Riverbed undercut multistage flexible protection test device and test method

Abstract

The invention provides a riverbed undercut multistage flexible protection test device and method, and the device comprises a forebay, the forebay is connected with a water inlet of a water tank through a pipeline, a water outlet of the water tank is connected with a flow stabilization device through a welding pipeline, and the flow stabilization device is connected with a water inlet of an experiment water tank. A tail gate is fixedly installed in the test water tank and located at the position of the water outlet, a water-sediment separation device is arranged at the position of the water outlet of the test water tank and located at the tail gate, a water storage pool is arranged at a water outlet of the water-sediment separation device, a water return channel is arranged on one side of the water storage pool, and the water return channel is connected with the forebay to form a circulating water supply system. Water recycling is achieved through the device. And by establishing a quantitative relation model of the aperture-protection effect of the protection net, the protection net with the optimal aperture is determined, the optimal riverbed undercutting inhibition effect is achieved, the protection thought of the multi-stage protection net is adopted, the net face is arranged at the 2B interval, the optimal protection effect can be achieved, and the pit flushing development scale is controlled to the maximum degree.

Description

Technical Field

[0001] The invention relates to riverbed maintenance, and in particular to a riverbed undercut multi-stage flexible protection test device and a test method. Background Art

[0002] Riverbed incision is driven by both natural evolution and human activities. Disasters such as earthquakes trigger landslides, and sediment replenishment alters the water-sand balance, leading to dramatic changes in the riverbed. Indiscriminate sand mining destroys the coarsening layer, triggering upstream and downstream scour, ultimately triggering overall riverbed adjustments. Incision poses a serious threat to river ecology and engineering safety: scouring at the toe of the slope leads to bank collapse and landslide risks, while falling water levels cause water quality deterioration and saltwater intrusion, disrupting fish habitats. Furthermore, exposed bridge piers lead to structural damage, putting pipelines through the river at risk of collapse and directly impacting the stability of water-related structures. Riverbed incision endangers life and property through multiple pathways, making strengthening the protection of incised river channels of great practical significance.

[0003] The existing patents for flexible bed consolidation technology for protecting riverbed undercutting still have the following deficiencies: the overall structural stability is poor, and while suppressing riverbed undercutting, it will aggravate the scouring and damage of the downstream riverbed by the water flow. In addition, the existing patent CN115467290A provides an ecological bed consolidation component, a test device and a test method thereof, in which the ecological bed consolidation technology uses a single-stage flexible net for protection, which has the following problems: (1) This technology does not consider the influence of the mesh hole diameter on the protection effect. The test found that when the mesh hole diameter is close to 0, the mesh surface will be severely scoured upstream, and the mesh body will be scoured to the depth of burial in a short time, resulting in the "net peeling" phenomenon and protection failure; when the mesh hole diameter is infinite, the mesh surface loses the movement constraint on the erosion base bed sediment and has almost no protection effect. Therefore, the uncertainty of the hole diameter becomes one of the constraints for the actual production and use of flexible net technology. (2) The protection distance of the upstream river channel of this technology is limited, and as the mesh surface continues to grow, the protection effect is difficult to improve, and large scouring pits will be generated downstream. The continued development of the scouring pit may cause the mesh surface to become unstable rapidly.

[0004] In addition, the water-sand separation technology used in existing riverbed experiments still has significant limitations in adapting to the needs of moving bed flume tests: first, the cylinder-type or filter-type separation devices used in the agricultural irrigation field are designed to remove fine particles in the water, which cannot meet the separation needs of bed load sand in moving bed tests; second, although the vibration screening equipment used in river dredging projects can achieve the separation of large-particle sand, there are problems such as large equipment footprint, increased system complexity and failure probability due to the mechanical vibration unit; third, the traditional devices such as sand collecting funnels and sand collecting baskets commonly used in current laboratories rely solely on the passive separation mode of static water sedimentation, which has the disadvantages of low separation efficiency (requiring tens of minutes of sedimentation time) and frequent manual sand removal operations that increase the workload of the test; in addition, the improved cyclone sedimentation device in the sewage treatment field can achieve efficient collection of suspended sand, but after being connected to the flume, traditional separation devices still need to be additionally configured for bed load with larger particle sizes, resulting in low system integration. Summary of the Invention

[0005] The purpose of the present invention is to solve the defects of the prior art in that the protective net fails due to the aperture and the water-sand separation technology used in the riverbed experiment is not suitable for the requirements of the moving bed water tank test.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A multi-stage flexible protection test device for riverbed undercutting includes a forepool, which is connected to the water inlet of a water tank through a pipe, the water outlet of the water tank is connected to a flow stabilizing device through a welded pipe, the flow stabilizing device is connected to the water inlet of an experimental water flume, a tail gate is fixedly installed in the test water flume at the position of the water outlet, a water-sand separation device is provided at the water outlet of the test water flume and at the tail gate, a water reservoir is provided at the drain outlet of the water-sand separation device, a return water channel is provided on one side of the water reservoir, and the return water channel is connected to the forepool to form a circulating water supply system.

