Experimental method for researching sediment erosion characteristics under complex hydrodynamic conditions

Abstract

The invention provides an experimental method for researching sediment erosion characteristics under complex hydrodynamic conditions. The experimental method comprises the following steps: collecting sediment samples on site through a columnar sampler; carrying out an annular water tank simulation experiment according to the sediment sample to obtain a bottom shear force on a sediment-water interface in the simulation experiment; hydrodynamic parameters in a real state are monitored and collected, shearing force of hydrodynamic acting on the sediment-water interface in a natural state is calculated, the bottom shearing force of the sediment-water interface in a simulation experiment is compared with the shearing force of the hydrodynamic acting on the sediment-water interface in the natural state, and erosion characteristics in the natural state are predicted. According to the invention, the sampler and the annular water tank are combined for use, so that the disturbance to a sediment-water interface in the sampling and experiment process is reduced, the integrity of a surface sediment structure is ensured, and the accuracy of a simulation experiment is improved.

Description

Technical Field

[0001] The invention belongs to the field of water conservancy engineering, and in particular relates to an experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions. Background Art

[0002] The study of sediment erosion characteristics involves disciplines such as sediment movement mechanics, sedimentation dynamics, and rock and soil mechanics. Its core indicators mainly include sediment critical shear stress and erosion rate. The hydrodynamics of estuarine and coastal areas are jointly controlled by rivers, tides, and wave effects. Due to the complex dynamic field conditions and sediment characteristics in estuarine and coastal areas, the initiation, sedimentation, and transport processes of sediment often show large differences in time and space, which may lead to a series of scouring and silting problems, such as a decrease in the silting rate of coastal tidal flats and wetlands, or even erosion retreat, and silting of waterways. Therefore, the study of the anti-erosion capacity of sediments in estuarine and coastal areas has important research significance and application value.

[0003] The initiation law of fine-grained viscous sediment in estuary and coastal areas is more complicated due to its viscosity and complex hydrodynamic conditions. Whether it is theoretical research, basic experiments or field measurements, the research process faces the difficulty of restoring the real situation. The research results are mostly empirical or semi-empirical in nature, and there is a relative lack of quantitative experiments on the physical processes of undisturbed samples and interpretation of physical mechanisms. Summary of the invention

[0004] The purpose of the invention is to provide an experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions.

[0005] The technical solution of the present invention is as follows:

[0006] An experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions, including:

[0007] Sediment samples were collected in situ by column samplers;

[0008] Conducting an annular flume simulation experiment based on the sediment sample to obtain the bottom shear force on the sediment-water interface in the simulation experiment;

[0009] Monitor and collect hydrodynamic parameters under real conditions, calculate the shear force of hydrodynamic force on the sediment-water interface under natural conditions, compare the bottom shear force on the sediment-water interface in the simulated experiment with the shear force of hydrodynamic force on the sediment-water interface under natural conditions, and predict the sediment erosion characteristics under natural conditions.

[0010] Furthermore, the specifications of the columnar sampler are: outer diameter 200mm, inner diameter 190mm, height 500mm.

[0011] Furthermore, the sediment depth is not less than 300 mm.

[0012] Furthermore, the bottom shear force calculation formula on the sediment-water interface in the simulation experiment is:

[0013]

[0014] Where: C1 is the proportional constant, which is 0.19; ρ is the liquid density; u'(t), v'(t), and w'(t) are the pulsating flow velocities in the x, y, and z directions, respectively.

[0015] Furthermore, the hydrodynamic parameters under the real state include flow velocity, wave height and period.

[0016] Furthermore, the shear force calculation formula of the hydrodynamic force acting on the sediment-water interface under the natural state is:

[0017] τ cw =τ c +τ w

[0018] In the formula, τ c is the shear force at the bottom of the water flow, τ w is the shear stress caused by the waves.

[0019] Furthermore, the water flow bottom shear force calculation formula is:

[0020]

[0021] In the formula, is the friction velocity (m / s); k is the Kaman coefficient, u z is the flow velocity at the near-bed height z (m / s), and z0 is the absolute roughness.

[0022] Furthermore, the shear stress caused by the wave is calculated as follows:

[0023]

[0024] Where ρ is the density of water, kg / m 3 ;f w is the wave friction coefficient, and its value is related to the bottom roughness and the Reynolds number; u m It is the maximum horizontal velocity of the water particle near the bottom, m / s.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] The present invention combines the sampler with the annular water tank, thereby reducing the disturbance to the sediment-water interface during sampling and experiment, ensuring the integrity of the surface sediment structure and improving the accuracy of the simulation experiment.

[0027] The present invention can gradually raise the height of the column core through a lifting device, so that the sediment-water interface always coincides with the bottom of the water tank, thereby realizing long-term continuous sediment erosion experimental research.

[0028] The small and medium-sized annular water tank of the present invention has low cost, small occupation of space and simple operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings generally illustrate various embodiments by way of example and not limitation, and together with the description and claims, serve to illustrate the embodiments of the invention. Where appropriate, the same reference numerals are used throughout the drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the present apparatus or method.

