Editorial

5 pages, 170 KiB

Open AccessEditorial

Fluvial Hydraulics in the Presence of Vegetation in Channels

by Hossein Afzalimehr and Jueyi Sui

Water 2023, 15(16), 2907; https://doi.org/10.3390/w15162907 - 11 Aug 2023

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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 [...] Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

Research

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16 pages, 2545 KiB

Open AccessArticle

Comparison of Velocity and Reynolds Stress Distributions in a Straight Rectangular Channel with Submerged and Emergent Vegetation

by Mohammad Reza Tabesh Mofrad, Hossein Afzalimehr, Parsa Parvizi and Sajjad Ahmad

Water 2023, 15(13), 2435; https://doi.org/10.3390/w15132435 - 01 Jul 2023

Cited by 4 | Viewed by 1247

Abstract

Vegetation in rivers and streams plays an important role in preventing erosion and improving bank stability. Comparison between emergent vegetation (bank vegetation) and submerged vegetation, in terms of velocity and Reynolds stress distributions, for the same aspect ratio and flow discharge, has received [...] Read more.

Vegetation in rivers and streams plays an important role in preventing erosion and improving bank stability. Comparison between emergent vegetation (bank vegetation) and submerged vegetation, in terms of velocity and Reynolds stress distributions, for the same aspect ratio and flow discharge, has received limited attention in the literature. This study investigates the velocity and Reynolds stress, as well as the log law for submerged and emergent vegetation in a laboratory flume and compares the results for a different set up with different sediment size and aspect ratio but the same discharge. The results indicate that the influence of submerged vegetation on the secondary currents generation is less than emergent vegetation. In addition, the log law application is valid for both submerged and emergent vegetation cases, however, it is valid up to y/h = 0.75 for emergent vegetation (in which the vegetation cover in banks is partly out of the water) but up to y/h = 0.25 for vegetation bank. For both submerged and emergent vegetation, Reynolds stress distribution presents a convex form but with a different turning point. Comparison of the results with those in an artificial pool over submerged vegetation and low aspect ratio (<5) keeps almost the same form for velocity and Reynolds stress distributions but decreases the turning point in Reynolds stress distribution. For the submerged vegetation cover, the location of zero shear stress superposes that of maximum velocity, but for the emergent vegetation approaching the bank vegetation and shifting the maximum velocity towards the bed, the location of zero shear stress approaches the bed. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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22 pages, 5764 KiB

Open AccessArticle

Turbulence Kinetic Energy and High-Order Moments of Velocity Fluctuations of Flows in the Presence of Submerged Vegetation in Pools

by Mohammad Reza Tabesh Mofrad, Parsa Parvizi, Hossein Afzalimehr and Jueyi Sui

Water 2023, 15(12), 2170; https://doi.org/10.3390/w15122170 - 08 Jun 2023

Cited by 1 | Viewed by 1445

Abstract

The flow in arid and semi-arid regions changes significantly during seasons, letting many vegetation patches develop in different parts of rivers. In the presence of aquatic plants in streams, different flow structures have resulted. When the water level increases in these rivers, the [...] Read more.

The flow in arid and semi-arid regions changes significantly during seasons, letting many vegetation patches develop in different parts of rivers. In the presence of aquatic plants in streams, different flow structures have resulted. When the water level increases in these rivers, the presence of vegetation patches influences the turbulent flow structures, which may considerably change the estimation of key hydraulic parameters. The results of earlier investigations indicated that a wide range of submerged and non-submerged vegetation influences the hydrodynamic features of flows in rivers and streams. In the present investigation, two pools with various slopes of entry and exit sections were used to conduct eight independent experiment runs. In addition, a vegetation patch over the entire pool section has been set up to investigate the effects of the vegetation patch on flow structures in pools. The effect of two slopes of 5 and 10 degrees for both entrance and exit of the pools on flow structure has been investigated. Considering two aspect ratios of 2.0 and 2.7, the distributions of flow velocity, Reynolds normal and shear stresses, turbulence intensities, turbulent kinetic energy (TKE), quadrant analysis, and spectral analysis have been studied at the trailing edge of the vegetation patch along an artificial pool. Results show that, for large entrance and exit slopes (10 degrees), the TKE distribution profiles have no specific form. However, the TKE values have a convex-shaped distribution pattern with the maximum TKE value near the bed when the slopes of the entrance and exit sections of the pool are small (5 degrees). Results showed that both ejections and sweeps govern the turbulence structures and coherent motions at the trailing edge of the vegetation patch along the pool. The geometry, entrance, and exit slopes of the pool have no effect on the validation of power spectral function compared to the presence of a vegetation patch in a flatbed. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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23 pages, 4283 KiB

