Sediment motion characteristic analysis method for river channel with floating vegetation

Abstract

The invention belongs to the field of hydraulics and river dynamics, and discloses a sediment motion characteristic analysis method for a river channel with floating vegetation, which comprises the following steps of: firstly, determining near-bottom turbulent kinetic energy at different positions along a water flow direction; determining the Hiltz number of sediment particles at different positions along the water flow direction and the critical Hiltz number of sediment resuspension, then determining the deposition probability, and finally obtaining the on-way distribution of the sediment deposition amount. According to the method, the influence of the floating vegetation on the river flow turbulence characteristic and the sediment deposition is considered, the obtained along-way near-bottom turbulence energy and sediment deposition distribution of the river with the floating vegetation are closer to the real river condition, and the prediction precision is high.

Description

Technical Field

[0001] The present invention belongs to the field of hydraulics and river dynamics, and relates to a method for predicting near-bottom turbulent energy and sediment deposition along a river channel based on flow velocity, vegetation density and vegetation relative water depth, and in particular to a method for analyzing sediment movement characteristics in a river channel with floating vegetation. Background Art

[0002] Floating vegetation is an important component of river ecosystems, playing an important role in adjusting water flow structure, changing river flow velocity, affecting material and sediment transport and diffusion, and purifying water quality. Under appropriate climatic and environmental conditions, some floating vegetation species (such as water hyacinth) reproduce in large numbers, with their roots intertwined and consolidated, forming a stable covering structure on the river surface. For example, water hyacinth invaded some waters in the lower reaches of the Yangtze River Delta, such as the Taihu Lake Basin, significantly affecting the sediment deposition and resuspension process in the river.

[0003] Due to the obstruction of vegetation, the water flow is deflected vertically downward from the inside of the floating vegetation to the free water area below it. g ) The velocity gradually decreases, and the free water flow area below the vegetation (h g ≥z≥0) the velocity increases accordingly. Low velocity vegetation area (H>z>h g ) and the high velocity area below the vegetation (h g ≥z≥0) forms a shear layer at the bottom of the vegetation. If the shear is strong enough, the Kelvin-Helmholtz vortex (KH vortex) will develop downstream along the bottom of the vegetation, which can drive the momentum exchange between the vegetation area and the free water area below it, thereby affecting the sediment and pollutant transport process and changing the riverbed morphology. In view of this fact, it is particularly important to explore the impact of floating vegetation on the turbulent characteristics of river flow and sediment deposition, which can provide technical guidance for river environmental governance and ecological restoration.

[0004] However, sediment deposition in floating vegetation rivers is affected by many conditions, such as the average flow velocity, vegetation density, and relative water depth. Under natural conditions, changes in upstream flow and relative water depth directly affect the sediment deposition measurement results, making the measurement results unrepresentative; at the same time, sediment deposition measurement requires a lot of manpower, material and financial resources to carry out continuous sampling work in natural floating vegetation rivers. Under laboratory conditions, although a constant and uniform water flow can be obtained and the sediment deposition along the river with floating vegetation can be collected, data measurement and analysis still require a lot of funds and time. Generally speaking, a 1-meter-wide and 4-meter-long floating vegetation area is constructed in a 1-meter-wide and 13-meter-long test flume. Relevant technicians use slides to measure the sediment deposition. The running time of each set of sediment deposition tests is 4 hours. In addition, after the flume operation, the slides are dried for 8 hours and weighed. It takes a month or even longer to measure the sediment deposition in the floating vegetation area in detail. Therefore, a simple and practical method is urgently needed to predict the distribution of sediment deposition along the river with floating vegetation.

[0005] Below the floating vegetation area is a free water flow area, and the evolution of the near-bottom turbulent energy along the river is closely related to the change of the flow velocity below the floating vegetation area. However, due to the lack of a prediction model for the near-bottom turbulent energy of the river channel with floating vegetation, the distribution of sediment deposition along the river channel under the action of floating vegetation is unknown. Summary of the invention

[0006] In view of the current technical status that there is a lack of research on sediment deposition in rivers with floating vegetation, the purpose of the present invention is to provide a method for analyzing the sediment movement characteristics of rivers with floating vegetation, taking into account the average flow velocity, vegetation density and relative water depth of the vegetation, to achieve the analysis of the sediment movement characteristics of rivers with floating vegetation (including the along-the-line prediction of near-bottom turbulent energy and sediment deposition), and to provide a theoretical basis for further studying the interactive influence relationship between floating vegetation and riverbed evolution.

[0007] The present invention is applicable to the working condition of a river channel with floating vegetation and a water flow velocity greater than 0 cm / s. The water flow adjustment only occurs in the water flow direction and vertical direction (perpendicular to the water flow direction), so the water flow is considered to be a two-dimensional flow. In the present invention, x and z are used to represent the water flow direction and vertical direction respectively. x=0 is the front edge of the floating vegetation; z=0 is the riverbed surface.

