Bank collapse prediction method based on quantitative development degree of undercut groove

Abstract

The invention discloses a bank collapse prediction method based on quantitative undercut groove development degree, and relates to the technical field of river evolution and geological disaster prevention and control engineering, and the method comprises the steps: drawing a river channel two-dimensional profile map and a river water level height; calculating the river scouring strength according to the water body weight, the river gradient and the river water level height; according to the soil body weight, the water body weight and the soil body median particle size, the anti-scouring strength of the bank slope is calculated; when the river scouring strength is smaller than the bank slope anti-scouring strength, it is judged that the undercut groove is not formed, and otherwise, the following steps are carried out; obtaining a river channel scouring time-shear strength curve according to actual measurement data for many years or indoor tests; according to a river channel scouring time-shear strength curve, calculating the boundary of the undercut groove at the current moment; and changing the shape of the bank slope according to the undercut groove boundary at the current moment, and importing the undercut groove boundary at the current moment into numerical software to calculate a safety coefficient. The method solves the problem that the existing bank collapse calculation method has limitation and does not consider the influence of the undercut groove on the bank collapse scale.

Description

Technical Field

[0001] The invention relates to the technical field of river evolution and geological disaster prevention and control engineering, and in particular to a bank collapse prediction method based on quantitative erosion groove development degree. Background Art

[0002] The partial or even complete collapse of the river bank slope due to the scouring of water flow is usually called bank collapse. Bank collapse is a natural evolution process of the river.

[0003] At present, the theoretical derivation of bank collapse calculation method mainly includes the following two aspects:

[0004] 1. Based on soil mechanics, the method is derived from the perspective of soil slope stability. Similar to the calculation method of slope stability in the field of engineering geology, the slope stability is calculated by the ratio of the anti-sliding force at the toe of the slope to the sliding force. The stability of the slope is determined by the size of the safety factor Fs. This method has certain limitations, such as the single sliding surface shape and the need to pass through the toe of the slope.

[0005] 2. Based on river dynamics, the theory is derived from the perspective of sediment initiation. This theory believes that water flow is the main factor leading to bank collapse, and proposes a series of criteria for judgment: starting shear stress, starting power, starting probability, etc. The measured, experimental data or formula calculation results are compared with the starting basis to judge whether the bank slope is eroded or not. However, this method mainly focuses on the direct widening of the river, and lacks a scientific and unified theoretical solution for the calculation of indirect widening such as bank collapse.

[0006] In addition, the above two commonly used methods do not consider the impact of erosion grooves on the scale of bank collapse. For cohesive soil slopes, it is very easy to form erosion grooves near the river water level during river erosion, thereby reducing the potential shear outlet of bank collapse (potential sliding surface 1 → potential sliding surface 2), resulting in a larger amount of bank collapse, such as Figure 1 shown. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, the present invention provides a bank collapse prediction method based on quantifying the degree of development of erosion grooves, which solves the problem that the existing bank collapse prediction methods have limitations and do not consider the impact of erosion grooves on the scale of bank collapse.

[0008] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: a bank collapse prediction method based on quantifying the degree of development of erosion grooves, comprising the following steps:

[0009] S1: Determine the river bank slope, bank material composition and bank strength parameters based on the measured river section data, flow data and flow velocity data, and draw the two-dimensional cross-section of the river and the river water level;

[0010] S2: Based on the two-dimensional profile of the river channel, the river scour intensity is calculated according to the water mass, river gradient and river water level;

[0011] S3: Calculate the anti-scour strength of the slope based on the soil weight, water weight and soil median particle size;

[0012] S4: when the river scouring intensity is less than the bank slope anti-scouring intensity, it is determined that the erosion groove will not be formed, and the process returns to step S1, otherwise it goes to step S5;

[0013] S5: Obtain a river scouring time-shear strength curve based on years of measured data or indoor tests, and use the slope of the river scouring time-shear strength curve as the soil softening coefficient;

[0014] S6: Calculating the boundary of the erosion groove at the current moment according to the river scouring time-shear strength curve;

[0015] S7: changing the bank slope shape according to the erosion groove boundary at the current moment, and importing the erosion groove boundary at the current moment into the numerical software to calculate the safety factor. When the safety factor is less than 1, it is determined that the bank collapse occurs, otherwise returning to step S6 to calculate the erosion groove boundary at the next moment.