Open AccessArticle

Double Parameters Generalization of Water-Blocking Effect of Submerged Vegetation

by Chunlin Qiu, Jiesheng Huang, Shihe Liu and Wenhao Pan

Water 2023, 15(4), 764; https://doi.org/10.3390/w15040764 - 15 Feb 2023

Cited by 1 | Viewed by 988

Abstract

Submerged vegetation has strong vitality, and the root system is highly developed. Because this vegetation has a good bank-solidifying-and-beautifying effect, it is widely used in ecological river construction. However, the open channel flow field and water-blocking mechanism of submerged vegetation are complicated. It [...] Read more.

Submerged vegetation has strong vitality, and the root system is highly developed. Because this vegetation has a good bank-solidifying-and-beautifying effect, it is widely used in ecological river construction. However, the open channel flow field and water-blocking mechanism of submerged vegetation are complicated. It is not convenient to use this kind of original model directly in engineering calculation, but it can be much more convenient if the original model is generalized into a simple model. However, there are not many generalization models, so it is necessary to propose a simple generalization model of the water-blocking effect of submerged vegetation to facilitate engineering calculation. Upon theoretical analysis, numerical calculation and experiment data analysis, the following conclusions are obtained: As the basis of generalization, in order to make up for the deficiency of experimental results, a new numerical simulation model for the flow field of submerged vegetation open channel flow was firstly proposed. For the purpose of this research, a simple generalization model of the water-blocking effect of submerged vegetation was proposed. Finally, two parameters of generalized roughness coefficient and virtual channel elevation were obtained to reflect the water-blocking effect. They can be substituted directly into a planar two-dimensional model in engineering. It achieves the ultimate goal of convenient engineering calculation. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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15 pages, 3453 KiB

Open AccessArticle

Characteristics of Shallow Flows in a Vegetated Pool—An Experimental Study

by Parsa Parvizi, Hossein Afzalimehr, Jueyi Sui, Hamid Reza Raeisifar and Ali Reza Eftekhari

Water 2023, 15(1), 205; https://doi.org/10.3390/w15010205 - 03 Jan 2023

Cited by 4 | Viewed by 1537

Abstract

Pools are often observed in gravel-bed rivers, together with the presence of vegetation patches. In the present study, a conceptual model of a gradual varied flow with both convective deceleration and acceleration flow sections has been constructed in a flume to study turbulent [...] Read more.

Pools are often observed in gravel-bed rivers, together with the presence of vegetation patches. In the present study, a conceptual model of a gradual varied flow with both convective deceleration and acceleration flow sections has been constructed in a flume to study turbulent flow structures. Vegetation patches with extended canopies were planted in the pool sections in order to increase the thickness of the boundary layer inside the inner zone. The effects of different flows (namely decelerating, uniform and accelerating flows) along an artificial pool on flow velocity, shear stress and bursting events have been investigated. In addition, due to the occurrence of secondary currents in shallow streams, the characteristics of turbulent shallow flow have been investigated along two axes that are parallel to the sidewall of the flume. The results showed that the application of the log law should be used with care to estimate shear velocity along a pool with a vegetated bed. The presence of a vegetation patch causes an increase in Reynolds shear stress, especially along the entrance section of the pool where the flow decelerates. The results of the quadrant analysis reveal that the sweep and ejection events have the most dominant influence over the vegetation patch in the pool; however, the contributions of outward and inward events increase near the bed, especially in the entrance section of the pool where the flow is decelerating. The distribution of stream-wise RMS of turbulence intensity along the pool generally presents a convex shape. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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23 pages, 5625 KiB

Open AccessFeature PaperArticle

Effects of Submerged Vegetation Arrangement Patterns and Density on Flow Structure

by Mahboubeh Barahimi and Jueyi Sui

Water 2023, 15(1), 176; https://doi.org/10.3390/w15010176 - 01 Jan 2023

Cited by 3 | Viewed by 2169

Abstract

Aquatic vegetation appears very often in rivers and floodplains, which significantly affects the flow structure. In this study, experiments have been conducted to investigate the effects of submerged vegetation arrangement patterns and density on flow structure. Deflected and non-bending vegetation is arranged in [...] Read more.