[0008] The idea of ​​the present invention is to analyze the flow velocity and near-bottom turbulent energy below the floating vegetation area, and establish a near-bottom turbulent energy prediction model for the river channel with floating vegetation based on the flow velocity prediction method of the free water flow area below the floating vegetation. Further, combined with the near-bottom turbulent energy evolution along the river channel with floating vegetation and the sediment deposition probability model, a sediment deposition prediction model suitable for the river channel with floating vegetation is proposed.

[0009] The study found that in the water flow adjustment area (XD >x>0), due to the resistance of vegetation, the average flow velocity in the inner water depth of the floating vegetation area decreases (H>z>h g ), while the average velocity of the free flow area under the vegetation area increased (h g ≥z≥0), until the flow is fully developed (x>X D ), the flow velocity in the floating vegetation area and the free water flow area below it no longer changes. Water flow adjustment length X D It is defined as the distance from the leading edge of the floating vegetation area to the position where the average velocity of the water depth inside the vegetation area decreases to a constant position, which can be calculated using the following formula:

[0010]

[0011] In the formula, h c Indicates the height of floating vegetation; C D represents the vegetation drag coefficient; a represents the floating vegetation water blocking area per unit water body, a=nd, n represents the vegetation density per unit area, d represents the vegetation diameter; h g It indicates the height of the free water flow area below the floating vegetation area; H indicates the water depth of the river.

[0012] For a river channel with floating vegetation, the near-bottom turbulent kinetic energy (TKE) is closely related to the evolution of the flow velocity in the free water area below the floating vegetation area, which in turn determines the sediment deposition and resuspension. The basis for predicting the amount of sediment deposition along a river channel with floating vegetation is to understand the variation of the near-bottom turbulent kinetic energy below the floating vegetation area along the course.

[0013] Based on the above invention ideas, the present invention provides a method for analyzing the characteristics of sediment movement in a river channel with floating vegetation, comprising the following steps:

[0014] S1 determines the near-bottom turbulent energy k at different positions (x direction) along the water flow direction according to the following formula: t (x):

[0015]

[0016] In the formula, k t (x) represents the near-bottom turbulent energy at position x below the floating vegetation area; τ b (x) represents the shear stress of the riverbed; ρ represents the water density; C f represents the friction coefficient of the bed surface, C f =[5.75log(2H / d 50 )] -2 , H represents the river depth, d 50 Indicates the average particle size of the sediment on the riverbed surface. b (x) represents the depth-averaged velocity in the free-flow area below the floating vegetation area; ω (= 0.2 ± 0.01) represents the proportionality parameter;

[0017] S2 determines the Shields number Θ(x) of sediment particles at different positions (x direction) along the water flow direction based on the change of near-bottom turbulent energy along the flow. The calculation formula is as follows:

[0018]

[0019] In the formula, ρ s represents the density of sediment; g represents the acceleration of gravity; d s represents the sediment particle size;

[0020] S3 determines the critical Shields number θ for sediment resuspension c ;

[0021] S4 determines the deposition probability p(x) of sediment particles at different positions (x direction) along the water flow direction according to the following formula:

[0022]

[0023] Where, K(=1) is the friction coefficient;

[0024] S5 determines the sediment deposition without resuspension, that is, the sediment deposition probability p = 1 per unit bed surface sediment deposition amount Dep (p＝1) ;

[0025] S6 determines the sediment deposition per unit bed surface Dep(x) at different positions (x direction) according to the following formula:

[0026]

[0027] In the above step S1, based on the water flow continuity equation, the average water velocity (h g ≥z≥0), U b (x), the calculation formula is as follows:

[0028]

[0029] Where U c (x) is the average velocity of the water depth in the floating vegetation area; U 0 is the average flow velocity of the river at the front edge of the floating vegetation area; φ represents the volume percentage of vegetation in a unit water body.

[0030] U c (x) is obtained according to the following calculation steps:

[0031] (1) Determine the water flow adjustment length X D (Formula (1));

[0032] (2) When X D>x>0, the average velocity of water depth at x in the floating vegetation area is U c (x) is:

[0033]

[0034]

[0035] Where U 0 is the average velocity of the river channel at the leading edge of the floating vegetation area; the parameters α, β, A', B' and C' are defined as:

[0036]

[0037] In the formula, φ represents the volume percentage of vegetation in a unit water body; C f Indicates the friction coefficient of the bed surface; C * (=0.076±0.025) represents the shear coefficient, which represents the vertical momentum exchange intensity between the floating vegetation area and the free water flow area below it.

[0038] (3) When x ≥ X D When U c (x) is the average velocity U in the fully developed area of ​​water flow inside the floating vegetation cf , can be calculated by the following formula:

[0039]

[0040] In the above step S3, θ c is the critical Shields number for determining whether sediment resuspension occurs, which is determined by the dimensionless particle size. Determine, where v(=0.01cm 2 / s) represents kinematic viscosity. * <0.3, Θ c =0.5tan30°; when 0.3≤d * ≤19, Θ c =0.25d * -0.6 tan30°; when 19 <d * When ≤50, Θ c =0.013d * 0.4 tan30°; when d * >50, Θ c =0.06tan30°.

