A method for characterizing the Manning roughness coefficient in a braided reach and a method for calculating the resistance coefficient based on the same

Abstract

The present invention provides a method for characterizing the Manning roughness coefficient in a braided reach and a method for calculating the resistance coefficient based on the same. The Froude number is selected to reflect the flow intensity; the relative water depth is selected to reflect the bed roughness; the cross-sectional river facies coefficient is selected to reflect the cross-sectional shape; the ratio of the cross-sectional area of the braided section to the cross-sectional area of the single upstream section is used as a parameter to reflect the degree of cross-sectional contraction and expansion of the braided reach; an empirical formula for the Manning roughness coefficient in the braided reach is constructed. The empirical formula for the Manning roughness coefficient of the braided river in the present invention can better reflect the comprehensive effect of different influencing factors on the resistance of the braided river, and has a higher calculation accuracy. Applying the empirical formula for the Manning roughness coefficient to the mathematical model of the braided reach can accurately determine the roughness values under different topographic boundaries and different discharges, and can better make up for the deficiency that the roughness cannot be automatically adjusted according to the changes in riverbed scouring and silting and water flow conditions in the past mathematical models, and improve the simulation accuracy of the mathematical model.

Description

Technical Field

[0001] The present invention relates to the technical field of water conservancy and water transportation engineering, and specifically relates to a method for characterizing the Manning roughness coefficient of a braided reach and a method for calculating the resistance coefficient based on the same. Background Art

[0002] Channel resistance determines the loss of mechanical energy during the movement of water flow, is closely related to water flow turbulence and sediment movement, is a basic issue in the study of river dynamics, and is also an important basis for river numerical simulation. Previous researchers have mainly carried out a large number of studies from two aspects: the composition and calculation of channel resistance. According to the different sources of frictional resistance, the resistance of alluvial rivers can be divided into bed resistance, bank and floodplain resistance, river channel form resistance (also known as river regime resistance), and artificial structure resistance, etc. Existing movable bed resistance calculation methods include the resistance division method and the comprehensive resistance method. In previous studies, few people have proposed corresponding resistance calculation formulas for different river types, and there are few studies on the calculation of movable bed resistance in braided reaches. Even if the observed data of braided reaches are used, the differences and similarities in the resistance distribution between braided reaches and other reaches are not considered separately. Generally speaking, the existing movable bed resistance calculation formulas have poor adaptability in braided reaches. Summary of the Invention

[0003] The present invention provides a method for characterizing the Manning roughness coefficient of a braided reach and a method for calculating the resistance coefficient based on the same, so as to solve the problems that the existing typical movable bed resistance formula reflects the resistance change law of the braided reach poorly and the calculation accuracy is low. The resistance coefficient calculation formula of the braided reach provided by the present invention is established based on considering the main influencing factors of the resistance of the braided reach, namely water flow intensity, bed surface roughness, cross-sectional shape, and the degree of contraction and expansion of the braided section.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] A method for characterizing the Manning roughness coefficient of a braided reach includes the following steps:

[0006] Step S1, select the Froude number Fr to reflect the water flow intensity;

[0007] Step S2, select the relative water depth to reflect the bed surface roughness;

[0008] Step S3, select the cross-sectional river phase coefficient to reflect the cross-sectional shape;

[0009] Step S4. Regarding the influence of the contraction and expansion of the braided section on the resistance, use the ratio of the cross-sectional area A of the braided section to the cross-sectional area A0 of the upstream single section as a parameter to reflect the degree of contraction and expansion of the braided section;

[0010] Step S5. Construct an empirical relationship between n and Fr, where n is the Manning roughness coefficient;

[0011] Step S6. On the basis of the empirical relationship between n and Fr in Step S5, further add factors reflecting bed surface roughness, cross-sectional morphology, and the degree of cross-sectional contraction and expansion in the braided section to construct a general form of the calculation formula for the Manning roughness coefficient in the braided river reach.

