Method suitable for evaluating transportation rate of soil body under wave and wave flow combined action condition

Abstract

The invention belongs to the field of seabed sediment scouring research, and discloses a method suitable for evaluating the transportation rate of a soil body under the condition of the combined action of waves and wave flows. Local scour of a seabed structure is closely related to the transport rate and critical shear stress of a soil body. At present, forecast about the transport rate and critical shear stress of a soil body is established on the basis of loose and uniform sand, and large errors can be given to relevant forecast about the soil body with the viscous effect. According to the method, a parameter description method for the transport rate and the critical starting stress of any type of soil under the complex wave and wave flow combined condition is established on the basis of a physical experiment of the apparent scouring rate of the soil, and the defect that an existing empirical formula can only be used for loose and uniform sand is overcome. The method provided by the invention can accurately forecast the transport rate and the critical shear stress of any type of soil under the wave and wave flow combined condition, and can provide scientific basis and technical guarantee for local scour evaluation and safety design of a seabed structure.

Description

Technical Field

[0001] The invention belongs to the research field of seabed sediment scouring, and relates to a method suitable for evaluating soil transport rate under the conditions of wave and wave-current combined action. Background Art

[0002] The route of a submarine cable is generally long, such as tens of kilometers to hundreds of kilometers, and its route area may experience different seabed sediment conditions, such as silt, fine sand, medium sand and medium-coarse sand, silt, silty silty clay, and silty silt. These soil types have significantly different critical starting conditions and transport modes (single suspended load, single bedload, and suspended and bedload), which makes the local scouring characteristics around the submarine cable also vary greatly. In addition, in the depth direction of the seabed, the seabed soil often has certain stratification characteristics, which brings huge technical challenges to the engineering prediction of local scouring of submarine cables and the dynamic evolution of sand waves and sand ridges. For sediments with insignificant cohesive effects between particles, such as silt, fine sand, and medium sand, the critical starting stress can be analyzed by the empirical formula established by Soulsby (Dynamics of marine sands: a manual for practical applications, 1997, Thomas Telford), and has good accuracy; however, for the critical starting stress of soils with obvious cohesive effects, such as silt, silty silty clay, and silty silt, the empirical formula established by Soulsby (1997) often gives an underestimation, resulting in a large error between the predicted results and the actual results. At present, there is no mature theoretical formula for predicting the critical starting stress of cohesive soils.

[0003] The local scour of any type of submarine structure is closely related to the critical starting stress and transport rate of the soil. At present, for cohesive loose sand, relevant scholars have established a relatively complete method to evaluate its transport rate and critical starting stress, but there are still major deficiencies in the relevant research work on the soil with significant viscosity effect commonly seen in engineering. Mohr et al. (2016) (Mohr H., Draper S., Cheng L., White DJ Predicting the rate of scour beneath subsea pipelines in marine sediments under steady flow conditions, Coastal Engineering, 2016, 110: 111-126.) took the lead in developing a physical experimental method for the apparent scour rate of soil in the world. The experiment only considers unidirectional flow conditions. In the experiment, by applying different unidirectional flow velocities to the soil samples, the expansion rate of the soil in the depth direction was measured, and finally the functional relationship between the soil transport rate η and the seabed shear stress τ caused by the flow was established, that is, η=M(τ-τ cr ) n , where M and n are constant coefficients, τ cr represents the critical shear stress of the soil. These parameters can be obtained by fitting the experimental data with the least square method. For local scour of submarine structures, the physical meaning of this formula is that when the shear stress of the seabed around the structure is greater than the critical shear stress of the soil, local scour will occur. Therefore, after obtaining the soil transport rate, the development process of local scour and the equilibrium profile of any structural form can be accurately predicted in theory.

[0004] The research work of Mohr et al. (2016) was conducted for unidirectional flow. Under unidirectional flow conditions, the seabed shear stress caused by it is independent of time. However, for more complex wave and wave-current combined conditions, the seabed shear gravity caused by it is closely related to time. Therefore, the soil transport rate formula under unidirectional flow conditions established by Mohr et al. (2016) will not be applicable.

