Two-dimensional flow velocity prediction method for river with floating vegetation

Abstract

The present application belongs to the field of hydraulics and river dynamics, and discloses a two-dimensional flow velocity prediction method for river flow in a channel with floating vegetation. The open channel with floating vegetation is divided into four regions along the vertical direction, which are, in order from the water surface z=H to the riverbed bottom z=0: (1) an internal vegetation area with uniform flow velocity distribution; (2) an external vegetation area; (3) a non-vegetation area affected by shear stress; and (4) a channel bottom boundary layer constrained by the riverbed. The two-dimensional flow velocity of the four subareas of the river flow in the channel with floating vegetation is calculated based on the internal flow adjustment length of the floating vegetation, the flow velocity at the interface between the internal vegetation area and the external vegetation area, and the maximum flow velocity in the entire water depth direction. The present application takes into account the influence of floating vegetation on the flow structure and flow velocity distribution, and verifies the model vegetation community with a real vegetation form as a case; and provides a theoretical basis for further research on the flow evolution in a channel with floating vegetation.

Description

Technical Field

[0001] This invention belongs to the fields of hydraulics and river dynamics, and relates to a two-dimensional flow velocity calculation method for rivers based on average river velocity, vegetation density, and relative water depth. It is particularly suitable for two-dimensional flow velocity prediction technology in rivers with floating vegetation. Background Technology

[0002] Floating vegetation is commonly found in natural rivers and wetlands, playing a vital role in river ecosystems. It promotes the oxidative decomposition of pollutants and nutrients, thereby purifying water and maintaining biodiversity. The leaves and flowers of floating vegetation (such as water hyacinth) are above the water surface, while their roots float in the water. When temperatures are suitable, some floating vegetation species reproduce rapidly, their roots intertwining and binding together to form a stable cover structure on the river surface, sometimes even covering the entire river surface. Due to the flow-blocking effect of floating vegetation, water flow is controlled within the internal flow regulation zone (0... <x<X D X D (Adjusting the length for internal water flow within vegetation) from the floating vegetation area (h) g <z<H,h g The free flow region below the riverbed (distance from the bottom of the vegetation) (0≤z≤h) g Vertical deflection causes the water flow structure within and around floating vegetation to continuously adjust along both the longitudinal (x-direction) and vertical (z-direction). Specifically, the water velocity decreases within the floating vegetation, while the velocity increases accordingly from the bottom of the floating vegetation to the riverbed due to the continuity of the river flow. The exchange of mass and momentum between the low-velocity vegetation zone and the high-velocity zone below the vegetation promotes the formation of a shear layer at the bottom of the vegetation. When the shear layer is sufficiently strong, a Kelvin-Helmholtz vortex street is generated at the bottom of the floating vegetation. The complex spatial variations in river velocity caused by floating vegetation influence the distribution of solid material deposition, pollutant transport and diffusion, re-suspension of riverbed sediment, and riverbed evolution. Therefore, elucidating how floating vegetation affects velocity evolution and accurately predicting the velocity distribution in rivers with floating vegetation is of great significance and is a current hot topic in river wetland ecological restoration projects.

[0003] To further study the impact of floating vegetation on river flow velocity distribution and improve the quality of vegetation-based ecological restoration projects, it is urgent to understand the water flow changes within vegetation communities, especially the two-dimensional flow velocity of rivers with floating vegetation.

[0004] However, the distribution of river flow velocity is affected by many factors, including the average river velocity, vegetation density, and relative water depth. This is especially true considering that the flow beneath floating vegetation is constrained by both the vegetation boundary and the riverbed boundary, making the vertical variation of the flow even more complex. Under natural conditions, changes in upstream flow and relative water depth directly affect the flow measurement results, leading to unrepresentative results. Furthermore, it is difficult to continuously measure the flow velocity in rivers with floating vegetation over a long period and with significant manpower and resources. While constant and uniform flow can be obtained and detailed two-dimensional flow velocity measurements of rivers with floating vegetation can be taken in the laboratory, the data measurement and analysis require substantial financial and time investment. Typically, constructing a 4-meter-long floating vegetation community in a 16-meter-long, 1-meter-wide, and 0.5-meter-deep experimental flume, and using a Micro-ADV instrument, requires a sampling time of at least 2 minutes and 30 seconds at each point (sampling frequency of 50 Hz), and detailed two-dimensional flow velocity measurements can take a month or even longer.

