Interactions between vegetation, water flow and sediment transport: A review*

WANG Chao （王超）, ZHENG Sha-sha （郑莎莎）, WANG Pei-fang （王沛芳）, HOU Jun （侯俊）

Key Laboratory of Integrated Regulation and Resource Department on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China, E-mail: cwang@hhu.edu.cn

Interactions between vegetation, water flow and sediment transport: A review*

WANG Chao （王超）, ZHENG Sha-sha （郑莎莎）, WANG Pei-fang （王沛芳）, HOU Jun （侯俊）

Key Laboratory of Integrated Regulation and Resource Department on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China, E-mail: cwang@hhu.edu.cn

(Received May 28, 2014, Revised January 6, 2015)

The vegetation, as one of the most important components, plays a key role in the aquatic environment. This paper reviews recent progress on the complex interaction between the vegetation and the water flow. Meanwhile, the relationships between the vegetation and the sediment transport are discussed. The vegetation characteristics, such as the shape, the flexibility and the height, have significant effects on the flow structures. The density and the arrangement of the vegetation influence the flow velocity in varying degrees and the flow resistance increases with the increase of the plant density. In turns, the growth of aquatic plants is influenced by the water flow via the direct effect (stretching, breakage, uprooting, etc.) and the indirect effect (changes in gas exchange, bed material distribution, sediment resuspension etc.). Numerical models were developed and widely used for the flow through vegetated waterways, and the results could be applied to solve engineering problems in practice. The sediment is essential for the survival of most vegetation. The existence of the vegetation helps to resist the deformation and the erosion of the bed sediment, to maintain the bed stability and to improve the water quality by removing suspended particles. Additionally, the effects of the sediment transport on the growth of the vegetation mainly consist of the reduction of their photosynthetic capacity by decreasing the water transparency and hindering the exchange of gas and nutrients between plants and water by attaching particles to plant leaves. Therefore, the interaction between the vegetation and the sediment transport is great and complicated. In order to establish a healthy aquatic ecosystem, it is important to study the relationships between the vegetation, the water flow and the sediment transport.

interaction, vegetation, water flow, sediment transport

Introduction

The aquatic ecosystems include the water, the sediment and various aquatic organisms. The vegetation contributes to the sustainable development of the aquatic environment. It provides food and shelter to many organisms and controls the ecological system in rivers, lakes, estuaries and coastal areas[1,2]. On one hand, the vegetation changes the flow structure and the sediment transport[3,4], on the other hand, the vegetation can purify the polluted water and beautify the environment[5-7]. According to the existing ways, aquatic plants are generally divided into the following classes: the emerged plants, the floating plants and the submerged plants. In the past, the vegetation used to be considered as a cause of river blockages and subsequently a cause of flooding[8], so in the river management, the vegetation is to be removed. Recently, some suggestions are gradually made to retain the vegetation to avoid costly and ecological damaging procedures of removing vegetation. The vegetation’s positive effects on the water quality become to be known, such as removing the contaminant, increasing the bed stability, assisting the water restoration/rehabilitation, controlling the flow velocity, reducing the water turbidity, as well as diversifying the habitat[9-11]. Moreover, the vegetation offers a local flow resistance by increasing the drag force to reduce the velocity, while simultaneously decreasing the shear stress as responsible for the sediment transport and the erosion[12]. Recently,the flow structures in vegetated waterways were widely studied by field observations or indoor experiments[13-15]. Accordingly, numerical models simulating these phenomena were developed[16-18], including the use of the Reynolds averaged Navier-Stokes equations (RANS) and the large eddy simulation (LES).

The sediments are traditionally viewed as a sink for the contaminants in the water ecosystem, and they can also act as a source of the contaminants when they are disturbed, moved or relocated[19]. The sediment resuspension might be resulted from a range of natural processes (e.g., wind wave, storm and bioturbation) and the anthropogenic activities (e.g., dredging, shipping and trawling). The persistent sediment transport would bring about changes of riverbed[20]. Many kinds of aquatic plants covering the bottom sediment can reduce the sediment transport significantly, which is very important to keep the stability of the riverbed. The vegetation can substantially reduce the water flow velocity and the turbulence as compared with that in non-vegetations areas[21,22]. The disperse water flow weakens the bed shear stress and reduces the sediment transport[23]. Fine particles can be adsorbed by plant stems and leaves to decrease the water turbidity. The developed root systems forming a barrier at the sediment-water surface can increase the particle adsorption and restrain the sediment resuspension[24]. In addition, the exudates of the root systems have a strong ability to absorb different kinds of compounds[25]. Generally speaking, the vegetation has physical and biochemical effects on the sediment resuspension.