[0005] Therefore, the present invention will develop an evaluation method for soil transport rate under the combined action of waves and wave-currents. This method further enriches and develops the theory of sediment movement mechanics, and also makes it theoretically possible to accurately predict the local scouring characteristics of seabed structures under more complex flow conditions. Summary of the invention

[0006] In order to solve the above-mentioned problems existing in the prior art and provide an accurate evaluation method for the transport rate of clay under complex flow conditions, the purpose of the present invention is to provide a method suitable for evaluating the soil transport rate under the conditions of wave and wave-current combined action, so as to overcome the problem that the existing research work can only consider the soil transport rate under unidirectional flow conditions, but cannot consider the transport rate of clay under more complex wave and wave-current combined action, thereby providing a scientific basis and technical guarantee for the local scour evaluation and safety design of submarine structures.

[0007] The technical solution of the present invention:

[0008] A method for evaluating soil transport efficiency under wave and wave-current combined conditions includes the following steps:

[0009] (1) Determination of critical shear stress of soil

[0010] For underwater structures, they face a variety of seabed soil types. For loose cohesionless sand, its critical shear stress can be predicted by the empirical formula established by Soulsby (1997). However, for soil types with significant cohesive effects, the empirical formula of Soulsby (1997) will give an underestimation, resulting in a large error in the assessment of local scour of the structure. For cohesive soil, a reliable theoretical and empirical prediction formula has not yet been established, and it is impossible to quickly predict the critical shear stress of cohesive soil.

[0011] Theoretically, the critical shear stress of soil is an inherent property and has nothing to do with flow conditions. Therefore, physical experiments on the apparent scour rate of soil under unidirectional flow conditions were carried out to establish the following quantitative relationship between soil transport rate and seabed shear stress and critical shear stress:

[0012] η=M(τ-τ cr ) n (1)

[0013] Among them, η represents the transport rate of the soil, and its physical meaning is the scouring depth of the soil per unit time; τ represents the shear stress of the seabed caused by the flow. For unidirectional flow, it can be predicted by the logarithmic rate formula. M and n represent constant coefficients, reflecting the transport capacity of the soil, τ cr The critical shear stress of the soil can be obtained by fitting the experimental data with the least square method;

[0014] (2) Determination of seabed shear stress caused by different flow conditions

[0015] From formula (1), we can see that the core of the soil transport rate formula is to determine the seabed shear stress caused by different flow conditions. For unidirectional flow, the distribution of horizontal flow velocity along the water depth satisfies the following logarithmic rate formula:

[0016]

[0017] Where u(z) represents the horizontal velocity at depth z; u* represents the bottom friction velocity; κ = 0.4, represents the Karman constant coefficient; z s It represents the seabed roughness length. For sandy seabed, its value is d 50 / 12,d 50 represents the median particle size of sediment;

[0018] It can be seen from formula (2) that for unidirectional flow, the seabed shear stress τ is independent of time. When the horizontal flow velocity u(z) at space z is measured and the bottom friction velocity u* is calculated, the calculation formula for the seabed shear stress caused by this flow velocity is τ = ρu* 2 Where ρ represents the water density;

[0019] Different from unidirectional flow, under wave and current and wave and current combined conditions, the movement of water particles has obvious periodic characteristics, which leads to obvious periodic characteristics of the corresponding seabed shear stress. Therefore, it is impossible to directly use formula (1) to evaluate the sediment transport rate under wave and wave and current combined conditions. Regarding the calculation of sediment transport under wave conditions, the periodic average value of the absolute value of instantaneous seabed shear stress is usually used to represent the characteristic seabed shear stress τ w (Zou Zhili, Coastal Dynamics, 4th edition, 2009, People's Communications Press), and then used for the calculation and analysis of sediment transport. Therefore, the characteristic seabed shear stress τ w The accurate calculation of the seabed shear stress under wave conditions requires the cyclical variation characteristics of the seabed shear stress under wave conditions to be clarified first; the calculation of the temporal and spatial distribution of the seabed shear stress under wave action is carried out using the following formula:

[0020]

[0021] Among them, ω represents the wave circular frequency, U represents the amplitude of the velocity of the wave water particles outside the boundary layer, ρ represents the density of the water body, and i is a unit imaginary number. represents the seabed shear stress; 1 , 2 、p 1 and p 2 is the characteristic parameter, and its calculation formula is:

[0022]

[0023] lg(λ 2 / k s )=0.97lg(a / k s )-1.97 (5)

[0024] p 1=46.17×[lg(a / k s ·Re w )] -2.22 +0.1 (6)

[0025] p 2 =-0.27×[0.33lg(a / k s )+lgRe w ] 0.77 +1.82 (7)