[0016] Furthermore, in S1, the two-dimensional cross-section of the river channel is drawn according to the two-dimensional cross-section above the water surface of the bank slope and the river bottom cross-section;

[0017] The two-dimensional profile of the bank slope above the water surface is obtained by collecting topographic data of the river section;

[0018] The riverbed profile is obtained by calculating the riverbed width and the river water level height, and the formula is:

[0019]

[0020] Among them, S is the cross-sectional area of ​​the water flow, V is the flow rate, and U is c is the flow velocity, h is the river water level, B T is the width of the water surface, B B is the river bottom width, α L is the left bank slope, α R It is the right bank slope.

[0021] Furthermore, the river scouring intensity in S2 is represented by water flow shear stress τ, and the formula is:

[0022] τ=γ w h

[0023] Among them, γ w is the water mass density and J is the river gradient.

[0024] Furthermore, the anti-scour strength of the bank slope in S3 is calculated by the starting shear stress τ of the bank slope soil. c Said, the formula is:

[0025]

[0026] Among them, γ s is the soil mass, d 50 is the median particle size of the soil, k b is the comprehensive coefficient.

[0027] Furthermore, the river scouring time-shear strength curve in S5 is expressed as:

[0028] τ f (t 1 ) = kt 1 +τ f (0)

[0029] Among them, τ f (t 1 ) is the soil at t 1 The shear strength corresponding to time, k is the soil softening coefficient, τ f (0) is the initial shear strength of the soil.

[0030] Furthermore, the S6 includes the following sub-steps:

[0031] S61: Based on the measured profile, calculate the overlying water pressure and overlying soil pressure on the particles on the slope surface, and obtain the slope failure time at t i Shear strength at time τ f (t i ), the formula is:

[0032] τ f (t i ) = kt i +τ f (0)

[0033]

[0034] Among them, P w is the overlying water pressure on the slope particles, P s is the overlying soil pressure on the slope particles, τ z is the shear stress of water flow decomposed in the vertical direction, is the internal friction angle, C is the cohesion;

[0035] S62: Break the bank slope at t i Shear strength at time τ f (t i ) is subtracted from the scouring force of the water flow, and the point where the difference is 0 is the boundary of the erosion groove at the current moment.

[0036] Furthermore, the boundary of the etched groove at the current moment in S62 is:

[0037]

[0038] Among them, h i and h j is the boundary of the erosion groove, τ y is the water shear stress decomposed in the plane direction.

[0039] The beneficial effects of the present invention are:

[0040] (1) The present invention combines field exploration, indoor tests and numerical simulations. The calculation process takes into account the influence of various factors on the degree of bank collapse, such as water flow scouring force, bank slope morphology, and bank slope strength. The calculation formula is relatively simple.

[0041] (2) The present invention can calculate the bank slope erosion position and the development scale of the erosion groove at any flow rate and any time, so that the bank collapse range obtained thereby is more scientific and accurate.

[0042] (3) The present invention i When i is infinite, the calculation result is most accurate in theory. This process can be realized through programming, and numerical calculation software can be written based on it. It has wide application and good development prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of how the formation of an erosion groove will reduce the potential shear outlet of a landslide.

[0044] Figure 2 The present invention is a flow chart of the bank collapse prediction method based on quantifying the degree of development of erosion grooves.

[0045] Figure 3 This is the calculation diagram of riverbed profile parameters.

[0046] Figure 4 This is the shear strength attenuation curve of the slope soil.

[0047] Figure 5 This is a diagram of the stress state of any point underwater on the bank slope of a straight river.

[0048] Figure 6 This is the safety factor diagram after the Ganhaizi landslide was eroded for 1 hour to form an erosion groove. DETAILED DESCRIPTION

[0049] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0050] like Figure 2 As shown, a bank collapse prediction method based on quantifying the degree of erosion groove development includes the following steps:

[0051] S1: Determine the river bank slope, bank material composition and bank strength parameters based on the measured river section data, flow data and flow velocity data, and draw the two-dimensional cross-section of the river and the river water level;

[0052] S2: Based on the two-dimensional profile of the river channel, the river scour intensity is calculated according to the water mass, river gradient and river water level;

[0053] S3: Calculate the anti-scour strength of the slope based on the soil weight, water weight and soil median particle size;

[0054] S4: when the river scouring intensity is less than the bank slope anti-scouring intensity, it is determined that the erosion groove will not be formed, and the process returns to step S1, otherwise it goes to step S5;

[0055] S5: Obtain a river scouring time-shear strength curve based on years of measured data or indoor tests, and use the slope of the river scouring time-shear strength curve as the soil softening coefficient;

[0056] S6: Calculating the boundary of the erosion groove at the current moment according to the river scouring time-shear strength curve;

[0057] S7: changing the bank slope shape according to the erosion groove boundary at the current moment, and importing the erosion groove boundary at the current moment into the numerical software to calculate the safety factor. When the safety factor is less than 1, it is determined that the bank collapse occurs, otherwise returning to step S6 to calculate the erosion groove boundary at the next moment.