[0005] Therefore, there is an urgent need for a simple and practical method to predict the two-dimensional flow velocity of river channels, especially those with floating vegetation, and to provide support for further development of river ecological restoration technologies. Summary of the Invention

[0006] In view of the current situation where existing technologies are difficult to effectively predict the flow velocity distribution of rivers with floating vegetation, the purpose of this invention is to provide a two-dimensional flow velocity prediction method for rivers, which takes into account the influence of many factors such as average river velocity, vegetation density and relative water depth, and realizes the prediction of two-dimensional flow velocity of rivers with vegetation (especially floating vegetation).

[0007] This invention is applicable to river channels with floating vegetation where the flow velocity is greater than 0 cm / s. Although floating vegetation and submerged vegetation have similar geometries, their influence on flow structure and velocity distribution differs significantly due to different boundary conditions. The surface resistance of submerged vegetation is generally less than the vegetation drag force, so the effect of surface resistance on the flow velocity within the vegetation is negligible. However, the flow beneath floating vegetation is constrained by both the vegetation boundary and the riverbed boundary, and the maximum vertical flow velocity always occurs in the unvegetated area between the floating vegetation and the riverbed. Therefore, the calculation model for the velocity distribution of submerged vegetation cannot be directly applied to river channels with floating vegetation.

[0008] The present invention provides a two-dimensional velocity prediction method for river flow in floating vegetation channels. The open channel under the action of floating vegetation is divided into four regions vertically, from the water surface z = H to the bottom of the riverbed z = 0, which are: (1) the internal vegetation zone h with uniform velocity distribution. g +δ e ≤z≤H,δ e (2) External vegetation area h g ≤z≤hg +δ e (3) Unvegetated areas affected by shear stress δ b ≤z≤h g δ b The thickness of the channel bottom boundary layer; and (4) the channel bottom boundary layer constrained by the riverbed 0≤z≤δ b ;

[0009] (1) The two-dimensional flow velocity in the internal vegetation zone is uniformly distributed along the vertical direction. At this time, the two-dimensional flow velocity is the flow velocity along the x direction.

[0010] (2) The two-dimensional flow velocity in the external vegetation zone is calculated according to the following formula:

[0011]

[0012] In the formula, K represents the flow velocity at the interface between the internal and external vegetation zones. II C is a proportionality constant; D ρ is the drag coefficient; a is the water-blocking area of ​​the floating vegetation; h g =Hh c H represents the river depth, h c This refers to the height of the floating vegetation.

[0013] (3) The two-dimensional flow velocity in the unvegetated area affected by shear stress is calculated according to the following formula:

[0014]

[0015] In the formula, S is the water surface gradient; K III It is a proportionality constant; The maximum water flow velocity across the entire water depth direction;

[0016] (4) The two-dimensional flow velocity of the channel bottom boundary layer constrained by the riverbed is calculated according to the following formula:

[0017]

[0018] In the formula, K IV It is a proportionality constant.

[0019] (I) Internal Vegetation Zone

[0020] In the internal vegetation zone (h g +δ e ≤z≤H)( Figure 1 In the middle region ①), the Reynolds stress is very small and can be ignored (τ1(z)=0).