[0026] Among them, k s represents the seabed roughness height, for sandy seabed k s =2.5d 50 , a represents the displacement amplitude of wave water particles outside the boundary layer, Re w =aU / ν represents the wave Reynolds number, ν represents the kinematic viscosity coefficient of the fluid, and ω represents the circular frequency of the wave; through the above formula, the temporal and spatial distribution of the seabed shear stress under wave conditions is obtained, and then the characteristic seabed shear stress τ is obtained w ;

[0027] For the sediment transport problem under the combined action of waves and currents, the seabed shear stress τ wc The following formula is used for calculation (Zou Zhili, Coastal Dynamics, 4th edition, 2009, People's Communications Press):

[0028] τ wc =τ w +τ c (8)

[0029] Among them, τ c represents the seabed shear force caused by unidirectional flow; by obtaining the characteristic seabed shear stress τ under wave conditions w and the characteristic seabed shear stress τ under the combined action of waves and currents wc , and put it into formula (1), we can achieve accurate prediction of soil transport rate under the combined action of waves and wave-current.

[0030] Beneficial effects of the present invention: The method established by the present invention can be used to evaluate the transport rate and critical starting stress of cohesive soil under complex wave and wave-current combined conditions, thereby overcoming the major defect that the existing empirical formula is mainly for loose cohesive sand and cannot accurately predict the transport rate of soil with significant cohesive effect. The method established by the present invention can provide a scientific basis and technical guarantee for local scour assessment and safety design of submarine structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the physical experiment setup for apparent scour rate of soil;

[0032] Figure 2 is the quantitative relationship between the scouring rate of the soil and the shear stress of the seabed;

[0033] Figure 3 Comparison between the critical shear stress of loose sand measured experimentally and the empirical formula;

[0034] Figure 4 Comparison between the critical shear stress of cohesive soil measured experimentally and the empirical formula;

[0035] Figure 5 Comparison between the temporal and spatial distribution of seabed shear stress calculated by the formula and the experimental results of others.

[0036] In the figure: 1 physical experiment terrain; 2 slope; 3 soil sample box; 4 laser Doppler flowmeter; 5 three-dimensional laser terrain scanner. DETAILED DESCRIPTION

[0037] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0038] First, we conduct a physical experiment on the apparent scour rate under unidirectional flow conditions, and then establish a quantitative relationship between the soil transport rate and the seabed shear stress and critical shear stress. The relevant physical experiment settings are as follows: Figure 1 As shown. 1 represents the physical experimental terrain, which can be formed by pouring concrete. In order to ensure that the incoming flow can smoothly transition to the experimental section, a 1:10 slope 2 is arranged at both ends of the experimental terrain. 3 represents the soil sample box, which is used to arrange the test soil. In the experiment, the surface of the soil needs to be flat and flush with the surrounding terrain. The length of the soil sample box is 20 cm, and the depth and width are both 10 cm. A laser Doppler flowmeter 4 is arranged at a height of 5 cm above the soil sample box 3 to measure the incoming flow velocity, and then calculate the long shear stress caused by the incoming flow through formula (2). 5 represents a three-dimensional laser terrain scanner, which is used to measure the elevation change of the soil. Assuming that the scouring depth in time t is s, the scouring rate of the soil η = s / t.

[0039] Figure 2 is the result obtained through physical experiment of soil apparent scour rate. The black dots represent the results obtained through physical experiment measurement, and the black line represents the result obtained through least squares fitting. The relevant results can be expressed by formula (1), and then the critical seabed shear stress τ is obtained. cr Through physical experimental analysis, the formula for the apparent scour rate of soil is η = 1.697 × 10 -4 (τ-0.189) 1.263 , that is, the critical seabed shear stress is 0.189Pa.

[0040] Figure 3The figure shows the comparison between the critical shear stress of loose sand obtained by physical experiment and the empirical formula established by Soulsby (1997). The points are experimental measurement results, and the black line is the predicted result of Soulsby (1997) empirical formula. It can be seen from the figure that for loose sand, the results obtained by physical experiment measurement are consistent with the predicted results of the empirical formula, which verifies the reliability of the relevant physical experiment method.

[0041] Figure 4 The figure shows the comparison between the critical shear stress of cohesive soil obtained by physical experiment and the empirical formula established by Soulsby (1997). The points are experimental measurement results, and the black line is the prediction result of the empirical formula of Soulsby (1997). It can be seen from the figure that the critical starting stress of cohesive soil is significantly higher than the critical starting stress of loose sand under the same median particle size conditions. Therefore, using the empirical formula established by Soulsby (1997) to predict the critical starting stress of cohesive soil will give a lower estimate.

