Method for analyzing sediment movement characteristics of river with floating vegetation

Abstract

This invention belongs to the fields of hydraulics and river dynamics, and discloses a method for analyzing sediment transport characteristics in river channels with floating vegetation. First, the near-bottom turbulent kinetic energy at different locations along the flow direction is determined. Then, based on the variation of the near-bottom turbulent kinetic energy along the flow direction, the Shields number of sediment particles at different locations along the flow direction and the critical Shields number for sediment resuspension are determined. Finally, the deposition probability is determined, and the distribution of sediment deposition along the flow direction is obtained. This invention considers the influence of floating vegetation on the turbulent characteristics of river flow and sediment deposition, and the obtained near-bottom turbulent kinetic energy and sediment deposition distribution along the flow direction of river channels with floating vegetation are closer to the actual river conditions, resulting in high prediction accuracy.

Description

Technical Field

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

[0002] Floating vegetation is an important component of river ecosystems, playing a vital role in regulating water flow structure, altering river flow velocity, influencing material transport and sediment dispersion, and purifying water quality. Under suitable climatic and environmental conditions, some floating vegetation species (such as water hyacinth) proliferate in large numbers, their roots intertwining and binding together to form a stable cover structure on the river surface. For example, water hyacinth has invaded parts of the lower Yangtze River Delta, such as the Taihu Lake basin, significantly impacting river sediment deposition and resuspension processes.

[0003] Due to the flow-blocking effect of vegetation, the water flow is deflected vertically downwards from the interior of the floating vegetation to the free flow area below it. (H>z>h) g The flow velocity gradually decreases, and the free flow area below the vegetation (h) g (≥z≥0) The flow velocity increases accordingly. Low flow velocity vegetation zone (H>z>h) g ) and the high-velocity zone beneath the vegetation (h g The shearing forces generated by vegetation (≥z≥0) create a shear layer at the base of the vegetation. If the shearing is strong enough, the Kelvin-Helmholtz eddy (KH eddy) develops downstream along the vegetation base, driving momentum exchange in the vegetated area and the free flow zone below, thus affecting sediment and pollutant transport processes and altering riverbed morphology. Given this fact, investigating the impact of floating vegetation on river flow turbulence and sediment deposition is particularly important, providing technical guidance for river environmental management and ecological restoration.

[0004] However, sediment deposition in floating vegetation channels is influenced by numerous factors, including average river flow velocity, vegetation density, and relative water depth. Under natural conditions, variations in upstream flow and relative water depth directly affect sediment deposition measurement results, rendering them unrepresentative. Furthermore, sediment deposition measurements require substantial manpower, resources, and funding for continuous sampling in naturally floating vegetation channels. While laboratory conditions allow for a constant and uniform water flow and the collection of sediment deposits along floating vegetation channels, data measurement and analysis still require significant financial and time investment. Typically, a 1-meter-wide and 4-meter-long floating vegetation area is constructed within a 1-meter-wide, 13-meter-long test flume. Technicians measure sediment deposition using glass slides. Each sediment deposition experiment takes 4 hours to run, and after the flume runs, the slides require 8 hours to dry and are weighed. Therefore, detailed measurements of sediment deposition in floating vegetation areas can take a month or even longer. Thus, a simple and practical method is urgently needed to predict the distribution of sediment deposition along floating vegetation channels.

[0005] Below the floating vegetation zone lies a free-flow area, and the evolution of near-bottom turbulent kinetic energy along the course of the river is closely related to the changes in flow velocity below the floating vegetation zone. However, due to the lack of predictive models for near-bottom turbulent kinetic energy in channels with floating vegetation, the distribution of sediment deposition along the course of the river under the influence of floating vegetation is also unknown. Summary of the Invention

[0006] In view of the current lack of technical research on sediment deposition in river channels with floating vegetation, the purpose of this invention is to provide a method for analyzing the sediment movement characteristics of river channels with floating vegetation. This method considers the average flow velocity, vegetation density, and relative water depth of the vegetation to analyze the sediment movement characteristics of river channels with floating vegetation (including near-bottom turbulent kinetic energy and longitudinal prediction of sediment deposition), providing a theoretical basis for further research on the interaction between floating vegetation and riverbed evolution.