[0041] Furthermore, the critical starting turbulent energy k for sediment resuspension is t(c)Therefore, if the bed shear stress just reaches the critical condition for driving sediment resuspension, then it can be considered that the bed shear stress τ b(c) The near-bottom turbulent energy generated is equal to the critical starting turbulent energy k t(c) :

[0042]

[0043] Critical starting turbulent energy k t(c) The corresponding flow velocity is the critical starting velocity for resuspension of sediment, which can be calculated by the following formula:

[0044]

[0045] In the above step S5, if the average flow velocity of the river channel is less than the critical flow velocity U for resuspension of sediment c(bare) (Equation (11)) indicates that the sediment is only deposited and not resuspended, and the sediment deposition probability p = 1. At this time, the measured sediment deposition per unit bed surface is defined as Dep (p＝1) .

[0046] In the above step S6, the mass flux of sediment deposited on the riverbed surface per unit time is expressed as:

[0047]

[0048] Where Dep is the sediment deposition per unit bed surface; p is the probability that sediment particles remain deposited on the riverbed surface; w s is the sediment particle settling velocity; C is the sediment concentration. It is worth noting that when the river sediment supply is not limited, that is, (L / U bf ) / (H / w s )<1, the sediment concentration C of the entire river channel (including the floating vegetation area and the surrounding non-vegetated area) can be considered to be equal along the entire river channel, where L represents the length of the floating vegetation area, U bf represents the fully developed mean velocity in the free water area below the floating vegetation area, U bf It is calculated by the water flow continuity equation (Equation (6)).

[0049] By integrating equation (12) over the duration t of the entire deposition process, the sediment deposition per unit bed surface is calculated as:

[0050]

[0051] Dep (p＝1) represents the amount of sediment deposited per unit bed surface where only deposition occurs. t represents the duration of the entire deposition process, and C(t) represents the sediment concentration in the river at different times t. In the present invention, when only deposition occurs, the situation of continuous sediment deposition is mainly considered, and C(t) does not change significantly with time and space, that is, the sediment concentration in the river does not change much; the sediment deposition at different locations in the river is equal. Therefore, it can be obtained by sampling in the prototype river or in the laboratory (see the slide weighing method given in the embodiment to measure the sediment deposition). If the sediment deposited below the floating vegetation area is resuspended, p(x)<1. Substituting formula (4) and Dep (p＝1) Substituting into the above formula (5), the sediment deposition per unit bed surface Dep(x) at different x positions can be calculated.

[0052] It is worth noting that Dep (p＝1) The corresponding average flow velocity of the river is U 0 Less than the critical velocity U for resuspension of sediment c(bare) (Formula (11)).

[0053] If the average flow velocity of the river is U 0 ≥U c(bare) The above-mentioned sediment movement characteristics analysis method for a river channel with floating vegetation can also determine the unit bed sediment deposition Dep(x) at different positions (x direction) along the river channel through the following steps:

[0054] S7 Determine the probability of sediment deposition in the area without vegetation cover in the river channel P (0) and sediment deposition Dep (0) ;

[0055] S8 determines the unit bed sediment deposition Dep(x) at different positions (x direction) along the river channel according to the following formula:

[0056]

[0057] In the above step S7, the near-bottom turbulent energy k of the area without vegetation coverage in the river channel is first calculated. t(0) :

[0058]

[0059] Then determine the Shields number Θ of sediment particles in the area without vegetation cover in the river channel (0) and its sediment deposition probability P (0) :

[0060]

[0061]

[0062] Dep (0)The amount of sediment deposition can be obtained by sampling in a prototype river channel or in a laboratory (see the glass slide weighing method given in the example for measuring the amount of sediment deposition).

[0063] In the above step S8, according to the sediment deposition amount Dep in the area without vegetation coverage in the river channel, (0) Calculate the sediment deposition per unit bed surface Dep(x) at different positions (x direction) of the floating vegetation area in the river channel. p(x) can be calculated according to the above steps S1 to S4.

[0064] Compared with the prior art, the method for analyzing the sediment movement characteristics of a river channel with floating vegetation provided by the present invention has the following beneficial effects:

[0065] (1) The present invention considers the influence of floating vegetation on the turbulent characteristics of river flow and sediment deposition, proposes a method for predicting the near-bottom turbulent energy and sediment deposition below the floating vegetation area, and uses the natural water hyacinth river channel as a case to verify the method. This provides a theoretical basis for further research on the sediment movement characteristics and riverbed evolution involved in natural rivers, lakes and wetlands with floating vegetation, and can provide technical support for river management and water ecological restoration.

