Method for calculating disaster forming threshold value of slope type debris flow

Abstract

The invention provides a slope type debris flow disaster forming threshold value calculation method for areas where rainfall monitoring is difficult to carry out and historical rainfall data is lacked, that is, for slope type debris flow, the slope stability in a debris flow basin is calculated through a TRIGRS model, and distribution of unstable slope units under different rainfall conditions is obtained. And then dynamic process simulation is carried out on the unstable slope units under different rainfall conditions by utilizing a MassFlow model, the deposition condition of the unstable slope units at the ditch opening is simulated, and statistics is carried out on the rainfall characteristics of the debris flow which can rush out of the ditch and cause damage to the disaster-bearing body, so that the rainfall disaster-forming threshold value of the debris flow in the drainage basin is established. The method has the beneficial effects that the problem that the rainfall threshold value is difficult to establish in a region lacking rainfall and debris flow observation data is solved, and the defect that the traditional debris flow rainfall threshold value is insufficient in consideration of the debris flow scale and the existing debris flow prevention and treatment measures is overcome by establishing the debris flow disaster rainfall threshold value; the false alarm rate of the debris flow can be greatly reduced, and the debris flow early warning precision is improved.

Description

Technical field:

[0001] The invention relates to the technical field of debris flow early warning and forecast application, and in particular to a method for calculating a debris flow disaster threshold based on numerical simulation. Background technology:

[0002] Debris flow is a temporary fluid between flood and soil-rock slide, containing a large amount of sediment and rocks. It is highly destructive and dangerous, and seriously endangers the safety of mountain residents and public facilities (Xu Hengli, 2009). Debris flow can be divided into two types: slope type and channel type. Among them, slope type debris flow is caused by rainfall causing the shallow surface soil of the slope to become unstable, and the slope material is converted into a mixed fluid to form a debris flow (Kang Zhicheng et al., 2008). The formation of this type of debris flow is closely related to the rainfall process, so the establishment of a basin rainfall threshold is the key to predicting the occurrence of debris flow.

[0003] The study of rainfall thresholds for slope debris flows can be traced back to the 1980s, when Caine proposed a clearly defined rainfall threshold for debris flows caused by the instability of shallow landslides (Caine, 1980). Subsequently, in the 1980s, research on rainfall thresholds for debris flows became active worldwide. In the 1990s and the early 21st century, many prediction and early warning models for shallow landslides and debris flows based on numerical simulation were proposed. In the past decade, there have been many studies on landslide prediction in China, but few studies on debris flow prediction based on numerical simulation, and more measurement data and rainfall parameters are often required. For example, CN114596689B discloses a shallow landslide-type valley debris flow early warning method based on basin topographic factors, geological characteristics, and rainfall observation data, which takes into account the role and mutual influence of the two major factors of topography and hydrology that trigger debris flows, thereby improving the early warning efficiency; CN106157541A discloses a valley-type debris flow early warning method based on geological factors, basin clay mineral characteristics, and previous rainfall data, which takes into account the influence of previous rainfall and soil clay particles on water content. These methods require more rainfall data and complex parameters. However, it is very difficult to obtain a large amount of rainfall background data and parameters for debris flow basins in vast mountainous areas. The method based on numerical simulation can establish debris flow rainfall thresholds for vast mountainous areas that lack rainfall and debris flow monitoring data, making up for the shortcomings of traditional methods that rely on a large amount of rainfall empirical statistical data.

[0004] At present, there are many studies on the rainfall threshold for debris flow initiation. However, just because a debris flow is initiated does not necessarily mean it will cause disasters on a certain scale. In many mountainous areas of China, drainage measures have been established for debris flow gullies, which can safely discharge debris flow materials of a certain scale. Only when the flow rate of the debris flow exceeds the drainage capacity of the treatment measures can the debris flow cause certain harm. Therefore, it is very necessary to construct the disaster-causing threshold of debris flow. On the one hand, it can reduce the false alarm rate of debris flow. On the other hand, combined with the initiation threshold of debris flow, it can be used to establish debris flow early warning and forecasting at different levels, improving the accuracy of debris flow early warning.

