristics in bare channels to vegetated channels. In the last three decades, to address issues regarding river restoration and water conservation projects, the effects of vegetation in vegetated rivers have attracted a lot of attention from researchers to conduct studies based on both experimental investigations and numerical simulations. However, our knowledge about the impacts of vegetation patches on the estimation of hydraulic parameters is still limited for engineers in the practice of river restoration projects, especially for cases of vegetated meandering channels and pool–riffle sequences.

The aim of this Special Issue is to bring together research results that improve our understanding of fluvial hydraulics in vegetated channels, including the effects of submerged and emergent vegetation on velocity distribution, validation of the logarithmic law, Reynolds stress, turbulence intensities and bursting events for different aspect ratios (the ratio of width to flow depth) over flat gravel beds and 2D and 3D bedforms. Results showed that the determination of key hydraulic parameters such as the roughness coefficient and drag coefficient is influenced by vegetation arrangements and variation in bedforms.

This Special Issue calls for research work in environmental hydraulics for improving our knowledge of the fluvial hydraulics of vegetated channels. Among the topics of interest in this Special Issue are:

• Turbulent flow characteristics in the presence of submerged and emergent vegetation;
• The water blockage effect of submerged vegetation;
• The influence of vegetation arrangement and density on the flow structure;
• The hydrodynamic effect of emergent vegetation in a U-shape open channel confluence flow with partially rigid emergent vegetation;
• The CDF simulation of flow structures in vegetated channels;
• The influence of bedform and vegetation on flow characteristics;
• The determination of erosion intensity and sedimentation rate from a watershed by means of empirical models.

2. Overview of This Special Issue

Many manuscripts received for this Special Issue show interesting and impressive results. The review process for each manuscript has been completed strictly following the journal requirements. In total, eleven (11) papers have been accepted and published in this Special Issue; they present the last results of fluvial hydraulics in vegetated channels based on laboratory experiments and numerical simulations.

Vegetation in rivers and streams plays an important role in preventing erosion and improving bank stability. Tabesh Mofrad et al. [1] compared the impacts of emergent vegetation on velocity and Reynolds stress distributions to those of submerged vegetation under conditions of the same aspect ratio and flow discharge. It is found that the influence of submerged vegetation on the generation of secondary currents is less than that of emergent vegetation. The application of the logarithmic law is valid for flow up to the depth of y/h = 0.75 for emergent vegetation (in which the vegetation cover in banks is partly out of the water) but up to the flow depth of y/h = 0.25 for vegetation bank. For submerged vegetation patches, the location of zero-shear stress superposes that of the maximum velocity. However, for emergent vegetation, near the zone of emergent vegetation in channel banks, the maximum velocity is shifted towards the channel bed, and thus the location of the zero-shear stress approaches the bed. The distribution of Reynolds stress shows a convex shape for submerged and emergent vegetation covers in channel beds. However, approaching the bank vegetation (emergent case), the power of secondary currents increases, and thus the maximum Reynolds stress is shifted towards the bed. The results of this study indicate that the estimation of key parameters of fluvial hydraulics such as velocity and Reynolds stress is influenced by the feature of vegetation in a channel including the arrangement patterns and submerged or emergent status.

Vegetation patches are often present in pool–riffle sequences in natural streams. Based on laboratory experiments conducted using two different vegetated pools with various slopes of their entry and exit sections, the impact of the vegetation patch on flow structures in pools has been investigated. The effect of the slopes of both entrance and exit sections of the pools on flow structure has been studied by Tabesh Mofrad et al. [2]. Results show that, for large entrance and exit slopes, the distribution profiles of turbulent kinetic energy (TKE) have no specific shape. However, the maximum TKE value is located near the bed when the slopes of the entrance and exit sections of the pool are small. Both ejections and sweeps govern the turbulence structures and coherent motions at the trailing edge of the vegetation patch. In addition to the location of the pool (entrance, middle pool and exit section), the bedform slopes and features of vegetation patches affect the velocity profile, Reynolds shear stresses, TKE distribution and the contribution of each bursting event on turbulent flow structures. For a pool with a larger entrance and exit slope, the TKE distribution has no specific form. However, for a pool with a small entrance and exit slope, the TKE distribution has a convex shape with the maximum value near the bed. The sweep motion occurs in a narrower zone above the vegetation canopy. The sweep motions of bursting events are the dominant processes directly above the vegetation canopy, while an outward motion with slightly positive values has been observed at the front edge of the vegetation patch. The Kolmogorov -5/3 power law rests valid for the 3D vegetation patch. The shedding frequency is affected by the changes in the bedform slopes and the presence of vegetation, resulting in higher values than those reported in the literature.