[0021] δ e The formula for predicting the depth of water flow penetration is: and

[0022] The water flow velocity is uniformly distributed vertically in this region, as calculated by the following formula:

[0023] U1(x,z=z1)=U1(x,z=z2)=…=U1(x,z=z n )

[0024] In the formula: x is the vertical position, z1, z2…z n For z = h g +δ e The vertical position between z and H. That is, the two-dimensional flow velocity in the internal vegetation zone is taken as the flow velocity distribution along the x-direction, specifically divided into the average flow velocity in the water flow regulation zone inside the vegetation and the average flow velocity in the water flow fully developed zone inside the vegetation.

[0025] The distance from the front of the floating vegetation (x=0) to the point where the water flow is fully developed (the flow velocity no longer changes along the path) is defined as the water flow adjustment length X within the vegetation. D (X D <Length L of the floating vegetation zone), dimensional analysis revealed that the internal water flow regulation length X of the floating vegetation zone D With vegetation density (a) and relative water depth (h) g / H) closely related (Guietal., 2023), X D The following formula can be used to calculate:

[0026]

[0027] In the water flow regulation zone within the vegetation (0 <x<X D ), calculate the average flow velocity U inside the floating vegetation. c (x):

[0028]

[0029]

[0030] In the formula, U0 is the average flow velocity at the front of the floating vegetation channel; parameters α, β, A', B', and C' are defined as follows:

[0031]

[0032] In the formula, C f is the coefficient of friction of the bed surface; C (=0.076±0.025) is the shear coefficient, characterizing the intensity of vertical momentum exchange between the floating vegetation zone and the free flow zone below; h c C represents the height of floating vegetation. D ρ is the drag coefficient; a is the water-blocking area of ​​floating vegetation; H is the river depth, h g =Hhc φ represents the vegetation volume fraction within a unit of water body.

[0033] The average flow velocity (U) in the fully developed water flow zone within vegetation cf The following formula can be used to calculate:

[0034]

[0035] Based on the water flow continuity equation, the average flow velocity U in the unvegetated area below the floating vegetation is... b (x) is calculated using the following formula:

[0036]

[0037] (II) External Vegetation Area

[0038] In the external vegetation zone (h g ≤z≤h g +δ e ()( Figure 1 In the middle region (②), the velocity difference between the floating vegetation area and the unvegetated area below forms a shear layer at the bottom of the vegetation. In this region, due to vertical momentum exchange, the water velocity gradually increases vertically. At each x-position, the momentum equation can be written as:

[0039]

[0040] In the formula, g is the acceleration due to gravity; S is the water surface gradient; Let τ be the drag force of the vegetation; τ be the Reynolds stress. Integrate equation (6) from any vertical position z in the external vegetation zone to point C2 (z = h). g +δ e , Figure 1 ),get:

[0041]

[0042] In the formula, δ e The depth through which the water flows; point C2 (z = h) g +δ e Located at the interface between the inner and outer vegetation zones, the Reynolds stress is continuous here, ie,τ2(z=h g +δ e )=τ1(z=h g +δ e Equation (7) can be rewritten as: ) = 0.

[0043]

[0044] External vegetation zone (h g ≤z≤h g +δ eThe Reynolds stress τ(z) is directly related to the flow velocity U, and its specific expression is shown in equation (9):

[0045]

[0046] In the formula, l II (=K II (h g +δ e –z)) is the mixing length of this region, K II Let (9) be a proportionality constant; substituting equation (9) into equation (8), we get:

[0047]

[0048] Integrating equation (10) along any vertical position z in the external vegetation zone to point C2 at different x positions, the vertical velocity distribution in this region is as follows:

[0049]

[0050] In the formula, The velocity at the interface between the inner and outer vegetation zones (point C2, z = h) g +δ e This location satisfies the continuity of water flow velocity, i.e., To predict the vertical distribution of water flow velocity in the external vegetation zone, U2(x,z), It must be determined.

