Method for simulating migration and retention of flexible aquatic vegetation fragments in river channel

Abstract

The invention discloses a method for simulating migration and retention of flexible aquatic vegetation fragments in a river channel. The method comprises the following steps: collecting topographic data, hydro-meteorological data and aquatic vegetation data of a simulation area where the aquatic vegetation fragments are accumulated; an aquatic vegetation model after generalization of the simulation area is constructed, diffusion characteristic, collision and retention characteristic tests are carried out, the correlation among the diffusion characteristics, the collision and retention characteristics and water flow characteristics of different types of aquatic vegetation is established, and the motion characteristics of the aquatic vegetation are obtained; and establishing and coupling a two-dimensional hydrodynamic calculation model and a flexible aquatic vegetation fragment movement calculation model of the simulation area, and carrying out migration and retention calculation on the flexible aquatic vegetation fragments of the simulation area, and analyzing the migration and retention conditions of the flexible aquatic vegetation fragments in the river channel under different levels of flood conditions in the flood season in the simulated area. The method is suitable for all water taking projects, pump stations and river type reservoirs facing the flexible aquatic vegetation fragment blocking risk.

Description

Technical Field

[0001] The invention relates to the technical field of river channel vegetation ecological management in water conservancy and hydropower engineering construction, and in particular to a method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel. Background Art

[0002] In recent decades, floods caused by extreme weather have occurred frequently, especially during the flood season. The violent scouring of floods has caused the aquatic vegetation in the river to be broken. The water flow will carry a large amount of broken aquatic vegetation fragments in the basin and gather in the slow-flow area of ​​the river, pump stations and in front of reservoirs. The accumulation of aquatic vegetation fragments has led to an increasing risk of pump stations and hydropower stations shutting down. Since the drift and aggregation process of flexible aquatic vegetation fragments in the river is unclear, the key prevention areas for aquatic vegetation are also unclear. In order to reduce the amount of aquatic vegetation fragments gathered and the retention time, reduce the cost of aquatic vegetation management, reduce the sudden pressure of grass disasters during the flood season, and ensure the safety of facilities and property, it is necessary to study and analyze the movement and retention of flexible aquatic vegetation fragments in the river. Summary of the invention

[0003] The purpose of the present invention is to provide a method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel in view of the problems existing in the prior art.

[0004] To achieve the above object, the technical solution adopted by the present invention is: A method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel comprises the following steps: Step 1: Obtain terrain data, hydrological and meteorological data, and aquatic vegetation data of the simulated area where flexible aquatic vegetation debris accumulation occurs; Step 2: Conduct simulation tests on the flexible aquatic vegetation fragments after generalization of the simulation area, including at least a diffusion characteristic test of the flexible aquatic vegetation fragments and a collision and retention characteristic test of the flexible aquatic vegetation fragments, obtain diffusion characteristic parameters, collision and retention characteristic parameters, establish the correlation between the diffusion characteristics, collision and retention characteristics of different types of flexible aquatic vegetation fragments and water flow characteristics, obtain the motion characteristics of the flexible aquatic vegetation fragments, and provide parameters and verification for the calculation model; Step 3: Establish and couple a two-dimensional hydrodynamic calculation model and a flexible aquatic vegetation fragment movement calculation model for the simulation area, perform calculations on the migration and retention of flexible aquatic vegetation fragments in the simulation area, and simulate the migration and retention of the flexible aquatic vegetation fragments in the river channel under different levels of flood conditions in the simulation area during the flood season, so as to obtain the changes in the location and area of ​​flexible aquatic vegetation fragment aggregation over time, and the time when the flexible aquatic vegetation fragments arrive in front of the dam.

[0005] Specifically, in step 1, the topographic data and hydroclimatic data of the simulation area are collected through literature, including at least the river topography and typical cross-sectional data of the simulation area, inlet section flow and outlet water level data, and wind direction and speed data.

[0006] Specifically, in step 1, the method for collecting data on flexible aquatic vegetation fragments is: after the flood, aquatic vegetation survey plots are set up through drone photography combined with manual field investigations, and the types of flexible aquatic vegetation fragments in the simulated area, the areas and areas of accumulation and retention, and the biomass of flexible aquatic vegetation fragments are obtained through salvage.