[0007] This invention is applicable to river channels with floating vegetation where the water flow velocity is greater than 0 cm / s. Water flow adjustments occur only in the direction and vertical (perpendicular to the water flow direction), therefore the water flow is considered two-dimensional. In this invention, x and z represent the direction and vertical of the water flow, respectively. x = 0 represents the leading edge of the floating vegetation; z = 0 represents the riverbed surface.

[0008] The present invention proposes the following approach: Analyzing the flow velocity and near-bottom turbulent kinetic energy beneath floating vegetation areas, and establishing a flow velocity prediction model for near-bottom turbulent kinetic energy in channels with floating vegetation based on a flow velocity prediction method for free flow areas beneath floating vegetation. Furthermore, combining this with a flow velocity evolution model for near-bottom turbulent kinetic energy and a sediment deposition probability model for channels with floating vegetation, a sediment deposition prediction model suitable for channels with floating vegetation is proposed.

[0009] Research has found that in the water flow adjustment zone (X)D (where x>0), due to vegetation resistance, the average velocity at depth within the floating vegetation zone decreases (H>z>h). g The average flow velocity in the free flow zone below the vegetation zone increased (h). g ≥z≥0), until the water flow is fully developed (x>X) D The flow velocity in the floating vegetation area and the free flow area below it no longer changes. (Flow adjustment length X) D Defined as the distance from the leading edge of the floating vegetation zone to the point where the average current velocity decreases to a constant value within the vegetation zone, it can be calculated using the following formula:

[0010]

[0011] In the formula, h c Indicates the height of floating vegetation; C D The vegetation drag coefficient is represented by 'a', where 'a' represents the floating vegetation surface area per unit volume of water, a = nd, 'n' represents the vegetation density per unit area, and 'd' represents the vegetation diameter. h g H represents the height of the free-flowing area below the floating vegetation zone; H represents the river depth.

[0012] For channels with floating vegetation, the near-bottom turbulent energy (TKE) is closely related to the evolution of flow velocity in the free flow area below the floating vegetation zone, thus determining sediment deposition and resuspension. Predicting sediment deposition along a channel with floating vegetation relies on understanding the changes in near-bottom turbulent energy below the floating vegetation zone.

[0013] Based on the above-mentioned inventive concept, the present invention provides a method for analyzing sediment transport characteristics in river channels with floating vegetation, comprising the following steps:

[0014] S1 is determined by the following formula for the near-bottom turbulent kinetic energy k at different positions (x direction) along the water flow direction. t (x):

[0015]

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

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

[0018]

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

[0020] S3 determines the critical Shield 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] In the formula, K (=1) is the coefficient of friction;

[0024] S5 determines the sediment deposition rate per unit bed surface (Dep) based on the principle that sediment only settles and does not resuspend, i.e., the sediment deposition probability p = 1. (p＝1) ;

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

[0026]

[0027] In step S1 above, based on the water flow continuity equation, the average flow velocity (h) of the water depth in the free flow region below the floating vegetation area is... g ≥z≥0), U b (x), the calculation formula is as follows:

[0028]

[0029] In the formula, U c (x) represents the average flow velocity at the depth of the floating vegetation zone; U0 represents the average flow velocity of the channel at the leading edge of the floating vegetation zone; φ represents the percentage of the volume occupied by vegetation in a unit of water.

[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 When x>0, the average flow velocity U at depth x in the floating vegetation area is...c (x) is:

[0033]

[0034]

[0035] In the formula, U0 is the average channel velocity at the leading edge of the floating vegetation zone; parameters α, β, A', B', and C' are defined as follows:

[0036]

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

[0038] (3) When x≥X D At that time, U c (x) represents the average flow velocity U in the zone where water flow is fully developed within the floating vegetation. cf It can be calculated using the following formula:

[0039]

[0040] In step S3 above, Θ c The Shields number is the critical number used to determine whether sediment has been resuspended. It is determined by the dimensionless particle size. Determined, where v(=0.01cm) 2 / s) represents kinematic viscosity. When d * When <0.3, Θ c =0.5tan30°; when 0.3≤d * When ≤19, Θ c =0.25d * -0.6 tan30°; when 19 <d * When ≤50, Θ c =0.013d * 0.4 tan30°; when d * When >50, Θ c =0.06tan30°.