[0012]

[0013] where a, b, c, d, and e are empirical coefficients, where H is the average cross-sectional water depth, D 50 is the median diameter of the bed sediment, ε is the cross-sectional river facies coefficient, A is the cross-sectional flow area of the braided section, and A0 is the cross-sectional flow area of the upstream single section;

[0014] Step S7. Substitute the prototype observation data into the above formula to calibrate and obtain the empirical coefficients a, b, c, d, and e, and finally obtain the empirical formula for the Manning roughness coefficient.

[0015] Furthermore, in Step S2, the relative water depth expression is H / D 50 , and the larger the H / D50, the smaller the bed surface roughness.

[0016] Furthermore, in Step S3, select the cross-sectional river facies coefficient ε = B 1 / 2 / H, substitute it into the river facies coefficient ε' = B 1 / 2 / h max and H / h max of the maximum cross-sectional water depth, analyze their relationship with n. All three cross-sectional morphology parameters have a certain degree of influence on n. As ε, ε', and H / h max increase, n shows a decreasing trend, that is, the resistance of the wide and shallow braided section is less than that of the narrow and deep braided section. However, among the three morphology parameters, the correlation between ε and n is the best, followed by ε', and the relationship between n and H / hmax is relatively scattered, with the worst correlation. Therefore, the cross-sectional river facies coefficient is selected to reflect the cross-sectional morphology.

[0017] Furthermore, in Step S4, the ratio of the cross-sectional flow area A of the braided section to the cross-sectional flow area A0 of the upstream single section is expressed as A / A0.

[0018] Furthermore, in Step S5, there is a good power function relationship between the Manning roughness coefficient n and Fr.

[0019] Taking n as the dependent variable and Fr as the independent variable, propose the basic form of the n~Fr relationship formula:

[0020] n = aFr b .

[0021] The present invention also provides a calculation method for the channel resistance coefficient of a braided river reach based on the Manning roughness coefficient, which is characterized in that it includes the following steps:

[0022] Step I. According to the Manning roughness coefficient characterization method for braided river reaches, an empirical formula for the Manning roughness coefficient of the study reach is calculated.

[0023] Step II. The comprehensive resistance coefficient of the braided river reach is calculated according to the empirical formula of the Manning roughness coefficient.

[0024] Further, in Step II, the Manning roughness coefficient is substituted into to calculate the resistance coefficient λ, where U represents the average flow velocity, U * represents the friction velocity, C represents the Chezy coefficient, g represents the acceleration due to gravity, and R represents the hydraulic radius.

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] 1. The calculation formula of the comprehensive resistance coefficient in the present application has improved adaptability in the braided river reach compared with the existing movable bed resistance coefficient formula, and its calculation accuracy is relatively high.

[0027] 2. Applying the empirical formula of the Manning roughness coefficient in the present application to calculate the roughness in the mathematical model has the ability to automatically adjust the roughness value according to the changes in riverbed erosion and deposition and the inlet flow rate. Compared with the traditional method of calibrating and inversely calculating the roughness from measured data, the simulation accuracy of the mathematical model is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 shows the relationship between the measured value of the Manning roughness coefficient n and the Froude number Fr in the embodiment of the present invention;

[0029] Figure 2 shows the relationship between the measured value of the Manning roughness coefficient n and the relative water depth H / D 50 in the embodiment of the present invention;

[0030] Figure 3 shows the relationship between the measured value of the Manning roughness coefficient n and the cross-sectional shape parameter ε in the embodiment of the present invention, where ε = B 1 / 2 / H;

[0031] Figure 4 shows the relationship between the measured value of the Manning roughness coefficient n and the cross-sectional shape parameter ε' in the embodiment of the present invention, where ε' = B 1 / 2 / h max ;

[0032] Figure 5 shows the relationship between the measured value of the Manning roughness coefficient n and the cross-sectional shape parameter H / h max in the embodiment of the present invention;

[0033] Figure 6Relationship between the measured value of Manning roughness coefficient n and cross-sectional shape parameter ε under the same flow rate in the embodiments of the present invention;