[0005] The present invention mainly designs a calculation method for the rainfall threshold of debris flow disasters for slope debris flows transformed from shallow landslides. This method couples the transient rainfall infiltration slope stability analysis model (TRIGRS) and the debris flow movement process model (MassFlow). Based on TRIGRS, slope stability analysis is carried out under different rainfall conditions (rain intensity, rainfall history) to estimate the volume of slope instability. On this basis, the MassFlow model is used to simulate the movement process and inundation range of the landslide body from the slope surface to the gully and the basin outlet. Finally, statistical analysis is carried out on the rainfall conditions corresponding to the debris flow process that can rush out of the gully and cause burial of surrounding disaster-causing bodies to obtain the rainfall threshold for debris flow disasters in this basin. Summary of the Invention:

[0006] The purpose of the present invention is to provide a calculation method for the disaster-causing threshold of slope debris flows based on numerical simulation, so as to overcome the problems of traditional methods that require a large amount of long-term rainfall observation data and cannot predict the scale of debris flows, and can establish thresholds for some areas that are difficult to investigate or have no historical observation data.

[0007] Therefore, based on the methods of the MassFlow numerical simulation model and the TRIGRS slope stability model, the present invention establishes the disaster-causing threshold of debris flow and conducts an example demonstration.

[0008] The present invention is realized through the following technical processes:

[0009] (1) Obtain the basic data of the study area through methods such as field investigation, data collection, and remote sensing image recognition, including the thickness, particle size distribution, and permeability of loose materials on the slope surface, as well as parameters related to the drainage capacity of the debris flow gully (width and height of the drainage trough, longitudinal slope of the gully bed, etc.), and determine the disaster-causing scale of the debris flow.

[0010] (2) Set different rainfall characteristic conditions (rainfall duration and rain intensity), and calculate and analyze the slope stability of the study area based on the TRIGRS model. On this basis, identify the slope instability units under different rainfall conditions and estimate the volume of slope instability.

[0011] (3) The MassFlow model is used to simulate the process of slope instability units from initiation to movement to final accumulation under different rainfall conditions. Finally, the slope instability volume corresponding to the debris flow event that can cause a certain disaster is selected and calculated, and converted into the number of instability grids.

[0012] (4) Based on the TRIGRS model, the calculated number of slope instability grids is used to calculate the rainfall intensity and corresponding duration that will cause a debris flow event of a certain scale, thereby establishing the rainfall threshold for debris flow disasters in the basin.

[0013] Preferably, the method for determining the scale of debris flow disaster in the process (1) specifically includes: determining the debris flow fan accumulation range, debris flow fluid inundation range, debris flow depth, debris flow velocity and other data through real debris flow disaster events, and using this as the basis for verifying model parameters; and determining the debris flow fan accumulation range, debris flow fluid inundation range, flow depth and flow velocity and other factors corresponding to the formation of the disaster based on an investigation of the drainage capacity at the debris flow outlet and the disaster-bearing body, and using these as indicators to limit the disaster standard.

[0014] Preferably, the specific steps of calculating the slope stability based on TRIGRS in the process (2) are:

[0015] (1) The DEM (digital elevation model) of the study area was processed and used to fill depressions, analyze flow direction, analyze slope, perform attribute zoning, and input into the model through the GIS platform;

[0016] (2) Setting various water and soil mechanics parameters as input, where the methods for obtaining water and soil mechanics parameters include: obtaining the soil thickness or volume of the source area through field surveys; testing the permeability and mechanical properties of the soil in the source area through field experiments; testing the bulk density of debris flow fluid and the friction properties of the channel through field experiments; or obtaining the lithology and soil properties of the basin by searching for data, and then finding the corresponding water and soil mechanics parameters or empirical formulas for calculation parameters from the properties;

[0017] (3) Input the files and parameters required for simulation and conduct slope stability analysis under different rainfall characteristics.