[0051] The method for determining it is as follows:

[0052] First, the water flow velocity distribution in both the internal and external vegetated areas is integrated along the water depth direction (z). The average flow velocity in the floating vegetated area, U... c (x) is represented as:

[0053]

[0054] Secondly, let h g +δ e –z=b2, then dz=-db2. According to equation (12), It can be calculated using the following formula (13):

[0055]

[0056] when Once determined, substitute it into equation (11) to determine the external vegetation zone (h). g ≤z≤h g +δ eThe vertical distribution of the water flow velocity is calculated using U2(x,z).

[0057] (III) Vegetationless areas affected by shear stress and channel bottom boundary layers constrained by riverbed

[0058] In the unvegetated area (0≤z≤h) g Vegetation drag term in the momentum equation This can be ignored, and equation (6) simplifies to:

[0059]

[0060] In the unvegetated area affected by shear stress (z=δ) b ~h g ()( Figure 1 The middle region ③) and the channel bottom boundary layer constrained by the riverbed (z=0~δ) b ()( Figure 1 In the middle region ④), by integrating equation (14) along the vertical direction, the expression for the Reynolds stress τ(z) is written as:

[0061] -[τ(z)-τ(z=δ b )]=ρgS(z-δ b ) (δ b ≤z≤h g (15a)

[0062] -[τ(z=δ b )-τ(z)]=ρgS(δ b -z) (0≤z≤δ b (15b)

[0063] In the formula, δ b The thickness of the boundary layer at the bottom of the canal is predicted by the formula: δ b =0.024h g The maximum flow velocity occurs at z = δ b (Point C4, Figure 1 At point ), the Reynolds stress is zero, i.e., τ(z=δ b ) = 0. Therefore, equation (15) can be rewritten as:

[0064] τ(z)=-ρgS(z-δ b ) (δ b ≤z≤h g (16a)

[0065] τ(z)=ρgS(δ b -z) (0≤z≤δ b (16b)

[0066] Combining equation (16) with the mixed length theory, the flow velocity distribution in regions ③ and ④ can be calculated as follows.

[0067] In region ③ (δ) b ≤z≤h g The Reynolds stress τ(z) is defined as:

[0068]

[0069] This region is affected by Reynolds stress, therefore the mixing length l III Represented as: l III =K III (h g +δ e –z), K III It is a proportionality constant.

[0070] The velocity gradient in region ③ can be derived from equation (17):

[0071]

[0072] Integrating equation (18) vertically, the vertical distribution of the water flow velocity in this region at different x positions, U3(x,z), is:

[0073]

[0074] In the formula, The maximum water flow velocity along the entire depth direction (z-direction) is located at the interface between regions ③ and ④ (point C4, z = δ). b (x)).

[0075] The determination method is as follows:

[0076] The water flow velocity is at the bottom of the vegetation (point C3, z = h) g The continuity condition is satisfied, therefore U3(z=h) g )=U2(z=h g Let z = h g Calculate using equation (11) As shown in equation (20a). Furthermore, z = h g Substitute into equation (19), The expression can also be written as equation (20b):

[0077]

[0078]

[0079] Combined equations (20a) and (20b), for:

[0080]

[0081] In region ④ (δ) b ≤z≤h g The Reynolds stress τ(z) is defined as:

[0082]

[0083] In the formula, l IV It is the mixing length of region ④, assuming l IV =K IV z, K IV It is a proportionality constant. The velocity gradient in this region is derived from equation (22):

[0084]

[0085] Integrating equation (24) vertically at different x positions yields the vertical distribution of the water flow velocity in region ④. The formula for calculating U4(x,z) is:

[0086]

[0087] Currently, there is no model for calculating the two-dimensional flow velocity of rivers with floating vegetation, especially considering the influence of the drag force of floating vegetation on the changes in flow velocity. The two-dimensional flow velocity prediction method for rivers with floating vegetation provided by this invention has the following beneficial effects:

[0088] 1. This invention considers the influence of floating vegetation on water flow structure and velocity distribution, and uses a model vegetation community with real vegetation morphology as a case study for verification; it provides a theoretical basis for further research on the evolution of water flow in rivers with floating vegetation.