[0034] Figure 7 Relationship between the measured value of Manning roughness coefficient n and A / A0 under the same flow rate in the embodiments of the present invention, where A is the cross-sectional area of the bifurcated section and A0 is the cross-sectional area of the single upstream section;

[0035] Figure 8 Comparison between the calculated value and the measured value of the resistance coefficient obtained from the empirical relationship of n~Fr in the embodiments of the present invention;

[0036] Figure 9 Comparison between the calculated value and the measured value of the resistance coefficient of the empirical formula of the comprehensive resistance coefficient in the bifurcated river reach in the embodiments of the present invention;

[0037] Figure 10 Comparison between the calculated value and the measured value of the resistance coefficient obtained from the comprehensive resistance coefficient formula of the bifurcated river reach for a single section in the embodiments of the present invention;

[0038] Figure 11 Comparison between the calculated value and the measured value of the resistance coefficient obtained from the comprehensive resistance coefficient formula of the bifurcated river reach for the bifurcated section in the embodiments of the present invention;

[0039] Figure 12 Schematic diagram of the river regime and related calculation parameters of Taipingkou Waterway in the embodiments of the present invention, where -5m represents 5m above the navigation datum elevation, 0m represents the navigation datum, and 3m and 5m respectively represent 3m and 5m below the navigation datum elevation;

[0040] Figure 13 Schematic diagram of the calculation grid division of the mathematical model of Taipingkou Waterway in the embodiments of the present invention;

[0041] Figure 14 Flow chart of determining the roughness coefficient by the resistance formula method in the embodiments of the present invention;

[0042] Figure 15 Comparison between the calculation result of determining the roughness coefficient by the resistance formula method and the measured value in the embodiments of the present invention;

[0043] Figure 16 Comparison of the calculation results of the water surface profile along the way in the embodiments of the present invention. Specific Embodiments

[0044] The technical solution of the present invention will be described in detail below with reference to the drawings, embodiments, and comparative examples.

[0045] Embodiment

[0046] A method for characterizing the Manning roughness coefficient in a bifurcated river reach includes the following steps:

[0047] Step S1, selecting the Froude number Fr to reflect the flow intensity;

[0048] Step S2: Select the relative water depth to reflect the bed surface roughness;

[0049] Step S3: Select the cross-sectional river facies coefficient to reflect the cross-sectional shape;

[0050] Step S4: Regarding the influence of the cross-sectional contraction and expansion of the braided reach on the resistance, use the ratio of the cross-sectional flow area A of the braided section to the cross-sectional flow area A0 of the upstream single section as a parameter to reflect the degree of cross-sectional contraction and expansion of the braided reach;

[0051] Step S5: Construct an empirical relationship between n and Fr, where n is the Manning roughness coefficient;

[0052] Step S6: On the basis of the empirical relationship between n and Fr in Step S4, further add factors reflecting the bed surface roughness, cross-sectional shape, and degree of cross-sectional contraction and expansion of the braided reach to construct a general form of the calculation formula for the Manning roughness coefficient of the braided reach

[0053]

[0054] where a, b, c, d, and e are empirical coefficients, where H is the cross-sectional average water depth, D 50 is the median diameter of the bed sediment, ε is the cross-sectional river facies coefficient, A is the cross-sectional flow area of the braided section, and A0 is the cross-sectional flow area of the upstream single section;

[0055] Step S7: Substitute the prototype observation data into the above formula to calibrate and obtain the empirical coefficients a, b, c, d, and e, and finally obtain the empirical formula for the Manning roughness coefficient.

[0056] The present invention also provides a calculation method for the comprehensive resistance coefficient of a braided reach based on the Manning roughness coefficient, which is characterized in that it includes the following steps:

[0057] Step I: Calculate the empirical formula for the Manning roughness coefficient of the study reach according to the characterization method of the Manning roughness coefficient of the braided reach;

[0058] Step II: Calculate the comprehensive resistance coefficient of the braided reach according to the empirical formula of the Manning roughness coefficient.