[0018] Preferably, the specific steps of using MassFlow in the process (3) to simulate the instability process of the slope unit under different rainfall conditions are:

[0019] (1) Construct the surface elevation topography and simulate the topography of the study area through DEM in order to simulate the subsequent dynamic process of fluid on the surface elevation topography;

[0020] (2) Constructing the volume of moving matter, by setting the volume of the material source to simulate the movement of the material as a fluid, which is generally achieved by identifying and delineating the material source area that forms the debris flow (such as shallow landslides formed by slope deposits, collapsed rock piles, etc.);

[0021] (3) Set specific parameters in the simulation;

[0022] (4) Input the constructed base topography and moving materials into MassFlow for simulation.

[0023] Preferably, in the above step (3), the Voellmy model is selected as the friction model in the MassFlow model to better simulate the movement of debris flow, wherein the dynamic friction coefficient should be between 0.01 and 0.2, and the turbulence coefficient should be between 500 and 1000.

[0024] Preferably, in the process (3), the specific method of calculating and selecting the slope instability volume corresponding to the debris flow event that can cause a certain disaster and converting it into the number of unstable grids is: assuming that all unstable grids in the stability analysis based on TRIGRS can be transformed into debris flow materials. Then, the results of the MassFlow simulation that can correspond to a certain scale of debris flow events are used as a reference, and the simulated debris flow material volume is used as the corresponding slope instability volume, and it is converted with the grid area to obtain the number of grids corresponding to the debris flow material, which is used as the standard for judging the debris flow disaster in TRIGRS.

[0025] Preferably, the specific method of establishing the rainfall threshold using the number of unstable grids in the process (4) is:

[0026] (1) The number of unstable grids calculated is used as the criterion for judging debris flow disaster in TRIGRS. That is, when the number of unstable grids in the TRIGRS simulation reaches this number, the corresponding simulated rainfall conditions (rainfall intensity and corresponding duration) are the critical point for disaster in the study area.

[0027] (2) Finally, by setting a series of different rainfall conditions, the critical points of different rainfall intensities and their corresponding durations are obtained according to the aforementioned judgment criteria;

[0028] (3) Use the ID threshold model formula to fit the rainfall critical point data, obtain the rainfall threshold curve, and establish the rainfall threshold formula for the study area.

[0029] Furthermore, the MassFlow model mentioned above calculates the momentum and density of the fluid based on the simplified three-dimensional Navier-Stokes equations (Iverson et al., 2015), and the formula is as follows:

[0030]

[0031]

[0032]

[0033] Where vx and vy are the components of the velocity vector in the x and y directions; β is the momentum distribution coefficient; k ap is the geopressure coefficient; τ is the shear stress; ρ is the fluid density; u is the fluid velocity vector; τ is the fluid shear stress tensor; g is the gravity vector; h is the fluid height.

[0034] Furthermore, the TRIGRS model determines slope stability based on the Fs (factor of safety) value. The specific formula when the soil depth is Z is as follows (Baum, 2008):

[0035]

[0036] In the formula, τ f is the shear strength; τ d is the shear stress at equilibrium; C is the soil cohesion under effective stress; is the soil friction angle; δ is the slope angle; γ w is the unit weight of water; s is the unit weight of soil; Ψ(Z, t) is the pressure head.

[0037] The calculation formula of pressure head Ψ(Z, t) is as follows:

[0038]

[0039] Where t is time, d wt is the depth of the initial steady-state groundwater level measured in the Z direction, β = λcosα, where λ = cosα-[I Z / K Z Here I Z is the long-term (steady-state) surface flux in the Z direction, K Z is the saturated hydraulic conductivity in the Z direction, I nZ is the surface flux of a given intensity in the nth time interval, d LZ is the soil depth measured in the Z direction. N is the total number of time intervals, H(tt n ) is a heavy step function until t = t n , t n is the current computation step size.