Aquatic vegetation appears very often in rivers and floodplains, which significantly affects the flow structure. In this study, experiments have been conducted to investigate the effects of submerged vegetation arrangement patterns and density on flow structure. Deflected and non-bending vegetation is arranged in square and staggered configurations in the channel bed of a large-scale flume. Results showed that the staggered configuration leads to intensified streamwise velocity, turbulence kinetic energy (TKE), and Reynolds shear stress (RSS) compared to the square configuration. When vegetation density is low (λ = 0.04 and λ = 0.07), the produced wake in the rear of the vegetation is more expansive than that with high vegetation density (λ = 0.09 and λ = 0.17) because the velocity in the center of four vegetation elements is lower than that in the middle of two vegetation elements with low vegetation density. Results of TKE in the wake zone of the deflected vegetation indicate that the maximum root-mean-square velocity fluctuations of flow occur at the sheath section (z/H = 0.1) and the top of the vegetation (z/H = 0.4). In the wake zone behind the vegetation elements, the maximum value of the RSS 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. Within the range of vegetation density in this study (0.04 < λ ≈< 0.23), as the vegetation density increases, the negative and positive values of RSS throughout the flow depth increase. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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20 pages, 14001 KiB

Open AccessFeature PaperArticle

Numerical Investigation of Hydrodynamics in a U-Shaped Open Channel Confluence Flow with Partially Emergent Rigid Vegetation

by Zhengrui Shi and Sheng Jin

Water 2022, 14(24), 4027; https://doi.org/10.3390/w14244027 - 09 Dec 2022

Cited by 2 | Viewed by 1444

Abstract

The effects of partially emergent rigid vegetation on the hydrodynamics of a curved open-channel confluence flow were simulated using OpenFOAM. The numerical model using the Volume of Fluid method and the RNG k-ε turbulence model in the Reynolds-averaged Navier–Stokes equations was first validated [...] Read more.

The effects of partially emergent rigid vegetation on the hydrodynamics of a curved open-channel confluence flow were simulated using OpenFOAM. The numerical model using the Volume of Fluid method and the RNG k-ε turbulence model in the Reynolds-averaged Navier–Stokes equations was first validated by existing experimental data with good agreement. Then the characteristics of hydrodynamics were analyzed in aspects of separation zone, water level, streamwise velocities, secondary flows, bed shear stress and flow resistance. Some main conclusions can be drawn from the results. Compared to the non-vegetated cases, the separation zones in vegetated cases are smaller in both length and width. With higher vegetation Solid Volume Fraction (SVF), 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 after the confluence. The main circulation cell shrinks and the circulation near the concave bank moves towards the channel midline. The differences in velocities and bed shear stress between the convex and concave banks become larger with a higher SVF. Under the same SVF, a larger vegetation density has more disturbance on the tributary than a larger stem diameter. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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18 pages, 5313 KiB

Open AccessArticle

Dynamic Roughness Modeling of Seasonal Vegetation Effect: Case Study of the Nanakita River

by André Araújo Fortes, Masakazu Hashimoto, Keiko Udo, Ken Ichikawa and Shosuke Sato

Water 2022, 14(22), 3649; https://doi.org/10.3390/w14223649 - 12 Nov 2022

Cited by 2 | Viewed by 1528

Abstract

Hydraulic models of rivers are essential for vulnerability assessment in disaster management. This study simulates 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 [...] Read more.