[0007] Furthermore, a transition section and a test section are sequentially provided in the experimental water flume along the direction of water flow. The transition section is an inverse slope paved with test sand; the test section is sequentially provided with a protection section, a protection net laying section, and a flushing pit observation section along the direction of water flow. The protection net laying section is used for laying the protection net.

[0008] Furthermore, acrylic plates and foam plates are provided in the test water tank at the location of the water inlet to reduce the energy of the water flow.

[0009] Furthermore, the water-sand separation device includes a water inlet, a vertical spiral pipe is installed at the bottom of the water inlet, a support frame is provided on one side of the vertical spiral pipe, a drain hose A is installed at the bottom of the vertical spiral pipe, a drain hose B is installed on one side of the drain hose A, and a sand receiving box is installed at the end of the drain hose B away from the drain hose A.

[0010] Furthermore, the vertical spiral pipe is divided into an acceleration section and a separation section from top to bottom.

[0011] Furthermore, a vibration component is installed at the bottom end of the sand box; Furthermore, the sandbox is configured as a transparent structure with scales.

[0012] A test method for multi-stage flexible protection of riverbed undercutting, comprising: S1. Model making: Survey the river channel where the protective net needs to be laid, and obtain the water flow intensity Q*, river channel length l, river channel width B, riverbed slope J, bed sand gradation, and test sand characteristic particle size D by measuring. 50 Based on the data, a model of the ecological bed reinforcement test device was made by reducing the proportion of the river channel where the flexible protection net needs to be laid; S2. Selection of sand for the test: gravel is used to lay on the bottom of the experimental water tank, and the median particle size of the gravel is d 50 If the particle size is larger than 2mm, the color bed sand of the test flume riverbed shall be selected according to the test sand grading curve; S3. Design of water flow intensity: According to the river channel conditions of the protection net, the slope range of the sand covering in the test is determined to be 5.5‰, the sand covering thickness at the outlet of the test flume is 8cm, and the sand covering thickness at the water inlet of the test flume should be 13.6cm; according to the shear stress τ0 of the riverbed sediment being greater than the critical starting shear stress τ c , define the dimensionless water flow intensity Q * =τ 0 / τ c , determine the water flow conditions in the test: Q*=τ0 / τ c 1.1~1.5; S4. Flexible protective net setting: according to the particle size of bed sand d 50 Size, select three protective nets with a mesh size of dn and lay them in the protective net laying section of the experimental flume; S5. Multi-level protection net setting: Select the protection net with the optimal aperture and lay it inside the experimental water tank; The total length of the test flume is 18m. The starting point of the test section is x=-4m, the starting point of the upstream water flow adjustment section is x=-1.2m, the starting point of the test section is x=0m, and the starting point of the downstream water flow adjustment section is x=8m. The first protective net is laid at x=3.6m. The length of the protective net along the water flow direction is 0.5B, and the length perpendicular to the water flow direction is B, where B is the width of the flume, B=0.5m. The distance between the second-level protective net and the first-level protective net is L, which is set to 2B, 3B, and 4B, i.e. 1m, 1.5m, and 2m respectively. The distance between the second-level protective net and the starting point of the test section is x=4.85m, x=5.35m, and x=5.85m respectively. S6. Test instrument: A self-made data measuring instrument was used to measure the water level and riverbed elevation during the test; S7. Measurement time: During the test, the water level and bed elevation were measured at intervals of 15, 30, and 60 minutes during the first hour. After one hour, the measurement interval was changed to every 30 minutes. The vertical measurement interval was 20 cm, and the measurement range was the test section. The center point on the cross section of each paved section was used as the measurement data for that section. The bed elevation data obtained by measurement was calculated to obtain the unit incision rate (mm / h) per hour. When the unit incision rate per hour was less than 2.0 mm, the overall adjustment of the riverbed was considered to have reached equilibrium. S8. Record the water level changes and equilibrium water depth under various working conditions during the test, and perform calculations and analysis on the bed adjustment process and equilibrium bed data under the protection of the flexible protection net to evaluate the protective effect of the flexible protection net under different working conditions; S9. Analyze the undercutting balance process of the riverbed and calculate the overall average relative undercutting depth of the riverbed. The calculation formula is Δh=h o -h b ;h o is the initial bed height of each section, h b The bed height of each section when the scouring reaches equilibrium; definition is the protective capacity of the flexible protective net, among which Average relative cutting depth of the protection section under the no-net condition (cm); The average relative cutting depth (cm) of the protective section under the condition of laying the protective net; the larger the η value, the higher the protective efficiency of the flexible protective net.