[0059] Further, in Step II, substitute the Manning roughness coefficient into to calculate the resistance coefficient λ, where U represents the average flow velocity, U * represents the friction velocity, C represents the Chezy coefficient, g represents the acceleration due to gravity, and R represents the hydraulic radius.

[0060] In this embodiment, as Figure 1 shown, there is a good power function relationship between the Manning roughness coefficient n and Fr. n decreases with the increase of Fr. When Fr is greater than a certain value, the change of n with Fr tends to be gentle. AsFigure 2 As shown, although the distribution of the point group in the relationship diagram between n and H / D 50 is relatively scattered, it can still be seen that whether it is a single cross-section or a bifurcated cross-section, n tends to decrease with the increase of H / D 50 , that is, the smaller the bed roughness, the smaller the resistance of the water flow. As Figures 3 - 5 shown, the river facies coefficient ε = B 1 / 2 / H of the cross-section is selected and substituted into the river facies coefficient ε' = B 1 / 2 / h max of the maximum water depth of the cross-section and H / h max These three parameters are used to analyze their relationship with n. The three cross-sectional shape parameters all have a certain degree of influence on n. With the increase of ε, ε' and H / h max , n shows a decreasing trend, that is, the resistance of the wide and shallow bifurcated section is smaller than that of the narrow and deep bifurcated section. However, among the three shape parameters, the correlation between ε and n is the best, followed by ε', and the relationship between n and H / h max is relatively scattered and the correlation is the worst. In addition, since ε will also change at the same cross-section under different discharges, in order to eliminate the influence of discharge, the variation of n with ε under the same discharge is plotted, and it can still be seen that n decreases with the increase of ε. As Figure 6 shown, it is the relationship between the measured value of the Manning roughness coefficient n and the cross-sectional shape parameter ε under the same discharge in different river reaches.

[0061] As Figure 7 shown, aiming at the influence of the cross-section contraction and expansion on the resistance in the bifurcated section, the ratio A / A0 of the cross-sectional area A of the bifurcated section to the cross-sectional area A0 of the upstream single section is used as a parameter to reflect the degree of cross-section contraction and expansion in the bifurcated section. The resistance coefficient increases rapidly with the increase of the cross-section contraction and expansion coefficient. In the branch channel with a large increase in cross-sectional area (such as Guanzhou branch channel), A / A0 = 1.8 - 2.2, the water flow is sharply dispersed, the turbulence is strong, the proportion of local losses increases, and the n value can reach 0.05. When A / A0 is equal to 1, the n value is the smallest. At this time, there is no sudden contraction or sudden expansion change along the cross-section, the local loss is significantly reduced, and the resistance coefficient is small.

[0062] In the embodiment of the present invention, taking n as the dependent variable and Fr as the independent variable, the basic form of the n~Fr relational expression is proposed:

[0063] n = aFr b

[0064] where a and b are empirical coefficients obtained by calibrating with measured data.

[0065] Based on 327 groups of prototype observation data of some bifurcated river reaches from Yichang to Jiujiang in the middle reaches of the Yangtze River from 2003 to 2015, the empirical relational expressions of n~Fr are obtained as follows:

[0066]

[0067] Calculate the value of \(n\) for the braided reach in the middle reaches of the Yangtze River using the \(n\sim Fr\) empirical relationship and compare it with the measured value of \(n\). Substitute the measured value and the calculated value of \(n\) into Calculate the measured value and the estimated value of the resistance coefficient \(\lambda\). The results are shown in Figure 8 as follows. Figure 8 In (a), it is the comparison diagram of the calculated value of \(n\) and the measured value of \(n\). Figure 8 In (b), it is the comparison diagram of the calculated value and the measured value of the resistance coefficient \(\lambda\). Use statistical parameters such as the root mean square \(RMS\), geometric standard deviation \(GSD\), and root mean square error \(RMSE\) to evaluate the degree of agreement between the calculated value and the measured value. The results show that the \(n\sim Fr\) relationship established based on the measured data of the braided reach can basically reflect the variation law of the resistance of the braided reach with the flow intensity. At the same time, it is also found that due to the lack of consideration of other factors affecting the resistance of the braided reach, the degree of deviation of the calculation results is relatively large, and the calculation accuracy needs to be further improved.