[0007] Specifically, the step 2 includes the following steps: The simulation model is subjected to test classification, including at least two tests: a test of the diffusion characteristics of flexible aquatic vegetation fragments and a test of the collision and retention characteristics of flexible aquatic vegetation fragments; Design the test conditions of the two tests, establish the test model according to the gravity similarity criterion, determine the test conditions of the laboratory according to the length scale, velocity scale and flow scale, and select the bionic blade; The test model is arranged and carried out in a self-circulating water tank system, which at least includes a water tank, a tailgate and an electromagnetic flowmeter. Water is pumped from a downstream reservoir to the upstream water tank. The electromagnetic flowmeter is arranged at the water inlet of the water tank to control the water inlet flow, and the water level is adjusted by the downstream tailgate. A high-speed camera is mounted on the mobile measuring platform on the top of the water tank, a plastic net is arranged in front of the tailgate at the end of the water tank to recycle the bionic blades, and a waterproof sticker is arranged at the bottom of the water tank. Preliminary test to determine the most easily identifiable color of the bionic leaf; The bionic leaf with the most easily identifiable color is released from the release point, and the next leaf is released after the bionic leaf passes through the shooting area of ​​the high-speed camera; the video shot by the high-speed camera is imported into the object capture software to automatically track the position of the bionic leaf and identify it, and obtain the identification elements; The parameters are calculated to obtain the diffusion coefficient K of the flexible aquatic vegetation fragments in two directions. x With K y , collision probability P c , cohesion probability P a , retention probability P r and the retention time T r ; Analyze the calculated data and establish the diffusion coefficient K x With K y , collision probability P c , cohesion probability P a , retention probability P rCorrelation with model test variables.

[0008] Furthermore, the test selects at least four water flow rates, including the multi-year average monthly maximum flow rate in the normal water period, the multi-year average water flow rate, the multi-year average monthly maximum flow rate in the flood period, and the flood flow rate that occurs once in ten years; and selects at least four types of bionic blades, including two planar blades and two clustered blades.

[0009] Further, in the experiment, the variables related to the shape of the bionic blade include blade type and blade complexity index DI, and the variables related to the arrangement of aquatic vegetation include vegetation density ρ and vegetation arrangement form; Among them, the blade complexity index is: , In the formula, C l represents the circumference of the bionic blade; A l Represents the area of ​​the bionic leaf.

[0010] Specifically, the elements to be identified include the position of the bionic blade at each moment after release, the speed of the bionic blade in multiple directions, and the rotation speed; The parameter calculation formula is as follows: , In the formula, is the standard deviation of the bionic blade position at time t1; is the standard deviation of the bionic blade position at time t2, N represents the total number of bionic blades released, N c Indicates the number of bionic blades that collided, N a Indicates the number of bionic leaves that have cohesion after collision, N r Indicates the number of bionic leaves that are retained after collision.

[0011] Specifically, in step 3, the two-dimensional hydrodynamic calculation model is constructed as follows: the two-dimensional hydrodynamic calculation model uses an unstructured grid to divide the calculation area, and for the shore boundary, the water flow no-slip condition is adopted, and the water surface wind stress tensor is considered at the same time, and the wind stress is obtained through an empirical formula.

[0012] Specifically, in step 3, the flexible aquatic vegetation fragment motion calculation model is constructed as follows: an aquatic vegetation source is introduced, and the motion of the flexible aquatic vegetation fragments includes a translation process caused by wind and water flow and a random diffusion process of the flexible aquatic vegetation fragments in the water, and also considers the collision, cohesion and retention process of the flexible aquatic vegetation fragments during the motion process; At each time step, the displacement of flexible aquatic vegetation fragments in two directions is calculated as follows: , Among them, β represents the followability coefficient of flexible aquatic vegetation fragments to water flow, It represents the flow velocity in the x direction in the grid calculated by the two-dimensional hydrodynamic calculation model of the simulation area. It represents the flow velocity in the y direction of the grid calculated by the two-dimensional hydrodynamic calculation model of the simulation area. It means taking a random number between the interval (-1,1); During the movement, the bionic blades are generalized into two circles. When the distance d between the two bionic blades is less than or equal to 2 times the blade radius r, the bionic blades will have P c The probability of collision, P a The probability of agglomeration occurs, and after agglomeration occurs, the mass and area of ​​the two bionic leaves are superimposed to form a leaf agglomerate; When the bionic blade or the blade aggregate passes through the shallow water area or the edge of the river, the flexible aquatic vegetation fragments will collide with the water according to the probability P c The probability of colliding with the river bank boundary and / or the river island and passing through the retention is P r The reduction of flexible aquatic vegetation leaves occurred, and the retention time was consistent with the time distribution T in the experiment. r , and then continue to move downstream. When the bionic blades or the blade aggregates encounter obstacles when moving downstream, the bionic blades or the blade aggregates immediately stop moving, and the displacement step calculation of these bionic blades or the blade aggregates is stopped. , Where M t represents the amount of flexible aquatic vegetation debris remaining after passing through the retention zone, and M0 represents the amount of the initially introduced flexible aquatic vegetation debris source.