The aquatic plants modify the aquatic environment through their development and metabolic activity. Conversely, the properties of the aquatic environment have direct (stretching, breakage, uprooting, etc.) and indirect effects (changes in gas exchange, bed material distribution, sediment resuspension, etc.) on the growth, the development and the reproduction of the aquatic plants[26]. Fast currents bring about mechanical damages on the vegetation[27,28]. Additionally, fast currents also increase the sediment resuspension in sparsely vegetated areas, which further reduces the light available for the growth of the vegetation[29]. However, the water movements help to disperse the seeds or the vegetative fragments of the aquatic plants. Besides the water flow, the sediment transport is another important factor that affects the vegetation survival. The surface sediment transport, which makes the root system of the plants unstable, would be unfavourable for the growth of the plants[30]. The suspended particles adhered to the plant stems and leaves reduce their photosynthetic capacity and block the exchange of gas and nutrients between the plants and the water[31]. In view of the key functions of the vegetation in the aquatic ecosystem, it would be beneficial to preserve such communities in the water body.

The interactions between the vegetation, the water flow and the sediment transport are important in the aquatic environment and they involve the hydraulics, the river dynamics, the sedimentology, the environmental science, the plant ecology and other interdisciplinary fields. The hydrodynamics studies mainly focus on the flow characteristics under the influence of the vegetation[32-35]. The research methods include the field observation and the flume experiment. Using artificial rigid and flexible plants in the laboratory experiment would result in deviations as compared with using natural plants. Meanwhile, less attention was paid to the sediment resuspension. The studies of the sediment mainly focus on the physical and chemical characteristics of the sediment and the transport processes of the sediment[36-38], with the interaction between the sediment and the attached living things being ignored. The studies in the environmental science concentrate on the release of chemical pollutants during the sediment resuspension process and the removal of the pollutants by the vegetation to improve the water quality[39-41]. Field sampling and laboratory analysis are the major tools in these researches. However, without a due consideration of hydrodynamic characteristics, the obtained laboratory results may not be consistent with the field results. Studies in the plant ecology are interested in the biological and ecological characteristics of the vegetation[42-44]. Great importance may be attached on the vegetation’s influence on the flow characteristics and the sediment transport.

The review of this paper focuses on the following two aspects: (1) the interaction between the vegetation and the water flow, (2) the interaction between the vegetation and the sediment transport. In addition, the type and the role of the vegetation in the aquatic environment are discussed at the beginning. Meanwhile, this review places emphasis on the deficiencies of the existing studies. At last, we make some suggestions for the future research.

1. Type and role of vegetation in aquatic environment

The vegetation plays a key role in the aquatic environment. The vegetation has some advantages, the high treatment efficiency for organics, nutrients and metals, the removal ability of suspended particulate matters, when the retention of suspended particulate matters is a main way of purifying the water body. The vegetation can retrain the sediment resuspension and change the characteristics of the sediment consiserably[23]. The aquatic plants are the main study objects in the water column. According to their different ways in the aquatic environment, the aquatic plants are generally divided into the emerged plants, the floating plants and the submerged plants.

The emerged plants refer to the plants with the roots being grown in the sediments and the stems andthe leaves reaching out of water to blossom in the air. This kind of plants has strong resistance and adaptability in the aquatic environment. Meanwhile, the emerged plants usually have a certain economic benefit because of large biomass and fast growth rate. The typical emerged plants are Phragmites australis, Typha angustifolia and Oenanthe javanica. The floating plants are those grown in shallow waters, with the leaves floating on the water surface and the roots growing in the sediments or floating in the water. Due to their own characteristics, e.g., thermophily and strong pollution tolerance, most of the floating plants have a positive effect on cleaning water. However, they possess strong reproductive ability, much more manpower, materials and financial resources are required to harvest. The common floating plants include Common duckweed, Eichhornia crassipes, and Nymphaea tetragona. The submerged plants refer to those with roots or fibrous roots fixing in the sediments and leaves growing under the water surface. Each part of the submerged plants can absorb nutrients to grow well. The developed aerenchyma is advantageous in exchanging gas in the water column, especially under the anoxic conditions. Elodea Canadensis, Vallisneria asiatica and Potamogeton crispus belong to the submerged plants. Table 1 shows the major roles of different tissues of plants in the aquatic environment (based on Vymazal[45]). Any parts of plants have different effects on the improvement of the water quality.