[0058] In S1, the two-dimensional cross-section of the river channel is drawn according to the two-dimensional cross-section above the water surface of the bank slope and the river bottom cross-section;

[0059] The two-dimensional profile of the bank slope above the water surface is obtained by collecting topographic data of the river section;

[0060] Since the riverbed profile is difficult to measure, it can be calculated based on measured data: First, use the flow rate V and flow velocity U c Calculate the cross-sectional area S; most impact river channels are trapezoidal in shape, wide at the top and narrow at the bottom. Based on the geometric relationship, the left bank slope α can be used to calculate the cross-sectional area S. L , right bank slope α R 、Water surface width B T , cross-sectional area S, and the following formula are combined to obtain the river bottom width B B and river water level h, such as Figure 3 As shown;

[0061] The riverbed profile is obtained by calculating the riverbed width and the river water level height, and the formula is:

[0062]

[0063] Among them, S is the cross-sectional area of ​​the water flow, V is the flow rate, and U is c is the flow velocity, h is the river water level, B T is the width of the water surface, B B is the river bottom width, α L is the left bank slope, α R It is the right bank slope.

[0064] At this point, by combining the profile above the water surface of the bank slope and the riverbed profile, the two-dimensional profile of the river channel used for calculation can be drawn.

[0065] The river scouring intensity in S2 is expressed by water flow shear stress τ, and the formula is:

[0066] τ=γ w h

[0067] Among them, γ w is the water mass density and J is the river gradient.

[0068] The anti-scour strength of the bank slope in S3 is based on the starting shear stress τ of the bank slope soil. c Said, the formula is:

[0069]

[0070] Among them, γ s is the soil mass, d 50 is the median particle size of the soil, k b is the comprehensive coefficient, with a value of 2.84×10 -4 N / m.

[0071] The river scouring time-shear strength curve in S5 is expressed as:

[0072] τ f (t 1 ) = kt 1 +τ f (0)

[0073] Among them, τ f (t 1 ) is the soil at t 1 The shear strength corresponding to time, k is the soil softening coefficient, τ f (0) is the initial shear strength of the soil.

[0074] The idea of ​​obtaining the shear strength-scour time curve is to record the time from the rise of the water level during the flood period to the destruction of the bank slope in the measured data, or the time from the discharge of water to the destruction of the bank slope in the indoor test. It is assumed that during the scour process, the shear strength of the bank slope soil decreases linearly. The slope of the scour starting point and the destruction point is connected on the shear strength-scour time coordinate system, and the softening coefficient k is obtained, such as Figure 4 shown.

[0075] The S6 comprises the following sub-steps:

[0076] S61: Based on the measured profile, calculate the overlying water pressure and overlying soil pressure on the slope surface particles. This term is the positive pressure in the Mohr-Coulomb strength formula. The slope failure time at t i Shear strength at time τ f (t i ), the formula is:

[0077] τ f (t i ) = kt i +τ f (0)

[0078] The stress on any single point particle A on the bank slope surface under the water level of the straight river is as follows: Figure 5 As shown, it is subjected to vertical downward water pressure P w At the same time, due to the existence of river gradient, the shear stress of water flow can be decomposed into τ in the plane direction y and τ in the vertical direction z When the particles on the slope surface are carried away, the erosion groove develops inward, and the particles are subjected to the overlying soil pressure P s The vertical force on the particles constitutes the normal stress in the Mohr-Coulomb strength formula, so the shear strength at any point at t = 0 is:

[0079]

[0080] Among them, P w is the overlying water pressure on the slope particles, P s is the overlying soil pressure on the slope particles, τ z is the shear stress of water flow decomposed in the vertical direction, is the internal friction angle, C is the cohesion;

[0081] Substitute the shear strength calculation result at t = 0 into τ f (t i ), the bank slope breaks at t i Shear strength at the moment;

[0082] S62: Break the bank slope at t i Shear strength at time τ f (t i ) is subtracted from the scouring force of the water flow, and the point where the difference is 0 is the boundary of the erosion groove at the current moment.

[0083] The boundary of the erosion groove at the current moment in S62 is:

[0084] Due to the water pressure P on the slope particles w and overlying soil pressure P s is inconsistent, resulting in different shear strengths, which are expressed as follows: from the water level downward, the water pressure P w Gradually increases, the P of the water level plane w =0; from the water level line to the slope, the overlying soil pressure P s Gradually increasing, the P of the slope surface particles and the bottom of the erosion groove s is 0. Then the calculation formula for the development limit of the erosion groove can be obtained:

[0085]

[0086] Among them, h i and h j is the boundary of the erosion groove, τ y is the water shear stress decomposed in the plane direction.

