Method for rapidly evaluating mud-rock flow siltation soil forming time

Abstract

The invention relates to the technical field of debris flow siltation soil forming time evaluation, and provides a method for quickly evaluating debris flow siltation soil forming time, which comprises the following steps of: respectively acquiring 210Pbex area activity of debris flow siltation soil and background point soil in an acquisition area; and calculating the formation time of the debris flow siltation soil according to the 210Pbex area activity of the debris flow siltation soil and the background point soil. The method has the advantages that the problems that at present, debris flow event observation data are lost, and debris flow deposited soil is difficult to disrupt can be solved, and operation is easy and convenient.

Description

Technical Field

[0001] The present invention relates to the technical field of debris flow alluvial soil formation time assessment, and in particular to a method for rapidly assessing the debris flow alluvial soil formation time. Background Art

[0002] Due to global warming and the impact of human activities, debris flows and major floods in mountainous areas have become more frequent over the past 100 years. Information on the timing of debris flows and major floods is urgently needed to improve understanding of the impacts of climate change and human activities on fragile mountain ecosystems and to assess the risks of these hazards at local and global scales.

[0003] Traditionally, the timing of debris flow occurrences has been determined using historical data or remote sensing methods. However, historical data is often difficult to collect and covers only limited areas where debris flow occurs, especially in remote mountainous areas. Meanwhile, remote sensing satellites have been used since the 1970s to interpret changes in land use patterns, thus also enabling debris flow monitoring. However, the low accuracy of satellite remote sensing imagery in the last century has made its application difficult.

[0004] Radioactive fallout nuclides, (such as 137 Cs and 210 Pb) has been widely used in sediment dating. 137 Cs, a radioactive isotope released into the environment as a byproduct of nuclear weapons testing, primarily from the 1950s to the 1970s, has been shown to be a reliable nuclide with a half-life of approximately 30.17 years. 137 The rate of Cs deposition peaked in 1963 and subsequently declined to very low values ​​in the Northern Hemisphere by 1972 due to the ban on thermonuclear weapons testing in the atmosphere. 137 Cs concentrations are gradually decreasing, so it will be difficult to detect Cs in soil in the future. 137 Cs concentration.

[0005] Patent authorization number CN110161215B, titled "A method for utilizing atmospheric generation 10 Be and 26 The Chinese invention patent "Method for determining the age of loess sediments by Al dual nuclide ratio method" discloses a method using atmospheric genesis 10 Be and 26 The Al dual-nuclide ratio method for dating loess sediments has the advantage of being unaffected by formation rates, the Earth's magnetic field, and climate change, and boasts a wide dating range, encompassing the entire Quaternary. However, this method is primarily suitable for long-term chronological studies of aeolian sediments such as loess and is not suitable for dating the formation time of recent alluvial soils.

[0006] Chinese invention patent application number CN202310652178.3, entitled "A Method for Reconstructing the Developmental Stages and Catastrophic History of Large-Scale Debris Flow Fans," discloses a method for reconstructing the developmental stages and catastrophic history of large-scale debris flow fan deposits. The method includes field demarcation of debris flow sedimentary structures and stage units, geophysical analysis of debris flow thickness variations and boulder distribution, sampling along transverse and longitudinal profiles of the debris flow fan to conduct dating analysis and chronological constraints, and establishing the developmental stages and chronological sequences of the debris flow fan. This method allows for systematic dating of the top of the deposited fan and its overlying slope soils, as well as systematic dating of the base of the deposited fan and its fluvial and lacustrine sand layers. Through numerical simulation of debris flows, long-term debris flow developmental stages can be reconstructed, potentially supplementing the lack of short-term observational disaster data and meeting the practical needs of major debris flow fan history reconstruction and disaster prevention and mitigation projects. However, this method relies on geophysical analysis, multi-profile systematic sampling, and numerical simulation, resulting in heavy field and laboratory workload and difficulty in elucidating the formation time of debris flow deposits over a century.

[0007] Therefore, a method for quickly assessing the formation time of debris flow alluvial soil is proposed, which can solve the current problems of missing observation data of debris flow events and difficulty in dating debris flow alluvial soil, and is easy to operate. Summary of the Invention

[0008] The present invention aims to address at least one of the aforementioned deficiencies in the prior art. For example, one of the objectives of the present invention is to provide a method for rapidly assessing the formation time of debris flow alluvial soils. By utilizing naturally occurring stable isotopes, the method can rapidly assess the formation time of debris flow alluvial soils on a centennial scale through simple measurement and calculation. This method addresses the current challenges of lacking observational data on debris flow events and the difficulty in dating debris flow alluvial soils, and is simple to operate.