[0068] On the basis of the above \(n\sim Fr\) empirical relationship, further add factors reflecting the bed surface roughness, cross-sectional shape, and cross-sectional contraction and expansion degree of the braided section to construct the following general form of the Manning roughness coefficient calculation formula for the braided reach:

[0069]

[0070] where \(a\), \(b\), \(c\), \(d\), and \(e\) are empirical coefficients.

[0071] Substitute the same prototype observation data as above into the above formula, and finally obtain the following empirical formula for the Manning roughness coefficient of the braided reach:

[0072]

[0073] Use this empirical formula to calculate the value of \(n\) for the braided reach in the middle reaches of the Yangtze River, and substitute the calculated value of \(n\) into Calculate the calculated value of the resistance coefficient \(\lambda\). Compare the calculated values of \(n\) and \(\lambda\) with the measured values respectively. The results are shown in Figure 9 , Figure 9 In (a), it is the comparison diagram of the calculated value of \(n\) and the measured value of \(n\). Figure 9 In (b), it is the comparison diagram of the calculated value and the measured value of the resistance coefficient \(\lambda\). The measured data used is the same as the data for calibrating the empirical formula of the Manning roughness coefficient. Use statistical parameters such as the root mean square \(RMS\), geometric standard deviation \(GSD\), and root mean square error \(RMSE\) to evaluate the degree of agreement between the calculated value and the measured value. Generally speaking, this formula can better reflect the comprehensive effect of different factors on the resistance of the braided reach, and the calculation accuracy is relatively high.

[0074] As Figure 10 and 11As shown, they respectively correspond to the comparison results of the calculated values and measured values of the resistance coefficients obtained from the comprehensive resistance coefficient formula for the single-section and bifurcated-section of the embodiments of the present invention according to 68 sets of prototype observation data and 159 sets of physical model test measurement data from 2016 to 2019. The other measured data except the calibrated parameters are used. Figure 10 In (a), it is a comparison diagram of the calculated value of n for the single section and the measured value of n. Figure 10 In (b), it is a comparison diagram of the calculated value and measured value of the resistance coefficient λ for the single section. Figure 11 In (a), it is a comparison diagram of the calculated value of n for the bifurcated section and the measured value of n. Figure 11 In (b), it is a comparison diagram of the calculated value and measured value of the resistance coefficient λ for the bifurcated section.

[0075] See Figure 12 , in the embodiments of the present invention, the Taipingkou Waterway is selected as a typical river section to establish a mathematical model, as Figure 13 shown. The Taipingkou Waterway starts from Chenjiawan in the upper reaches and ends at Yuheping in the lower reaches, with a total length of about 18 km. There is the confluence of the Juzhang River on the left side of the river section inlet, and the Taipingkou diversion on the right side. From top to bottom, it can be divided into two branches: the Taipingkou straight bifurcation and the Sanbatan slightly curved bifurcation according to the different plane shapes. The verification river section is covered with a body-fitted orthogonal curvilinear grid of 220×100. The grid spacing in the water flow direction is 66 - 115 m, and the grid spacing perpendicular to the water flow direction is 15 - 30 m.

[0076] To compare the calculation effects of the Manning roughness coefficient empirical formula under different flow conditions, the present invention collected the topographic maps of the river channel and the measured hydrological section flow velocity and water level measurement data for three different measurement times: December 2014, August 2015, and August 2018. The inlet is respectively given three levels of flow rates: low, medium, and high (7020 m 3 / s, 17350 m 3 / s, 27500 m 3 / s) as the upper boundary conditions of the model; the outlet water level is obtained by interpolating the measured water level data of the upstream and downstream.