[0040] Furthermore, the ID threshold model formula mentioned above refers to the threshold relationship established by rainfall intensity I and duration D. The specific formula is as follows:

[0041] I=C+αD β

[0042] Where I is the average effective rainfall intensity (mm / h); C is a constant (≥0); D is the duration or duration of rainfall (h); α and β are statistical parameters (calculated from the data and vary with rainfall intensity and duration).

[0043] The beneficial effects of the present invention are as follows: (1) The present invention proposes a method for calculating the threshold value of debris flow disaster on slopes, which couples the transient rainfall infiltration slope stability analysis model (TRIGRS) and the debris flow dynamic process simulation model (MassFlow), simulates the whole process of rainfall-slope instability-debris flow movement, and then establishes the rainfall threshold value of debris flow disaster. The present invention provides a new theoretical method for the current situation that the traditional method relies on a large amount of long-term rainfall data to establish the rainfall threshold value of debris flow; (2) The rainfall threshold calculation method proposed by the present invention can predict the scale of debris flow formation, study the relationship between the rainfall threshold value and the scale of debris flow, and solve the problem that it is difficult to predict the scale of debris flow in the calculation of the rainfall threshold value of debris flow. Description of the drawings:

[0044] The accompanying drawings described herein are for the purpose of more clearly explaining the present invention and introducing the examples. The accompanying drawings used in the present invention will be briefly introduced as follows:

[0045] Figure 1 It is a flowchart of the specific implementation of the present invention, which clearly presents the implementation process of the entire method;

[0046] Figure 2 It is an interpretation map of the shallow landslide formed in the study area after the extreme rainfall event in August 2020 in the embodiment, and the range of the debris flow accumulation fan downstream of the study area;

[0047] Figure 3 This is the downstream debris flow accumulation height after the time step simulated by the MassFlow model in the embodiment ends.

[0048] In the accompanying drawings, the meanings of the various reference numerals are listed as follows:

[0049] 1. Shallow landslide body, 2. Watershed in the study area, 3. Debris flow accumulation fan, 4. Debris flow accumulation height is less than 1m, 5. Debris flow accumulation height is greater than 5m, 6. Debris flow accumulation height is 3 to 5m, 7. Debris flow accumulation height is 1 to 3m. Specific implementation method:

[0050] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with embodiments and drawings.

[0051] like Figure 1As shown in the figure, the main idea of ​​a calculation method for the threshold of slope debris flow disaster based on numerical simulation is: first, through remote sensing image processing and field and indoor data collection, the various parameters and files required for simulation are obtained, and the slope stability of the study area is analyzed based on the TRIGRS model. Secondly, the dynamic process of debris flow in the study area is simulated by the MassFlow model, and the volume of solid matter formed after the debris flow occurs is calculated, so as to calculate the number of disaster grids involved in the formation of debris flow. Then, by setting a series of rainfall intensities and durations, based on the number of disaster grids, TRIGRS is used to simulate the threshold critical points under different rainfall conditions, and thus the disaster rainfall threshold relationship of the study area is established.

[0052] Example

[0053] In order to verify the feasibility of the present invention and the reliability of the calculation, the Yangpingyagou debris flow in Longnan, Gansu Province was selected as the study area. Taking the extreme rainfall event in Longnan on August 17, 2020 as an example, the rainfall threshold for the debris flow event of this scale in Yangpingyagou was calculated. The specific steps are as follows:

[0054] 1. Using the GF-2 remote sensing images after the disaster, we can identify the distribution of the shallow landslide body 1 caused by this rainfall event ( Figure 2 ), through field investigation, determine the area of ​​the debris flow accumulation fan 3 and the height of the accumulation in the channel;