[0009] To achieve the above object, the present invention provides a method for rapidly evaluating the soil formation time of debris flow alluvial soil, comprising the following steps:

[0010] Obtain the debris flow alluvial soil and background soil in the collection area respectively 210 Pb ex Area activity;

[0011] According to the debris flow alluvial soil and background soil 210 Pb ex The formation time of debris flow alluvial soil is calculated based on area activity.

[0012] In a preferred embodiment of this solution, the debris flow deposit soil and background soil 210 Pb ex The area activity is used to calculate the formation time of debris flow alluvial soil, including formula (1):

[0013] A(t)=Aref (1-e -αt )+A′e -αt (1)

[0014] Where t is the formation time of debris flow alluvial soil, years; A(t) is the thickness of debris flow alluvial soil after t years of deposition. 210 Pb ex Area activity, Bq m -2 ; A ref For the background point of soil 210 Pb ex Area activity, Bq m -2 ; α is the annual soil 210 Pb ex The comprehensive loss ratio is dimensionless; A′ is the surface layer of soil in the initial stage of debris flow deposition. 210 Pb ex Area activity, Bq m -2 .

[0015] In a preferred embodiment of this solution, the surface layer of the soil in the initial stage of debris flow deposition 210 Pb ex The area activity A′ is characterized by the area activity of the soil in the debris flow impact gully that is developing in the collection area.

[0016] In a preferred embodiment of this scheme, when there is no developed debris flow impact gully in the collection area, the surface layer of the soil in the initial stage of debris flow deposition is 210 Pb ex The area activity A′ is 0.

[0017] In a preferred embodiment of this solution, the debris flow alluvial soil and background soil of the collection area are obtained respectively. 210 Pb ex Area activity includes:

[0018] Collect soil samples from the surface of debris flow alluvial soil or background soil, remove gravel and plant roots from the soil samples, then air-dry, grind and sieve them for determination 210 Pb ex Content, finally according to the measured 210 Pb ex The content calculation corresponds to 210 Pb ex Area activity.

[0019] In a preferred embodiment of this solution, the 210 Pb ex The content calculation corresponds to 210 Pb ex Area activity includes:

[0020] Correction according to formula (2) 210 Pb ex Mass activity, formula (2) is:

[0021] C=C′×2 t″ / T (2)

[0022] Where C is the nuclide mass activity of the soil sample at the time of sampling, Bq kg -1 ; C′ is the nuclide mass activity of soil samples tested in the laboratory, Bq kg -1 ; t″ is the time from soil sample collection to laboratory testing, years; T is 210 Pb half-life, 22.26 years;

[0023] The area activity is calculated according to formula (3):

[0024] A=C×BD i ×D i ×1000 (3)

[0025] Where A is the soil sample 210 Pb ex Area activity, Bq m -2 ; C is the nuclide mass activity of the soil sample at the time of sampling, Bq kg -1 BD is soil bulk density (g cm2) -3 ; D is the thickness of the surface soil, cm; 1000 is the unit correction factor.

[0026] In a preferred embodiment of this scheme, the background point is a grassland with no erosion and sedimentation for a long time, flat terrain and high vegetation coverage; and the background point is no more than 10 kilometers away from the debris flow area. 210 Pb ex Differences in sedimentation flux.

[0027] In a preferred embodiment of this solution, the surface soil sample is the soil 2 cm to 5 cm above the surface of the sampling point.

[0028] In a preferred embodiment of this scheme, the method for collecting soil samples from the surface layer of the debris flow alluvial soil is: using the plum blossom five-point sampling method or the S-shaped sampling method to collect debris flow alluvial soil from multiple points respectively, and then mixing them into one sample.

[0029] In a preferred embodiment of this scheme, a high purity germanium gamma spectrometer is used to measure the 210 Pb ex Mass activity.