[0030] Figure 1 A schematic diagram of a columnar sampler of the present invention is shown;

[0031] Figure 2 A schematic diagram of a small annular water tank according to the present invention is shown. DETAILED DESCRIPTION

[0032] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0033] The embodiment of the present invention provides an experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions:

[0034] Sample collection

[0035] In situ sediment core samples were collected with a columnar sampler (outer diameter 200 mm, inner diameter 190 mm, height 500 mm), where the sediment depth was not less than 300 mm. The core was sealed with a rubber stopper and a sealing film and then transported to the laboratory. During transportation, the core kept the sediment-water interface in an undisturbed state. See the schematic diagram of the columnar sampler for details. Figure 1 .

[0036] Small annular flume device for undisturbed sediment erosion experiments

[0037] In the past, sediment samples collected by mud buckets were quite different from the natural sediment-water interface. The sampling process also affected the sediment structure, so it was difficult to accurately reflect the erosion characteristics of the sediment. Therefore, it is necessary to develop a small annular water tank that can be used for undisturbed sediment erosion experiments. Figure 2 .

[0038] Experimental methods

[0039] The present invention combines the original sediment sampling column core with the annular water tank, so that the integrity and original state of the sediment-water interface are guaranteed to the greatest extent during the test, thereby improving the accuracy of the erosion parameter test. The annular water tank approximately simulates an infinitely long straight water tank to study the key characteristic parameters of the sediment erosion process (sediment erosion flux, critical starting flow rate, critical starting shear stress, etc.). The flow of water in the annular water tank generates shear force at the bottom of the water tank, which then acts on the sediment-water interface of the sediment column core, thereby simulating the erosion process of the sediment-water interface by water flow under natural conditions. During the test, an optical sensor is used to measure the turbidity and concentration of the sediment after resuspension, and the sediment erosion situation is judged based on this. Finally, by measuring the shear force values ​​at the bottom of the water tank at different flow rates, the corresponding erosion characteristic parameters under different shear forces are quantitatively analyzed, providing a scientific basis for sediment erosion research.

[0040] Function of components:

[0041] The motor, gearbox and transmission shaft form a transmission system that provides power for the annular water tank.

[0042] ADV current meter: measures the velocity of water near the bottom and uses this as a basis to calculate the bottom shear force using the TKE turbulent kinetic energy method.

[0043] OBS function: An optical back scattering turbidimeter (OBS) is installed on the water tank cylinder to monitor the changes in turbidity in the water body and determine the erosion of sediments.

[0044] Sediment core and lifting device: The sediment core is directly installed in the annular water tank, and the height of the core is adjusted to make the sediment-water interface parallel to the bottom of the water tank, so as to simulate the sediment erosion process in the natural environment.

[0045] Shear ring: Makes the shear force distribution at the bottom of the annular water tank more uniform.

[0046] Working steps:

[0047] (1) Fix the sediment column core to the bottom of the water tank and adjust the height. Install a top support lifting device under the sample column and fix it on the supporting device. By controlling the height of the lifting device, the sediment-water interface of the column core coincides with the bottom of the water tank. By ensuring the original state of the sediment-water interface, the accuracy of the simulation experiment is improved. And during the experiment, the height control device can be used to ensure that the sediment-water interface always coincides with the bottom of the water tank.

[0048] (2) Set the annular water tank speed to conduct simulation experiments. By setting different water tank flow rates, erosion forces of different intensities can be simulated.

[0049] (3) An optical backscattering turbidimeter (OBS) was used to monitor changes in water turbidity (sediment concentration) near the sediment-water interface, which was subsequently used to analyze sediment erosion characteristics.

[0050] (4) The hydrodynamic characteristic parameters are monitored by an acoustic Doppler velocimeter (ADV), which are subsequently used to analyze the bottom shear force.

[0051] Erosion dynamics analysis:

[0052] The bottom shear force is used as an indicator to represent the erosion dynamics. The profile distribution of the near-bottom flow velocity is monitored by a three-dimensional flow meter ADV, and the pulsating flow velocity is used to calculate the turbulent kinetic energy of the water flow at the measuring point (i.e., the TKE method). The bottom shear force calculation formula is:

[0053]

[0054] Where:

[0055] C1 is the proportionality constant, which is taken as 0.19 based on previous research results;

[0056] ρ is the liquid density;

[0057] u'(t), v'(t), and w'(t) are the pulsating flow velocities in the x, y, and z directions, respectively.

[0058] (4) Erosion characteristics under different shear forces

[0059] Erosion characteristics usually include critical shear stress and erosion rate.

[0060] The method for determining the critical shear stress is as follows: when the shear stress at the bottom of the water body is less than the critical scour shear stress, the bed surface will not be scoured; when the shear stress at the bottom of the water body is greater than the critical scour shear stress, the bed surface will be scoured; therefore, in the process of gradual increase in hydrodynamic intensity, the erosion force corresponding to the initial erosion of the sediment is the critical shear stress.