[0055] 2. Obtain various properties of the soil in the study area through field investigation and indoor experiments: obtain the cohesion and internal friction angle of different soils in the study area through a quadruple direct shear test, obtain the permeability properties of various soils in the study area through a field single-ring permeability test, obtain the thickness of the shallow landslide in the study area through a field investigation, and calculate the fluid density when the debris flow occurs through indoor analysis. The following table shows the various parameter values ​​used in the simulation of the embodiment:

[0056]

[0057] 2. Combining the previous survey results with the extreme rainfall conditions in the study area on August 17, 2020, the parameters required by the TRIGRS model were input for simulation, and the overall stability of the slopes in the study area was analyzed to provide a reference for subsequent MassFlow simulations.

[0058] 3. Use drones to obtain high-precision DEM data of the watershed. On this basis, build a debris flow calculation model, calculate the dynamic process of debris flow based on the MassFlow model, simulate the movement and accumulation process of debris flow caused by the rainfall event, and compare and verify it with the survey data in the actual situation. Figure 3 This is the calculation result of the MassFlow model. The simulated accumulation fan area accounts for about 80% of the real accumulation fan area, which shows that the model simulation results are good and applicable.

[0059] 4. Determine the volume of solid material required to form this debris flow event through the simulation results of the MassFlow model. Assuming that all materials on unstable slopes will be converted into debris flow materials, the volume of unstable materials required to form this scale of debris flow can be known from the MassFlow simulation results, and can be converted into the number of unstable grid units. The conversion result is that 39,901 unstable grids are required to form this scale of debris flow. If the grid number error is controlled within 1%, the unstable grid standard for disaster is between 39,502 and 40,300 grids;

[0060] 6. Based on the above disaster judgment criteria, the TRIGRS model is used to calculate the number of grids with safety factors less than 1 in the study area. When the number of grids with safety factors less than 1 reaches the unstable grid unit determined in the above steps, it can be determined that the corresponding rainfall condition in the simulation at this time is the critical point of the threshold; the calculated duration corresponding to different rainfall intensities is shown in the following table:

[0061]

[0062] 7. By setting a series of rainfall intensities and durations, the critical points for determining the scale of disaster under different rainfall conditions are obtained. Finally, a series of critical points are used to fit the curve based on the relationship between the ID rainfall threshold and the disaster rainfall threshold formula for the study area is calculated to establish the relationship between the debris flow disaster threshold. The calculated rainfall threshold for debris flow disaster in the study area is I = 74.255 × D -1.14 .

[0063] The disaster threshold calculation method proposed in the present invention combines the TRIGRS model and the MassFlow model, and takes into account the calculation of debris flow thresholds under a certain debris flow scale. It overcomes the difficulty of requiring a large amount of long-term rainfall observation data when establishing debris flow rainfall thresholds in the past, and provides a new idea for establishing rainfall thresholds for some river basins where rainfall observations are difficult to carry out and for areas without historical rainfall monitoring.

[0064] Finally, it should be noted that the above-described embodiments are only a more detailed description of the implementation steps of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit and principles of the present invention, any modifications, equivalent substitutions and improvements made to the present invention by any person skilled in the art should be within the scope of protection determined by the claims of the present invention.

Claims

1. A method for calculating the threshold of slope debris flow disaster. It is characterized in that include: (1) Obtain basic data of the study area through field investigation, data collection and remote sensing image recognition, including the thickness, particle size distribution and permeability of loose materials on the slope, as well as parameters related to the drainage capacity of debris flow channels (width and height of drainage channels, longitudinal gradient of ditch beds, etc.), and determine the scale of debris flow disasters; (2) Setting different rainfall characteristic conditions (rainfall duration and rainfall intensity), the slope stability of the study area was calculated and analyzed based on the TRIGRS model. On this basis, the slope instability units under different rainfall conditions were identified and the volume of the slope instability was estimated; (3) The MassFlow model is used to simulate the process of slope instability units from initiation to movement to final accumulation under different rainfall conditions. Finally, the slope instability volume corresponding to the debris flow event that can cause a certain disaster is selected and calculated, and converted into the number of instability grids. (4) Based on the TRIGRS model, the calculated number of slope instability grids is used to calculate the rainfall intensity and corresponding duration that will cause a debris flow event of a certain scale, thereby establishing the rainfall threshold for debris flow disasters in the basin.