Table1 Major roles of different tissues of plants in aquatic environment

2. Interaction between vegetation and water flow

Both the river flood and the water ecological restoration are related with the flow structure in the vegetated waterway. Hence, it is necessary and even urgent to study the ecological hydraulics and the flow characteristics. The resistance of the vegetation to the water flow, the turbulence characteristics of the vegetated flow and the characteristics of the vortex field have an important bearing in the flood control and the water ecological restoration[46].

A great importance was paid to the study of the flow characteristics influenced by the vegetation[47,48]. The vegetated flow is fairly ubiquitous in rivers. The vegetation causes changes in the motion behavior of the water flow. On the one hand, the water flow slows down owing to the resistance of the vegetation, and the pollutants are removed from the water, which helps the water purification. On the other hand, the vegetation reduces the deformation and the erosion of the riverbed, which helps the channel stability. However, during the flood period, the presence of the vegetation reduces the flow velocity, which might raise the water level and weaken the flood discharge capacity, and finally exacerbate the flood disaster.

3. Effects of vegetation on flow characteristics

The Karman-Prandtl logarithmic law is often used to describe the velocity profile in open channels. Once the vegetation establishes in the channels, the flow structures will be affected remarkably. Based on the study of Stephan and Gutknecht[49], a modified velocity profile equation (Eq.(1)) can be obtained, and the impact on the effective plant height is expressed as Eq.(2):

where u is the time averaged stream-wise velocity, u*is the friction velocity, κ is the Karman constant (=0.4κ as is obtained experimentally), z is the vertical coordinate,sk is the equivalent sand roughness, C is the integration constant (=8.5C as is obtained according to Nikuradse[50]), andeh is the effective plant height.

Although Eq.(2) can describe the velocity profile well, it is not convenient to use it to assess the flow rate in the channels. In view of the flood safety and in order to better estimate the flood discharge capacity, the calculation of the flow rate is very important. Chezy and Manning formulations are empirical equations used widely for estimating the flow rate or the flow velocity. Chow[51]analyzed and discussed the vegetated channels firstly in the last century. Some other attempts were made to specify the effect of the vegetation in these formulations[10,52]. The most widely used resistance parameter is the flow resistance coefficient, the Manning coefficient, and the equations are as follows:

where v is the mean flow velocity, Q is the mean flow rate, A is the discharge area, R is the hydraulic radius, S is the hydraulic gradient, and n is the Manning roughness coefficient[53]. In these equations, the value of n is estimated from experimental data. Genetally speaking, it is fairly subjective and may be highly inaccurate[54]. Moreover, those resistant coefficients are not specifically designed for vegetated channels, and therefore their resistance values are often underestimated[55]. The estimated n is greatly affected by the vegetation factors, including the stem flexibility, the plant height, and the vegetation porosity[56,57]. In the flow characteristics influenced by the vegetation, the drag force is a very important factor. The drag force is the friction drag as a result of the vegetation and acts in the opposite direction to the forthcoming flow. The drag force theory is based on the flow surrounding a cylinder[4]under emergent conditions as shown in Eq.(5)

where F is the drag force, CDis the drag coefficient, Avis the vegetation frontal area in the longitudinal direction, ρ is the water viscosity, and U is the mean longitudinal flow velocity. The relationship between the flow structure, the drag force and the vegetation characteristics was explored[58,59].

White and Corfield[60]proposed an empirical formula for calculating the drag coefficient under emergent conditions, and the formula is valid for Re≥1 000

Moreover, Tsihrintzis[61]included the biomechanical properties of the vegetation ()k and the vegetation porosity ()γ parameters as shown in the following equation

The Reynolds number for laminar and turbulent flows was defined by Serra et al.[62]based on the stem diameter for the emergent vegetation as

where V is the mean longitudinal flow velocity, D is the stem diameter, and the ν is the kinematic viscosity.

The vegetation characteristics have a remarkable impact on the flow structure. The vegetation porosity and the flexibility reduction may lead to a reduction of the layer transition size[63]. The studies of the flow resistance caused by the flexible vegetation in open channels show that the interaction between the flexible vegetation and the water flow is considerable[64-66]. The simulated experiments show that the density and the arrangement of the vegetation influence the flow velocity in some degrees and the flow resistance increases with the increase of the plant density[67,68]. The increase of the vegetation density affects the cross section, as well as significantly increases the flow resistance[52]. Although the velocity of the mean flow might be reduced with the increase of the density of the vegetation, the turbulence may remain unchanged or even increased[69]. In addition, the length, the height and the flexibility of the vegetation affect the flow structure differently[56,70].

