Method for calculating Manning roughness coefficient of rectangular river channel containing submerged flexible plants

Abstract

The invention discloses a method for calculating a Manning roughness coefficient of a rectangular river channel containing submerged flexible plants. The method comprises the following steps: acquiring the volume VSP of a single plant and the number D of plants in a unit area; measuring the width B, the length L and the water level Hc of the river containing the submerged flexible plant section; equivalently distributing the plant resistance on the bed surface, and calculating the water depth Hp and the hydraulic radius Rp after equivalent distribution; determining the relationship between the dimensionless deflection angle of the plant and the submerging degree and the water flow velocity; and calculating to obtain a Manning roughness coefficient of the rectangular river channel containing submerged flexible plants. According to the method, the complex flow velocity distribution containing the plant section is reflected through the deflection angle and the submerging degree of the plant, the problem that the flow velocity parameter is not easy to determine is solved, the calculation error is reduced, the measurement precision is improved, and the calculation convenience and accuracy are improved.

Description

Technical Field

[0001] The invention relates to river channel roughness calculation, and in particular to a method for calculating the Manning roughness coefficient of a rectangular river channel containing submerged flexible plants. Background Art

[0002] Building a plant ecosystem in the river has become a common solution. This ecosystem mainly restores and improves the water environment by planting submerged plants, which not only helps to stabilize and improve water quality, but also effectively stabilizes the riverbed and reduces erosion. However, the swaying and deformation of flexible submerged plants with the waves complicates the flow pattern of the water, which in turn significantly affects the roughness characteristics of the river. Since the roughness of the river is related to ecological governance and flood safety during floods, accurate calculation of river resistance is crucial for river ecological management and the design of flood control measures.

[0003] River channel resistance is usually characterized by the Manning coefficient, and the roughness of the river channel is usually estimated by the Manning coefficient in the Manning formula. However, when there is a large amount of flexible vegetation, the effectiveness and accuracy of the traditional method will be significantly reduced. This is because the flexible vegetation exhibits diverse resistance characteristics due to its dynamic behavior under different flow velocities and flow conditions, resulting in complex flow velocity distribution in the river section. The flow velocity parameters in the traditional formula are not easy to determine, resulting in low accuracy of the calculation results.

[0004] Therefore, a new technical solution is needed to solve this problem. Summary of the invention

[0005] Purpose of the invention: In order to overcome the deficiencies in the prior art, a method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants is provided, which reflects the complex flow velocity distribution of the plant section through the deflection angle and submergence degree of the plants, solves the problem that the flow velocity parameters are difficult to determine, reduces calculation errors, improves measurement accuracy, and enhances the convenience and accuracy of calculation.

[0006] Technical solution: To achieve the above purpose, the present invention provides a method for calculating the Manning roughness coefficient of a rectangular river channel containing submerged flexible plants, comprising the following steps:

[0007] S1: Get the volume V of a single plant SP and the number of plants per unit area, D;

[0008] S2: Measure the width B, length L and water level H of the river channel containing submerged flexible plants c ;

[0009] S3: Distribute the plant resistance equivalently on the bed surface and calculate the water depth H after equivalent distribution p And hydraulic radius R p ;

[0010] S4: Determine the relationship between the dimensionless deflection angle of the plant and the degree of submergence and the water flow velocity;

[0011] S5: Combining the data from steps S1 to S4, the Manning roughness coefficient of the rectangular river channel containing submerged flexible plants is calculated.

[0012] Furthermore, the method for obtaining the volume of a single plant in step S1 is: using the drainage method to measure multiple times and take the average value to obtain the volume of the plant; the number of plants per unit area is determined by selecting the number of plants within 1 square area.

[0013] Furthermore, the water level measurement method in step S2 is: starting from the point where the vegetation section is entered and ending at the point where the vegetation section is left, selecting sections at equal intervals in the middle, measuring three measuring points in each section, and calculating the hydraulic slope of the vegetation section based on the water level difference between the starting point and the end point.