Based on experiments and numerical simulations, Qiu et al. [3] studied the blockage effect of submerged vegetation. A new numerical simulation model for the flow field in the presence of submerged vegetation in an open channel was proposed. The influence of velocity integration and average treatment on the water-blocking effect generalization model caused by submerged vegetation has been studied. It is found that the relationship between velocity integration and the generalized roughness coefficient is positive, and the relationship between velocity integration and virtual channel elevation is negative. The average treatment and numerical simulation treatment have effects on the generalization of the roughness coefficient and virtual channel elevation value. In addition, the average treatment with or without water-blocking effect generalization makes little difference in the generalization of the roughness coefficient and virtual channel elevation.

The interaction between bedforms and vegetation patches in mountainous streams is a challenging subject for river engineers. Considering the importance of vegetation in stabilizing bedforms, Parvizi et al. [4] studied turbulent flow structures based on a gradual varied shallow flow in a flume with both convective deceleration and acceleration flow sections. The presence of vegetation patches in the pool bed resulted in an increase in the shear velocity, particularly along the entrance section of the pool where the flow decelerates. Results of the quadrant analysis reveal that sweep events play the most important role in different parts of the pool, followed by ejection events. However, near the bed, especially along the pool entrance section, the role of outward and inward events is considerable. The Reynolds shear stress and turbulence intensity distributions over the gravel bed (bare bed) were compared to those in the pool with a vegetation patch. It is found that, in the presence of vegetation in the channel bed of the pool, the velocity profiles show an S-shaped distribution via increasing the aspect ratio. For these velocity profiles, the velocity gradient in the upper part is very small, the velocity gradient is higher in the middle part, and the velocity follows a logarithmic distribution in the region near the bed.

Brahimi and Sui [5] carried out laboratory experiments in a large-scale flume to investigate the effects of submerged vegetation arrangement patterns and density on flow structure. Results of turbulent kinetic energy in the wake zone of the deflected vegetation indicate that the maximum root-mean-square velocity fluctuations of flow occur at the sheath section and the top of the vegetation. In the wake zone behind the vegetation elements, the maximum value of Reynolds shear stress occurred slightly above the interface between deflected vegetation and the non-vegetation layer, showing the Kelvin–Helmholtz instability that is associated with inflectional points of the longitudinal velocity. As the vegetation density increases, the negative and positive values of Reynolds shear stress throughout the flow depth increase. In the presence of non-bending vegetation in a shallow flow bed, a high velocity gradient appears near the channel bed from the depth of z/H = 0 to z/H = 0.1, reaching a peak velocity at the depth of z/H = 0.1, and a decreasing trend of velocity toward the water surface is noticeable. However, for the deeper flow, the peak velocity occurs at a higher location close to the water surface.

By means of the RNG k-ε numerical model coupled with the volume of fluid method, Shi and Jin [6] simulated the hydrodynamics of a U-shape open-channel confluence flow with partially rigid emergent vegetation. Compared to non-vegetated cases, the separation zones in vegetated cases are smaller in both length and width. With a higher vegetation solid volume fraction, the separation zone is divided into two parts, a smaller one right after the confluence point and a larger one on the second half of the curved reach downstream of the confluence. With the increase in the solid volume fraction, the differences in velocities and bed shear stress between the convex and concave banks become larger. With the same solid volume fraction, a larger vegetation density caused more disturbance on the tributary than that resulting from a larger stem diameter.

Fortes et al. [7] simulated the 2019 Typhoon Hagibis at the Nanakita River using a dynamic roughness model. The model estimates the roughness of the river on a pixel level from the relationship between the Manning roughness coefficient and the degree of submergence of vegetation. The dynamic roughness model showed that the water level profile increased by 7.03% on average. It is found that seasonal variations in vegetation area and height have a clear effect on the flow dynamics of the river. It could be concluded that the water level is proportional to the amount of vegetation in the riparian zones. The highest water level was obtained in the summer, when the vegetation volume is at its peak, and the lowest value during the winter, when there is less vegetation. The vegetation area and average height were demonstrated to have a good correlation with the simulated water levels, with average height achieving the strongest relationship. Therefore, considering only the vegetation area to estimate the water level, as does the static roughness model, would be less efficient. The method to obtain the vegetation location and its distributed height, with the consideration of the parameters in the dynamic roughness model proposed in this study, was proven to be applicable for the purpose of river management in the Nanakita River.