The turbulence intensity is an important parameter, that reflects the turbulence structure. Compared with the case of no vegetation, the turbulence intensity of the flow in the vegetated water body distributes obviously different. Nepf and Vivoni[71]suggested that the flexible vegetation determines the flow characteristics and the turbulence structure. The area above the vegetation is called the “vertical exchange zone” and the flow exchange in this area mainly in the verticaldirection. The Reynolds stress is found significantly increasing at the top of the canopy and has a positive correlation with the stem density[72]. The topmost vegetation under the submerged condition mostly leads to the highest shear stress. Folkard et al.[11]also found that the turbulence kinetic energy is increased at the top of the vegetation canopy. The “longitudinal exchange zone” represents the inner area of the vegetation. In this area, the horizontal flow in the longitudinal direction is the dominant flow pattern. The turbulence is mainly caused by the disturbance and the resistance of the vegetation and the pressure gradient of the water body. When the plants are under the non-submerged condition, the exchange of the turbulence is mainly in the longitudinal direction. Once the plants are submerged, the exchange of the turbulence is mainly in the vertical direction at the top of the plants. Moreover, with the increase of the water depth, the vertical turbulence becomes more and more acute[73].

In the vegetated waterway, a great deal of attention was paid to the velocity profile under non-submerged[12,73]and completely submerged conditions[52]. Normally, different structural characteristics of plants have different effects on the flow velocity. These structural characteristics include the uniformity[74], the arrangement[75], the planting slope[76,77], the planting density[78]and the flexibility[79]. The velocity profile above the canopy is normally assumed to follow the well-accepted logarithmic law, whereas that below the canopy follows the exponential law or the power law. However, the logarithmic, exponential, and power laws can be affected by some characteristics of the vegetation, resulting in a highly inaccurate description of the velocity distributions in the vegetated channels[80]. Most velocity profiles in the vegetated waterways are grouped into multilayers with different equations. Chen and Kao[81]pointed out that the velocity profiles in the vegetated channel under the completely submerged condition could be divided into four layers. The velocity distribution in the first zone, strongly affected by the channel bed, has the logarithmic boundary layer profile. In the second zone, among the vegetation, sufficiently far from both the channel bed and the top of the vegetation, the velocity profile is uniform. In the third zone, close to the top of the vegetation, the profile could be approximated expressed by an exponential function. Finally, in the fourth zone, the zero plane, the velocity distribution is in a logarithmic profile[82].

Wang and Wang[33]used an emerged plant (Acorus calamus) and a submerged plant (Ceratopteris thalictroides) to study the velocity profiles in the flume, respectively. Under the emerged condition, the velocity inflection point is found at the branch of the stem, and the velocity is decreased above the branch while increased gradually below the branch. Under the submerged condition, the velocity inflection point is at the plant canopy, and the velocity profile is in an inverse “S” shape. Above the canopy, the flow velocity is increased significantly. However, the flow velocity decreases quickly in the vegetation layer. Shi and Li[83]used two plants (Myriophyllum verticillatum and Hydrilla Rich) to simulate the velocity profiles in rivers under the submerged condition. They found that the Karman-Prandtl equation could be used to describe the velocity distribution above the canopy and the profile is in a “J” shape. Meanwhile, the velocity profile is in an “S” shape in the canopy, which indicates a reverse flow velocity gradient in the canopy layer.

4. Numerical simulation of flow in vegetated waterway

The numerical simulation is a routine but effective method to reveal the relations between the flow structure and the vegetation. For a flexible vegetation with a relatively high stiffness, Kutija and Hong[84]developed a 1-D model using the Timoshenko’s theory to determine the bending of the vegetation. Erduarn and Kutija[85]continued this work and developed a quasi-three-dimensional method. The deflection of the vegetation stems with a moderate flexibility was computed by Velasco et al.[86]by using the classical elastic beam equation. The vertical distributions of the velocity and the Reynolds stress were simulated by a 1-D model. A 2-D LES model is used to simulate the wavy motion of the flexible vegetation[87]. A complex“plant grid” is used in this model to track the movement of each stem and the equation of motion of each flexible stem is solved directly. This approach is sophisticated but computationally expensive.