[0087] According to the above calculation results of the erosion groove boundary, the slope shape is changed and imported into the numerical software to calculate the safety factor (here Geo-studio is used as an example, Figure 6 As shown in the figure, it is considered that all surfaces with a safety factor Fs < 1 have slipped, and the scale of bank collapse is obtained; otherwise, return to step S6 and calculate t i+1 The time corresponds to the parameter.

[0088] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the invention.

Claims

1. A bank collapse prediction method based on quantifying the degree of erosion groove development, characterized in that: The following steps are involved: S1: Determine the river bank slope, bank material composition and bank strength parameters based on the measured river section data, flow data and flow velocity data, and draw the two-dimensional cross-section of the river and the river water level; S2: Based on the two-dimensional profile of the river channel, the river scour intensity is calculated according to the water mass, river gradient and river water level; S3: Calculate the anti-scour strength of the slope based on the soil weight, water weight and soil median particle size; S4: when the river scouring intensity is less than the bank slope anti-scouring intensity, it is determined that the erosion groove will not be formed, and the process returns to step S1, otherwise it goes to step S5; S5: Obtain a river scouring time-shear strength curve based on years of measured data or indoor tests, and use the slope of the river scouring time-shear strength curve as the soil softening coefficient; S6: Calculating the boundary of the erosion groove at the current moment according to the river scouring time-shear strength curve; S7: changing the bank slope shape according to the erosion groove boundary at the current moment, and importing the erosion groove boundary at the current moment into the numerical software to calculate the safety factor. When the safety factor is less than 1, it is determined that the bank collapse occurs, otherwise returning to step S6 to calculate the erosion groove boundary at the next moment.

2. A bank collapse prediction method based on quantifying the degree of development of erosion grooves according to claim 1, characterized in that: In S1, the two-dimensional cross-section of the river channel is drawn according to the two-dimensional cross-section above the water surface of the bank slope and the river bottom cross-section; The two-dimensional profile of the bank slope above the water surface is obtained by collecting topographic data of the river section; The riverbed profile is obtained by calculating the riverbed width and the river water level height, and the formula is: Among them, S is the cross-sectional area of ​​the water flow, V is the flow rate, and U is c is the flow velocity, h is the river water level, B T is the width of the water surface, B B is the river bottom width, α L is the left bank slope, α R It is the right bank slope.

3. A bank collapse prediction method based on quantifying the degree of development of erosion grooves according to claim 2, characterized in that: The river scouring intensity in S2 is expressed by water flow shear stress τ, and the formula is: τ=γ w hJ Among them, γ w is the water mass density and J is the river gradient.

4. A bank collapse prediction method based on quantifying the degree of development of erosion grooves according to claim 3, characterized in that: The anti-scour strength of the bank slope in S3 is based on the starting shear stress τ of the bank slope soil. c Said, the formula is: Among them, γ s is the soil mass, d 50 is the median particle size of the soil, k b is the comprehensive coefficient.

5. A bank collapse prediction method based on quantifying the degree of development of erosion grooves according to claim 4, characterized in that: The river scouring time-shear strength curve in S5 is expressed as: t f (t1)=kt1+τ f (0) Among them, τ f (t1) is the shear strength of the soil at time t1, k is the softening coefficient of the soil, τ f (0) is the initial shear strength of soil.

6. A bank collapse prediction method based on quantifying the degree of development of erosion grooves according to claim 5, characterized in that: The S6 comprises the following sub-steps: S61: Based on the measured profile, calculate the overlying water pressure and overlying soil pressure on the particles on the slope surface, and obtain the slope failure time at t i Shear strength at time τ f (t i ), the formula is: t f (t i )=kt i +t f (0) Among them, P w is the overlying water pressure on the slope particles, P s is the overlying soil pressure on the slope particles, τ z is the shear stress of water flow decomposed in the vertical direction, is the internal friction angle, C is the cohesion; S62: Break the bank slope at t i Shear strength at time τ f (t i ) is subtracted from the scouring force of the water flow, and the point where the difference is 0 is the boundary of the erosion groove at the current moment.

7. A bank collapse prediction method based on quantifying the degree of development of erosion grooves according to claim 6, characterized in that: The boundary of the erosion groove at the current moment in S62 is: Among them, h i and h j is the boundary of the erosion groove, τ y is the water shear stress decomposed in the plane direction.

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