[0014] Furthermore, in step S3, the equivalent distribution of plant resistance on the bed surface is as follows: since the water depth increased by the presence of plants does not completely affect the roughness of the river channel, the plants are regarded as plant particles attached to the original riverbed surface, and the resistance of the plants is simplified to the uniform force on the particles on the riverbed. The height of the plant particles is

[0015] The specific characteristics of the height of plant particles are: the height of plant particles is actually a dynamic value, which is affected by vegetation density and flow velocity. When the density increases, the particle height also increases; when the flow velocity increases, the deflection angle of the plant becomes larger, and the plant volume within a certain cross-sectional area decreases, thereby reducing the particle height.

[0016] Furthermore, the equivalent water depth H in step S3 is p The calculation formula is: Hydraulic radius R p The calculation formula is:

[0017] Furthermore, the dimensionless deflection angle and submergence degree in step S4 are defined as: the deflection angle is defined as the angle between the line connecting the position of the top center point of the measured plant and the position of the bottom center point of the measured plant and the vertical direction; the submergence degree is defined as the ratio of the water depth at the measured plant to the height of the plant after deflection.

[0018] Furthermore, the process of determining the relationship between the dimensionless deflection angle of the plant and the degree of submergence and the water flow velocity in step S4 includes:

[0019] A1: Select plants of the same type as the river;

[0020] A2: Select a plant or a prototype river channel in the water tank and continuously record the flow rate, deflection angle and flooding degree data. The deflection angle is defined as the angle between the vertical direction and the line connecting the center point of the top of the plant and the center point of the bottom of the plant. The flooding degree is defined as the ratio of the water depth at the plant to the height of the plant after deflection.

[0021] A3: A large number of experiments have proved that there is a linear relationship between flow velocity, deflection angle and flooding degree. Where u is the flow velocity, α m , β m are the dimensionless deflection angle and submergence degree of the flexible plant and the linear coefficient of the flow velocity, k is the compensation flow velocity, where θ v is the measured deflection angle, θ 0 Defined as the characteristic angle, specify θ 0 =1rad, after the data is dimensionless, the three recorded data are fitted and analyzed using the multivariate linear fitting method;

[0022] A4: Measure and calculate the average deflection angle and submergence degree of the plants in the section. Since the roughness of a section of the river is to be calculated, it is not very meaningful to measure the deflection angle and submergence degree of a single plant. The average of the deflection angle and submergence degree of all plants should be used.

[0023] Furthermore, the calculation formula of the Manning roughness coefficient in step S5 is:

[0024]

[0025] Beneficial effects: Compared with the prior art, the present invention reflects the complex flow velocity distribution of the plant segment through the deflection angle and submergence degree of the plant, and fully takes the dynamic behavior of the flexible plant under different flow velocities and flow conditions into consideration in the calculation, thus solving the problem that the flow velocity parameters are difficult to determine, reducing calculation errors, improving measurement accuracy, and enhancing the convenience and accuracy of calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the calculation method of the present invention. DETAILED DESCRIPTION

[0027] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0028] like Figure 1As shown, the present invention provides a method for calculating the Manning roughness coefficient of a rectangular river channel containing submerged flexible plants, comprising the following steps:

[0029] S1: Get the volume V of a single plant SP and the number of plants per unit area, D;

[0030] Volume of a single plant V SP The method of obtaining the volume V of the plant is as follows: use the drainage method to measure multiple times and take the average value to get the volume V of the plant. SP ; The number of plants per unit area D is determined by selecting the number of plants in 1 square area

[0031] S2: Measure the width B, length L and water level H of the river channel containing submerged flexible plants c ;

[0032] Since it is a rectangular river channel, the vertical distance between the two banks is measured using a distance measuring instrument to obtain the river channel width B;

[0033] Mark the two intersection sections of the river section with plants and the river section without plants, that is, the point where the river enters the vegetation section and the point where the river leaves the vegetation section. Use a distance measuring instrument to measure the vertical length of the two marked points, that is, the length of the vegetation section, to obtain the river length L.