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

[0042]

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

[0044]

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

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

[0047]

[0048] In the formula, Dep represents the sediment deposition per unit bed surface; p represents the probability that sediment particles remain deposited on the riverbed surface; w s denoted as L / U, where ρ is the sediment particle settling velocity; C is the sediment concentration. It is worth noting that when the riverbed sediment supply is unrestricted, i.e. (L / U) bf ) / (H / w s If ) < 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 course, where L represents the length of the floating vegetation area, and U bf U represents the fully developed average flow velocity in the free flow region below the floating vegetation area. bf It is calculated from the water flow continuity equation (Equation (6)).

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

[0050]

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

[0052] It is worth noting that Dep (p＝1) The corresponding average flow velocity U0 in the river channel is less than the critical flow velocity U for the resuspension of sediment. c(bare) (Equation (11)).

[0053] If the average flow velocity of the river channel U0 ≥ U c(bare) The above-mentioned method for analyzing sediment transport characteristics in river channels with floating vegetation can also determine the amount of sediment deposited per unit bed (Dep(x)) at different locations (x direction) along the river channel through the following steps:

[0054] S7 determines the probability P of sediment deposition in unvegetated areas of the river channel. (0) and sediment deposition Dep (0) ;

[0055] S8 determines the amount of sediment deposited per unit bed surface, Dep(x), at different locations (x direction) along the river channel according to the following formula:

[0056]

[0057] In step S7 above, the near-bottom turbulent kinetic energy k of the unvegetated area in the river channel is first calculated. t(0) :

[0058]

[0059] Then determine the Shield number Θ of sediment particles in the unvegetated areas of the river channel. (0) and its sediment deposition probability P (0) :

[0060]

[0061]

[0062] Dep (0) The sediment was collected from prototype riverbeds or in the laboratory (see the glass slide weighing method given in the examples for measuring sediment deposition).

[0063] In step S8 above, based on the sediment deposition amount Dep in the unvegetated areas of the river channel... (0) Calculate the sediment deposition per unit bed surface, Dep(x), at different locations (x direction) of the floating vegetation zone in the river channel. p(x) can be calculated according to steps S1 to S4 described above.

[0064] Compared with existing technologies, the method for analyzing sediment transport characteristics in river channels with floating vegetation provided by this invention has the following advantages:

[0065] (1) This 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 kinetic energy and sediment deposition along the course of floating vegetation areas, and verifies it using a natural water hyacinth river as a case study. This provides a theoretical basis for further research on sediment movement characteristics and riverbed evolution in natural rivers, lakes and wetlands with floating vegetation, and can provide technical support for river management and water ecological restoration.

[0066] (2) The river sediment deposition distribution prediction model under the action of floating vegetation constructed in this invention combines the evolution of near-bottom turbulent kinetic energy along the river and the sediment deposition probability model, which satisfies the physical laws of sediment transport and deposition; this invention considers the average river velocity (U0), vegetation density (a) and relative water depth (h) g Many variables, such as / H), directly affect the sediment deposition along the river channel. Therefore, the near-bottom turbulent kinetic 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 the prediction accuracy is high.

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

[0068] Figure 1 (a) is a schematic diagram of the layout of a river channel with floating vegetation; (b) is a schematic diagram of the adjustment of river flow under the action of floating vegetation, where the vertical lines represent floating vegetation communities constructed by individual plants, L represents the length of the floating vegetation, and h represents the length of the floating vegetation. c h represents the height of floating vegetation. g X represents the height of the free-flowing water area below the floating vegetation zone. D Dep(x) represents the length of the water flow adjustment, and Dep(x) represents the distribution of sediment deposition along the course beneath the floating vegetation zone. (0)(a) shows the amount of sediment deposited upstream of the floating vegetation area; (b) is a top view of the experimental setup for hydrodynamics and sediment deposition in the floating vegetation channel, where circles indicate the position of a single floating vegetation plant, U0 represents the average flow velocity of the channel, and B represents the width of the flume (floating vegetation area); (c) is a schematic diagram of the cross-section of the channel with floating vegetation. Under still water conditions (U0 = 0 cm / s), clean glass slides (7.5 cm × 2.5 cm) are placed in two rows along the center line of the flume at the target location; (d) shows the measurement of water flow velocity and turbulent kinetic energy at the characteristic regions 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 This is a schematic diagram of the method for analyzing sediment transport characteristics in a river channel with floating vegetation, as provided in Embodiment 1 of the present invention.