[0066] (2) The prediction model of the distribution of sediment deposition along the river channel under the action of floating vegetation constructed by the present invention combines the evolution of the near-bottom turbulent energy along the river channel and the sediment deposition probability model, which meets the physical laws of sediment transport and deposition; the invention takes into account the average flow velocity (U 0 ), vegetation density (a) and relative water depth (h g / H) and many other variable factors, which directly affect the sediment deposition along the river channel; therefore, the near-bottom turbulent energy and sediment deposition distribution along the river channel with floating vegetation obtained by this prediction method are closer to the actual river channel conditions and have high prediction accuracy;

[0067] (3) The present invention does not require a large number of measurements. It can accurately predict the near-bottom turbulent energy and relative sediment deposition distribution of a river with floating vegetation based on the basic parameters of the river water flow and floating vegetation (including the average flow velocity of the river, the water depth of the river, the vegetation density, the vegetation height, the vegetation drag coefficient and the riverbed friction coefficient). This method can not only reduce the research cost, but also is applicable to natural rivers (river areas that are inconvenient for staff to reach), and has wide applicability and versatility. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 Schematic diagram of the river channel layout with floating vegetation; (a) is a schematic diagram of the river flow adjustment under the action of floating vegetation. The vertical line represents the floating vegetation community constructed by a single plant, L represents the length of the floating vegetation, and h c Indicates the height of floating vegetation, h g represents the height of the free water flow area below the floating vegetation area, XD represents the water flow adjustment length, Dep(x) represents the distribution of sediment deposition along the floating vegetation area, and Dep (0) represents the sediment deposition in the upstream of the front end of the floating vegetation area; (b) is a top view of the experimental arrangement of hydrodynamics and sediment deposition in the floating vegetation river channel, where the circle represents the position of a single floating vegetation plant, and U 0 represents the average flow velocity of the river channel, B represents the width of the water channel (floating vegetation area); (c) is a schematic diagram of the cross section of the river channel with floating vegetation. Under static water conditions (U 0 =0cm / s), clean slides (7.5cm×2.5cm) are placed in two rows at the target position along the center line of the water tank; (d) is to measure the water flow velocity and turbulent energy at the characteristic areas y=0, y=-dy / 8 and y=dy / 8, where dx and dy are the longitudinal and transverse distances between adjacent wooden sticks, respectively;

[0069] Figure 2 A schematic flow chart of a method for analyzing sediment movement characteristics in a river channel with floating vegetation provided in Example 1 of the present invention;

[0070] Figure 3 is the velocity U of the free water flow area below the vegetation area in the river with floating vegetation under different working conditions b Comparison of measured and predicted values ​​of (x), where the measured velocity values ​​below the vegetation area are represented by hollow diamonds, and the predicted values ​​and their uncertainties are represented by solid lines and shaded strips;

[0071] Figure 4 The distribution of near-bottom turbulent energy and sediment deposition along the river channel with floating vegetation under different working conditions; (a)-(f) Comparison of the predicted and measured values ​​of the near-bottom turbulent energy distribution along the river channel under the action of vegetation in the rigid floating model, where the measured values ​​of the near-bottom turbulent energy are represented by hollow circles, the predicted values ​​and their uncertainties are represented by solid lines and shaded strips, and the dotted line represents the critical starting turbulent energy k t(c) , X D represents the water flow adjustment length; (g)-(l) relative sediment deposition distribution under the action of vegetation in the rigid floating model Dep(x) / Dep (p＝1) Comparison of predicted values ​​with measured values, where the measured values ​​of relative sediment deposition are represented by hollow circles, the predicted values ​​and their uncertainties are represented by solid lines and their shaded ranges, and the horizontal stripes represent the relative sediment deposition Dep upstream of the floating vegetation area. (0) / Dep (p＝1) , X D Indicates the water flow adjustment length;

[0072] Figure 5 Dep(x) / Dep is the relative sediment deposition distribution along the river channel with natural water hyacinth vegetation under different working conditions (p＝1)Comparison of predicted and measured values; the measured values ​​of relative sediment deposition are represented by hollow circles, the predicted values ​​and their uncertainties are represented by solid lines and their shaded ranges, and the horizontal stripes represent the relative sediment deposition Dep upstream of the floating vegetation area. (0) / Dep (p＝1) , X D Indicates the water flow adjustment length;

[0073] Figure 6 A schematic flow chart of another method for analyzing sediment movement characteristics in a river channel with floating vegetation provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0074] The following will clearly and completely describe the technical solutions of various embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative work are within the scope of protection of the present invention.

[0075] Example 1

[0076] Based on the flume test results, this embodiment provides a detailed description of the prediction of the near-bottom turbulent energy and sediment deposition along the river channel with floating vegetation.

[0077] (1) Purpose of the test

[0078] The flume test is used to measure the turbulent energy and net sediment deposition along the free water flow area below the floating vegetation in detail. The sediment deposition along the river channel with floating vegetation under different average flow velocities, vegetation density and vegetation relative water depth conditions is determined, and the measured values ​​of turbulent energy and sediment deposition near the bottom along the river channel are obtained. The measured near-bottom turbulent energy and sediment deposition are compared with the near-bottom turbulent energy and sediment deposition data predicted by the present invention to verify the accuracy of the sediment movement characteristics analysis method for the river channel with floating vegetation provided by the present invention.