[0030] In a preferred embodiment of the present invention, the method for collecting the surface soil sample of the background point surface soil is as follows: after collecting the soil of N background points respectively using the plum blossom five-point sampling method or the S-shaped sampling method, the soil is mixed into one sample;

[0031] The calculation formula of N is:

[0032]

[0033] Where t′ is the t value of the 95% confidence interval; C v is the coefficient of variation; AE is the allowable error (usually 10%).

[0034] In a preferred embodiment of this scheme, when the vegetation in the collection area recovers well after the debris flow siltation (the vegetation coverage is above 75%), the soil erosion amount in the collection area is 0, and α is 210 The natural decay factor of Pb is 0.031 year -1 .

[0035] In a preferred embodiment of this scheme, the vegetation recovery after the debris flow in the collection area is not good (the vegetation coverage is less than 75%), and the soil erosion in the collection area is not 0. The α is: λ+h / H, where h is the soil erosion amount, kg m -2 ; H is the mass of surface soil, kg m -2 .

[0036] In a preferred embodiment of this solution, the calculation formula of h is:

[0037] h=R×K×LS×C m ×P / 10(5)

[0038] Where h is the amount of soil erosion, kg / (m 2 ·a); R is the rainfall erosivity factor, (MJ·mm) / (hm 2 ·h·a); K is the soil erodibility factor, (t·hm 2 ·h) / (MJ·mm·hm 2 ) ; LS is the slope length and slope factor, dimensionless; C m is the coverage and management factor, dimensionless; P is the maintenance measure factor, dimensionless.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The present invention utilizes stable isotopes existing in nature 210 Pb exThrough simple measurement and calculation, the soil formation time of debris flow alluvial soil on a century-long scale can be quickly evaluated. The proposed method has a solid physical basis and improves the evaluation efficiency. Compared with the existing technology, this method is suitable for the determination of the soil formation time of alluvial soil in recent years, and the method proposed in the present invention does not require numerical simulation of debris flow and is easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The above and other objects and / or features of the present invention will become more apparent from the following description in conjunction with the accompanying drawings, in which:

[0042] Figure 1 A schematic diagram of a typical debris flow historical gully and sediment deposition area showing an exemplary embodiment of a method for rapidly assessing the soil formation time of debris flow alluvial soil according to the present invention.

[0043] Figure 2 A typical technical flow diagram of a method for rapidly evaluating the formation time of debris flow alluvial soil is shown, which is an exemplary embodiment of the method of the present invention for rapidly evaluating the formation time of debris flow alluvial soil.

[0044] Figure 3 An exemplary embodiment of a method for rapidly evaluating the soil formation time of debris flow alluvial soil according to the present invention is shown. 210 Pb ex The simulated curve of area activity over time and the measured sedimentation soil 210 Pb ex Comparison chart of the data. DETAILED DESCRIPTION

[0045] Hereinafter, a method for rapidly evaluating the soil formation time of debris flow alluvial soil according to the present invention will be described in detail with reference to exemplary embodiments.

[0046] Exemplary embodiments

[0047] A method for rapidly assessing the soil formation time of debris flow alluvial soil comprises the following steps:

[0048] Step 1: Obtain the debris flow alluvial soil in the collection area 210 Pb ex Area activity;

[0049] Step 2: Obtain the background soil of the collection area 210 Pb ex Area activity;

[0050] Step 3: Based on the debris flow alluvial soil and background soil 210 Pb ex The formation time of debris flow alluvial soil is calculated based on area activity.

[0051] 210Pb ex yes 238 A natural product of the U decay series. 238 U decays twice and becomes gaseous 222 Rn, part of which remains in the soil and decays into 210 Pb, and 226 Ra balance, called "compensation" 210 Pb(supported 210 Pb, that is 210 Pb supported ); another part 222 Rn escapes into the atmosphere and decays into 210 Pb settles to the surface and is adsorbed by the surface soil. 210 Pb is called "excess" 210 Pb(excess 210 Pb, that is 210 Pb ex ).

[0052] Debris flow sediments 210 Pb ex The area activity increases gradually with the increase of sedimentation time, and is related to the 210 Pb ex The area activity is related to the amount of soil erosion after deposition. 210 Pb ex Area activity, debris flow 210 Pb ex The abundance and erosion of 210 Pb ex The time of the most recent debris flow was calculated using the isotope deposition-decay law.