[0089] 2. This invention takes into account the water flow regulation length X within floating vegetation, which was neglected in previous studies. D With vegetation density (a) and relative water depth (h) g The river has many variables such as / H), and these conditions directly affect the changes in river flow velocity. Therefore, the two-dimensional flow velocity model of the river with floating vegetation obtained by this invention is closer to the real situation and has high prediction accuracy.

[0090] 3. This invention only requires the basic parameters of water flow and floating vegetation (including water depth H, water surface gradient S, length of floating vegetation zone L, water-blocking area of ​​vegetation a, and vegetation resistance coefficient C). D Vegetation height h c This allows for the prediction of two-dimensional flow velocity in river channels, which not only reduces research costs but is also applicable to river areas that are inaccessible or difficult to measure, making it highly versatile. Attached Figure Description

[0091] Figure 1 This is a schematic diagram illustrating the changes in river flow structure under the influence of floating vegetation.

[0092] Figure 2 The setup for the hydrodynamic test of the river channel under the influence of floating vegetation is shown in (a) as a top view and (b) as a side view. dx and dy are the longitudinal and transverse distances between adjacent wooden sticks, respectively. The vertical black dashed line represents the position of each measurement section. The ADV probe is placed vertically to collect flow velocity data. The vertical distribution of flow velocity at 7 sections (x = 50, 100, 150, 200, 250, 300 and 350 cm) was measured along the river. The circles represent the measurement points.

[0093] Figure 3 The embodiment of the present invention provides a dimensionless water flow velocity U in a river channel with floating vegetation. m Comparison of predicted and measured values ​​of / U0; vertical dashed lines represent measurement sections at different x positions (x = 50, 100, 150, 200, 250 and 300 cm). Detailed Implementation

[0094] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are part of the present invention.

[0095] Example 1

[0096] This embodiment provides a detailed explanation of the results obtained from the two-dimensional flow velocity prediction method for rivers with floating vegetation through a flume test.

[0097] Flume tests were conducted to measure the flow velocity in a river channel with floating vegetation, as well as the upstream flow velocity. Detailed velocity measurements were also performed under selected operating conditions. The river flow velocity under different vegetation densities was determined and dimensionlessly converted (U0). m The measured data obtained are compared with the river flow velocity obtained by using the prediction model to verify the accuracy of the prediction method provided by the present invention.

[0098] The main experimental equipment used in this embodiment is shown in Table 1 below.

[0099] Table 1 shows the experimental setup with floating vegetation in a water tank.

[0100]

[0101]

[0102] This experiment is carried out in a straight horizontal flume with a length of 16 m, a width of 1 m and a height of 0.5 m. A reservoir is provided at each of the head and tail of the flume, and the tail end is equipped with a variable-speed electric pump, which is used to transport water from the downstream water tank to the upstream water tank to complete circulation. A flow straightener is arranged at the inlet of the flume, so that the water flow entering the flume has a uniform and stable transverse distribution of flow velocity; an adjustable tail gate is installed at the end of the flume to control the test water depth. The water flow rate and water depth are obtained by readings from a flowmeter and rulers fixed at the front and rear ends of the vegetation community, respectively.