[0042] Figure 5 The temporal and spatial distribution of seabed shear stress predicted by formula (3) to (7) is compared with the physical experimental results of Jensen et al. (1989). As can be seen from the figure, the results predicted by the formula are well compared with the physical experimental results, which verifies the effectiveness of the method proposed in this invention. By using formula (3) to (7), the temporal and spatial distribution of seabed shear stress under wave conditions can be predicted, and then the characteristic seabed shear stress τ can be obtained. w , is substituted into formula (1) and combined with formula (8), the accurate prediction of soil transport rate under the combined action of waves and wave-current can be achieved.

Claims

1. A method for evaluating soil transport efficiency under wave and wave-current combined conditions, characterized in that: The following steps are involved: (1) Determination of critical shear stress of soil Physical experiments on the apparent scouring rate of soil under unidirectional flow conditions were carried out to establish the following quantitative relationship between soil transport rate and seabed shear stress and critical shear stress: η=M(τ-τ cr ) n (1) Among them, η represents the transport rate of soil, and its physical meaning is the scouring depth of soil per unit time; τ represents the seabed shear stress caused by flow; for unidirectional flow, it is predicted by the logarithmic rate formula; M and n represent constant coefficients, reflecting the transport capacity of soil; τ cr It represents the critical shear stress of soil and can be obtained by fitting the experimental data with the least square method; (2) Determination of seabed shear stress caused by different flow conditions From formula (1), we can see that the core of the soil transport rate formula is to determine the seabed shear stress caused by different flow conditions. For unidirectional flow, the distribution of horizontal flow velocity along the water depth satisfies the following logarithmic rate formula: Where u(z) represents the horizontal flow velocity at depth z; u * represents the bottom friction velocity; κ = 0.4, represents the Karman constant coefficient; z s It represents the seabed roughness length. For sandy seabed, its value is d 50 / 12,d 50 represents the median particle size of sediment; It can be seen from formula (2) that for unidirectional flow, the seabed shear stress τ is independent of time. When the horizontal flow velocity u(z) at space z is measured, the bottom friction velocity u is calculated: * , then the calculation formula for the seabed shear stress caused by the flow velocity is τ=ρu * 2 Where ρ represents the water density; Different from unidirectional flow, under wave-current and wave-current combined conditions, the movement of water particles has obvious periodic characteristics, which leads to obvious periodic characteristics of the corresponding seabed shear stress. Therefore, it is impossible to directly use formula (1) to evaluate the sediment transport rate under wave-current and wave-current combined conditions; the characteristic seabed shear stress τ w The accurate calculation of the seabed shear stress under wave conditions requires the cyclical variation characteristics of the seabed shear stress under wave conditions to be clarified first; the calculation of the temporal and spatial distribution of the seabed shear stress under wave action is carried out using the following formula: Among them, ω represents the wave circular frequency, U represents the amplitude of the velocity of the wave water particles outside the boundary layer, ρ represents the density of the water body, and i is a unit imaginary number. represents the seabed shear stress; λ1, λ2, p1 and p2 are characteristic parameters, and their calculation formula is: lg(λ2 / k s )=0.97lg(a / k s )-1.97 (5) p1=46.17×[lg(a / k s ·Re w )] -2.22 +0.1 (6) p2=-0.27×[0.33lg(a / k s )+lgRe w ] 0.77 +1.82 (7) Among them, k s represents the seabed roughness height, for sandy seabed k s =2.5d 50 , a represents the displacement amplitude of wave water particles outside the boundary layer, Re w =aU / ν represents the wave Reynolds number, ν represents the kinematic viscosity coefficient of the fluid, and ω represents the circular frequency of the wave; through the above formula, the temporal and spatial distribution of the seabed shear stress under wave conditions is obtained, and then the characteristic seabed shear stress τ is obtained w ; For the sediment transport problem under the combined action of waves and currents, the seabed shear stress τ wc The calculation is performed using the following formula: t wc =t w +t c (8) Among them, τ c represents the seabed shear force caused by unidirectional flow; by obtaining the characteristic seabed shear stress τ under wave conditions w and the characteristic seabed shear stress τ under the combined action of waves and currents wc , and put it into formula (1), we can achieve accurate prediction of soil transport rate under the combined action of waves and wave-current.

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