[0077] When determining the roughness using the Manning roughness coefficient empirical formula established by the present invention in the mathematical model, a trial calculation is required, and the trial calculation steps are as follows:

[0078] ① At the beginning of the calculation, a unified initial roughness value n = n0 is assigned to the entire calculation range, and the initial flow field distribution and river channel section geometry (A, B, U, A / A0, etc.) are solved and substituted into the right side of the comprehensive resistance coefficient empirical formula for the bifurcated river section to obtain the calculated value n* of the roughness.

[0079] ② Compare n* and n. If the difference between n* and n is large, then based on the difference between the two, n is updated according to the principle of gradual approximation and then substituted into the model for recalculation.

[0080] ③Substitute the calculation result of the model into the right side of the comprehensive resistance coefficient formula again to solve for n*, and compare it with the updated n. Repeat this process multiple times until the difference between n* and the updated n is within a certain error range. Then it is considered that n has been updated to the true roughness value, and the solution of the model roughness is completed.

[0081] The calculation flow chart is shown in Figure 14 .

[0082] Let the iteration calculation error control value e = 0.05. After calculating the roughness of the above three measurement times respectively by the above method, output the final calculated values of the cross-sectional water area, river width, cross-sectional average velocity and cross-sectional average water level (denoted as A*, B*, U* and H* respectively) and compare them with the measured values (A’, B’, U’ and H’). The results are shown in Figure 15 , Figure 15 . In (a), it is the calculated value and measured value of the cross-sectional average water level. In (b), it is the calculated value and measured value of the cross-sectional average velocity. In (c), it is the calculated value and measured value of the water area. In (d), it is the calculated value and measured value of the river width. The error ranges of each variable are shown in Table 1.

[0083] Control example

[0084] In the mathematical model, the roughness calibrated through measured data can also accurately describe the river channel resistance under verification conditions, but it cannot be automatically adjusted according to the changes in riverbed scouring and silting and water flow conditions during the calculation process. The roughness determined by the formula method can better make up for this deficiency.

[0085] The present invention first calibrates the river channel roughness under this terrain condition through the measured water level and velocity data in December 2014. The calibration of the roughness is carried out in segments, that is, multiple roughness intervals are divided along the way. After assigning a unified roughness value to each interval, the water surface profile along the way is solved by using the mathematical model, and the calculated water level and measured water level of each measured hydrological section (the position is shown in Figure 12 ) are compared. According to the deviation between the two, the value of the roughness is adjusted until the difference between the calculated water level and the measured water level is controlled within ±5 cm. Then it is considered that the calibrated roughness can reflect the true river channel resistance.

[0086] Assign the calibrated roughness value to the model and calculate the velocity distribution under the terrain conditions in August 2015 and August 2018 respectively, and compare it with the calculation results of determining the roughness by the resistance formula method in the previous section.

[0087] Figure 16 The comparison of the water surface profiles along the way under each calculation condition is given, where Figure 16 (a) is the comparison chart for the measurement in December 2014, Figure 16 (b) is the comparison chart for the measurement in August 2015, Figure 16(c) is the comparison chart for the measurement in August 2018. Tables 1 and 2 further give the error ranges and mean deviations of each variable (cross-sectional flow area A, river width B, cross-sectional average velocity U, and cross-sectional average water level H). The mean deviation is represented by the statistic Re = (∑(Mi - Ni)2 / n)1 / 2 (where i represents the number of data, and M and N represent the calculated value and the measured value respectively).

[0088] Obviously, since the channel resistance will be adjusted accordingly with the changes in riverbed scouring and silting and flow conditions, the traditional method of calibrating roughness using measured data cannot reflect the variation characteristics of the channel resistance. When applying the Manning roughness coefficient empirical formula established in the present invention to the mathematical model, the roughness value can be automatically adjusted according to the changes in riverbed scouring and silting and the inlet flow rate, enabling the determination of roughness and the solution of the flow field to interact with each other. This not only avoids the cumbersome process of calibrating with measured data but also can reflect the dynamic adjustment of the channel resistance with the changes in flow and channel boundary conditions, improving the simulation accuracy of the mathematical model.