[0103] The model vegetation is fixed in a staggered arrangement on 8 PVC plates each with a length of 1 m, a width of 0.5 m and a thickness of 1 cm. The PVC plates are suspended above the water surface with the vegetation facing downward, and are placed side by side, with the adjacent boundary line of the two PVC plates aligned with the center line of the flume. The floating vegetation zone designed in the experiment is 4 m long, which is slightly longer than the internal flow adjustment length of the vegetation (L>X D , X D calculated by Equation (1)), so the spatial evolution of the flow velocity in the floating vegetation area is divided into an internal flow adjustment zone (0<x≤X D ) and a fully developed flow zone (X D <x). The width of the floating vegetation zone is the same as the width of the flume, therefore, when water flows through the floating vegetation, structural changes only occur in the flow direction (x direction) and the vertical direction (z direction), and no structural changes occur in the transverse direction (y direction). In all working conditions, the submergence height of the floating vegetation remains unchanged (h c =14 cm), which is consistent with the root length of natural Eichhornia crassipes (E.crassipes) (Downing-Kunz and Stacey, 2011, 2012; Penfound and Earle, 1948). The height of the free flow zone below the floating vegetation (h g ) is defined as the distance from the river bed to the bottom of the vegetation, which can be obtained by h g =H–h cCalculations were performed. x, y, and z represent the direction of water flow, the transverse direction, and the vertical direction of water flow, respectively. x = 0 represents the leading edge of floating vegetation, y = 0 represents the position of the floating vegetation and the centerline of the channel, and z = 0 represents the riverbed surface. Accordingly, the components of the water flow velocity in the three coordinate directions are defined as u(x,y,z) = (U,V,W). For each point, the sampling time and frequency were 120s and 50Hz, respectively. The method of Goring and Nikora (2002) was used to filter out raw velocity data with correlation below 70% and signal-to-noise ratio less than 15dB. Instantaneous velocity was decomposed into time-averaged velocity (U, V, W) and fluctuating velocity (u′, v′, w′) using MATLAB code. To reduce the influence of spatial flow anisotropy on velocity measurement, a characteristic region was considered at each measuring point. The average value of the velocity at the three transverse positions (y = 0, y = -dy / 8, and y = dy / 8) within the characteristic region was used as the time-averaged velocity at that measuring point. In all operating conditions, the complete vertical velocity distribution was measured at each x-point, with velocity data collected sequentially from the water surface (z = H) to the riverbed bottom (z = 0) at 2cm intervals. The specific locations of the velocity measurement points are as follows: x = 50, 100, 150, 200, 250, 300, 350cm. The actual velocity measurement results are as follows: Figure 2 and Figure 3 As shown.

[0104] The reliability of the two-dimensional flow velocity calculation method for rivers with floating vegetation proposed in this patent is verified through experimental cases. The measured working parameters are shown in Table 2.

[0105] Based on the two-dimensional velocity prediction method for river channels with floating vegetation given above, the flow regulation length X within the floating vegetation is first determined. D Flow velocity at the interface between internal and external vegetation zones Maximum water flow velocity across the entire depth direction (z-direction) Then, based on the formulas (2), (4), (11), (19), and (24) given above, the two-dimensional flow velocities of the four sections of the drifting vegetation channel are obtained. The prediction results are shown in [the table below]. Figure 3 As shown.

[0106] The difference between the calculated and measured values ​​of the two-dimensional flow velocity is evaluated using the root mean square error (RMSE).

[0107]

[0108] In the formula, M is the number of measurement sections in the direction of water flow (x direction), N is the number of measured values ​​in the vertical direction (z direction), and U m (z) and U p (z) represents the measured and predicted flow velocity values, respectively. The calculation results are shown in Table 2.

[0109] Table 2 Experimental parameters *

[0110]

[0111] *d is the diameter of a single floating vegetation plant; D is the total diameter of the root system of a single vegetation plant; L is the length of the floating vegetation zone; n is the density of floating vegetation; a (=nD) is the water-blocking area of ​​vegetation per unit water body; φ is the solid volume fraction of vegetation; other parameters are defined in the same way as in Table 1.

[0112] Experiments revealed that the predicted and measured values ​​of the vertical distribution of water flow velocity (along the z-direction) showed good agreement at different locations along the river flow (along the x-direction), with the dimensionless root mean square error (RMSE / U0) between the predicted and measured values ​​being less than 15%. The largest dimensionless RMSE occurred at the vegetation front, because real vegetation roots are typically flexible, and vertical diversion may cause root deformation and slight bending at the vegetation front, thus reducing the submerged height of the roots. Experimental results indicate that the proposed model is also applicable to two-dimensional water flow velocity calculation in channels with floating vegetation. Furthermore, this model can capture both vertical and longitudinal velocity variations in the internal regulation zone of the flow, and also reflects vertical velocity variations in the fully developed flow zone.