[0070] Figure 3 The flow velocity U in the free flow area below the vegetation zone in a river channel with floating vegetation under different working conditions. b The measured value and the predicted value of (x) are compared. The measured value of the flow velocity below the vegetation area is represented by a hollow rhombus, and the predicted value and its uncertainty are represented by a solid line and a shaded strip.

[0071] Figure 4 This section describes the distribution of near-bottom turbulent kinetic energy and sediment deposition along the course of a river with floating vegetation under different working conditions. (a)-(f) compare the predicted and measured values ​​of near-bottom turbulent kinetic energy distribution under vegetation action in a rigid floating model. The measured near-bottom turbulent kinetic energy is represented by hollow circles, while the predicted values ​​and their uncertainties are represented by solid lines and shaded stripes. The dashed line represents the critical initiation turbulent kinetic energy k. t(c) X D Represents the length of water flow adjustment; (g)-(l) Relative sediment deposition distribution under vegetation action in the rigid floating model: Dep(x) / Dep (p＝1) A comparison of predicted and measured values ​​is shown, with relative sediment deposition measurements represented by hollow circles, and predicted values ​​and their uncertainties represented by solid lines and their shaded areas. Horizontal bars represent the relative sediment deposition (Dep) upstream of the floating vegetation zone. (0) / Dep (p＝1) X D Indicates the length of water flow adjustment;

[0072] Figure 5 The relative sediment deposition along the course of natural water hyacinth vegetation in river channels under different working conditions: Dep(x) / Dep (p＝1) Comparison of predicted and measured values; where the relative sediment deposition is represented by hollow circles, the predicted value and its uncertainty are represented by solid lines and their shaded areas, and the horizontal bands represent the relative sediment deposition (Dep) upstream of the floating vegetation area. (0) / Dep (p＝1) XD Indicates the length of water flow adjustment;

[0073] Figure 6 This is a schematic diagram of another method for analyzing sediment movement characteristics in a river channel with floating vegetation, provided in Embodiment 2 of the present invention. Detailed Implementation

[0074] The technical solutions of various embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0075] Example 1

[0076] This embodiment provides a detailed description of the near-bottom turbulent kinetic energy and sediment deposition along the course of a river with floating vegetation, based on flume test results.

[0077] (1) Experimental Objective

[0078] A flume experiment was conducted to measure in detail the turbulent kinetic energy and net sediment deposition along the free flow zone beneath floating vegetation. The sediment deposition along the river in rivers with floating vegetation was determined under different average flow velocities, vegetation densities, and relative water depths. Measured values ​​of near-bottom turbulent kinetic energy and sediment deposition were obtained. The measured near-bottom turbulent kinetic energy and sediment deposition were compared with the predicted near-bottom turbulent kinetic energy and sediment deposition data obtained in this invention to verify the accuracy of the sediment transport characteristic analysis method for rivers with floating vegetation provided by this invention.

[0079] (2) Test equipment

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

[0081] Table 1 shows the experimental setup with floating vegetation in a water tank.

[0082]

[0083] (3) Test conditions

[0084] This embodiment uses floating model vegetation and natural water hyacinth vegetation to conduct flume experiments under nine different conditions (different average river flow velocity, water depth, vegetation density, and relative water depth of vegetation) to verify the effectiveness of the prediction method provided by this invention. The circulating flume used in the experiment is 13m long and 1m wide, with a 6m long test section. A ruler was used to read the water depth along the test path, an electromagnetic flowmeter was used to read the upstream flow rate, and a glass slide was used to measure the sediment deposition. The test water depth ranged from 20 to 30 cm. Based on the flow velocity range (U0 = 5–25 cm / s) in natural rivers, streams, and wetlands, the average river flow velocity in the experiment was considered to be U0 = 11.5–24.5 cm / s. In all conditions, the Reynolds number Re (=U0R / v) = 20,641–35,000, where R is the hydraulic radius and Froude number is... This indicates that the test water flow was turbulent and slow-moving, such as Figure 1 As shown in (a).