[0089] Calculation error ranges of each variable in Table 1

[0090]

[0091] Mean deviations of each variable in Table 2

[0092]

[0093] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Those skilled in the art to which the present invention pertains can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, without departing from the spirit of the present invention or exceeding the scope defined by the appended claims.

Claims

1. A method for characterizing the Manning roughness coefficient in a braided reach, characterized in that It includes the following steps: Step S1: Select the Froude number Fr to reflect the flow intensity; Step S2: Select the relative water depth to reflect the bed roughness; Step S3: Select the cross-sectional river facies coefficient to reflect the cross-sectional shape; Step S4: Regarding the influence of the cross-sectional contraction and expansion of the braided section on the resistance, use the ratio of the cross-sectional area A of the braided section to the cross-sectional area A0 of the single upstream section as a parameter to reflect the degree of cross-sectional contraction and expansion of the braided section; Step S5: Construct an empirical relationship between n and Fr, where n is the Manning roughness coefficient; Step S6: On the basis of the empirical relationship between n and Fr in Step S5, further add factors reflecting the bed roughness, cross-sectional shape, and cross-sectional contraction and expansion degree of the braided section to construct a general form of the calculation formula for the Manning roughness coefficient of the braided reach; where a, b, c, d, and e are empirical coefficients, where H is the cross-sectional average water depth, D 50 is the median grain size of the bed sediment, ε is the cross-sectional river facies coefficient, A is the cross-sectional area of the braided section, and A0 is the cross-sectional area of the single upstream section; Step S7: Substitute the prototype observation data into the above formula to calibrate and obtain the empirical coefficients a, b, c, d, e, and finally obtain the empirical formula for the Manning roughness coefficient.

2. The Manning roughness coefficient characterization method for a braided reach according to claim 1, wherein: In step S2, the relative water depth expression is H / D 50 , H / D 50 The larger H / D is, the smaller the bed surface roughness is 3. A method for characterizing the Manning roughness coefficient in a braided reach according to claim 1, characterized in that: In step S3, select the cross-section river facies coefficient ε = B 1 / 2 / H, substitute the river facies coefficient ε' = B 1 / 2 / h max and H / h max of the three parameters, analyze their relationship with n. The three cross-section shape parameters all have a certain degree of influence on n. As ε, ε' and H / h max increase, n shows a decreasing trend, that is, the resistance of the wide and shallow braided section is less than that of the narrow and deep braided section. However, among the three shape parameters, the correlation between ε and n is the best, followed by ε', and the relationship between n and H / hmax is relatively scattered, with the worst correlation. Therefore, the cross-section river facies coefficient is selected to reflect the cross-section shape.

4. A method for characterizing the Manning roughness coefficient in a bifurcated river reach according to claim 1, characterized in that: In Step S4, the expression for the ratio of the cross-sectional area A of the braided section to the cross-sectional area A0 of the single upstream section is A / A0.

5. A method for characterizing the Manning roughness coefficient in a bifurcated river reach, characterized in that: In Step S5, there is a good power function relationship between the Manning roughness coefficient n and Fr. Taking n as the dependent variable and Fr as the independent variable, propose the basic form of the n~Fr relationship: n = aFr b .

6. A calculation method for the channel resistance coefficient of a braided reach based on the Manning roughness coefficient, characterized in that: It includes the following steps: Step I: Calculate the empirical formula for the Manning roughness coefficient of the study reach according to the method for characterizing the Manning roughness coefficient of the braided reach described in any one of Claims 1 to 5; Step II: Calculate the comprehensive resistance coefficient of the braided reach according to the empirical formula for the Manning roughness coefficient.

7. A calculation method for the channel resistance coefficient of a bifurcated river reach based on the Manning roughness coefficient, characterized in that: In the step II, substituting the Manning roughness coefficient into calculate the resistance coefficient λ, where U represents the average flow velocity, U * represents the friction velocity, C represents the Chezy coefficient, g represents the acceleration due to gravity, and R represents the hydraulic radius.

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