[0061] The calculation formula for the sediment erosion rate under various erosion forces is as follows:

[0062]

[0063] E is the erosion rate, ρ is the average sediment concentration in the water body, V is the total volume of the water body, s is the sediment bed area, and t is the erosion time.

[0064] Calculation of wave and current hydrodynamics and evaluation of erosion characteristics under natural conditions

[0065] The above is an indoor experiment on the study of sediment erosion characteristics. It has not yet been linked to the erosion dynamics in the natural state, so it cannot be directly used to evaluate the erosion characteristics of sediments in the real state. So the next step is to monitor and collect hydrodynamic parameters in the real state, such as flow rate, wave height, period, etc., and use the bottom shear force on the sediment-water interface as a key indicator to analyze the erosion dynamics. At the same time, the indoor experiment is linked to the erosion characteristics in the natural state through the bottom shear force, which is convenient for direct comparison.

[0066] The hydrodynamic characteristics of estuarine and coastal areas are complex. The main sources of hydrodynamics include rivers, waves and tides. The following is the calculation formula for the bottom shear force caused by various hydrodynamics at the sediment-water interface.

[0067] Calculation of bottom shear force of water flow

[0068] The bottom shear force of water flow refers to the shear drag force of the bottom water flow on the bed surface, which is calculated using the following formula:

[0069]

[0070]

[0071] Where:

[0072] is the friction velocity (m / s);

[0073] k is the Kaman coefficient.

[0074] u z is the flow velocity at the near-bed height z (m / s).

[0075] z0 is the absolute roughness.

[0076] Wave bottom shear force calculation

[0077] The shear stress caused by waves on the water-sediment interface is calculated as follows:

[0078]

[0079] Where ρ is the density of water, kg / m 3 ;f w is the wave friction coefficient, and its value is related to the bottom roughness and the Reynolds number; u m is the maximum horizontal velocity of the water particle near the bottom, m / s;

[0080] Under turbulent conditions, f w The calculation formula is as follows:

[0081]

[0082] In the formula, Am is the maximum motion amplitude of the bottom water particle, m; k s It is the bottom roughness, generally 0.2mm is taken as the bottom roughness of the lake bed.

[0083] According to the micro-wave theory, the maximum motion amplitude of the water particle near the bottom is A m 、The maximum horizontal velocity of the water particle near the bottom u m and effective wavelength L s The calculation formula is as follows:

[0084]

[0085]

[0086] Among them, H s is the significant wave height, m; h is the water depth, m; T s is the effective wave period, s.

[0087] Prediction of erosion characteristics under natural conditions

[0088] The shear force τ acting on the sediment-water interface under natural conditions cw Calculate as follows:

[0089] τ cw =τ c +τ w

[0090] When the bed surface shear force is greater than the critical starting shear force of the bed surface sediment, the sediment starts to move. For fine-grained sediment, the sediment can be suspended after starting, and the bed surface is scoured; through τ cw The values ​​are compared with the experimental values ​​τ0 and the corresponding erosion characteristics, and the interpolation method is used to predict the erosion characteristics under natural conditions.

[0091] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. An experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions, characterized in that: include: Sediment samples were collected in situ by column samplers; Conducting an annular flume simulation experiment based on the sediment sample to obtain the bottom shear force on the sediment-water interface in the simulation experiment; Monitor and collect hydrodynamic parameters under real conditions, calculate the shear force of hydrodynamic force on the sediment-water interface under natural conditions, compare the bottom shear force on the sediment-water interface in the simulated experiment with the shear force of hydrodynamic force on the sediment-water interface under natural conditions, and predict the sediment erosion characteristics under natural conditions.

2. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 1, characterized in that: The specifications of the columnar sampler are: outer diameter 200mm, inner diameter 190mm, height 500mm.

3. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 1, characterized in that: The sediment depth is not less than 300 mm.

4. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 1, characterized in that: The calculation formula for the bottom shear force on the sediment-water interface in the simulation experiment is: Where: C1 is the proportional constant, which is 0.19; ρ is the liquid density; u'(t), v'(t), and w'(t) are the pulsating flow velocities in the x, y, and z directions, respectively.

5. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 1, characterized in that: The hydrodynamic parameters under the actual state include flow velocity, wave height and period.

6. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 1, characterized in that: The shear force calculation formula of the hydrodynamic force acting on the sediment-water interface under the natural state is: t cw =t c +t w In the formula, τ c is the shear force at the bottom of the water flow, τ w is the shear stress caused by the waves.

7. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 6, characterized in that: The calculation formula of the water flow bottom shear force is: In the formula, is the friction velocity (m / s); k is the Kaman coefficient, u z is the flow velocity at the near-bed height z (m / s), and z0 is the absolute roughness.

8. The experimental method for studying sediment erosion characteristics under complex hydrodynamic conditions according to claim 6, characterized in that: The shear stress caused by the waves is calculated as follows: Where ρ is the density of water, kg / m 3 ;f w is the wave friction coefficient, and its value is related to the bottom roughness and the Reynolds number; u m It is the maximum horizontal velocity of the water particle near the bottom, m / s.

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