[0079] (2) Test equipment

[0080] The main equipment is shown in Table 1 below

[0081] Table 1 Flume test apparatus with floating vegetation

[0082]

[0083] (3) Test conditions

[0084] This embodiment uses floating model vegetation and natural water hyacinth vegetation to carry out flume tests under 9 different working conditions (different average river flow velocity, water depth, vegetation density, and vegetation relative water depth) to verify the effectiveness of the prediction method provided by the present invention. The circulating water flume used in the test is 13m long and 1m wide, and the flume test section is 6m long. Use a ruler to read the water depth along the way, an electromagnetic flowmeter to read the upstream flow rate, and a glass slide to measure the sediment deposition. The test water depth varies from 20 to 30cm. According to the flow velocity range (U 0 =5~25cm / s), the average flow velocity of the river in the experiment is considered to be U 0 =11.5~24.5cm / s. In all working conditions, Reynolds number Re(=U 0 R / v) = 20,641-35,000, R is the hydraulic radius, Froude number It shows that the test water flow is turbulent and slow flow, such as Figure 1 (a) shown.

[0085] like Figure 1 As shown in (b), the floating model vegetation was fixed on a 1 cm thick PVC board in a staggered arrangement. The PVC board was suspended above the water surface in a way that the roots of the floating vegetation were vertically downward. The diameter of a single floating model plant d = 0.8 cm, which is consistent with the range of observed values ​​of natural floating vegetation (d = 0.1-1.5 cm). Two vegetation densities were considered in the experiment: n = 200 and 380 plants / m 2 The corresponding vegetation water blocking areas are a=1.6 and 3.0m -1 Floating vegetation height h c =4~14cm, the test water depth H = 20 and 28cm, so the vegetation relative water depth h g / H=0.3~0.8. Height of free water flow area below floating vegetation h g It is defined as the distance from the riverbed (z = 0) to the bottom of the floating vegetation (z = h g ), that is, h g =Hh c It is worth noting that the width of the floating model vegetation in the experiment is the same as the width of the flume, so no lateral water flow regulation is produced. The experimental parameters are summarized in Table 2.

[0086] Water hyacinth (E.crassipes) is commonly found in warm, humid aquatic environments. In this experiment, mature water hyacinth plants aged 3-4 months were randomly planted to cover the entire width of the water tank to simulate the real situation of a river with floating vegetation. The average height of the natural water hyacinth root system h c =7~8.5cm, water blocking area of ​​water hyacinth a * (=N * D, where N *= (D) = 2.4-2.8 m -1 The test parameters are summarized in Table 3.

[0087] Table 2 Parameters of floating model vegetation test

[0088]

[0089] Table 3 Test parameters of natural water hyacinth vegetation

[0090]

[0091] like Figure 1 (b) The coordinate system in the flume defines the flow direction (x), lateral direction (y) and vertical direction (z), where x = 0 cm is the front end of the floating vegetation area; y = 0 cm is the center line of the flume; z = 0 cm is the riverbed surface. A Nortek profile flow meter was used to measure the instantaneous flow velocity in three directions. The instantaneous flow velocity was collected for 120 s at each measuring point at a sampling frequency of 50 Hz. The method of Goring and Nikora (Goring, DG, Nikora, VI (2002). Despiking acoustic Doppler velocity meter data. Journal of Hydraulic Engineering, 128, 117-126) was used to eliminate the flow velocity data with a correlation of less than 70% and a signal-to-noise ratio of less than 15, and the remaining instantaneous flow velocity (u, v, w) was decomposed into the time-averaged flow velocity (U, V, W) and the instantaneous pulsating flow velocity (u', v', w'). The turbulent energy is defined as follows:

[0092]

[0093] In all working conditions, the complete vertical distribution of flow velocity was measured at each x position, and flow velocity data were collected from the water surface (z = H) to the riverbed (z = 0) at intervals of 2 cm. Figure 1 As shown in (d), for each measuring point, the flow velocity and turbulent energy are measured using a Nortek profile flowmeter in the characteristic area at y = 0, y = -dy / 8, and y = dy / 8, and the average value is taken as the time-averaged flow velocity at each measuring point z position. and the average turbulent energy Then, according to The near-bottom turbulent energy, k, at each x position is calculated. t Indicates the near-bottom area (H nb ) is the average turbulent energy at each vertical (z direction) position in the bottom region. nb =2cm, because for rivers with floating vegetation, the water flow structure and flow velocity distribution in this area are mainly affected by the roughness of the riverbed.

[0094] Bed surface friction coefficient C f The bed shear stress τ in the non-vegetated area b and the average flow velocity U 0 According to the method proposed by Liu and Shan (Liu, C., Shan, Y. (2019) Analytical model for predicting the longitudinal profiles of velocities in a channel with a model vegetation patch. Journal of Hydrology, 576, 561–574), the bed shear stress τ b Measured 1 cm above the PVC board. The friction coefficients of the bed surface in this embodiment are C f =0.004±0.001(operating conditions 1-6) and C f =0.006±0.001 (working condition AC). The floating vegetation roots form a pore structure in the river channel, which causes the water-passing section of the river channel to shrink. This embodiment takes into account the pore effect of the adjacent vegetation roots when estimating the vegetation drag coefficient, and calculates the average flow velocity of the shrinking section by the law of conservation of mass. Based on the formula R ec =U c ′d / ν, and the drag coefficient C is obtained D =1+10R ec -2 / 3 In all working conditions, the drag coefficient range C D =1.0~1.2, therefore, this embodiment adopts C D =1 is reasonable.