[0085] like Figure 1 As shown in (b), the floating model vegetation was fixed to a 1 cm thick PVC board in an alternating arrangement. The PVC board was suspended above the water surface with the roots of the floating vegetation pointing vertically downwards. The diameter of a single floating model plant was d = 0.8 cm, which is consistent with the observed range of natural floating vegetation (d = 0.1-1.5 cm). Two vegetation densities were considered in the experiment: n = 200 and 380 plants / m². 2 The corresponding vegetation water-blocking areas are a = 1.6 and 3.0 m², respectively. -1 Floating vegetation height h c = 4~14cm, the experimental water depth H = 20 and 28cm, therefore, the relative water depth h of the vegetation is 4~14cm. g / H = 0.3~0.8. Height h of the free-flowing area below the floating vegetation. g Defined as the distance from the riverbed surface (z=0) to the bottom of the floating vegetation (z=h) g The distance of ), i.e., h g =Hh c It is worth noting that the width of the vegetation on the floating model was the same as the width of the water tank in the experiment, therefore no lateral water flow regulation was generated. The experimental parameters are summarized in Table 2.

[0086] Water hyacinth (Eichhornia crassipes) is commonly found in warm, humid aquatic environments. In this experiment, 3-4 month old mature water hyacinth plants were randomly used to cover the entire width of a water tank, simulating a real-world riverbed with floating vegetation. The average root height h of naturally grown water hyacinth was [not specified]. c =7~8.5cm, water-blocking area a of water hyacinth * (=N * D, where N * (D represents the total diameter of the water hyacinth root system, where D is the density of the water hyacinth plant) = 2.4–2.8 m -1The experimental parameters are summarized in Table 3.

[0087] Table 2 Parameters of the floating model vegetation experiment

[0088]

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

[0090]

[0091] like Figure 1 (b) The coordinate system in the flume defines the flow direction (x), transverse (y), and vertical (z). x = 0 cm is the foremost point of the floating vegetation zone; y = 0 cm is the centerline of the flume; and z = 0 cm is the riverbed surface. Instantaneous velocities in all three directions were measured using a Nortek profile velocity meter. Instantaneous velocities were collected for 120 seconds at each measuring point at a sampling frequency of 50 Hz. Velocity data with a correlation less than 70% and a signal-to-noise ratio less than 15 were removed using the method of Goring and Nikora (Goring, DG, Nikora, VI (2002). Despiking acoustic Dopplervelocimeter data. Journal of Hydraulic Engineering, 128, 117-126). The remaining instantaneous velocities (u, v, w) were decomposed into time-averaged velocities (U, V, W) and instantaneous fluctuating velocities (u', v', w'). Turbulent kinetic energy is defined as follows:

[0092]

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

[0094] Bed surface friction coefficient Cf The bed shear stress τ in the unvegetated area b The average flow velocity U0 in the channel is determined by the riverbed shear stress τ. 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 The measurement was taken 1 cm above the PVC board. In this embodiment, the coefficient of friction of the bed surface was C. f =0.004±0.001 (operating conditions 1-6) and C f =0.006±0.001 (operating condition AC). The roots of floating vegetation form a porous structure in the river channel, causing the cross-sectional area of ​​the river to contract. In this embodiment, the influence of the porosity of adjacent vegetation roots is considered when estimating the vegetation drag coefficient, and the average flow velocity of the contracted cross-section is calculated using the law of conservation of mass. Based on formula R ec =U c ′d / ν, to obtain the drag coefficient C D =1+10R ec -2 / 3 In all operating conditions, the drag coefficient ranges from 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 and measured values ​​of near-bottom turbulent kinetic energy. The calculation formula is as follows:

[0096]