[0013] Specifically, in the step 3, it also includes verification of the two-dimensional hydrodynamic calculation model and the flexible aquatic vegetation fragment movement calculation model. The two-dimensional hydrodynamic calculation model uses measured hydrological test data to calibrate and verify the water surface line and flow velocity distribution of the river section in the simulated area, and the flexible aquatic vegetation fragment movement calculation model uses the location and area of ​​vegetation aggregation in the simulated area during the flood season for verification.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present method provides a method for simulating the movement of flexible aquatic vegetation fragments in a river channel, and fully considers the drift process, random diffusion process and collision retention process of aquatic vegetation during the movement of aquatic vegetation, and is suitable for all water intake projects, pumping stations and river-type reservoirs facing the risk of blockage by flexible aquatic vegetation fragments; 2. The present method simulates the aquatic vegetation migration and retention model coupled with the hydrodynamic calculation model and the flexible aquatic vegetation movement calculation model. The movement includes the translation process caused by wind and water flow and the random diffusion process of flexible vegetation in water. Considering the collision between flexible aquatic vegetation fragments and the flexible vegetation in the passing vegetation area, the migration and retention of flexible aquatic vegetation fragments in the simulated area during the flood season are simulated by setting different incoming flow conditions and flexible vegetation release positions; 3. Simulating the drift and aggregation process of flexible aquatic vegetation fragments in the river channel can not only clarify the key prevention areas of aquatic vegetation, reduce the aggregation amount and retention time of aquatic vegetation fragments, reduce the cost of aquatic vegetation management, alleviate the sudden pressure of grass disasters during the flood season, and ensure the safety of facilities and property, but also provide technical and data support for aquatic vegetation management and improve the ecological quality of the water environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A flow chart of a method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to the present invention; Figure 2 It is a schematic diagram of the layout of the flexible aquatic vegetation fragment diffusion characteristic model test device of the present invention; Figure 3 It is a schematic diagram of vegetation arrangement for the model test of the collision and retention characteristics of flexible aquatic vegetation fragments of the present invention; Figure 4 It is a schematic diagram showing the change of the followability index of the flexible aquatic vegetation fragments to the water flow along with the mass of the fragments; Figure 5 This is a schematic diagram of the statistical analysis of the trajectory of flexible aquatic vegetation fragments of the present invention; Figure 6 A schematic diagram of the critical distance judgment for collision between two flexible aquatic vegetation fragments of the present invention; Figure 7 The structure and input parameters of the flexible aquatic vegetation fragment simulation model of the present invention. DETAILED DESCRIPTION

[0016] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0017] Combination Figure 1 to Figure 7 As shown, a method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel comprises the following steps: Step 1: collecting topographic data, hydrological and meteorological data, and aquatic vegetation data of a simulated area where aquatic vegetation debris accumulation occurs; specifically, step 1 includes: Step 101, collecting topographic data and hydrological and climatic data of the river: collecting topographic data and hydrological and climatic data of the simulation area through literature, including at least river topography and typical cross-sectional data of the simulation area, inlet section flow and outlet water level data, and wind direction and speed data; Step 102, collect aquatic vegetation data: after the flood, use drone photography combined with manual field surveys to set up aquatic vegetation survey plots, and salvage to understand the types of flexible aquatic vegetation fragments in the simulation area, the areas and areas of accumulation and retention, and the biomass of flexible aquatic vegetation fragments.

[0018] Step 2: Construct a generalized aquatic vegetation model of the simulation area, and conduct diffusion characteristics, collision and retention characteristics tests, establish the correlation between the diffusion characteristics, collision and retention characteristics and water flow characteristics of different types of aquatic vegetation, and obtain the movement characteristics of aquatic vegetation.