[0095] This embodiment uses relative error (ERR) to quantitatively compare the difference between the predicted value and the measured value of the near-bottom turbulent energy, and the calculation formula is:

[0096]

[0097] Among them, X(m) and X(p) are the measured value and predicted value respectively; is the average value of the measured values; N represents the number of measuring points along the water flow direction.

[0098] The sediment particles used in the sediment deposition test were replaced by spherical glass beads with a particle size of d s =22μm, which is similar to the sediment particle size d observed in natural river wetlands, estuaries and coasts. s =8~63μm, the density of sediment is ρ s =2.65g / cm 3, and the density of natural sediment (2.65g / cm 3 ) are consistent. Place clean slides (7.5cm×2.5cm) in two rows along the center line of the tank (such as Figure 1 (c) to measure the net sediment deposition during the test. Before the test, weigh the slide and gently place it at each measuring point. Slowly turn on the water pump to slowly increase the flow rate to the flow rate set for the test conditions. Mix the sediment with water in a measuring cup and add it to the downstream water tank. The water and sediment are transported to the upstream through the water-sand circulation system. The water and sediment in the entire water tank are fully mixed within 5 minutes. The initial sediment concentration of all working conditions is C 0 =100g / cm 3 . The running time of each test is 4 hours. After 4 hours, the water pump is slowly turned off to prevent waves from being generated, and then the water is slowly drained until the tank is empty. The drainage time for all working conditions is controlled to 20 minutes. All slides are left in the water tank to dry for no less than 12 hours, and then moved into a 50°C oven for 8 hours to remove the remaining moisture. After the slides are completely dry, they are weighed again. The weight difference between the slides before and after the test is the net sediment deposition at each measuring point. Each working condition is repeated twice to determine the average deposition at each measuring point and its uncertainty.

[0099] The reliability of the method for analyzing the near-bottom turbulent energy and sediment movement characteristics of a river channel with floating vegetation (Formulas (2) and (5)) proposed in the present invention is verified by using test cases. The test parameters are shown in Tables 2 and 3.

[0100] This embodiment is based on the above working condition data, such as Figure 2 As shown in the figure, the following steps are followed to predict the near-bottom turbulent energy and sediment deposition distribution along the river channel with floating vegetation, and to analyze the sediment movement characteristics of the river channel with floating vegetation:

[0101] S1 determines the near-bottom turbulent energy k at different positions (x direction) along the water flow direction t (x).

[0102] In this step, we first combine formulas (7a), (7b), (8), and (9) to determine the average velocity U in the floating vegetation area: c (x), and then the average velocity U of the free water flow area below the floating vegetation area is calculated according to formula (6): b (x), and then calculate the near-bottom turbulent energy k at different positions (x direction) along the water flow direction according to formula (2): t (x).

[0103] The near-bottom turbulent energy k below the floating vegetation area t (x), and the average velocity U of the free water flow area below the floating vegetation area b (x), and the bed surface friction coefficient C f, is positively correlated. D When >x>0, the velocity U in the free water flow area below the floating vegetation area b (x), increases along the way, and the near-bottom turbulent energy gradually increases from the front end of the floating vegetation area (x = 0) to the maximum value. Since formula (6) takes into account many influencing factors such as the relative water depth of vegetation, vegetation density, and average flow velocity of the river channel, the measured flow velocity in the free water flow area below the vegetation area is in good agreement with the measured value, and the relative error ERR is only 9% ( Figure 3 ). Furthermore, the distribution of the near-bottom turbulent energy along the path predicted by equation (2) is consistent with the measured value, with a relative error ERR of 13% ( Figure 4 (a)–(f)).

[0104] S2 determines the Shields number Θ(x) of sediment particles at different positions along the water flow direction (x direction) based on the change of near-bottom turbulent energy along the flow.

[0105] In this step, the Shields number Θ(x) of the sediment particles at different positions (x direction) along the water flow direction is calculated according to the above formula (3).

[0106] S3 determines the critical Shields number θ for sediment resuspension c .

[0107] In this step, the dimensionless particle size d is first calculated. * , and then according to the dimensionless particle size d * Determine the critical Shields number θ for sediment resuspension c .

[0108] For the average particle size d selected in this embodiment s = 22 μm model sand, dimensionless particle size d * =d s [(ρ s -ρ)g / ρν 2 ] 1 / 3 =0.6, at 0.3 <d * <19, so the critical Shields number Θ c =0.25d * -0.6 tan30°=0.21. The critical starting turbulent energy k of sediment calculated by formula (10) is t(c) =3.7cm 2 / s 2 Therefore, the critical starting velocity of sediment in this embodiment is calculated according to formula (11) to obtain U c(bare) =13.5cm / s.

[0109] S4 determines the deposition probability p(x) of sediment particles at different positions (x direction) along the water flow direction.

[0110] In this step, according to the Shields number Θ(x) of sediment particles at different positions (x-direction) along the water flow direction determined in step S2 and the critical Shields number Θ at which sediment resuspension occurs c , the deposition probability p(x) of sediment particles at different positions (x-direction) along the water flow direction is calculated according to the formula (4) given above.