Numerical models were developed and widely used for the flow through a vegetation with rigid stems. The most typical models are the RANS models and the LES models. For the RANS models, oneequation or two-equation anisotropic turbulence models were used[88-91], or the multi-equation anisotropic turbulence models[92-94]. The LES models were also developed recently, for example, Patton et al.[95]used the one-equation -k l sub-grid turbulence model, Cui and Neary[96]used the Smargorinsky sub-grid scale turbulence model with a dynamic adjustment of the closure coefficient. Huai et al.[97]used the LES model to study the open channel flows with a non-submerged vegetation. Compared with the RANS models, the use of the LES models is limited in practice. The number of grids used in the LES models is over 100 times of that used in the RANS models. The LES models can provide more information about the three dimensional turbulent flow fields and the anisotropic Reynolds stress. The results of the vertical distribution of the velocity and the Reynolds shear stress calculated by the LES models are more reasonable than the results ob-tained by the RANS models.

The RANS models describe the conservation of mass and momentum of the fluid, and are as follows:

Continuity equation

Momentum equation

where xi(=x, y, z) are the coordinates in the longitudinal, transverse and vertical directions, respectively, ui(=u, v, w) are the time-averaged velocity components in x, y and z directions, respectively, t is the time, vmis the molecular viscosity, vsis the subgrid scale viscosity, ρ is the density of the fluid, p is the static reduced pressure, Fi(=Fx, Fy, Fz) are the resistance force components per unit volume induced by the vegetation in x, y and z directions, respectively, gi(=0,0,-9.8 m/s2) are the components of the gravifitational acceleration.

The sub-grid scale viscosity is generally specified by the Smargorinsky model as follows[98]

where Csis the model coefficient, Δ is the filter width, andSij=0.5(∂ui/∂xj+∂uj/∂xi) is the strain rate of the large scale field.

Although this approach is widely used in numerical simulations, its one drawback is that the grid convergence of the solution is difficult to attain as the governing equations are now grid dependent. To overcome this difficulty, the LES approach is adopted to do a modification. In this approach, only the very large scale turbulence eddies will be resolved and the smaller scale eddies will be modeled. The Smargorinsky model used in this approach is Eq.(11) with Cs=0.15, Δ=0.1Lzand Lzis the water depth above the vegetation.

The vegetation affects the water flow and generates a resistance force. The resistance force is determined by the quadratic friction law. The forceif per unit depth on a single vegetation stem is given as

whereDC is the drag coefficient of the stem,vb is the width of the stem. The average force per unit volume within the vegetation domain is

where N is the number density. The above force terms vary with the vegetation characteristics.

In order to improve the accuracy of the numerical simulations, it is necessary to study the model parameters through physical experiments. Recently, extensive field and laboratory studies of the flows in vegetated waterways were carried out[99-101]. Accordingly, the numerical simulation models could be improved to become more sophisticated. The revised models were used to simulate various vegetated flow structures, such as those with the flexible vegetation, the rigid vegetation, and the mixed flexible and rigid vegetations. After verifying the simulated results repeatedly, the useful results could be applied to solve engineering problems in practice.

In recent years, a large number of useful results were obtained through numerical simulations. Wang et al.[102]used depth-averaged 2-D hydrodynamic models to simulate the flow fields in vegetated and nonvegetated zones in the Nansi Lake. Huai et al.[103]applied the mixing length approach and improved its expression according to the Karman similarity theory to analyze the flow structure in the vegetated region of the flow with submerged and emerged rigid vegetations. They found that the flow is divided into four and two regions for submerged and emerged vegetations, respectively. Wang and Wang[18]used a 3-D hydrodynamic model to forecast the vertical distribution of the horizontal velocity through submerged vegetation regions in a shallow lake. Li and Xie[98]developped a 3-D numerical model to simulate the hydrodynamics of the submerged flexible vegetation with or without foliage, and the results show that the flexibility of the vegetation reduces both the vegetation-induced flow resistance force and the vertical Reynolds shear stress, while the presence of foliage further enhances these reduction effects. A depth-averaged model was used by Zhang et al.[104]to simulate the hydrodynamics of the free surface flows in watercourses with emerged and submerged vegetations. They found that the resistance due to the emerged or submerged vegetation can be represented accurately by the Manning roughness equation.