[0013] The beneficial effects of the present invention are: 1. Optimizing the design of protective net aperture parameters. By establishing a quantitative relationship model between net aperture and protective effectiveness, it was determined that the net loses its protective effectiveness when the mesh aperture approaches zero or when the mesh aperture is infinite. Experiments have revealed an inflection point in the relationship between the flexible net aperture and its effectiveness in suppressing riverbed incision. By selecting the optimal mesh aperture Dn, the corresponding mesh aperture dn can be set on a given riverbed to achieve optimal suppression of riverbed incision.

[0014] 2. Propose a segmented effect evaluation method to accurately evaluate the effect of the protection segment and reduce errors.

[0015] 3. It is proposed to adopt a multi-level protection net protection idea, and the use of 2B spacing to set the net surface can achieve the best protection effect and control the scale of the pit development to the greatest extent.

[0016] 4. The present invention utilizes a circulating system consisting of an experimental water flume, a water tank, a water-sand separation device, a water reservoir, a return channel, and a water pool to provide a continuous water supply for the experiment. The water-sand separation device utilizes a spiral flow channel and a curved circulation system. The secondary circulation of the spiral flow drives the migration of coarse sand from the bottom layer, and combined with an inner drop structure, efficient separation is achieved, solving the problem of low coarse particle separation efficiency of traditional methods. The present invention utilizes an unpowered, self-circulating vertical layout. The potential energy of the water and sand's own weight drives the internal circulation, eliminating the need for external mechanical power, reducing energy consumption and failure rates. The vertical structure also saves space. The curved circulation utilizes a stratified transport mechanism. Surface clean water continues to flow downward around the vertical spiral device through the outer diversion device, while the bottom bed material is laterally transported by the spiral flow to the inner side for a concentrated drop. This design features structural optimization and economic efficiency. The combination of the spiral flow, drop structure, and vertical layout enables efficient and stable operation of laboratory moving bed flume tests with low energy consumption and maintenance, breaking through the limitations of traditional planar equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a structural schematic diagram of the water tank experimental device provided by the present invention; Figure 2 This is a schematic structural diagram of the water-sand separation device provided by the present invention; Figure 3 This is a schematic diagram of the curved spiral flow provided by the present invention; Figure 4 This is a layout diagram of flexible protective nets with different spacings provided by the present invention; Figure 5 The present invention provides Q*=1.1 different flexible mesh aperture d n A line graph of the depth of incision along the bed under working conditions; Figure 6 The present invention provides Q*=1.3 different flexible mesh aperture d n A line graph of the depth of incision along the bed under working conditions; Figure 7 The present invention provides Q*=1.5 different flexible mesh aperture d n A line graph of the depth of incision along the bed under working conditions; Figure 8 It is a line graph showing the protection efficiency of the flexible protection net under various working conditions provided by the present invention; Figure 9 It is a line graph showing the protection efficiency changes with the aperture under different water flow intensities provided by the present invention; Figure 10 It is the segmented protection efficiency of the protection net under each working condition provided by the present invention; Figure 11 This is a line graph of the elevation change (L=2B) of the bed surface at the equilibrium moment under different laying conditions of the Q*=1.5 protection net provided by the present invention; Figure 12This is a line graph of the elevation change (L=3B) of the bed surface at the equilibrium moment under different laying conditions of the Q*=1.5 protection net provided by the present invention; Figure 13 This is a line graph of the elevation change (L=4B) of the bed surface at the equilibrium moment under different laying conditions of the Q*=1.5 protection net provided by the present invention; Figure 14 It is the protection efficiency of the flexible protection net provided by the present invention under different laying conditions.

[0018] Figure numerals: 1-front pool, 2-water tank, 3-flow stabilizing device, 4-test water tank, 5-tail gate, 6-water-sand separation device, 7-water reservoir, 8-return channel; 601-water inlet, 602-vertical spiral pipe, 603-sand connecting box, 604-drainage hose A, 605-drainage hose B, 606-vibration component, 607-support frame. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0020] The application principle of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0021] like Figure 1 As shown, a multi-stage flexible protection test device for riverbed undercutting includes a forebay 1, a water tank 2, a flow stabilizing device 3, a test flume 4, a tailgate 5, a water-sediment separator 6, a reservoir 7, and a return channel 8. The test flume 4 is 18 meters long and 0.5 meters wide at the bottom. The side walls of the test flume 4 are 0.42 meters high, and the bottom slope is 1.5‰. The forebay 1, water tank 2, flow stabilizing device 3, test flume 4, tailgate 5, water-sediment separator 6, reservoir 7, and return channel 8 together form a circulating water supply system that can provide a maximum flow rate of approximately 60 L / s, which is controlled by a valve.

[0022] The water inlet of the water tank 2 is connected to the front pool 1 through a water pipe, and a water pump is installed on the water pipe between the water tank 2 and the front pool 1; the water outlet of the water tank 2 is connected to the flow stabilizing device 3 through a welded pipe, and a small channel or grid structure is arranged in the flow stabilizing device 3 to cut, disperse, and frictionally dissipate large-scale vortices and strong pulsations, and convert them into smaller-scale, lower-energy turbulence, ultimately making the water flow out of the device smooth; the test water tank 4 is fixed to the ground by a steel pipe.