[0113] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A method for predicting two-dimensional flow velocity in river channels with floating vegetation, characterized in that, The open channel under the action of floating vegetation is divided into four regions vertically, from the water surface z = H to the bottom of the riverbed z = 0, namely: (1) the internal vegetation zone with uniform flow velocity h g + δ e ≤ z ≤ H, δ e (2) External vegetation area h g ≤ z ≤ h g + δ e (3) Unvegetated areas affected by shear stress δ b ≤ z≤ h g δ b The thickness of the channel bottom boundary layer; and (4) the channel bottom boundary layer constrained by the riverbed 0 ≤ z ≤ δ b ; (1) The two-dimensional flow velocity in the internal vegetation zone is uniformly distributed along the vertical direction. At this time, the two-dimensional flow velocity is the flow velocity along the x direction. (2) The two-dimensional flow velocity in the external vegetation zone is calculated according to the following formula: ； In the formula, K represents the flow velocity at the interface between the internal and external vegetation zones. II C is a proportionality constant; D ρ is the drag coefficient; a is the water-blocking area of ​​the floating vegetation; h g =Hh c H represents the river depth, h c The height of the floating vegetation; (3) The two-dimensional flow velocity in the unvegetated area affected by shear stress is calculated according to the following formula: ； In the formula, S is the water surface gradient; K III It is a proportionality constant; The maximum water flow velocity across the entire water depth direction; (4) The two-dimensional flow velocity of the channel bottom boundary layer constrained by the riverbed is calculated according to the following formula: ； In the formula, K IV It is a proportionality constant.

2. The method for predicting two-dimensional flow velocity in rivers with floating vegetation according to claim 1, characterized in that, δ e Calculated using the following formula: When 0 <x<X D , ;X D Adjusting the length for water flow within vegetation; ； When x≥X D , .

3. The method for predicting two-dimensional flow velocity in rivers with floating vegetation according to claim 1, characterized in that, δ b Calculated using the following formula: .

4. The method for predicting two-dimensional flow velocity in rivers with floating vegetation according to claim 2, characterized in that, The velocity distribution along the x-direction is specifically divided into the average velocity in the water flow regulation zone within vegetation and the average velocity in the water flow fully developed zone within vegetation. In the water flow regulation zone within the vegetation 0 <x<X D Calculate the average flow velocity U inside the floating vegetation. c (x): ； ； In the formula, U0 is the average flow velocity at the front of the floating vegetation channel; parameters α, β, ... , and Defined as: ； In the formula, C f φ is the coefficient of friction of the bed surface; C is the shear coefficient; φ is the vegetation volume fraction per unit water body. The average flow velocity U in the fully developed water flow zone within vegetation cf The following formula can be used to calculate: 。 5. The method for predicting two-dimensional flow velocity in rivers with floating vegetation according to claim 4, characterized in that, The method for determining it is as follows: First, the water flow velocity distribution in both the internal and external vegetated areas is integrated along the water depth direction z. The average flow velocity in the floating vegetated area, U... c (x) is represented as: ； Secondly, let h g + δ e – z = b2, then dz = -db2; according to the above formula, The following formula can be used to calculate: 。 6. The method for predicting two-dimensional flow velocity in rivers with floating vegetation according to claim 4, characterized in that, The determination method is as follows: The water flow velocity satisfies the continuity condition at the bottom of the vegetation, therefore U3 (z = h) g ) = U2 (z = h g Let z = h g ,calculate As shown in the following equation; at the same time, let z = h g The two-dimensional velocity calculation formula for the channel bottom boundary layer constrained by the riverbed is obtained. : ； ； Combining the above two equations, we get : 。

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