Hydraulic models of rivers are essential for vulnerability assessment in disaster management. This study simulates 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. This degree is defined as the ratio of water depth to plant height. After validating the model, the effect of vegetation on the water level in different seasons from April 2020 to March 2021 was assessed. The vegetation area and height were obtained on a pixel level using unmanned aerial vehicle photogrammetry. The dynamic roughness model showed that the water level profile increased by 7.03% on average. The seasonal effect of vegetation was observed, revealing a strong correlation between variations in the vegetation conditions and water level profile. This approach may help mitigate flood damage by indicating the factors that can increase the risk of flooding. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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16 pages, 3185 KiB

Open AccessArticle

An Analytical Solution to Predict the Distribution of Streamwise Flow Velocity in an Ecological River with Submerged Vegetation

by Jiao Zhang, Zhangyi Mi, Wen Wang, Zhanbin Li, Huilin Wang, Qingjing Wang, Xunle Zhang and Xinchun Du

Water 2022, 14(21), 3562; https://doi.org/10.3390/w14213562 - 05 Nov 2022

Cited by 1 | Viewed by 1339

Abstract

Aquatic submerged vegetation is widespread in rivers. The transverse distribution of flow velocity in rivers is altered because of the vegetation. Based on the vegetation coverage, the cross-section of the ecological channels can be divided into the non-vegetated area and the vegetated area. [...] Read more.

Aquatic submerged vegetation is widespread in rivers. The transverse distribution of flow velocity in rivers is altered because of the vegetation. Based on the vegetation coverage, the cross-section of the ecological channels can be divided into the non-vegetated area and the vegetated area. In the vegetated area, we defined two depth-averaged velocities, which included the water depth-averaged velocity, and the vegetation height-averaged velocity. In this study, we optimized the ratio of these two depth-averaged velocities, and used this velocity ratio in the Navier–Stokes equation to predict the lateral distribution of longitudinal velocity in the open channel that was partially covered by submerged vegetation. Based on the Navier–Stokes equations, the term “vegetation resistance” was introduced in the vegetated area. The equations for the transverse eddy viscosity coefficient ξ, friction coefficient f, drag force coefficient C_d, and porosity α were used for both the non-vegetated area and the vegetated area, and the range of the depth-averaged secondary flow coefficient was investigated. An analytical solution for predicting the transverse distribution of the water depth-averaged streamwise velocity was obtained in channels that were partially covered by submerged vegetation, which was experimentally verified in previous studies. Additionally, the improved ratio proposed here was compared to previous ratios from other studies. Our findings showed that the ratio in this study could perform velocity prediction more effectively in the partially covered vegetated channel, with a maximum average relative error of 4.77%. The improved ratio model reduced the number of parameters, which introduced the diameter of the vegetation, the amount of vegetation per unit area, and the flow depth. This theoretical ratio lays the foundation for analyzing the flow structure of submerged vegetation. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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17 pages, 7399 KiB

Open AccessArticle

A Comparative Study on 2D CFD Simulation of Flow Structure in an Open Channel with an Emerged Vegetation Patch Based on Different RANS Turbulence Models

by Songli Yu, Huichao Dai, Yanwei Zhai, Mengyang Liu and Wenxin Huai

Water 2022, 14(18), 2873; https://doi.org/10.3390/w14182873 - 15 Sep 2022

Cited by 5 | Viewed by 2165

Abstract

Aquatic plants widely exist in rivers, which can affect the flow structure in rivers and have an important impact on the evolution of river morphology. The emerged vegetation is an important member of aquatic vegetation in the river, so studying the flow structure [...] Read more.

Aquatic plants widely exist in rivers, which can affect the flow structure in rivers and have an important impact on the evolution of river morphology. The emerged vegetation is an important member of aquatic vegetation in the river, so studying the flow structure around the emerged vegetation patches is of great significance. Computational fluid dynamics (CFD) simulation provides support for the related research works. Applying the appropriate turbulence model is crucial to achieving realistic numerical simulation results. In this study, two-dimensional numerical simulations were carried out and compared with experimental data by six different Reynolds-Averaged Navier–Stokes (RANS) turbulence models, i.e., Standard k-ε model, Renormalization group (RNG) k-ε model, Realizable k-ε model, Standard k-ω model, Shear-stress transport (SST) k-ω Model, and the Reynolds stress model (RSM). CFD is an effective research method, and the results showed that there are different simulation performances with different turbulence models. The shear stress transport k-ω model achieves the most consistent numerical simulation results with the experimental data for the longitudinal mean flow velocity distribution at the centerline, and the Reynolds stress model provides the least consistent numerical simulation with the experimental data. Then the performance of the six models in simulating the flow field characteristics and longitudinal outflow after vegetation patch was compared. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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17 pages, 4735 KiB

Open AccessArticle

Interaction of Irregular Distribution of Submerged Rigid Vegetation and Flow within a Straight Pool

by Kourosh Nosrati, Hossein Afzalimehr and Jueyi Sui

Water 2022, 14(13), 2036; https://doi.org/10.3390/w14132036 - 25 Jun 2022

Cited by 4 | Viewed by 1528

Abstract

The interaction of bedform and vegetation cover significantly affects the turbulent flow parameters. To investigate this interaction, experiments were carried out in both a gravel-bed river and a laboratory flume. The purpose of field investigations was to find the slopes for both the [...] Read more.