[0023] The outlet of the flow stabilizing device 3 is connected to the water inlet of the experimental water flume 4. A transition section and a test section are sequentially arranged in the test water flume 4 along the direction of water flow. The transition section of the test water flume is paved with test sand to form a relatively gentle reverse slope, which fully ensures that the water flows into the test section in a stable state. The test section of the test water flume is sequentially divided into a protection section, a protection net laying section, and a flushing pit observation section along the direction of water flow; the protection net section is used for laying the protection net. The protection net structure used in this application is the same as the protection net in the existing patent CN115467290A. The material of the protection net of this application is nylon thread; the protection section is the upstream section where the protection net is laid. The significance of studying this section is to judge the effect of the protection net; the flushing pit section is the downstream section where the protection net is laid.

[0024] Acrylic and foam panels are installed within the test water tank 4, located at the water inlet, to reduce water flow energy. A water pump draws water from the forebay 1 to supply the test water tank 4. At the front end of the test section, the water flows through the permeable acrylic and foam panels, reducing its energy and allowing it to enter the test section after it has stabilized sufficiently.

[0025] A tailgate 5 is fixedly installed within the test water tank 4, at the water outlet. This tailgate 5, a wooden board the same width as the tank, raises the water level within the tank to saturate the test sand. This tailgate 5 controls pre-test saturation of the test sand, minimizing test errors caused by flooding at the start of the test and significant drops after the test.

[0026] A water-sand separation device 6 is provided at the tailgate 5 below the end of the test water tank 4 to facilitate the collection, recovery and reuse of the mud and sand washed out during the test. The water and sand are separated by the water-sand separation device 6, and the water directly flows into the water reservoir for recycling.

[0027] like Figure 2 As shown, the water-sand separation device 6 includes a water inlet 601, a vertical spiral pipe 602, a sand receiving box 603, a drain hose A604, a drain hose B605, a vibrating component 606, and a support frame 607. The water inlet 601 of the water-sand separation device 6 is fixedly connected to the tailgate 5. The vertical spiral pipe 602 is mounted at the bottom of the water inlet 601. The vertical spiral pipe 602 is divided into an acceleration section and a separation section from top to bottom. The top of the separation section of the vertical spiral pipe 602 is open, similar to the structure of a spiral slide. A support frame 607 is provided on one side of the vertical spiral pipe 602. A drain hose A604 is mounted at the bottom of the vertical spiral pipe 602. A drain hose B605 is mounted on one side of the drain hose A604. A sand receiving box 603 is mounted on the end of the drain hose B605 away from the drain hose A604. A vibrating component 606 is mounted at the bottom of the sand receiving box 603.

[0028] The water-sand mixture in the test water tank 4 directly enters the water-sand separation device 6 through the tailgate 5. In this device, the sand is spirally separated into the sand receiving box 603, and the clean water in the sand receiving box 7 and the clean water in the vertical spiral pipe 602 are discharged into the water reservoir 7 through the drain pipe A604 and the drain pipe B605 respectively. The water in the water reservoir 7 flows back to the front pool 1 through the underground return channel 8, completing the water circulation.

[0029] The water-sediment mixture in the tailwater of bedload test flume 4 enters the acceleration section of the vertical spiral pipe 602 through the water inlet 601. The purpose of acceleration is to convert gravitational potential energy into kinetic energy. The slope of the acceleration section of the vertical spiral pipe 602 is greater than that of the separation section of the vertical spiral pipe 602. After the acceleration section of the vertical spiral pipe 602 ends, the water-sediment mixture reaches the separation section of the vertical spiral pipe 602. In this area, the bend circulation phenomenon ( Figure 3 ) Water and sand will separate. The bend circulation phenomenon refers to the unique motion state of a bend, where the surface water flows outward (concave bank) and the bottom water flows inward (convex bank). This is the three-dimensional spiral flow motion unique to bends. In a bend, the surface water velocity is greater than the bottom water velocity, and the centrifugal force on the surface water is greater than that on the bottom water. Therefore, the surface water flows outward under the centrifugal force. Due to the principle of continuity, the bottom water flows from the outside to the inside, forming a horizontal bend circulation when viewed from the cross-section of the bend. The bend circulation separation path is designed: the surface clear water flows outward and continues to flow in the vertical spiral pipe. The bedload sand is transported laterally inward by the spiral flow and then freely falls into the sand receiving box 603. The sand receiving box 603 is a fully transparent structure with a scale printed on the wall. Different sizes can be selected according to the specific experimental conditions.