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

[0098] In the sediment deposition experiment, the sediment particles used were replaced by spherical glass microspheres with a particle size of d. s =22μm, which is similar to the sediment particle size d observed in natural river wetlands, estuaries, and coastlines. s = 8~63μm consistent, sediment density is ρ s =2.65g / cm 3 The density of natural silt (2.65 g / cm³) 3Consistent. Place clean glass slides (7.5cm × 2.5cm) in two rows along the center line of the water tank (e.g., ...). Figure 1 (c) shows the measurement of net sediment deposition during the test. Before the test, the glass slides were weighed and gently placed at each measuring point. The water pump was slowly turned on to gradually increase the flow rate to the set flow rate for the test conditions. The sediment was mixed with water in a measuring cup and added to the downstream tank, and the water and sediment were transported upstream through the water-sand circulation system. The water and sediment in the entire tank were fully mixed within 5 minutes, and the initial sediment concentration for all conditions was C0 = 100 g / cm³. 3 Each test run lasted 4 hours. After 4 hours, the water pump was slowly shut off to prevent wave generation, and the water tank was slowly drained until empty. The draining time for all conditions was controlled to be 20 minutes. All glass slides were left in the water tank to dry for at least 12 hours, and then transferred to a 50°C oven to bake for 8 hours to remove residual moisture. After the glass slides were completely dry, they were weighed again. The weight difference of the glass slides before and after the test was the net sediment deposition at each measuring point. Each condition was repeated twice to determine the average deposition at each measuring point and its uncertainty.

[0099] The reliability of the proposed method for analyzing the near-bottom turbulent kinetic energy and sediment movement characteristics of floating vegetation river channels (formulas (2) and (5)) is verified through experimental cases. The experimental parameters are shown in Tables 2 and 3.

[0100] This embodiment is based on the above-mentioned operating condition data, such as Figure 2 As shown, the near-bottom turbulent kinetic energy and sediment deposition along the course of a river with floating vegetation are predicted by following the steps below, thus realizing the sediment transport characteristics analysis of a river with floating vegetation:

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

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

[0103] Near-bottom turbulent kinetic energy k beneath the floating vegetation zone t (x), and the average flow velocity U of the free flow zone below the floating vegetation area. b (x), and the coefficient of friction of the bed surface C f They are positively correlated. When X D When x>0, the free flow velocity U in the region below the floating vegetation areab (x), as the distance increases, the near-bottom turbulent kinetic energy also gradually increases to its maximum value from the front end of the floating vegetation area (x=0). Since equation (6) considers many influencing factors such as relative water depth of vegetation, vegetation density, and average river velocity, the measured value of the free flow velocity in the area below the vegetation area matches the actual value well, and the relative error ERR is only 9%. Figure 3 Furthermore, the near-bottom turbulent kinetic energy distribution 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 Shield number Θ(x) of sediment particles at different positions (x direction) along the flow direction based on the variation of near-bottom turbulent kinetic energy along the flow path.

[0105] In this step, the Shield 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 Shield number Θ for sediment resuspension. c .

[0107] In this step, the dimensionless particle size d is first calculated. * Then, based on the dimensionless particle size d * Determining the critical Shield number Θ for sediment resuspension c .

[0108] For the average particle size d selected in this embodiment s Model sand with a diameter of 22 μm and a dimensionless particle size d * =d s [(ρ s -ρ)g / ρν 2 ] 1 / 3 =0.6, at 0.3 <d * The critical Shield number Θ is within the range of <19. c =0.25d * -0.6 tan30°=0.21. The critical initiation turbulent kinetic energy k of sedimentary sediment is calculated by formula (10). 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 locations (x direction) along the water flow direction.

[0110] In this step, based on the Shield number Θ(x) of sediment particles at different positions (x direction) along the water flow direction determined in step S2 and the critical Shield number Θ for sediment resuspension,... 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 rate per unit bed surface (Dep) based on the principle that sediment only settles and does not resuspend, i.e., the sediment deposition probability p = 1. (p＝1) .

[0112] In this embodiment, in low flow rate condition 1 ( Figure 4 (g), U0 = 11.5 cm / s, a = 1.6 m -1 ,h g / H=0.5 and H=28cm), the sediment deposition in the floating vegetation zone x=0~2m section and the sediment deposition upstream of the floating vegetation zone Dep (0) Equal amounts indicate that only sediment deposition occurs in the river channel at this time, and the spatial average deposition within this range is defined as Dep. (p＝1) =3.0±0.2mg / cm 2 This is consistent with the trend of near-bottom turbulent kinetic energy variation along the path, that is, in the floating vegetation zone x = 0 to 2m section, the near-bottom turbulent kinetic energy is less than or equal to the critical initiation turbulent kinetic energy (k) of the sediment. t(c) =3.7cm 2 / s 2 , Figure 3 (a)).