2. The method for determining the scale of debris flow disaster according to claim 1, Features: Through real debris flow disaster events, the data such as the debris flow fan accumulation range, debris flow fluid inundation range, debris flow depth, and debris flow velocity are determined, which serve as the basis for verifying the model parameters; based on the investigation of the drainage capacity and disaster-bearing body at the debris flow outlet, the debris flow fan accumulation range, debris flow fluid inundation range, flow depth and flow velocity corresponding to the formation of the disaster are determined as indicators to limit the standard for disasters.

3. The slope stability analysis based on TRIGRS according to claim 1, Features: The process of step (2) specifically includes: (1) Process the study area dem, fill depressions, analyze flow direction, analyze slope, perform attribute partitioning, and input it into the model through the GIS platform; (2) Setting various water and soil mechanics parameters as input, where the methods for obtaining water and soil mechanics parameters include: obtaining the soil thickness or volume of the source area through field surveys; testing the permeability and mechanical properties of the soil in the source area through field experiments; testing the bulk density of debris flow fluid and the friction properties of the channel through field experiments; or obtaining the lithology and soil properties of the basin by searching for data, and then finding the corresponding water and soil mechanics parameters or empirical formulas for calculation parameters from the properties; (3) Input the files and parameters required for simulation and conduct slope stability analysis under different rainfall characteristics.

4. According to claim 1, the method for simulating the instability process of slope units under different rainfall conditions by using MassFlow, Features: The process of step (3) specifically includes: (1) Construct the surface elevation topography and construct the simulated topography of the study area through DEM (digital elevation model) to simulate the subsequent dynamic process of fluid on the surface elevation topography; (2) Constructing the volume of moving matter, by setting the volume of the material source to simulate the movement of the material as a fluid, which is generally achieved by identifying and delineating the material source area that forms the debris flow (such as shallow landslides formed by slope deposits, collapsed rock piles, etc.); (3) Set specific parameters in the simulation; (4) Input the constructed base topography and moving materials into MassFlow for simulation.

5. According to claim 1, the slope instability volume corresponding to the debris flow event that can cause a certain disaster is selected and calculated, and converted into the number of instability grids, Features: Assuming that all unstable grids in the stability analysis based on TRIGRS can be transformed into debris flow materials, the results of the MassFlow simulation that can correspond to a certain scale of debris flow events are used as a reference, and the simulated debris flow material volume is used as the corresponding slope instability volume, which is converted with the grid area to obtain the number of grids corresponding to the debris flow material, which is used as the standard for judging debris flow disasters in TRIGRS.

6. The method of establishing a rainfall threshold by using the number of unstable grids according to claim 1, Features: The process of step (4) specifically includes: (1) The number of unstable grids calculated is used as the criterion for judging debris flow disaster in TRIGRS. That is, when the number of unstable grids in the TRIGRS simulation reaches this number, the corresponding simulated rainfall conditions (rainfall intensity and corresponding duration) are the critical point for disaster in the study area. (2) Finally, by setting a series of different rainfall conditions, the critical points of different rainfall intensities and their corresponding durations are obtained according to the aforementioned judgment criteria; (3) Use the ID threshold model formula to fit the rainfall critical point data, obtain the rainfall threshold curve, and establish the rainfall threshold formula for the study area.

7. The specific parameters of setting according to claim 4, Features: The Voellmy model is selected as the friction model to better simulate the movement of debris flow. The dynamic friction coefficient should be between 0.01 and 0.2, and the turbulence coefficient should be between 500 and 1000.

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