[0030] In the test, most of the water is directly discharged into the underground water tank 7 through the drainage hose A604, and a small amount of water falls into the sand receiving box 603 along with the sand. The sand receiving box 603 is designed with a drainage hose B605 and a vibration component 606. When the amount of water in the sand receiving box 603 is too much and exceeds the elevation of the overflow hole, it is discharged through the drainage hose B605. Through the vibration device 606 at the bottom of the sand receiving box 603, the sand pile that originally naturally fell in the sand receiving box 603 can be flattened and distributed more evenly in the sand receiving box 603, and the sand surface appears level. During the test, the change in the height of the sand layer △h is read through the sand receiving box 603, and the sand transport rate of the bed load can be calculated using the volume method. The calculation formula is:

[0031] (2) Where: qs: sediment transport rate (unit: kg / s or t / s); A: bottom area of the sediment receiving pool (unit: m 2); △h: height of sand layer change (m); △t: time of sand layer change (s); n: porosity (dimensionless, ranging from 0 to 1); ρs: density of sand solid particles (kg / m 3 ) A test method for multi-stage flexible protection of riverbed undercutting, including 1. Model making: Model making, survey the river channel where the protective net needs to be laid, and obtain the water flow conditions Q*, river channel length l, river channel width B, riverbed slope J, bed sand gradation, and test sand characteristic particle size D through measurement. 50 By reducing the proportion of the river channel where the protective net needs to be laid, a model of the ecological bed fixing test device is made.

[0032] 2. Selection of sand for the test: Similar starting behavior of bed load in a river is an important condition for ensuring similar bed load movement and scouring and deposition. Therefore, gravel was selected as the sand for this test. The median particle size d 50 It should be greater than 2mm. According to the bed sand gradation, the characteristic particle size of the riverbed sand in the test flume can be obtained, where d 16 =1.87mm, d 50 =2.54mm, d 84 =2.96mm, the formula for the geometric standard deviation of sediment =1.24<1.3, the sand used in the test meets the conditions of uniform sand.

[0033] 3. Water flow intensity design: There is no sediment supply upstream of the flume, and the downstream outlet condition is a free outflow condition. At the same time, taking into account the control variables in the experiment and the conditions of natural mountain rivers, the slope range of the sand spreading in the experiment was determined to be 5.5‰. The thickness of the sand spread at the test outlet is 8cm. According to this slope, the thickness of the sand spread at the upstream water inlet should be 13.6cm. This experiment studies the incised river channel, so the design flow conditions must meet the characteristics of the incised river channel flow, that is, the water shear stress τ0 on the riverbed sediment is greater than the critical starting shear stress τ of the sediment. c , define the dimensionless water flow intensity Q * =τ 0 / τ c , which is the dimensionless water flow intensity. By considering the initial state of the riverbed sediment in the preliminary test before the formal test and the factors such as the force on the side wall of the test flume, the water flow condition in the test is finally determined: Q*=τ0 / τ c =1.1~1.5.

[0034] 4. Flexible protection net setting: According to the size of bed sand particles, the test selected three protection net apertures based on the similar d50 particle size and the size of the front and rear apertures, dn = 1.80mm / 2.35mm / 3.35mm, to explore the effect of the flexible protection net aperture size on the protection effect. The protection net laying position is as follows: Figure 1As shown. The aperture dn of the flexible protective net is equal to the median particle size D of the bed sand. 50 The ratio is defined as the relative aperture Dn. By analyzing the relative cutting depth Δh of the bed surface under different Dn conditions, the protection effect is judged, and the protection net with the optimal aperture is obtained.

[0035] 5. Protection effect of multi-level protection nets. In order to explore the impact of the number of protection net laying levels on the movement of water and sand in the protection section, the flexible protection net aperture with the best protection effect was selected for testing.

[0036] The test flume is 18 meters long. The upstream flow adjustment section begins at x = -4 m (range: -4 m to 0 m), the sand-topped section begins at x = -1.2 m (range: -1.2 m to 9.8 m), and the test section begins at x = 0 m (range: 0 m to 8 m). A flow adjustment section is maintained downstream of the test section to prevent the impact of the drop at the end of the sand-topped section and the backwater at the tailgate on the test section. The downstream flow adjustment section begins at x = 8 m (range: 8 m to 9.8 m). The first protective net is laid at x = 3.6 m. The length of the net along the flow is 0.5B, and the length perpendicular to the flow is B, where B is the flume width (B = 0.5 m). The length of the net is half the flume width, and the length of the net is set to match the flume width. The distance between the second level protection net and the first level protection net is L, which is set to 2B, 3B, 4B, i.e. 1m, 1.5m, 2m. The distance between the second level protection net and the starting point of the test section is x=4.85m, x=5.35m, and x=5.85m respectively. The spacing position diagram of the three sets of protection nets is shown in the figure below. Figure 4 shown.

[0037] 6. Test instrument: The homemade data measuring instrument is used to measure the water level and riverbed elevation during the test.