[0113] S6 determines the amount of sediment deposited per unit bed surface, Dep(x), at different locations (x direction).

[0114] In this step, the deposition probability p(x) determined in step S4 and the sediment deposition amount per unit bed surface Dep determined in step S5 with a sediment deposition probability p=1 are used. (p＝1) Substituting into formula (5), the amount of sediment deposited per unit bed surface, Dep(x), can be calculated after 4 hours of operation for each group of working conditions.

[0115] To verify the accuracy of the proposed method for analyzing near-bottom turbulent kinetic energy and sediment transport characteristics in rivers with floating vegetation, this embodiment uses nine sets of flume test data (U0 = 11.5–24.5 cm / s, h) collected from floating model vegetation and natural water hyacinth vegetation. g / H=0.3~0.8, a=1.6~3.0m -1 The prediction results were then tested. Figure 4 (g)-4(l) and Figure 5The predicted and measured values ​​of relative sediment deposition along the riverbed beneath the floating model vegetation and natural water hyacinth vegetation areas are presented respectively. The results show that the predicted and measured values ​​of sediment deposition are in good agreement within the uncertainty range. As the average river velocity U0 increases ( Figure 4 (h)-4(l)), because the near-bottom turbulent kinetic energy exceeds the critical turbulent kinetic energy k for the resuspension of sediment. t(c) The net sediment deposition below the floating vegetation zone is less than the reference value Dep for sediment deposition upstream of the vegetation zone. (0) As the distance the water flows over the floating vegetation increases (the distance between the water flow and the vegetation front at x=0 increases), the near-bottom turbulent kinetic energy in the free flow area below the vegetation zone increases along the way. Correspondingly, the amount of sediment deposition below the floating vegetation zone decreases along the way until a new water flow equilibrium is reached, and the amount of sediment deposition remains at its minimum value and no longer changes.

[0116] In summary, this invention combines the predicted near-bottom turbulent kinetic energy of the free flow zone below the floating vegetation area (Formula (2)) with the sediment deposition probability model, providing an effective method for predicting the distribution of sediment deposition along the course of rivers with floating vegetation (Formula (5)). The prediction accuracy is high and the applicability is strong, which can provide 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 transport characteristics in river channels with floating vegetation, including the following steps:

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

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

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

[0122] In this step, the Shield 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 Shield number Θ for sediment resuspension.c .

[0124] In this step, the dimensionless particle size d is first calculated. * Then, based on the dimensionless particle size d * Determining the critical Shield number Θ for sediment resuspension c .

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

[0126] In this step, based on the Shield number Θ(x) of sediment particles at different positions (x direction) along the water flow direction determined in step S2 and the critical Shield number Θ for sediment resuspension,... 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.

[0127] S7 determines the probability P of sediment deposition in unvegetated areas of the river channel. (0) and sediment deposition Dep (0) .

[0128] In this step, the near-bottom turbulent kinetic energy k of the unvegetated area in the river channel is first calculated according to the above formula (15). t(0) Then, determine the Shield number Θ of sediment particles in the unvegetated areas of the river channel according to (16) and (17) above. (0) and its sediment deposition probability P (0) .

[0129] This step follows the method given in Example 1, measuring the sediment deposition (Dep) in an area without vegetation cover using a glass slide. (0) .

[0130] S8 determines the amount of sediment deposited per unit bed surface at different locations (x direction) along the river channel.

[0131] In this step, the deposition probability p(x) determined in step S4 and the sediment deposition probability P in the unvegetated area of ​​the river channel determined in step S7 are used. (0) and sediment deposition Dep (0) Substituting into formula (14), the sediment deposition amount Dep(x) per unit bed surface at different locations (x direction) can be calculated.