[0019] The specific steps include: Step 201, determine the test type of the aquatic vegetation model: classify the test of the flexible aquatic vegetation fragment model, including at least two tests: a flexible aquatic vegetation fragment diffusion characteristic test and a flexible aquatic vegetation fragment collision and retention characteristic test. The flexible aquatic vegetation fragment diffusion characteristic test provides diffusion characteristic parameters for simulation calculation, and the collision and retention test provides collision and retention characteristic parameters for model calculation.

[0020] Step 202, design the test conditions of the two tests, establish the flexible aquatic vegetation fragment model according to the gravity similarity criterion, determine the test conditions of the laboratory according to the length scale, velocity scale and flow scale, the retention and accumulation of flexible aquatic vegetation fragments in the river or reservoir often occur during the biological flood season, so the test selects 4 flow rates, namely: the multi-year average monthly maximum flow rate in the normal water period, the multi-year average water flow rate, the multi-year average monthly maximum flow rate in the flood season, and the flood flow rate once in ten years. According to the survey data of aquatic plants in the simulated test area, 4 types of bionic flexible vegetation leaves with appropriate lengths and areas are selected according to the length scale, that is, the bionic leaves, including two types of flat leaves and two types of clustered leaves, so as to facilitate repeated tests.

[0021] Furthermore, as shown in Table 1, the test on the diffusion characteristics of flexible aquatic vegetation fragments adopted 4 water flow conditions, 4 leaf types, and a total of 16 test conditions, and each condition was repeated 60 times.

[0022] Furthermore, the collision and retention characteristics of flexible aquatic vegetation fragments were investigated using 4 flow conditions, 4 blade types, 4 vegetation arrangement densities, 2 vegetation arrangement methods, and 4 release distances, for a total of 512 test conditions, with each condition repeated 200 times.

[0023] Table 1 Variables of aquatic vegetation model test

[0024] In the experiment, the variables related to the shape of the bionic blade include the blade type and the blade complexity index DI, and the variables related to the aquatic vegetation arrangement include the vegetation density ρ and the vegetation arrangement form; Among them, the blade complexity index is:

[0025] In the formula, C l represents the circumference of the bionic blade; A l Represents the area of ​​the bionic leaf.

[0026] Step 203, arrange the flexible aquatic vegetation fragment model: the laboratory model is carried out in a self-circulating water tank system, which at least includes structures such as a water tank, a tailgate and an electromagnetic flowmeter. Water is pumped from a downstream reservoir to the upstream water tank. The electromagnetic flowmeter is installed at the water inlet of the water tank to control the water inlet flow, and the water level is adjusted by the downstream tailgate. A high-speed camera is mounted on the mobile measuring platform on the top of the water tank, and a plastic net is arranged in front of the end tailgate of the water tank to recycle the bionic blades. The bottom of the water tank is covered with white waterproof stickers, which can reduce the interference of external light or natural light on the video quality of the high-speed camera.

[0027] Step 204, pre-experiment: adjust the high-speed camera to a suitable shooting height through a lifting platform, release bionic leaves of various colors from a preset release point, and select the color that is most easily recognized by the software. According to the results of the pre-experiment, the bionic leaves are finally dyed red to enhance their recognition ability in image processing.

[0028] Step 205, formal test process: release the bionic blade from the release point, and after the bionic blade passes through the entire shooting area, release the next bionic blade to prevent the previous bionic blade from affecting the movement of the next bionic blade. Each time the bionic blade is released, use pointed tweezers to place it gently to minimize the disturbance of the bionic blade release process to the blade movement.

[0029] Furthermore, 10 bionic blades are grouped together for experiments, and the videos taken by the high-speed camera are numbered.

[0030] Furthermore, the video taken by the telescopic high-speed camera is spliced ​​and corrected at the same time to reduce the error caused by shooting. After the video is processed, the video is imported into the object capture software to automatically track the position of the bionic blade. For images that are difficult to track automatically, manual tracking is used to identify the motion trajectory of the bionic blade. Since the bionic blade has a shape, the position is identified by the center of mass of the bionic blade. The identified elements include the position of the bionic blade at each moment after release, the x and y velocities Vx, Vy of the bionic blade and the rotation speed W.