[0053] Specifically, the specific operations of step one include:

[0054] Use a bulk density drill to collect surface samples of debris flow alluvial soil (the top 2 cm, 3 cm, or 5 cm can be used). Use the five-point plum blossom sampling method or the S-shaped sampling method to collect alluvial soil from multiple locations and combine them into a single sample. The sample weight should be at least 1000 g. Remove gravel and plant roots from the soil sample and air-dry it for later use.

[0055] Use the quartering method to take about 500g of soil sample, grind it, pass it through a 2mm sieve, and use it for 210 Pb ex Content determination: A high-purity germanium gamma spectrometer (ORTEC, USA) was used. After preamplification and digital conversion, it was connected to a multi-channel analyzer. The channel boundary method was used for determination. The test time was not less than 50,000 seconds, and the repeated test error was less than 5%. 210Pb ex Mass activity equals 210 The mass activity value of the full peak area of ​​Pb at 46.5keV ray is subtracted from 226 Ra is the mass activity value of 351.9keV rays.

[0056] Bulk density: Determined by the original sample drying method.

[0057] Since there is a time interval between the nuclide content test and the soil sample collection, the test value needs to be corrected to the mass activity at the time of sampling: In this exemplary embodiment, the correction is performed according to formula (2): 210 Pb ex Mass activity, formula (2) is:

[0058] C=C′×2 t″T (2)

[0059] Where C is the nuclide mass activity of the soil sample at the time of sampling, Bq kg -1 ; C′ is the nuclide mass activity of soil samples tested in the laboratory, Bq kg -1 ; t″ is the time from soil sample collection to laboratory testing, years; T is 210 Pb half-life, 22.26 years.

[0060] soil 210 Pb ex The calculation formula of area activity is:

[0061] A=C×BD i ×D i ×1000(3)

[0062] Where A is the soil sample 210 Pb ex Area activity, Bq m -2 ; C is the nuclide mass activity of the soil sample at the time of sampling, Bq kg -1 BD is soil bulk density (g cm2) -3 ; D is the thickness of the surface soil, cm; 1000 is the unit correction factor.

[0063] Furthermore, in this exemplary embodiment, the background point in step 2 is a grassland with no erosion and sedimentation for a long time, flat terrain, and high vegetation coverage; and the background point is no more than 10 kilometers away from the debris flow area. 210 Pb ex Differences in sedimentation flux.

[0064] In order to overcome the errors caused by spatial variability, a standardized sampling method is adopted to scientifically determine the number of background sampling points. The number of background point sampling points N can be determined by the following formula:

[0065]

[0066] Where t′ is the t value of the 95% confidence interval; C v is the coefficient of variation; AE is the allowable error.

[0067] In this exemplary embodiment, background point soil sampling and soil sample 210 Pb ex The analysis method of area activity is consistent with that of debris flow alluvial soil mentioned above.

[0068] Furthermore, in this exemplary embodiment, in step 3, the method according to the debris flow deposit soil and background point soil 210 Pb ex The area activity is used to calculate the formation time of debris flow alluvial soil, including formula (1):

[0069] A(t)=A ref (1-e -αt )+A′e -αt (1)

[0070] Where, t is the formation time of debris flow alluvial soil, years; A(t) is the thickness of debris flow alluvial soil after t years of deposition. 210 Pb ex Area activity, Bq m -2 ; A ref For the background point of soil 210 Pb ex Area activity, Bq m -2 ; α is the annual soil 210 Pb ex The comprehensive loss ratio is dimensionless; A′ is the surface layer of soil in the initial stage of debris flow deposition. 210 Pb ex Area activity, Bq m -2 .

[0071] In this exemplary embodiment, the surface layer of the soil in the initial stage of debris flow deposition 210 Pb ex The area activity A' is characterized by the area activity of the soil in the debris flow impact gully that is developing in the collection area. When there is no debris flow impact gully in the collection area, the surface soil of the initial debris flow deposition is 210 Pb ex The area activity A′ is 0.

[0072] In this exemplary embodiment, when the vegetation in the collection area recovers well after the debris flow siltation, the soil erosion amount in the collection area is 0, and α is 210 The natural decay factor of Pb is 0.031 year -1 .