[0038] 7. Measurement time: During the test, the water level and bed elevation are measured at intervals of 15, 30, and 60 minutes in the first hour. After one hour, the measurement interval is changed to every 30 minutes. The longitudinal measurement interval is 20 cm, and the measurement range is the test section. The center point on the cross section of each paved section is used as the measurement data of this section. The bed elevation data obtained by measurement is calculated to obtain the unit cutting rate (mm / h) per hour. When the unit cutting rate per hour is less than 2.0 mm, it is considered that the overall adjustment of the riverbed has reached a balanced state.

[0039] 8. Record the water level changes and equilibrium water depth under various working conditions during the test, and perform calculations and analysis on the bed adjustment process and equilibrium bed data under the protection of the flexible protective net to evaluate the protective effect of the flexible protective net under different working conditions.

[0040] 9. Analyze the undercutting balance process of the riverbed and calculate the overall average relative undercutting depth of the riverbed. The calculation formula is Δh=h o -h b ;h o is the initial bed height of each section, h b The bed height of each section when the scouring reaches equilibrium definition is the protective capacity of the flexible protective net, among which Average relative cutting depth of the protection section under the no-net condition (cm); The average relative cutting depth (cm) of the protective section under the condition of laying the protective net; the larger the η value, the higher the protective efficiency of the flexible protective net.

[0041] Verification of the effectiveness of the aperture of the flexible permeable net ecological bed protection: Twenty-five sets of indoor flume tests verified the effectiveness and reliability of this technology in selecting permeable net sizes. The test conditions are shown in Table 1. In this table, n represents the number of flexible net installation stages, dn represents the permeable net aperture, S0 represents the initial riverbed slope (5.5‰), Q* represents the test flow intensity, and T represents the test duration. The relative aperture Dn is defined as the ratio of the net aperture dn to the median particle size D50 of the bed sediment. In the test numbers, A, B, C, D, and E represent different flexible net designs. The tests analyzed the impact of flexible nets on riverbed water and sediment movement and riverbed scouring, and quantitatively evaluated the protective effectiveness of five different aperture sizes.

[0042] Table 1. Summary of test conditions

[0043] Research Results: The water depth distribution characteristics along the riverbed show that the laying of the flexible protection net has adjusted the local riverbed roughness. The changes in water depth and flow velocity along the riverbed do not conform to the law of monotonic change. From the actual bed adjustment of the riverbed upstream of the protection net, it can be observed that the scouring changes along the riverbed are not uniform, especially in the section before the net where the scouring is aggravated. In order to more clearly show the impact of the laying of the flexible protection net on the adjustment of the riverbed morphology, the bed surface undercut depth is introduced. , ,in is the initial bed height, The bed height when the scouring reaches equilibrium. The smaller it is, the better the protection effect of the river section is; conversely, the worse the protection effect is.

[0044] Figure 5-Figure 7 is the depth of the bed surface cut under different flexible mesh apertures dn at different water flow intensities. Taking the working condition as an example, the relative incision depth of the riverbed upstream of the protective net when the scouring of the riverbed reaches equilibrium under different flexible protective net laying conditions is used. Figure 5-Figure 7 WN in the middle represents the test condition of the riverbed without the protection of the flexible protection net. Indicates the test conditions of three different mesh diameters when the riverbed is protected by a flexible protective net. Figure 7 As can be seen, the bed incision depth values at equilibrium in the test conditions with flexible protective nets were all lower than those in the conditions without them. In the test conditions with flexible protective nets, the incision depth values for different mesh apertures showed the following relationship: 1.80mm > 2.35mm > 3.35mm. These test results demonstrate that the protective net has a significant inhibitory effect on riverbed incision, and this effect is related to the relative aperture size. However, when the mesh aperture is infinitely large, the net loses its ability to constrain the movement of sediment from the eroding base bed, resulting in virtually no protective effect. Therefore, there is an inflection point in the curve between mesh aperture and protective effect.

[0045] Add two mesh diameter working conditions , try to find the critical value of the mesh aperture. ( =1.80~4.0mm) and the median particle size of bed sand The ratio is defined as the relative pore size Compare the average protection efficiency of the upstream river section of the flexible protection net under different series of working conditions. Figure 8 It can be seen that as the water flow intensity increases, the protection efficiency of the flexible protection net decreases. Figure 9 It can be seen that the upstream average protection efficiency first increases with the mesh diameter, and then decreases. (the mesh hole diameter is 3.5mm) to achieve the best protection effect.

[0046] The above test results show that the flexible protection net can suppress riverbed incision, and the protection efficiency is higher when the water flow intensity is small. At the same time, the optimal mesh diameter can be obtained according to the above test and analysis methods to ensure that the flexible protection net achieves the best protection efficiency.

[0047] Segmental evaluation of protective net efficiency: Compared with the amount of incision in the river without any protective measures, the amount of reduction can be regarded as the protective effect of the protection net. Under the same water flow conditions, compared with the working condition without net, the ratio of the reduced incision depth value when the net is laid to the incision depth value without net is defined as the protection efficiency:

[0048] (1) Where η is the protection efficiency, is the average relative cutting depth of the protection section when there is no net, It is the average relative cutting depth of the protective section when laying the net.