[0031] Step 206: Calculate the parameters and calculate the diffusion coefficient K of the flexible aquatic vegetation fragments in two directions according to the bionic leaf trajectory. x With K y , collision probability P c , cohesion probability P a , retention probability P r and the retention time T r ; The parameter calculation formula is as follows: , In the formula, is the standard deviation of the bionic blade position at time t1; is the standard deviation of the bionic blade position at time t2, N represents the total number of bionic blades released, N c Indicates the number of bionic blades that collided, N a Indicates the number of bionic leaves that have cohesion after collision, N r Indicates the number of bionic leaves that are retained after collision.

[0032] Step 207: Analyze the calculated data and establish the diffusion coefficient K x With K y , collision probability P c , cohesion probability P a , retention probability P r Correlation with model test variables.

[0033] Step 3: Establish and couple a two-dimensional hydrodynamic calculation model and a flexible aquatic vegetation fragment movement calculation model for the simulation area, perform calculations on the migration and retention of flexible aquatic vegetation fragments in the simulation area, and analyze the migration and retention of the flexible aquatic vegetation fragments in the river channel under different levels of flood conditions in the simulation area during the flood season.

[0034] The specific steps include: Step 301, calculate the two-dimensional hydrodynamic calculation model of the river simulation area: use unstructured grid to divide the calculation area, and for the bank boundary, use the no-slip condition of water flow, and consider the water surface wind stress tensor and wind stress It can be obtained through the following empirical formula: , in, is the air density, is the air resistance coefficient, also called the drag coefficient. is the wind speed 10 meters above the water surface, , and They are the wind speeds in the longitude and latitude directions respectively. For example, the longitude wind speed is positive when it is heading north and negative when it is heading south; the latitude wind speed is positive when it is heading east and negative when it is heading west.

[0035] Step 302, constructing a calculation model for the motion of flexible aquatic vegetation fragments. The vegetation source is introduced, and the motion of flexible aquatic vegetation fragments includes the translation process caused by wind and water flow and the random diffusion process of flexible vegetation in water, while considering the collision, aggregation and retention process of flexible aquatic vegetation fragments during motion. When the mass of flexible aquatic vegetation fragments is small, the translation speed of flexible aquatic vegetation fragments has a good followability to the water flow speed. With the collision and retention of flexible aquatic vegetation fragments, the mass of aggregated flexible aquatic vegetation fragments gradually increases, and the followability with the water flow gradually decreases. It is assumed that during the simulation process, the leaves, i.e., the leaf aggregates, always float on the surface of the water flow, that is, no matter what state of motion they are in, the bionic leaves and leaf aggregates will not dive or settle. In each time step, the displacement of flexible aquatic vegetation fragments in two directions (X and Y) is calculated as follows: , Among them, β represents the followability coefficient of flexible aquatic vegetation fragments to water flow, It represents the flow velocity in the x direction in the grid calculated by the two-dimensional hydrodynamic calculation model of the simulation area. It represents the flow velocity in the y direction of the grid calculated by the two-dimensional hydrodynamic calculation model of the simulation area. Indicates a random number between the interval (-1,1), K x With K y is the diffusion coefficient in two directions, and Δt represents the time step.

[0036] Furthermore, during the movement, the bionic blades are generalized into two circles. When the distance d between the two bionic blades is less than or equal to 2 times the blade radius r, the bionic blades will have P c The probability of collision, P a After the aggregation occurs, the mass and area of ​​the two bionic leaves are superimposed to form a leaf agglomerate.

[0037] Furthermore, when the bionic blade or the blade aggregate passes through the shallow water area or the edge of the river, the flexible aquatic vegetation fragments collide with each other according to the probability P c The probability of colliding with the river bank boundary and / or the river island and passing through the retention is P r The reduction of flexible aquatic vegetation leaves occurred, and the retention time was consistent with the time distribution T in the experiment. r , and then continue to move downstream. When the bionic blades or the blade aggregates encounter obstacles (such as trash drains, trash nets, etc.) when moving downstream, the bionic blades or the blade aggregates immediately stop moving, and the displacement step calculation of these bionic blades or the blade aggregates is stopped.

[0038] , Among them, M t It represents the amount of vegetation debris remaining after passing through the retention zone, and M0 represents the amount of vegetation debris source initially introduced.

