A method for calculating the roughness coefficient of riverbed with vegetation

Abstract

The present invention discloses a method for calculating the roughness coefficient of a river channel beach with vegetation, the method comprising the following steps: S1: calculating the characteristics of the beach vegetation, including vegetation type, vegetation characteristic length d, number of vegetation per unit area N, vegetation density a, vegetation height h c ; S2: Calculate the roughness coefficient n0 of the beach without vegetation; S3: Establish the water flow dynamic balance equation of the river beach with vegetation, and deduce the calculation formula applicable to the vegetation roughness coefficient of the river with beach; S4: Determine the water flow state of vegetation, calculate the beach water depth H, and combine the vegetation height h c , determine the submerged state of vegetation; S5: according to the submerged state of vegetation, use the calculation formula obtained in step S3 to calculate the roughness coefficient of the river beach containing vegetation. The present invention can quickly calculate the roughness coefficient of the river beach containing vegetation, and the calculation accuracy is high. It can be applied to the calculation of the roughness coefficient of the beach with various types of vegetation and various water flow states, avoiding the limitations of the existing methods with narrow application scope and single application situation.

Description

Technical Field

[0001] The invention relates to the technical field of water conservancy projects, and in particular to a method for calculating the roughness coefficient of a river channel beach containing vegetation. Background Art

[0002] The roughness coefficient is a coefficient that comprehensively reflects the roughness of the river channel, and is generally expressed as the Manning coefficient. The roughness coefficient is widely used in water conservancy projects, water transport projects and other fields. It is an important parameter required for the analysis of the relationship between water level and flow in the river channel, the calculation of water surface lines, and the simulation of hydrodynamics and riverbed deformation. In recent years, with the commissioning of water conservancy and hydropower hub projects in the upper reaches of major rivers, the frequency of low water levels in the plain rivers downstream of the hubs has increased, the frequency of high water levels has decreased, and various types of vegetation have grown luxuriantly on the river beach. At the same time, with the improvement of the requirements for river ecological management, vegetation such as reeds, shrubs and trees are also increasingly used in the ecological management of river beach land. While vegetation improves the ecological environment of the river bank, how much influence it has on water flow resistance, especially on major floods, is a scientific issue worthy of attention.

[0003] At present, there are some methods for calculating the roughness coefficient of riverbeds with vegetation, but in general the calculation process is relatively complicated, there are many calculation parameters, it is difficult to obtain the parameters, and the application scope of the calculation method is relatively narrow. How to simply, quickly, scientifically and reasonably calculate the roughness coefficient of riverbeds with vegetation has become a technical problem that needs to be solved urgently. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the above-mentioned background technology and provide a method for calculating the roughness coefficient of a river channel beach containing vegetation. The method can conveniently and quickly calculate the roughness coefficient of a river channel beach containing vegetation with high measurement accuracy.

[0005] To achieve the above object, the present invention provides a method for calculating the roughness coefficient of a river channel beach with vegetation, comprising the following steps:

[0006] S1: Calculate the characteristics of beach vegetation, including vegetation type, vegetation characteristic length d, number of vegetation per unit area N, vegetation density a, vegetation height h c ;

[0007] S2: Calculate the roughness coefficient n0 of the beach without vegetation;

[0008] S3: Establish the dynamic balance equation of water flow in riverbeds with vegetation, and derive the calculation formula for the roughness coefficient of vegetation in riverbeds with vegetation;

[0009] S4: Determine the vegetation flow state, calculate the beach water depth H, and combine the vegetation height h c , determine the submergence status of vegetation;

[0010] S5: Calculate the roughness coefficient of the river floodplain with vegetation using the calculation formula obtained in step S3 according to the vegetation inundation state.

[0011] In the above technical solution, in step S1, the vegetation types are generally divided into herbs, shrubs, and trees.

[0012] In the above technical solution, in step S1, the vegetation density a is the projected area of the vegetation perpendicular to the water flow direction per unit volume, and is calculated by the following formula:

[0013] a = Nd

[0014] In the formula, N is the number of vegetation corresponding to the unit area, and d is the vegetation characteristic length.

[0015] In the above technical solution, in step S1, if the vegetation stem is circular, the vegetation characteristic length d is the diameter of the stem; if the vegetation stem is of other shapes, the vegetation characteristic length d is the width perpendicular to the water flow direction; the vegetation height h c is the actual height of the vegetation above the floodplain surface.

[0016] In the above technical solution, in step S2, the roughness coefficient n0 of the floodplain without vegetation is obtained by querying the hydraulic calculation manual or by referring to relevant engineering experience.