[0049] The effectiveness of flexible protection nets in preventing river incision was previously measured by averaging the entire protection section. Figure 8 Through the experimental phenomenon, it is found that the depth of the bed surface is different at different distances from the net surface. This patent proposes to use segmented protection efficiency to quantify the net surface protection effect. The upper reaches of the protection net are divided into four protection segments, and their protection efficiency is calculated separately, such as Figure 10 Among them, sections 1-6 are the first section, sections 7-12 are the second section, sections 13-15 are the third section, and sections 16-18 are the fourth section.

[0050] like Figure 11-13 When the aperture of the protective net is the same, in the first, second and third sections, the closer the distance between the protection position and the laying position of the protective net is, the greater the protection efficiency is. However, in the fourth section closest to the front end of the protective net, the protection efficiency is lower. This is because a local undercut occurs at the front end of the protective net, and the Δh of the fourth section increases, while ΔH does not change significantly. Therefore, the protection efficiency in the fourth section is lower than that in the first three sections. In addition, the protection efficiency of the first three protection sections is higher than that of water flow intensities 1.3 and 1.5 under the working condition of water flow intensity 1.1; but in the fourth section, the protection efficiency of the three water flow intensities decreases rapidly, and the protection efficiency decreases particularly significantly when the water flow intensity is 1.1. Therefore, the protection efficiency of the front end of the protective net is lower under low water flow intensity. By calculating the protection effect in a segmented manner, it can be found through the data more clearly that the previous method of measuring the protection effect by averaging the entire protection section has a large error.

[0051] Verification of the protection effect of multi-level protection net: Think Take no net, single net and double net spacing L=2B as an example, Figure 14 In order to test the riverbed elevation along the way when reaching the downcut equilibrium moment, the upstream riverbed elevation under the two-stage flexible protection net was higher than the riverbed elevation under the single-stage flexible protection net and no net conditions, indicating that compared with the single-stage protection net, the double-stage protection net has a better protective effect on the upstream protection net section.

[0052] Downstream of the flexible protective net, when the net is single-stage, the scouring at the end of the flexible protective net gradually evolves into a scouring pit. Under the double-stage net working condition, the scouring at the end of the upper-level flexible protective net is greatly reduced, indicating that the second-level flexible protective net has a significant inhibitory effect on the upstream scouring pit.

[0053] By comparing the elevations of the riverbed along the river when it reaches equilibrium at different spacings with the same water flow intensity, it can be found that when L=2B, the difference between the upstream bed surface and the single net is greater; when L=2B, the depth of the scour pit generated downstream of the first-level flexible protection net is the smallest, and the inhibition effect is the greatest.

[0054] Research has shown that multi-level flexible protection nets can work together with upper-level flexible protection nets to protect the upstream riverbed, achieving enhanced protection. They also significantly inhibit scour pits generated at the end of the upper-level flexible protection net, with the strongest inhibitory effect occurring when L=2B. The installation of multi-level flexible protection nets extends the protection range of the flexible protection nets.

[0055] The following figure shows the protection efficiency under different working conditions. Figure 12 It can be found that under various water flow intensities, the protection efficiency of a single net is lower than that of a double net. Under the same laying conditions, the protection efficiency decreases with increasing water flow intensity.

[0056] The results show that adding a multi-level flexible protective net not only reduces the scour pits caused by scour at the end of the previous level of flexible protective net and the severe scour and incision at the front end of the flexible protective net, thereby stabilizing the previous level of flexible protective net, but also suppresses bed sand in the upstream protection section, which has a stronger inhibitory effect on riverbed incision and better protection. At a spacing of 2B, the protection effect is better than that of a single net and other spacing conditions. Therefore, this patent proposes that when using multi-level protective nets for bed protection, the net spacing should be 2B.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A riverbed undercut multi-stage flexible protection test device, characterized in that: It includes a front pool, which is connected to the water inlet of the water tank through a pipe, the water outlet of the water tank is connected to the flow stabilizing device through a welded pipe, the flow stabilizing device is connected to the water inlet of the experimental water tank, a tail gate is fixedly installed in the test water tank at the position of the water outlet, a water-sand separation device is provided at the water outlet of the test water tank and at the tail gate, a water reservoir is provided at the drain outlet of the water-sand separation device, a return water channel is provided on one side of the water reservoir, and the return water channel is connected to the front pool to form a circulating water supply system.

2. A riverbed undercut multi-stage flexible protection test device according to claim 1, characterized in that: The experimental water flume is equipped with a transition section and a test section along the direction of water flow. The transition section is a reverse slope paved with test sand; the test section is equipped with a protection section, a protection net laying section, and a flushing pit observation section along the direction of water flow. The protection net laying section is used to lay the protection net.