The interaction of bedform and vegetation cover significantly affects the turbulent flow parameters. To investigate this interaction, experiments were carried out in both a gravel-bed river and a laboratory flume. The purpose of field investigations was to find the slopes for both the entrance section and exit section of pools, the grain size of the bed material, and the flow condition. Based on field data, without considering any scaling analysis, a straight pool was constructed in a laboratory flume that was 0.9 m wide, 0.6 m deep, and 14 m long. The entry and exit slopes of the straight pool were 7.4° and 4°, respectively. The straight pool had vertical side walls and a gravel bed with a median grain size of d₅₀ = 23.3 mm. Plastic cylinders planted in an irregular pattern in the channel beds were used to model rigid submerged vegetation. The velocity components were recorded by using an ADV at 200 Hz. In this study, the distributions of velocity, Reynolds stress, and TKE were investigated for flows in the presence of submerged rigid vegetation in channel beds with various area densities of vegetation. Results show that the shape of Reynolds stress distribution depends on the entrance and exit slopes of the pool, as well as the irregular distribution pattern of vegetated elements. Inside the pool with the presence of submerged vegetation in the channel bed, the maximum TKE appears above the bed surface with a larger distance depending on the area density of vegetation in the channel bed. However, the momentum exchange and turbulent energy are likely influenced by the secondary circulation of the flows associated with the irregular distribution of vegetated elements in the channel bed. Results of the quadrant analysis show that the momentum between the flow, bedform, and vegetated elements is mostly transferred by sweep and ejection events. The outward event tends to grow toward the water surface, reaching the highest amount near the water surface. At the pool entrance section where the flow is decelerating, the ejection event is dominant near the bed while the sweep event is strong near the water surface. With the decrease in the vegetation density in the pool bed, both the ejection and outward events become dominant. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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19 pages, 3664 KiB

Open AccessArticle

Assessment of Annual Erosion and Sediment Yield Using Empirical Methods and Validating with Field Measurements—A Case Study

by Ehsan Shahiri Tabarestani, Hossein Afzalimehr and Jueyi Sui

Water 2022, 14(10), 1602; https://doi.org/10.3390/w14101602 - 17 May 2022

Cited by 5 | Viewed by 2111

Abstract

To implement soil conservation approaches, it is necessary to estimate the amount of annual sediment production from a watershed. The purpose of this study was to determine the erosion intensity and sedimentation rate from a watershed by employing empirical models, including the modified [...] Read more.

To implement soil conservation approaches, it is necessary to estimate the amount of annual sediment production from a watershed. The purpose of this study was to determine the erosion intensity and sedimentation rate from a watershed by employing empirical models, including the modified Pacific Southwest Inter-Agency Committee (MPSIAC), the erosion potential method (EPM), and Fournier. Moreover, the accuracy of these empirical models was studied based on field measurements. Field measurements were conducted along two reaches of Babolroud River. Total sediment transport, including suspended load and bed load, was predicted. Bed load transport rate was measured using a Helly–Smith sampler, and suspended load discharge was calculated by a sediment rating curve. The results of this study indicate that the erosion intensity coefficient (Z) of the Babolroud watershed is 0.54, with a deposition rate of 166.469 m³/(km²_.year). Due to the existence of unusable crops, the highest amount of erosion appeared in the northern region of the watershed. The results using the EPM and MPSIAC models were compared with field measurements and indicated that both models provided good accuracy, with differences of 22.42% and 20.5% from the field results, respectively. Additionally, it could be concluded that the Fournier method is not an efficient method since it is unable to consider the erosion potential. Full article

(This article belongs to the Special Issue Fluvial Hydraulics in the Presence of Vegetation in Channels)

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