[0073] In this exemplary embodiment, the vegetation recovery after the debris flow in the collection area is not good, and the soil erosion amount in the collection area is not 0. The α is: λ+h / H, where h is the soil erosion amount, kg m -2 ; H is the mass of surface soil, kg m -2 ; Wherein, the calculation formula of h is:

[0074] h=R×K×LS×C m ×P / 10(5)

[0075] Where h is the amount of soil erosion, kg / (m 2 ·a); R is the rainfall erosivity factor, (MJ·mm) / (hm 2 ·h·a); K is the soil erodibility factor, (t·hm 2 ·h) / (MJ·mm·hm 2 ) ; LS is the slope length and slope factor, dimensionless; C m is the coverage and management factor, dimensionless; P is the maintenance measure factor, dimensionless.

[0076] Example

[0077] Alluvial soil formed in debris flow alluvial fan (ref. Figure 1 Surface soil samples (2-5 cm) are collected layer by layer using a bulk density drill. Using the five-point plum blossom sampling method or the S-shaped sampling method, soil samples are collected from multiple locations and combined into a single sample weighing at least 1000 g. Gravel and plant roots are removed from the soil samples, which are then air-dried for later use.

[0078] The evaluation process of soil formation time of debris flow alluvial soil is as follows: Figure 2 As shown. Grasslands with long-term absence of erosion and sedimentation, flat terrain, and high vegetation coverage were selected as background points. The sampling method was the same as that for debris flow alluvial soil. To overcome the errors caused by spatial variability, a standardized sampling method was adopted to scientifically determine the number of background sampling points. The number of background sampling points can be determined using the following formula:

[0079]

[0080] Where N is the number of background point samples; t′ is the t value of the 95% confidence interval; C v is the coefficient of variation; AE is the allowable error. The number of sampling points should not be less than 3.

[0081] The original soil samples were collected by the ring knife method, and the bulk density of debris flow alluvial soil and background soil was measured by drying.

[0082] 210Pb ex Mass activity determination: 500 g of soil sample was ground by quartering method and passed through 2 mm sieve. 210 Pb ex Content determination: A high-purity germanium gamma spectrometer (ORTEC, USA) was used. After preamplification and digital conversion, it was connected to a multi-channel analyzer. The channel boundary method was used for determination. The test time was not less than 50,000 seconds, and the repeated test error was less than 5%. 210 Pb ex Mass activity equals 210 The mass activity value of the full peak area of ​​Pb at 46.5keV ray is subtracted from 226 Ra is the mass activity value of 351.9keV rays.

[0083] At sampling time 210 Pb ex The mass activity is calculated as follows:

[0084] C=C′×2 t″ / T (2)

[0085] Where C is the sample nuclide mass activity at the time of sampling, Bq kg -1 ; C′ is the sample nuclide mass activity detected in the laboratory, Bq kg -1 ; t″ is the time from soil sample collection to testing, a; T is 210 Pb half-life, 22.26a.

[0086] soil 210 Pb ex The calculation formula of area activity is:

[0087] A=C×BD i ×D i ×1000 (3)

[0088] Where A is the surface soil sample of debris flow alluvial soil 210 Pb ex Area activity, Bq m -2 ; C is the nuclide mass activity of the soil sample at the time of sampling ( 210 Pb ex Mass activity), Bq kg -1 BD is the bulk density of debris flow alluvial soil, g cm -3 ; D is the thickness of the surface soil, cm; 1000 is the unit correction coefficient. Similarly, taking the background point soil as the soil sample, the background point soil can be obtained by using formulas (2) and (3): 210 The area activity of Pbex is A ref (Bq m -2 ).

[0089] Then the formation time t of debris flow alluvial soil is calculated using the following formula:

[0090] A(t)=A ref (1-e -αt )+A′e -αt (1)

[0091] Where, t is the formation time of debris flow alluvial soil, years; A(t) is the thickness of debris flow alluvial soil after t years of deposition. 210 Pb ex Area activity, Bq m -2 ; A ref For the background point of soil 210 Pb ex Area activity, Bq m -2 ; α is the annual soil 210 Pb ex The comprehensive loss ratio is dimensionless; A′ is the surface layer of soil in the initial stage of debris flow deposition. 210 Pb ex Area activity, Bq m -2 .