3. A riverbed undercut multi-stage flexible protection test device according to claim 1 or 2, characterized in that: Acrylic plates and foam plates are installed in the test water tank at the water inlet to reduce water flow energy.

4. The multi-stage flexible protection test device for riverbed undercutting according to claim 1 is characterized in that: The water-sand separation device includes a water inlet, a vertical spiral pipe is installed at the bottom of the water inlet, a support frame is provided on one side of the vertical spiral pipe, a drain hose A is installed at the bottom of the vertical spiral pipe, a drain hose B is installed on one side of the drain hose A, and a sand receiving box is installed at the end of the drain hose B away from the drain hose A.

5. A riverbed undercut multi-stage flexible protection test device according to claim 4, characterized in that: The vertical spiral pipe is divided into an acceleration section and a separation section from top to bottom, and the top end of the vertical spiral pipe of the separation section is open.

6. The riverbed undercut multi-stage flexible protection test device according to claim 4, characterized in that: A vibration component is installed at the bottom of the sand box.

7. The multi-stage flexible protection test device for riverbed undercutting according to claim 4 is characterized in that: The sandbox is set to a transparent structure with scales.

8. A test method for multi-stage flexible protection of riverbed undercutting, characterized in that: The riverbed undercut multi-stage flexible protection test device according to any one of claims 1 to 7 comprises: S1. Model making: Survey the river channel where the protective net needs to be laid, and obtain the water flow intensity Q*, river channel length l, river channel width B, riverbed slope J, bed sand gradation, and test sand characteristic particle size D by measuring. 50 Based on the data, a model of the ecological bed reinforcement test device was made by reducing the proportion of the river channel where the flexible protection net needs to be laid; S2. Selection of sand for test: choose the median particle size d 50 Gravel larger than 2 mm is laid on the bottom of the experimental flume. According to the bed sand gradation, the characteristic particle size of the riverbed sand of the experimental flume is selected to lay the colored bed sand. S3. Design of water flow intensity: According to the river channel conditions of the protection net, the slope range of the sand covering in the test is determined to be 5.5‰, the sand covering thickness at the outlet of the test flume is 8cm, and the sand covering thickness at the water inlet of the test flume should be 13.6cm; according to the shear stress τ0 of the riverbed sediment being greater than the critical starting shear stress τ c , define the dimensionless water flow intensity Q * =τ 0 / τ c , determine the water flow conditions in the test: Q*=τ0 / τ c 1.1~1.5; S4. Flexible protective net setting: according to the particle size of bed sand d 50 Size, choose the protective net with aperture dn = 1.80mm / 2.35mm / 3.35mm and lay it in the protective net laying section of the experimental water tank; 50 The ratio is defined as the relative aperture Dn. By analyzing the relative undercut depth Δh of the bed under different Dn conditions, the protection effect is judged and the protection net with the optimal aperture is obtained; S5. Multi-level protection net setup: Select a protection net with the optimal aperture and lay it in the experimental flume. The total length of the experimental flume is 18m. The starting point of the test section is x = -4m, the starting point of the upstream water flow adjustment section is x = -1.2m, the starting point of the test section is x = 0m, and the starting point of the downstream water flow adjustment section is x = 8m. The first protection net is laid at x = 3.6m. The length of the protection net along the water flow direction is 0.5B, and the length perpendicular to the water flow direction is B, where B is the flume width, B = 0.5m. The three sets of protection nets are laid, and the spacing L between the second-level protection net and the first-level protection net is set to 2B, 3B, and 4B respectively. The distances between the second-level protection net and the starting point of the test section are x = 4.85m, x = 5.35m, and x = 5.85m respectively. S6. Test instrument: A self-made data measuring instrument was used to measure the water level and riverbed elevation during the test; S7. Measurement Time: During the test, the water level and bed elevation were measured at the 15th, 30th, and 60th minute intervals during the first hour. After one hour, the measurement interval was changed to every 30 minutes. The vertical measurement interval was 20 cm, and the measurement range was the test section. The center point on the cross section of each paved section was used as the measurement data for that section. The bed elevation data obtained by measurement was calculated to obtain the unit incision rate per hour. When the unit incision rate per hour was less than 2.0 mm, the overall adjustment of the riverbed was considered to have reached a state of equilibrium. S8. Record the water level changes and equilibrium water depth under various working conditions during the test, and perform calculations and analysis on the bed adjustment process and equilibrium bed data under the protection of the flexible protection net to evaluate the protective effect of the flexible protection net under different working conditions; S9. Analyze the undercutting balance process of the riverbed and calculate the overall average relative undercutting depth of the riverbed. The calculation formula is Δh=h o -h b ;h o is the initial bed height of each section, h b The bed height of each section when the scouring reaches equilibrium; definition is the protective capacity of the flexible protective net, among which Average relative cutting depth of the protection section under the no-net condition (cm); The average relative cutting depth (cm) of the protective section under the condition of laying the protective net; the larger the η value, the higher the protective efficiency of the flexible protective net.

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