[0034] Water level H c The measurement method is: starting from the point where the water enters the vegetation section and ending at the point where the water leaves the vegetation section, selecting sections at equal intervals in the middle, measuring three measuring points in each section, and calculating the hydraulic slope of the vegetation section based on the water level difference between the starting point and the end point.

[0035] S3: Distribute the plant resistance equivalently on the bed surface and calculate the water depth H after equivalent distribution p And hydraulic radius R p ;

[0036] The equivalent distribution of plant resistance on the bed surface is as follows: Since the presence of plants increases the water depth but does not completely affect the roughness of the river channel, the plants are regarded as plant particles attached to the original riverbed surface. The resistance of the plants is simplified to the uniform force on the particles on the riverbed. The height of the plant particles is

[0037] The specific characteristics of the height of plant particles are: the height of plant particles is actually a dynamic value, which is affected by vegetation density and flow velocity. When the density increases, the particle height also increases; when the flow velocity increases, the deflection angle of the plant becomes larger, and the plant volume within a certain cross-sectional area decreases, thereby reducing the particle height.

[0038] Equivalent water depth H p The calculation formula is: Hydraulic radius R pThe calculation formula is:

[0039] S4: Determine the relationship between the dimensionless deflection angle of the plant and the degree of submergence and the water flow velocity;

[0040] The process includes:

[0041] A1: Select plants of the same type as the river;

[0042] A2: Select a plant or a prototype river channel in the water tank and continuously record the flow rate, deflection angle and flooding degree data. The deflection angle is defined as the angle between the vertical direction and the line connecting the center point of the top of the plant and the center point of the bottom of the plant. The flooding degree is defined as the ratio of the water depth at the plant to the height of the plant after deflection.

[0043] A3: Through a large number of experiments, it is found and proved that there is a linear relationship between flow velocity, deflection angle and flooding degree. Where u is the flow velocity, α m , β m are the linear coefficients of the dimensionless deflection angle and submergence degree of the flexible plant and the flow velocity, which are obtained by multivariate linear fitting of observational or experimental data; k is the compensation flow velocity, which is obtained by linear fitting and is defined as the compensation flow velocity, which has the dimension of velocity; θ v is the measured deflection angle, θ 0 Defined as the characteristic angle, specify θ 0 =1rad, after the data is dimensionless, the three recorded data are fitted and analyzed using the multivariate linear fitting method;

[0044] A4: Measure and calculate the average deflection angle and submergence degree of the plants in the section. Since the roughness of a section of the river is to be calculated, it is not very meaningful to measure the deflection angle and submergence degree of a single plant. The average of the deflection angle and submergence degree of all plants should be used.

[0045] S5: Combine the data from steps S1 to S4 to calculate the Manning roughness coefficient of the rectangular river channel containing submerged flexible plants; the calculation formula is:

[0046]

[0047] The calculated Manning's roughness coefficient is mainly used in actual ecological river restoration projects or newly designed ecological channel projects.

[0048] Based on the above content, this embodiment also verifies the effectiveness and effect of the present invention through specific experiments, which are as follows:

[0049] Experimental process: (water tank experiment, plants are simulated by nylon material) The experiment was carried out in a transparent water tank, and the plants were fixed on the gray plastic board laid in the water tank. The plant density was 166 plants per square meter, and the length of the plant segment was 2m. During the experiment, the flow rate of water inlet was changed, thereby changing the water depth and flow rate. The deflection angle and submergence degree of the plants were recorded under each working condition, and the image recognition processing technology was used for automatic reading. During the reading process, abnormal points such as maximum and minimum points were removed; and a Doppler three-dimensional current meter was used to measure on the cross section. The cross section selection was selected every 40cm from the beginning of entering the vegetation section. There were a total of 6 cross sections, with three measuring points in each cross section. Each measuring point started from 2 cm from the bottom, and a measuring point was set every 3 cm until it was 5 cm from the water surface. When collecting flow rate data, the sampling frequency of each measuring point was 200Hz, and the average flow rate of the cross section was calculated, and finally the average flow rate of the entire vegetation section was obtained. The average deflection angle and average submergence degree of the flexible plants during this period were obtained. Then the fitting was performed according to the multivariate linear regression method. The experimental data are shown in Table 1:

[0050] Table 1

[0051]

[0052] The relationship between flow velocity, deflection angle and flooding degree was obtained by multivariate linear fitting of experimental data:

[0053]

[0054] Final calculation formula:

[0055]

[0056] Compared with the measured values, the specific values ​​are shown in Table 2:

[0057] Table 2

[0058]

[0059]

[0060] It can be seen from the data in Table 2 that the calculated values ​​in the five experimental sequences are very close to the measured values, which verifies that the calculation method of the present invention has good calculation accuracy.

Claims

1. A method for calculating the Manning roughness coefficient of a rectangular river channel containing submerged flexible plants, characterized in that: The steps include: S1: Get the volume V of a single plant SP and the number of plants per unit area, D; S2: Measure the width B, length L and water level H of the river channel containing submerged flexible plants c ; S3: Distribute the plant resistance equivalently on the bed surface and calculate the water depth H after equivalent distribution p And hydraulic radius R p ; S4: Determine the relationship between the dimensionless deflection angle of the plant and the degree of submergence and the water flow velocity; S5: Combining the data from steps S1 to S4, the Manning roughness coefficient of the rectangular river channel containing submerged flexible plants is calculated.

2. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 1, characterized in that: The method for obtaining the volume of a single plant in step S1 is: using the drainage method to measure multiple times and take the average value to obtain the volume of the plant; the number of plants per unit area is determined by selecting the number of plants within 1 square area.

3. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 1, characterized in that: The water level measurement method in step S2 is: starting from the point where the vegetation section is entered and ending at the point where the vegetation section is left, selecting sections at equal intervals in the middle, measuring three measuring points in each section, and calculating the hydraulic slope of the vegetation section based on the water level difference between the starting point and the end point.

4. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 1, characterized in that: The method of equivalently distributing the plant resistance on the bed surface in step S3 is: the plants are regarded as plant particles attached to the original riverbed surface, and the resistance of the plants is simplified to the uniform force of the particles on the riverbed. The height of the plant particles is 5. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 4, characterized in that: The water depth H after equivalent in step S3 p The calculation formula is: Hydraulic radius R p The calculation formula is:

6. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 1, characterized in that: The dimensionless deflection angle and flooding degree in step S4 are defined as follows: the deflection angle is defined as the angle between the line connecting the position of the top center point of the measured plant and the position of the bottom center point of the measured plant and the vertical direction; the flooding degree is defined as the ratio of the water depth at the measured plant to the height of the plant after deflection.

7. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 6, characterized in that: The process of determining the relationship between the dimensionless deflection angle of the plant and the degree of submergence and the water flow velocity in step S4 includes: A1: Select plants of the same type as the river; A2: Select a plant or a prototype river channel in the water tank and continuously record the flow rate, deflection angle and flooding degree data. The deflection angle is defined as the angle between the vertical direction and the line connecting the center point of the top of the plant and the center point of the bottom of the plant. The flooding degree is defined as the ratio of the water depth at the plant to the height of the plant after deflection. A3: There is a linear relationship between flow velocity, deflection angle and flooding degree. Where u is the flow velocity, α m , β m are the dimensionless deflection angle and submergence degree of the flexible plant and the linear coefficient of the flow velocity, k is the compensation flow velocity, where θ v The measured deflection angle, θ0 is defined as the characteristic angle, and θ0 = 1 rad is specified. After the data are dimensionless, the three recorded data are fitted and analyzed using the multivariate linear fitting method; A4: Measure and calculate the average deflection angle and submergence degree of the plants in the segment.

8. The method for calculating the Manning's roughness coefficient of a rectangular river channel containing submerged flexible plants according to claim 1, characterized in that: The calculation formula of the Manning roughness coefficient in step S5 is:

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