It is well known that the velocity profile of the flow in a vegetated waterway can mainly determinethe accuracy of the calculated discharge. Meanwhile, the velocity profile is important to estimate the effects of the vegetation on the river bed load transportation and the related erosion, deposition, and bed evolution. In addition, the pollutant transport is also affected by the velocity profile. The velocity profiles in a vegetated waterway are related not only to the bed shear stress, but also to the vegetation drag force[105,106]. The profiles show variance with different types of plants. Huai et al.[107,108]used submerged and floating rigid vegetations to simulate the velocity profiles in open channels. For the submerged rigid vegetation (Fig.1(a)), the vertical water layer is divided into three parts, including the non-vegetated layer (Zone I), the outer vegetated layer (Zone II) and the inner vegetated layer (Zone III). For the floating rigid vegetation (Fig.1(b)), the vertical water layer is divided into four parts, including the outer vegetated layer (Zone I), the inner vegetated layer (Zone II) and the non-vegetated layer (Zone III and Zone IV). The full vertical velocity profiles can be described by different formulas with a three-layer analytical model. The analytical predictions of the velocity profile over the whole flow depth were compared with the results obtained by experiments and other researchers, and the good agreement shows that the three-layer model can be used to predict the velocity distribution of the open channel flow with submerged and floating rigid vegetations.

Fig.1 Schematic diagram of the velocity profile in open channel flow with submerged (a) and floating (b) rigid vegetations

5. Effect of water flow on vegetation growth

Compared to the studies of the vegetation affecting the flow characteristics, the research findings about the vegetation growth influenced by the water flow are fewer. The effects of the water flow on the aquatic plants include the direct one (the stretching, the breakage, the uprooting, etc.) and the indirect one (the changes in the gas exchange, the bed material distribution, the sediment resuspension, etc.)[26]. Both of them could affect the vegetation growth, development and reproduction in a complex way.

The ability of the aquatic plant to withstand the water flow without suffering mechanical damages relies either on maximizing its resistance of breakage and uprooting or minimizing the hydrodynamic forces[109,110]. In order to adapt to different kinds of water flows, the vegetation would change its shape to adjust to the increasing flow velocity through flattening close to the substrate, aligning its shoots in the flow direction and compacting its leaves, etc.[111]. The water flow can affect the growth of the vegetation significantly. For example, the submerged plants might appear in streamlined shapes, with strap-like leaves and flat shoots as opposed to a rigid stem, and broad leaves to survive better in the flowing water[112,113]. In addition, the growth, the development and the reproduction of the vegetation are all influenced by the flow water directly.

Table2 Reduction rate (%) of SPM by the vegetation in the aquatic environment

For instance, some morphological changes, including the reduced plant height, the reduced leaf size and the increased belowground organs, can be frequently observed in the plant growthg under hydrodynamic conditions[114,115].

Compared with the direct effects, the water flow has various indirect effects on the plant growth. The light and nutrients are two key factors for plant photosynthesis. Under hydrodynamic conditions, the water stirring reduces the thickness of the boundary layer and leads to enhanced nutrients[116], however, the sediment resuspension induced by the water movement can potentially limit the light penetration in the water[117]. Besides these, the dispersal of seeds or vegetative fragments of the aquatic plants relies on the water movement[118,119]. Some species of plants break and produce viable shoot fragments, which would disperse, while protecting the belowground organs against movement[120,121]. Due to the interaction of the water flow and the aquatic plants, it is very important to have a reasonable vegetation distribution in keeping the flood discharge ability for establishing a healthy aquatic ecosystem.

6. Interaction between vegetation and sediment transport

6.1 Effects of vegetation on sediment transport

Coarse-grained bed sediments and fne-grained suspended sediments involve some important information of erosion and weathering in the source area, which is important in understanding the supergene geological processes[122]. Once the vegetation occurs in a river, the flow structure and the sediment transport become very complicated. When the water flows through the vegetated waterways, the stem, the branch and the leaf of the vegetation spread the flow through the function of shunt and block. The disperse water flow reduces the bed shear stress and weakens the sediment transport. Meanwhile, the existence of the vegetation decreases the flow velocity due to the blocking effect. Then the part of larger particles coming from upstream settle down, which leads to the rise of the bed surface. At the same time, if the plants concentrate in the main stream, the increased flow velocity near the river edge and slope would aggravate the slope erosion (except for rock slope), which makes the river more shallow and wide[20].

The existence of plants not only changes the riverbed landform, but also influences the water ecological environment. The sediment resuspension and the consequent increase in the water turbidity have numerous effects on the primary producers, the zooplankton communities and various predator-prey interactions[123]. The aquatic plants significantly reduce the sediment resuspension and erosion[124]. One of the main mechanisms behind the reductive effect of the vegetation on the sediment movements is their effect on the hydrodynamics. The vegetation can substantially reduce the water flow velocity and turbulence as compared with those in non-vegetation areas[22]. The existing studies show that the submerged and emergent plants have considerable effects on the hydrodynamics, thus reduce the sediment resuspension[22,124].