[0111] S5 determines the sediment deposition amount Dep per unit bed surface where sediment only deposits and no resuspension occurs, that is, the sediment deposition probability p = 1 (p＝1) .

[0112] In this embodiment, in low flow rate condition 1 ( Figure 4 (g), U 0 = 11.5 cm / s, a = 1.6 m -1 , h g / H = 0.5 and H = 28 cm), the sediment deposition amount in the section of x = 0 - 2 m in the floating vegetation area is equal to the sediment deposition amount Dep (0) upstream of the floating vegetation area, indicating that only sediment deposition occurs in the river channel at this time. The spatial average deposition amount within this range is defined as Dep (p＝1) = 3.0 ± 0.2 mg / cm 2 . This is consistent with the trend of the variation of near-bottom turbulent kinetic energy along the course, that is, in the section of x = 0 - 2 m in the floating vegetation area, the near-bottom turbulent kinetic energy is less than or equal to the critical incipient motion turbulent kinetic energy of the deposited sediment (k t(c) = 3.7 cm 2 / s 2 , Figure 3 (a)).

[0113] S6 determines the sediment deposition amount Dep(x) per unit bed surface at different positions (x-direction).

[0114] In this step, substituting the deposition probability p(x) determined in step S4 and the sediment deposition amount Dep per unit bed surface with sediment deposition probability p = 1 determined in step S5 (p＝1) into formula (5), the sediment deposition amount Dep(x) per unit bed surface at different positions (x-direction) after each group of working conditions run for 4 hours can be calculated.

[0115] To verify the accuracy of the analysis method for near-bottom turbulent kinetic energy and sediment movement characteristics in a river channel with floating vegetation proposed by the present invention, 9 groups of flume test data collected in floating model vegetation and natural water hyacinth vegetation are used in this embodiment (U 0 = 11.5 - 24.5 cm / s, h g / H = 0.3 - 0.8, a = 1.6 - 3.0 m -1 ) to test the prediction effect. Figure 4 (g)-4(l) and Figure 5The predicted values ​​of relative sediment deposition along the riverbed under the floating model vegetation and the natural water hyacinth vegetation area were compared with the measured values. The results show that the predicted values ​​of sediment deposition are consistent with the measured values ​​within the uncertainty range. 0 improve( Figure 4 (h)-4(l)), because the turbulent energy near the bottom exceeds the critical turbulent energy k of sediment resuspension t(c) The net sediment deposition below the floating vegetation area is less than the reference value of sediment deposition upstream of the vegetation area Dep (0) As the distance that the water flows through the floating vegetation increases (the distance between the water flow and the front edge of the vegetation at x = 0 increases), the near-bottom turbulent energy of the free water flow area below the vegetation area increases along the way, and accordingly, the amount of sediment deposition below the floating vegetation area decreases along the way until a new water flow equilibrium state is reached, and the sediment deposition remains at the minimum value and does not change.

[0116] In summary, the present invention combines the predicted value of the near-bottom turbulent energy of the free water zone below the floating vegetation zone (Formula (2)) with the sediment deposition probability model, and provides an effective method for predicting the distribution of sediment deposition along the river channel with floating vegetation (Formula (5)). The method has high prediction accuracy and strong applicability, and can provide a theoretical basis and technical support for river water environment management and ecological restoration projects.

[0117] Example 2

[0118] like Figure 6 As shown, this embodiment provides another method for analyzing sediment movement characteristics in a river channel with floating vegetation, comprising the following steps:

[0119] S1 determines the near-bottom turbulent energy k at different positions (x direction) along the water flow direction t (x).

[0120] In this step, we first combine formulas (7a), (7b), (8), and (9) to determine the average velocity U in the floating vegetation area: c (x), and then the average velocity U of the free water flow area below the floating vegetation area is calculated according to formula (6): b (x), and then calculate the near-bottom turbulent energy k at different positions (x direction) along the water flow direction according to formula (2): t (x).

[0121] S2 determines the Shields number Θ(x) of sediment particles at different positions along the water flow direction (x direction) based on the change of near-bottom turbulent energy along the flow.

[0122] In this step, the Shields number Θ(x) of the sediment particles at different positions (x direction) along the water flow direction is calculated according to the above formula (3).

[0123] S3 determines the critical Shields number θ for sediment resuspension c .

[0124] In this step, the dimensionless particle size d is first calculated. * , and then according to the dimensionless particle size d * Determine the critical Shields number θ for sediment resuspension c .

[0125] S4 determines the deposition probability p(x) of sediment particles at different positions (x direction) along the water flow direction.

[0126] In this step, the Shields number θ(x) of the sediment particles at different positions (x direction) along the water flow direction determined in step S2 and the critical Shields number θ for sediment resuspension are calculated. c According to the formula (4) given above, the deposition probability p(x) of sediment particles at different positions (x direction) along the water flow direction is calculated.

[0127] S7 Determine the probability of sediment deposition in the area without vegetation cover in the river channel P (0) and sediment deposition Dep (0) .