[0017] In the above technical solution, in step S3, the hydrodynamic equilibrium equation of the river floodplain with vegetation is:

[0018]

[0019] In the formula, U is the vertical average velocity of the water flow in the floodplain with vegetation; U1 is the vertical average velocity of the water flow in the vegetation layer (when the vegetation is in the inundated state, U1 < U; when the vegetation is in the non-inundated state, U1 = U); a is the vegetation density; h is the height of the floodplain vegetation below the water surface; H is the floodplain water depth (when the vegetation is non-inundated, h = H); g is the acceleration due to gravity; f is the Darcy-Weisbach coefficient; S0 is the water surface slope; C D is the vegetation drag force coefficient.

[0020] In the above technical solution, in step S3, the water surface slope S0 and the Darcy-Weisbach coefficient f of the water flow in the floodplain with vegetation are calculated by the following formula:

[0021]

[0022] f = 8g / C 2

[0023] C = R 1 / 6 / n0

[0024] Where n is the roughness coefficient of the beach with vegetation; R is the hydraulic radius, which can be expressed by the beach water depth H; S0 is the water surface gradient; g is the gravity acceleration; C is the Xie Cai coefficient; n0 is the roughness coefficient of the beach without vegetation;

[0025] Combining the hydrodynamic balance equation of the river channel beach with vegetation with the above formula, the calculation formula of the roughness coefficient n of the river channel beach with vegetation is obtained:

[0026]

[0027] In the formula, C D is the vegetation drag coefficient; a is the vegetation density; h is the height of beach vegetation below the water surface; H is the beach water depth; U is the vertical average flow velocity of the beach vegetation water flow; U1 is the vertical average flow velocity of the vegetation layer water flow; g is the gravitational acceleration; n0 is the roughness coefficient of the beach without vegetation.

[0028] In the above technical solution, in step S4, the beach water depth H is determined by on-site measurement, or by calculation based on the existing beach topographic data and the measured beach water level; c By comparing it with the beach water depth H, the submerged state of vegetation can be determined.

[0029] In the above technical solution, when the beach water depth H < vegetation height h c , the water flow belongs to non-flooded vegetation flow; when the beach water depth H> vegetation height h c , the water flow belongs to the water flow that submerges vegetation.

[0030] In the above technical solution, in step S5, when the beach water flow belongs to the non-submerged vegetation water flow, U1=U, and the roughness coefficient n of the river beach with vegetation is calculated by the following formula:

[0031]

[0032] In the formula, C D is the vegetation drag coefficient, which can be taken as 1; a is the vegetation density; H is the beach water depth; g is the gravitational acceleration; n0 is the beach roughness coefficient without vegetation.

[0033] In the above technical solution, in step S5, when the beach water flow belongs to the water flow that submerges vegetation, the relationship between the vertical average flow velocity U1 of the vegetation layer water flow and the vertical average flow velocity U of the beach vegetation water flow can be expressed as:

[0034]

[0035] After conversion, the roughness coefficient n of the river beach with vegetation is calculated by the following formula:

[0036]

[0037] In the formula, C m is the water flow exchange coefficient between the vegetation layer and the non-vegetation layer under the submerged state, which can be taken as 0.07; C D is the vegetation drag coefficient, which can be taken as 1; a is the vegetation density; h is the height of the beach vegetation below the water surface; H is the beach water depth; g is the gravitational acceleration; n0 is the roughness coefficient of the beach without vegetation.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] Firstly, based on the dynamic balance equation of beach water flow, the present invention constructs a calculation formula for the beach roughness coefficient with vegetation characteristics as the main parameters. The method has a strong theoretical basis and can be applied to the calculation of beach roughness coefficients of various types of vegetation and various water flow states. Various types of vegetation include vegetation types such as grasses, shrubs and trees, and various water flow states include submerged vegetation states and non-submerged vegetation states, thus avoiding the limitations of existing methods such as narrow scope of application and single applicable conditions.

[0040] Secondly, the calculation process of the present invention is clear and concise, the calculation parameters are easy to obtain, and the technical solution is easy to use. The height of beach vegetation below the water surface, the water depth of the beach, the vegetation density, the roughness coefficient of the beach without vegetation, etc. can all be obtained through simple field measurements or consulting the hydraulic calculation manual. Other parameters, such as the vegetation drag coefficient and the water flow exchange coefficient, are already clear.

[0041] Thirdly, the roughness coefficient of vegetated riverbed beach calculated by the present invention is of high accuracy. It is verified by examples that the difference between the calculation results of the present invention and the test results is within 5%. The calculation accuracy is high and it can be used in scenes such as water level-flow relationship analysis, water surface line calculation, hydrodynamics and riverbed deformation simulation in vegetated river channels, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The present invention is a flow chart of a method for calculating the roughness coefficient of a river channel beach containing vegetation. DETAILED DESCRIPTION

[0043] The following is a detailed description of the implementation of the present invention in conjunction with the implementation cases, but they do not constitute a limitation of the present invention and are only given as examples. At the same time, the advantages of the present invention will become clearer and easier to understand through the description.