The large quantities of plants in the water decrease the wind speed near the water and sediment surfaces. This function is beneficial to the removal of the SPM and reduces the possibility of the sediment resuspension. It is shown that the aquatic plants promote the sedimentation of fine particles and the sediment accretion[125,126]. Generally speaking, the vegetation with its root system has a large contact area of the water, forming a barrier at the sediment-water interface. When the water flow passes it, some insoluble colloids, especially, the organic detritus, will be adsorbed by the root system and then settle down[24]. The aquatic plants not only provide a habitat for microorganisms and other organisms, but also make SPM effectively settle down. The adsorption of mineral elements by the plants helps these elements going from the nutrient cycling process into the geochemical cycling process.

In the aquatic environment, the sediment resuspension occurs frequently, which strongly influences the water transparency and material cycling between the sediments and the overlying water[127,128]. Most field investigations indicate that the vegetation decreases the suspended sediment concentrations, either by increasing the sediment deposition rates[129,130], or by direct trapping of the sediment on the stems and the leaves[131]. Table 2 shows the reduction rate of SPM by the vegetation in the aquatic environment. In this Table, 8 typical aquatic plants, including the emerged, floating and submerged plants, are selected to analyze the reduction rate of SPM. In general, the submerged plants are used widely in the inhibition of the sediment resuspension. Vallisneria natans and Acorus calamus are two effective species which reduce the SPM concentrations in overlying water.

6.2 Effect of sediment transport on vegetation growth

The vegetation plays a critical role in the structure and the function of the aquatic ecosystem, however, the survival environment of the vegetation is influenced by many factors. Among these factors, the sediment transport is generally accepted as playing an important role in controlling the environment[136,137]. The effects of suspended particles on the growth of the plants mainly include three aspects. First, the suspended solids reduce the water transparency and the effective light, which is not conducive to the normal photosynthesis of the aquatic plants[117]. Secondly, the partial suspended particles adhered to the plant leaves reduce their photosynthetic capacity directly, and moreover, the attached particles will hinder the exchange of gas and nutrients between the plants and the water, which affects the growth of the plants significantly[31]. Besides, the surface sediment transport makes the root system of the plants unstable, which is also not beneficial to the growth of the plants.

In recent years, some exploratory researches were carried out on the plant’s growth affected by the sediment transport. Xie et al.[138]studied the effects of the sediment resuspension on the growth of Potamogeton malainus and Vallisneria natants. It is shown that the biomass of the two plants under the condition of the sediment resuspension is significantly smaller than that grown in a limpid water, and the sediment transport reduces the number of tillers and tubers of the plants. Meanwhile, it is indicated that the negative effect of the sediment resuspension on Potamogeton malainus is smaller than that of Vallisneria natants. Zhang et al.[139]analyzed the effect It is shown that high loading suspended particles in the water body have a remarkable effect on the growth of Potamogeton crispus, especially in the fresh weight and the chlorophyll content. These studies advanced the understanding of the plant’s tolerance to suspended particles, and provided an important basis for selecting pioneer species for the plant restoration.

7. Deficiences in existing researches

The number of studies about the water flow and the sediment transport in vegetated waterways are enormous. However, there are some deficiencies in these studies, related to the study methods, the objective, the systems and the applications.

7.1 Study methods

So far only a few species for the vegetation were studied in laboratory experiments or field observations. They only account for a very small part of natural vegetation species. Therefore, we can use modern techniques to carry out classification studies of the aquatic plants, such as by using the fractal geometry theory. In addition, artificial plants are used more often than the natural plants in the previous studies, meanwhile, the indoor simulations are more often than the field observations evidently. Consequently, one sees the deviations between calculated values and experimentally determined values in the prediction of the relationships among the vegetation, the water flow and the sediment transport. Therefore, we should carry out more field observations to improve the existing results and then establish more reasonable predictions.

7.2 Study objective

At present, many studies are carried out about floodplain and wetland abroad, while the similar studies in China are relatively few. In China, a large number of people live in the places where there are dense rivers and lakes. So in the past for a long period of time, the flood discharge capacity of rivers was considered as the most important factor in the river management. By increasing the flood discharge capacity and preventing floods from entering into floodplains, a large number of lives can be saved and economic losses can be avoided. Consequently, related studies focus on the water flow patterns in rivers. In the past, the vegetation in the water body and the riparian zone used to be considered as a factor that might reduce the flood discharge capacity significantly, so the vegetation was not encouraged to be planted in rivers. But now, it has been realized that under the condition of ensuring the discharge capacity, it is advisable to rebuild a healthy aquatic ecosystem. A great effort has been made to recover the healthy ecological environment of rivers and riparian zones. Hence, we should expand the study objective to the whole region.