[0128] In this step, the near-bottom turbulent energy k of the area without vegetation coverage in the river channel is first calculated according to the above formula (15): t(0) ; Then, according to (16) and (17) above, determine the Shields number θ of sediment particles in the area without vegetation coverage in the river channel (0) and its sediment deposition probability P (0) .

[0129] In this step, the sediment deposition amount Dep in the area without vegetation coverage is measured by a glass slide according to the method given in Example 1. (0) .

[0130] S8 determines the sediment deposition per unit bed surface at different positions (x direction) along the river channel.

[0131] In this step, the sediment deposition probability p(x) determined in step S4 and the sediment deposition probability P(x) in the area without vegetation coverage in the river determined in step S7 are calculated. (0) and sediment deposition Dep (0) Substituting into formula (14), the sediment deposition per unit bed surface Dep(x) at different positions (x direction) can be calculated.

[0132] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A method for analyzing the characteristics of sediment movement in a river channel with floating vegetation, characterized in that: The following steps are involved: S1 determines the near-bottom turbulent energy k at different positions along the water flow direction according to the following formula: t (x): In the formula, k t (x) represents the near-bottom turbulent energy at position x below the floating vegetation area; τ b (x) represents the shear stress of the riverbed; ρ represents the water density; C f Indicates the friction coefficient of the bed surface; U b (x) represents the depth-averaged velocity in the free-flow area below the floating vegetation area; ω represents the proportionality parameter; S2 determines the Shields number Θ(x) of sediment particles at different positions along the flow direction based on the change of near-bottom turbulent energy along the flow. The calculation formula is as follows: In the formula, ρ s represents the density of sediment; g represents the acceleration of gravity; d s Indicates the sediment particle size; S3 determines the critical Shields number θ for sediment resuspension c ; S4 determines the deposition probability p(x) of sediment particles at different positions along the water flow direction according to the following formula: Where K is the friction coefficient; S5 determines the sediment deposition without resuspension, that is, the sediment deposition probability p = 1 per unit bed surface sediment deposition amount Dep (p＝1) ; S6 determines the unit bed sediment deposition Dep(x) at different locations according to the following formula:

2. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 1, characterized in that: In step S1, based on the water flow continuity equation, the free water flow area below the floating vegetation area h g Average velocity U at depth ≥z≥0 b (x), the calculation formula is as follows: Where U c (x) is the average flow velocity at the depth of the floating vegetation area; U0 is the average flow velocity at the front edge of the floating vegetation area; φ represents the volume percentage of vegetation in the unit water body.

3. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 2, characterized in that: U c (x) is obtained according to the following calculation steps: (1) Determine the water flow adjustment length X according to the following formula D : In the formula, h c Indicates the height of floating vegetation; C D represents the vegetation drag coefficient; a represents the floating vegetation water blocking area per unit water body, a=nd, n represents the vegetation density per unit area, d represents the vegetation diameter; h g It indicates the height of the free water flow area below the floating vegetation area; H indicates the water depth of the river; (2) When X D >x>0, the average velocity of water depth at x in the floating vegetation area is U c (x) is: Where U0 is the average velocity of the river channel at the leading edge of the floating vegetation area; the parameters α, β, A', B' and C' are defined as: In the formula, φ represents the volume percentage of vegetation in a unit water body; C f Indicates the friction coefficient of the bed surface; C * represents the shear coefficient; (3) When x ≥ X D When U c (x) is the average velocity U in the fully developed area of ​​water flow inside the floating vegetation cf , can be calculated by the following formula:

4. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 1, characterized in that: In step S3, θ c is the critical Shields number for determining whether sediment resuspension occurs, which is determined by the dimensionless particle size. Determine, where v represents the kinematic viscosity; when d * <0.3, Θ c =0.5tan30°; when 0.3≤d * ≤19, Θ c =0.25d * -0.6 tan30°; when 19 <d * When ≤50, Θ c =0.013d * 0.4 tan30°; when d * >50, Θ c =0.06tan30°.

5. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 1, characterized in that: In step S5, if the average flow velocity of the river channel is less than the critical flow velocity U for resuspension of sediment c(bare) , indicating that the sediment is only deposited and not resuspended, and the sediment deposition probability p = 1. At this time, the measured sediment deposition per unit bed surface is defined as Dep (p＝1) .

6. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 5, characterized in that: Critical velocity U for resuspension of sediment c(bare) , can be calculated by the following formula: In the formula, k t(c) Represents the critical starting turbulent energy.

7. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 1, characterized in that: If the average flow velocity of the river U0 ≥ U c(bare) , and the sediment deposition per unit bed surface Dep(x) at different locations along the river channel is determined by the following steps: S7 Determine the probability of sediment deposition in the area without vegetation cover in the river channel P (0) and sediment deposition Dep (0) ; S8 determines the unit bed sediment deposition Dep(x) at different locations along the river channel according to the following formula:

8. The method for analyzing sediment movement characteristics in a river channel with floating vegetation according to claim 7, characterized in that: First, calculate the near-bottom turbulent energy k in the area without vegetation coverage in the river channel t(0) : Then determine the Shields number Θ of sediment particles in the area without vegetation cover in the river channel (0) and its sediment deposition probability P (0) :

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