[0044] Example 1 (Roughness coefficient of riverbed with trees)

[0045] A method for calculating the roughness coefficient of a river channel with vegetation in this embodiment includes the following steps:

[0046] Step S1: Calculate the characteristics of beach vegetation, select representative areas within the research scope to conduct vegetation type, number of vegetation per unit area, characteristic length of vegetation, vegetation height and other vegetation characteristics surveys, and the number of vegetation in a representative area is generally not less than 10 plants. The vegetation type is tree. Through field investigation and measurement, it is determined that the average diameter d of each tree in the research area is 6 cm, and the number of vegetation per unit area N is 0.18 plants / m 2 , tree height h c is 10m, and the vegetation density a is 0.011m -1 ;

[0047] Step S2: Calculate the roughness coefficient of the beach without vegetation. By consulting the hydraulic calculation manual and drawing on relevant engineering experience, it is determined that the roughness coefficient of the beach without vegetation is 0.025;

[0048] Step S3: Establish a dynamic balance equation for the flow of riverbeds with vegetation, and deduce the calculation formula for the roughness coefficient of vegetation in riverbeds with vegetation:

[0049]

[0050] Step S4: Determine the vegetation flow state. The beach water depth H is about 3.2m and the vegetation height h c About 10m, the vegetation height is greater than the beach water depth, and the vegetation flow belongs to non-submerged vegetation flow;

[0051] Step S5: Since the vegetation flow belongs to non-submerged vegetation flow, the roughness coefficient of the river beach with vegetation can be simplified to

[0052]

[0053] Combined with vegetation density a=0.011m -1 , water depth H = 3.2m, C D =1.0, g=9.8m / s 2 , n0=0.025 and other parameters, the roughness coefficient of the river beach with vegetation is calculated to be n=0.057. The calculated roughness coefficient of the beach is basically consistent with the roughness coefficient of the beach deduced from the experiment of 0.06, with an error of only 5.0%.

[0054] Example 2 (Roughness coefficient of riverbed with shrubs)

[0055] Step S1: Calculate the characteristics of beach vegetation, select representative areas within the research scope to conduct vegetation type, number of vegetation per unit area, characteristic length of vegetation, vegetation height and other vegetation characteristics surveys, and the number of vegetation in a representative area is generally not less than 10 plants. The vegetation type is shrubs. Through field investigation and measurement, it was determined that the average stem diameter d of each shrub in the research area was 16 cm, and the number of vegetation per unit area N was 0.30 plants / m 2 , shrub height hc is 1.5m, and the vegetation density a is a=0.048m -1 ;

[0056] Step S2: Calculate the roughness coefficient of the beach without vegetation. By consulting the hydraulic calculation manual and drawing on relevant engineering experience, it is determined that the roughness coefficient of the beach without vegetation is 0.025;

[0057] Step S3: establishing a dynamic balance equation for water flow in a river channel with vegetation, and deriving a calculation formula for the roughness coefficient of vegetation in the river channel with vegetation;

[0058]

[0059] Step S4: Determine the vegetation flow state. The beach water depth H is about 3.2m and the vegetation height h c About 1.5m, the height of vegetation is less than the depth of beach water, and the vegetation flow belongs to the submerged vegetation flow;

[0060] Step S5: Since the vegetation flow belongs to the water flow that submerges vegetation, the roughness coefficient of the river beach with vegetation is expressed as:

[0061]

[0062] Combined with vegetation density a=0.048m -1 , beach water depth H = 3.2m, beach vegetation height below water surface h = 1.5m, C D =1.0, g=9.8m / s 2 , n0=0.025, C m =0.07 and other parameters, the roughness coefficient of the river beach with vegetation is calculated to be n = 0.085. The calculated beach roughness coefficient is basically consistent with the beach roughness coefficient of 0.083 deduced from the experiment, with an error of only 2.4%.

[0063] The above are only specific embodiments of the present invention. It should be pointed out that any changes or substitutions that can be easily thought of by any technician familiar with the field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention, and the rest not described in detail belong to the prior art.