[0039] Step 303, model verification. This includes verification of the two-dimensional hydrodynamic calculation model and the flexible aquatic vegetation fragment movement calculation model. The two-dimensional hydrodynamic calculation model uses measured hydrological test data to calibrate and verify the water surface line and flow velocity distribution of the river section in the simulated area. The flexible aquatic vegetation fragment movement calculation model uses the location and area of ​​vegetation aggregation in the simulated area during the flood season to verify.

[0040] Step 304, process simulation. According to the aquatic vegetation type in the simulation area, different types of vegetation sources (different input parameters) are input, different flood level flow conditions and flexible vegetation release positions are set, and the migration and retention of flexible aquatic vegetation fragments in the simulation area during the flood season are simulated, and finally the aggregation and retention positions of flexible aquatic vegetation in the river channel after a flood, as well as the aggregation area and biomass are obtained.

[0041] This method simulates the aquatic vegetation migration and retention model coupled with the hydrodynamic calculation model and the flexible aquatic vegetation movement calculation model. The movement includes the translation process caused by wind and water flow and the random diffusion process of flexible vegetation in the water. At the same time, the collision between the flexible aquatic vegetation fragments and the flexible vegetation in the vegetation area passing through is considered. By setting different incoming flow conditions and flexible vegetation release positions, the migration and retention of flexible aquatic vegetation fragments in the simulated area during the flood season are simulated. The present invention is applicable to all water intake projects, pumping stations and river-type reservoirs facing the risk of blockage by flexible aquatic vegetation fragments.

[0042] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel, characterized in that: The steps include: Step 1: Obtain terrain data, hydrological and meteorological data, and aquatic vegetation data of the simulated area where flexible aquatic vegetation debris accumulation occurs; Step 2: Conduct simulation tests on the flexible aquatic vegetation fragments after generalization of the simulation area, including at least a diffusion characteristic test of the flexible aquatic vegetation fragments and a collision and retention characteristic test of the flexible aquatic vegetation fragments, obtain diffusion characteristic parameters, collision and retention characteristic parameters, establish correlations between the diffusion characteristics, collision and retention characteristics of different types of flexible aquatic vegetation fragments and water flow characteristics, and obtain the motion characteristics of the flexible aquatic vegetation fragments; Step 3: Establish and couple a two-dimensional hydrodynamic calculation model and a flexible aquatic vegetation fragment movement calculation model for the simulation area, and perform calculations on the migration and retention of flexible aquatic vegetation fragments in the simulation area; simulate the migration and retention of the flexible aquatic vegetation fragments in the river channel under different levels of flood conditions in the simulation area during the flood season.

2. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 1, characterized in that: In step 1, the topographic data and hydroclimatic data of the simulation area are collected through literature, including at least the river topography and typical cross-sectional data of the simulation area, inlet section flow and outlet water level data, and wind direction and speed data.

3. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 1, characterized in that: In step 1, the method for collecting data on flexible aquatic vegetation fragments is as follows: after the flood, aquatic vegetation survey plots are set up through drone photography combined with manual field surveys, and the types of flexible aquatic vegetation fragments in the simulated area, the areas and areas of accumulation and retention, and the biomass of flexible aquatic vegetation fragments are obtained through salvage.

4. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 1, characterized in that: The step 2 comprises the following steps: The test model was established according to the gravity similarity criterion, the test conditions in the laboratory were determined according to the length scale, velocity scale and flow scale, and the bionic blades were selected; The test model is arranged and carried out in a self-circulating water tank system, which at least includes a water tank, a tailgate and an electromagnetic flowmeter. Water is pumped from a downstream reservoir to the upstream water tank. The electromagnetic flowmeter is arranged at the water inlet of the water tank to control the water inlet flow, and the water level is adjusted by the downstream tailgate. A high-speed camera is mounted on the mobile measuring platform on the top of the water tank, a plastic net is arranged in front of the tailgate at the end of the water tank to recycle the bionic blades, and a waterproof sticker is arranged at the bottom of the water tank. Preliminary test to determine the most easily identifiable color of the bionic leaf; Release the bionic leaf with the most easily identifiable color from the release point, and after the bionic leaf passes through the shooting area of ​​the high-speed camera, release the next leaf; import the video shot by the high-speed camera into the object capture software to automatically track the position of the bionic leaf and identify it, and obtain the identification elements; The diffusion coefficient K of the flexible aquatic vegetation fragments in two directions is obtained x With K y , collision probability P c , cohesion probability P a , retention probability P r and the retention time T r ; Analyze the calculated data and establish the diffusion coefficient K x With K y , collision probability P c , cohesion probability P a , retention probability P r Correlation with model test variables.

5. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 4, characterized in that: The experiment selects at least four water flow rates, including the multi-year average monthly maximum flow rate in the normal water period, the multi-year average water flow rate, the multi-year average monthly maximum flow rate in the flood period, and the flood flow rate that returns once in ten years; and selects at least four types of bionic blades, including two planar blades and two clustered blades.

6. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 4, characterized in that: In the experiment, the variables related to the shape of the bionic blade include the blade type and the blade complexity index DI, and the variables related to the aquatic vegetation arrangement include the vegetation density ρ and the vegetation arrangement form; Among them, the blade complexity index is: , In the formula, C l represents the circumference of the bionic blade; A l Represents the area of ​​the bionic leaf.

7. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 4, characterized in that: The elements identified include the position of the bionic blade at each moment after release, the speed of the bionic blade in multiple directions, and the rotation speed; The formula for parameter calculation is as follows: ， In the formula, is the standard deviation of the bionic blade position at time t1; is the standard deviation of the bionic blade position at time t2, N represents the total number of bionic blades released, and N c Indicates the number of bionic blades that collided, N a Indicates the number of bionic leaves that have cohesion after collision, N r Indicates the number of bionic leaves that are retained after collision.

8. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 1, characterized in that: In step 3, the two-dimensional hydrodynamic calculation model is constructed as follows: the two-dimensional hydrodynamic calculation model uses an unstructured grid to divide the calculation area, and for the bank boundary, the water flow no-slip condition is adopted, while considering the water surface wind stress tensor, and the wind stress is obtained through an empirical formula.

9. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 1, characterized in that: In step 3, the flexible aquatic vegetation fragment motion calculation model is constructed as follows: an aquatic vegetation source is introduced, and the motion of the flexible aquatic vegetation fragments includes a translation process caused by wind and water flow and a random diffusion process of the flexible aquatic vegetation fragments in the water, and also considers the collision, cohesion and retention process of the flexible aquatic vegetation fragments during the motion process; At each time step, the displacement of flexible aquatic vegetation fragments in two directions is calculated as follows: ， Among them, β represents the followability coefficient of flexible aquatic vegetation fragments to water flow, It represents the flow velocity in the x direction in the grid calculated by the two-dimensional hydrodynamic calculation model of the simulation area. It represents the flow velocity in the y direction of the grid calculated by the two-dimensional hydrodynamic calculation model of the simulation area. It means taking a random number between the interval (-1,1); During the movement, the bionic blades are generalized into two circles. When the distance d between the two bionic blades is less than or equal to 2 times the blade radius r, the bionic blades will have P c The probability of collision, P a The probability of agglomeration occurs, and after agglomeration occurs, the mass and area of ​​the two bionic leaves are superimposed to form a leaf agglomerate; When the bionic blade or the blade aggregate passes through the shallow water area or the edge of the river, the flexible aquatic vegetation fragments will collide with the water according to the probability P c The probability of colliding with the river bank boundary and / or the river island and passing through the retention is P r The reduction of flexible aquatic vegetation leaves occurred, and the retention time was consistent with the time distribution T in the experiment. r , and then continue to move downstream. When the bionic blades or the blade aggregates encounter obstacles when moving downstream, the bionic blades or the blade aggregates immediately stop moving, stop the displacement step calculation of these bionic blades or the blade aggregates, and calculate the amount of flexible aquatic vegetation fragments remaining after passing through the retention area: ， Where M t represents the amount of flexible aquatic vegetation debris remaining after passing through the retention zone, and M0 represents the amount of the initially introduced flexible aquatic vegetation debris source.

10. The method for simulating the migration and retention of flexible aquatic vegetation fragments in a river channel according to claim 1, characterized in that: In the step 3, it also includes verification of the two-dimensional hydrodynamic calculation model and the flexible aquatic vegetation fragment movement calculation model. The two-dimensional hydrodynamic calculation model uses measured hydrological test data to calibrate and verify the water surface line and flow velocity distribution of the river section in the simulated area, and the flexible aquatic vegetation fragment movement calculation model uses the location and area of ​​vegetation aggregation in the simulated area during the flood season for verification.

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