[0132] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A method for analyzing sediment transport characteristics in river channels with floating vegetation, characterized in that, Includes the following steps: S1 determines the near-bottom turbulent kinetic energy k at different locations along the flow direction according to the following formula. t (x): ； In the formula, k t (x) represents the near-bottom turbulent kinetic energy at position x below the floating vegetation area; This represents the shear stress on the riverbed surface; C represents the density of water. f U represents the coefficient of friction of the bed surface; b (x) represents the average flow velocity of the water depth in the free flow area below the floating vegetation area; Indicates the proportional parameter; S2 determines the Shield number of sediment particles at different locations along the flow direction based on the variation of near-bottom turbulent kinetic energy along the flow path. The calculation formula is as follows: ； In the formula, d represents the density of the sediment; g represents the acceleration due to gravity; d represents the acceleration due to gravity. s Indicates the particle size of sediment; S3 determines the critical Shelltz number for sediment resuspension. ; S4 determines the deposition probability p(x) of sediment particles at different locations along the water flow direction using the following formula: ； In the formula, K is the coefficient of friction; S5 determines the sediment deposition rate per unit bed surface (Dep) based on the probability of sediment deposition (p = 1), meaning the sediment only deposits and does not resuspend. (p=1) ; S6 determines the sediment deposition rate Dep(x) per unit bed surface at different locations using the following formula: 。 2. The method for analyzing sediment transport characteristics in river channels with floating vegetation according to claim 1, characterized in that, In step S1, based on the water flow continuity equation, the free flow region h below the floating vegetation area... g Average flow velocity U at depth ≥ z ≥ 0 b (x), the calculation formula is as follows: ； In the formula, U c (x) represents the average flow velocity at the depth of the floating vegetation zone; U0 represents the average flow velocity of the channel at the leading edge of the floating vegetation zone. H represents the percentage of water volume occupied by vegetation; H represents the river depth; h g Indicates the height of the free-flowing water area below the floating vegetation zone; h c 'z' represents the height of floating vegetation; 'z' represents the vertical direction of water flow.

3. The method for analyzing sediment transport characteristics in river channels 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, C D denoted by , where 'a' represents the floating vegetation drag coefficient; 'n' represents the water-blocking area of ​​a unit volume of water, a = nd; 'n' represents the vegetation density per unit area; and 'd' represents the vegetation diameter. (2) When X D When x > 0, the average flow velocity U at depth x in the floating vegetation area is... c (x) is: ； ； In the formula, U0 is the average flow velocity of the river at the leading edge of the floating vegetation zone; parameters α, β, ... , and Defined as: ； In the formula, C represents the percentage of water volume occupied by vegetation; f Indicates the coefficient of friction of the bed surface; C * Indicates the shear coefficient; (3) When x≥X D At that time, U c (x) represents the average flow velocity U in the zone where water flow is fully developed within the floating vegetation. cf It can be calculated using the following formula: 。 4. The method for analyzing sediment transport characteristics in river channels with floating vegetation according to claim 1, characterized in that, In step S3, The Shields number is the critical number used to determine whether sediment has been resuspended. It is determined by the dimensionless particle size. Determine, where v represents kinematic viscosity; when d * When <0.3, ; When 0.3 ≤ d * ≤ 19:00 When 19 <d * When ≤ 50, When d * > 50 o'clock, .

5. The method for analyzing sediment transport characteristics in river channels 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 the resuspension of sedimentary sediment... c(bare) This indicates that the sediment only settles and does not resuspend, with a sediment deposition probability p = 1. In this case, the measured sediment deposition per unit bed surface is defined as Dep. (p = 1) .

6. The method for analyzing sediment transport characteristics in river channels with floating vegetation according to claim 5, characterized in that, The critical flow velocity U for the resuspension of sedimentary sediments c(bare) It can be calculated using the following formula: ； ； In the formula, It represents the critical starting turbulent kinetic energy.

7. The method for analyzing sediment transport characteristics in river channels with floating vegetation according to claim 1, characterized in that, If the average flow velocity of the river channel U0 ≥ U c(bare) The following steps were used to determine the amount of sediment deposited per unit bed surface, Dep(x), at different locations along the river channel: S7 Determine the probability P of sediment deposition in areas without vegetation cover in the river channel. (0) and sediment deposition Dep (0) ; S8 determines the amount of sediment deposited per unit bed surface, Dep(x), at different locations along the river channel using the following formula: 。 8. The method for analyzing sediment transport characteristics in river channels with floating vegetation according to claim 7, characterized in that, First, calculate the near-bottom turbulent kinetic energy k of the unvegetated area in the river channel. t(0) : ； Then determine the Shields number of sediment particles in the unvegetated areas of the river channel. and its sediment deposition probability P (0) : ； 。

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