7.3 The systems

From the obtained results, the flow characteristics influenced by the vegetation were widely explored, and meanwhile, the studies of the sediment transport caused by the water movement also made some pro-gress. However, the interactions among the water flow, the vegetation, and the sediment transport are rarely studied. The systematic study of these three factors involves multiple fields, such as the hydraulics, the ecology, the environmental science, which increases complexity of the problem. Although the related research is still in an exploratory stage, it will be a hot and difficult topic in future studies. Generally speaking, the interactions between the water flow and the sediment, between the water flow and the vegetation and between the sediment and the vegetation are, respectively, well studied, but the systematic researches of the interactions among the water flow, the sediment and the vegetation are few.

7.4 Applications of obtained results

In the studies of the water flow, the sediment and the vegetation, a number of empirical formulas and parameters were obtained from the experimental results, but the accuracy of these formulas and parameters need to be verified. However, some empirical formulas are still applied widely, such as the Manning formulation which is the primary formula to calculate the flow rate and the flow velocity in open channels. But there are also many problems in these applications, for example, the Manning roughness coefficient is obtained based on the condition of the steady uniform flow, while many existing studies of the Manning roughness coefficient involve steady non-uniform flows. The obtained approximate value would have an effect on the accuracy of the calculated results. Hence, the acquired empirical formulas and parameters need to be further improved. At present, there are many research results that have not been verified repeatedly and applied widely. In order to predict the water flow and the sediment transport in vegetated waterways by numerical simulations, it is very urgent to validate and apply these results.

8. Future researches

Although some progresses have been made on the studies of the interactions among the vegetation, the water flow and the sediment transport, there is still a lot of work to do in the future. In view of the deficiencies of the studies in this field, future researches should be directed in the following three aspects: the short-term work, the long-term work and the difficult problems.

In the short-term work, the study methods of the water flow, the vegetation and the sediment transport should be improved to provide a theoretical basis for their interactions. And also, the study objective is to be expanded. It is important to use combined methods, including the field observations, the flume experiments, the environmental chemistry analyses and the numerical simulations, to study the interactions among the water flow, the sediment and the vegetation. Meanwhile, the obtained results are expected to be in a widespread use in the engineering practice and management.

In the long-term work, we should focus on the effects of the water flow and the water level variations on the growth and reproduction of aquatic plants. In the aquatic environment, the suitable density of the vegetation is very important. We need to find the optimum planting density of the vegetation to keep a balance between the flood discharge capacity and a healthy water environment. Moreover, the migration and the transformation of pollutants in the “water flow-sediment-vegetation” system should be studied in the future.

In addition, some difficult problems need to be solved in future. We should make more efforts to study the mechanisms of the suspended particle size change under different hydrodynamic conditions in vegetated and non-vegetated areas. Meanwhile, the numerical simulations of the sediment transport in vegetated regions need to be further explored. The effects of the water flow on the growth, reproduction, decline and fall processes of microorganisms in the“water flow-sediment-vegetation” system are important issues to study.

9. Conclusion

The water flow, the vegetation and the sediment are important elements in the aquatic environment. This paper mainly reviews studies of the interactions between the vegetation and the water flow, and between the vegetation and the sediment transport. The existing abundant studies show that the vegetation can affect the flow structure and the sediment transport, which in turns have direct and indirect effects on the vegetation growth. At present, the results of the flow characteristics influenced by the vegetation are abundant, and the study of the vegetation’s influence on the sediment transport has also made some progress. Meanwhile, the numerical simulations of the flow structure in vegetated waterway have obtained a large number of results. However, there are still some deficiencies in the existing researches, as discussed in the last part of this paper.

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10.1016/S1001-6058(15)60453-X

* Project supported by the National Science Funds for Creative Research Groups of China (Grant No. 51421006), the National Science Fund for Distinguished Young Scholars (Grant No. 51225901), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13061), the QingLan Project for Innovation Team of Jiangsu Province, and PAPD.

Biography: WANG Chao (1958-), Male, Ph. D., Professor

WANG Pei-fang,

E-mail: pfwang2005@hhu.edu.cn