Claims

1. A method for calculating the roughness coefficient of a river channel with vegetation, characterized in that: The steps include: S1: Calculate the characteristics of beach vegetation, including vegetation type, vegetation characteristic length d, number of vegetation per unit area N, vegetation density a, vegetation height h c ; S2: Calculate the roughness coefficient n0 of the beach without vegetation; S3: Establish the dynamic balance equation of water flow in riverbeds with vegetation, and derive the calculation formula for the roughness coefficient of vegetation in riverbeds with vegetation; Among them, the dynamic balance equation of water flow in riverbed with vegetation is: In the formula, U is the vertical average velocity of the flow over the floodplain vegetation; U1 is the vertical average velocity of the flow within the vegetation layer; when the vegetation is in a submerged state, U1 < U; when the vegetation is in a non-submerged state, U1 = U; a is the vegetation density; h is the height of the floodplain vegetation below the water surface; H is the depth of the floodplain; when the vegetation is non-submerged, h = H; g is the acceleration due to gravity; f is the Darcy-Weisbach coefficient; S0 is the water surface slope; C D is the vegetation drag force coefficient; The water surface gradient S0 and Darcy-Weisbach coefficient f of the vegetation flow in the beach are calculated by the following formula: f=8g / C 2 C=R 1 / 6 / n0 Where n is the roughness coefficient of the beach with vegetation; R is the hydraulic radius, which can be expressed by the beach water depth H; S0 is the water surface gradient; g is the gravity acceleration; C is the Xie Cai coefficient; n0 is the roughness coefficient of the beach without vegetation; Combining the hydrodynamic balance equation of the river channel beach with vegetation with the above formula, the calculation formula of the roughness coefficient n of the river channel beach with vegetation is obtained: In the formula, C D is the vegetation drag coefficient; a is the vegetation density; h is the height of the beach vegetation below the water surface; H is the beach water depth; U is the vertical average velocity of the beach vegetation water flow; U1 is the vertical average velocity of the vegetation layer water flow; g is the gravitational acceleration; n0 is the roughness coefficient of the beach without vegetation; S4: Determine the vegetation flow state, calculate the beach water depth H, and combine the vegetation height h c , determine the submergence status of vegetation; S5: Calculate the roughness coefficient of the river beach containing vegetation using the calculation formula obtained in step S3 according to the vegetation inundation state.

2. The method for calculating the roughness coefficient of a river channel with vegetation according to claim 1, characterized in that: In step S1, the vegetation density a is the vegetation projection area perpendicular to the water flow direction within a unit volume, which is calculated by the following formula: a=Nd Where N is the number of vegetation per unit area, and d is the characteristic length of vegetation.

3. The method for calculating the roughness coefficient of a river channel with vegetation according to claim 2, characterized in that: In step S1, if the stem of the vegetation is circular, the characteristic length d of the vegetation is the diameter of the stem; if the stem of the vegetation is other shapes, the characteristic length d of the vegetation is the width perpendicular to the water flow direction; the height h of the vegetation is c It is the actual height of vegetation above the beach surface.

4. The method for calculating the roughness coefficient of a river channel beach with vegetation according to claim 1, 2 or 3, characterized in that: In the step S2, the roughness coefficient n0 of the beach without vegetation is obtained by consulting the hydraulic calculation manual.

5. The method for calculating the roughness coefficient of a river channel with vegetation according to claim 4, characterized in that: In step S4, the beach water depth H is determined by on-site measurement, or by calculation based on the existing beach topographic data and the measured beach water level; c By comparing it with the beach water depth H, the submerged state of vegetation can be determined.

6. The method for calculating the roughness coefficient of a river channel with vegetation according to claim 5, characterized in that: In step S4, when the beach water depth H < vegetation height h c , the water flow belongs to non-flooded vegetation flow; when the beach water depth H> vegetation height h c , the water flow belongs to the water flow that submerges vegetation.

7. The method for calculating the roughness coefficient of a river channel with vegetation according to claim 6, characterized in that: In step S5, when the beach water flow is a non-submerged vegetation water flow, U1=U, and the roughness coefficient n of the river beach with vegetation is calculated by the following formula: In the formula, C D is the vegetation drag coefficient; a is the vegetation density; H is the beach water depth; g is the gravitational acceleration; n0 is the beach roughness coefficient without vegetation.

8. The method for calculating the roughness coefficient of a river channel with vegetation according to claim 7, characterized in that: In step S5, when the beach water flow belongs to the water flow that submerges vegetation, the relationship between the vertical average flow velocity U1 of the vegetation layer water flow and the vertical average flow velocity U of the beach vegetation water flow can be expressed as: After conversion, the roughness coefficient n of the river beach with vegetation is calculated by the following formula: In the formula, C m is the water flow exchange coefficient between the vegetation layer and the non-vegetation layer under the submerged state; C D is the vegetation drag coefficient; a is the vegetation density; h is the height of the beach vegetation below the water surface; H is the beach water depth; g is the gravitational acceleration; n0 is the roughness coefficient of the beach without vegetation.

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