Zhang et al. [8] optimized the ratio of the vegetation height-averaged velocity to the water depth-averaged velocity. In the vegetated area, two depth-averaged velocities which included the water depth-averaged velocity and the vegetation height-averaged velocity were defined. The N–S equation with an optimized velocity ratio can predict the lateral distribution of the water depth-averaged streamwise velocity in the open channel partially covered by the submerged vegetation. In both the non-vegetated area and submerged vegetation area, different parameters, including the transverse eddy viscosity coefficient, friction coefficient, porosity and the drag force coefficient, were introduced to determine the analytical solution. Additionally, the methods for calculating the parameters in different zones were discussed. Results showed that a relatively satisfactory prediction of the transverse distribution of water depth-averaged streamwise velocity in the channel flow with submerged vegetation was obtained using the proposed model.

Yu et al. [9] conducted two-dimensional numerical simulations and compared the simulation results to those of laboratory experiments. In their numerical simulations, six different Reynolds-averaged Navier–Stokes (RANS) turbulence models were used. It is found that the shear stress transport k-w model achieves the most consistent simulation results with those of experiments for the longitudinal mean flow velocity distribution at the centerline, and the Reynolds stress model provides the poorest results. Results showed that the turbulence intensity determined using the Reynolds stress model, standard k-w model and the Shear-stress transport k-w model is stronger than that calculated using the standard k-ε model, RNG k-ε model and realizable k-ε model. For the prediction of turbulent structure around rigid emerged vegetation via 2D numerical simulation, the Shear-stress transport k-w model can achieve better results than the standard k-ε model, RNG k-ε model, realizable k-ε model, standard k-w model and Reynolds stress model.

The interaction of the bedform and vegetation cover significantly affects turbulent flow parameters. Based on laboratory experiments with the presence of submerged rigid vegetation elements in the channel bed of a straight pool with different entry and exit slopes, Nosrati et al. [10] studied the turbulent flow field in pools with vegetation cover. It is found that the suitable method for estimating the shear velocity for the flow in the presence of submerged vegetation in the channel bed is the TKE method. The shape and the location of the maximum value of the Reynolds stress distribution depend on the slopes of the entrance and the exit section of the pool. In addition, the layout pattern of vegetation elements also affects the shape and the location of the maximum value of the Reynolds stress distribution. The maximum Reynolds stress and TKE occur at a larger distance above the bed surface, depending on the area density of vegetation. The irregular distributions of Reynolds stress and TKE result from the secondary circulations associated with the irregular distribution pattern of vegetation elements. The momentum between flow, bedform and vegetation elements is mostly transferred via sweep and ejection events. Toward the water surface, the outward event becomes the dominant event. In the presence of an irregular distribution of submerged vegetation elements in the pool bed, ejections become dominant, and then the outward event becomes stronger as the area density of vegetation decreases.

Shahiri Tabarestani et al. [11] carried out field measurements at seven cross-sections along two reaches of the Babolroad River and calculated the bed load transport rate. Suspended sediment discharge was calculated through applying sediment rating curves. The total sediment load was determined based on field measurements along these two reaches of the Babolroad River. On the other hand, they determined the erosion intensity and sedimentation rate from a watershed by means of empirical models, including the modified Pacific Southwest Inter-Agency Committee (MPSIAC), the erosion potential method (EPM) and Fournier. The results using the EPM and MPSIAC models were compared to those of the field measurements and indicated that both models provided good accuracy, with differences of 22.42% and 20.5% from the field results, respectively. Results showed that the Fournier method is not an efficient method since it is unable to consider the erosion potential.

3. Conclusions and Future Works

The application of the results of this Special Issue may help researchers to better understand some important factors in fluvial hydraulics in the presence of vegetation in channel beds, including the resistance to flow and flow velocity for sediment transport. This may help river engineers to apply results of these research works in engineering practice. A variety of experimental and numerical studies in eco-hydraulics must be carried out in order to improve our knowledge about the interaction of flow and vegetation under different conditions. For example, in the presence of vegetation in the channel bed, the effects of a moving bedform on the turbulent flow structures should be investigated through considering the different geometry of a channel under both open channel flow and ice-covered flow conditions. The impact of different types and densities of vegetation on the flow structure in hydraulic models needs to be clarified.