[0092] If the silt soil is relatively flat, without obvious slopes, and the vegetation coverage is above 75%, the soil erosion can be ignored. In this embodiment, the soil erosion of the silt soil cannot be ignored, and α is: λ + h / H, where h is the soil erosion, kg m -2 ; H is the mass of surface soil, kg m -2 The improved universal soil erosion equation RUSLE can be used to estimate the amount of soil erosion in the region:

[0093] h=R×K×LS×C m ×P / 10(5)

[0094] Where: h is the amount of soil erosion, kg / (m 2 ·a); R is the rainfall erosivity factor, (MJ·mm) / (hm 2 ·h·a); K is the soil erodibility factor, (t·hm 2 ·h) / (MJ·mm·hm 2 ) ; LS is the slope length and slope factor, dimensionless; C m is the coverage and management factor, dimensionless; P is the maintenance measure factor, dimensionless.

[0095] Finally, the following formula is used to calculate the debris flow alluvial soil formation time t:

[0096] A(t)=A ref (1-e -(λ+h / H)t )+A′e -(λ+h / H)t (6)

[0097] Where h is the amount of soil erosion, kg m -2 ; H is the mass of surface soil, kg m -2 ;A(t) and A ref Debris flow alluvial soil and surface soil of background point 210 Pb ex Area activity.

[0098] Since debris flows mainly originate from subsurface areas such as source gullies, A′ is generally much lower than the debris flow deposition area. Assuming A′ is 0, the above formula (6) can be rewritten as:

[0099] A(t)=A ref (1-e -(λ+h / H)t )(7)

[0100] According to formula (7), the soil formation time t of debris flow alluvial soil can be calculated.

[0101] In this embodiment, according to the above formula (6), we can get Figure 2 The " 210 Pb ex "Simulation curve of area activity over time"; such as Figure 3 As shown, six alluvial soils with known soil formation time are 210 Pb ex The data are marked in the graph, and it can be seen that the simulation curve provided by the present invention is consistent with the measured results and can be used to determine and evaluate the soil formation time of debris flow alluvial soil.

[0102] Although the present invention has been described above with reference to the exemplary embodiments and the accompanying drawings, it will be apparent to those skilled in the art that various modifications may be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for rapidly evaluating the soil formation time of debris flow alluvial soil, characterized in that: The following steps are involved: Obtain the debris flow alluvial soil and background soil in the collection area respectively 210 Pb ex Area activity; According to the debris flow alluvial soil and background soil 210 Pb ex The formation time of debris flow alluvial soil is calculated based on area activity.

2. The method according to claim 1, wherein: The debris flow alluvial soil and background soil 210 Pb ex The area activity is used to calculate the formation time of debris flow alluvial soil, including formula (1): A(t)=A ref (1-e -αt )+A′e -αt (1) Where, t is the formation time of debris flow alluvial soil, years; A(t) is the thickness of debris flow alluvial soil after t years of deposition. 210 Pb ex Area activity, Bq m -2 ; A ref For the background point of soil 210 Pb ex Area activity, Bq m -2 ; α is the annual soil 210 Pb ex The comprehensive loss ratio, dimensionless; A' is the surface layer of soil in the initial stage of debris flow deposition 210 Pb ex Area activity, Bq m -2 .

3. The method according to claim 2, wherein: The surface of the soil in the initial stage of debris flow deposition 210 Pb ex The area activity A′ is characterized by the area activity of the soil in the debris flow impact gully that is developing in the collection area.

4. The method according to claim 2, wherein: When there is no debris flow impact gully in the collection area, the surface soil of the soil in the initial stage of debris flow deposition is 210 Pb ex The area activity A′ is 0.

5. The method according to claim 1, wherein The debris flow alluvial soil and background soil of the acquisition area are obtained respectively. 210 Pb ex Area activity includes: Collect soil samples from the surface of debris flow alluvial soil or background soil, remove gravel and plant roots from the soil samples, then air-dry, grind and sieve them for determination 210 Pb ex Content, finally according to the measured 210 Pb ex The content calculation corresponds to 210 Pb ex Area activity.

6. The method according to claim 5, characterized in that: The measured 210 Pb ex The content calculation corresponds to 210 Pb ex Area activity includes: Correction according to formula (2) 210 Pb ex Mass activity, formula (2) is: C=C′×2 t″T (2) Where C is the nuclide mass activity of the soil sample at the time of sampling, Bq kg -1 ; C′ is the nuclide mass activity of soil samples tested in the laboratory, Bq kg -1 ; t″ is the time from soil sample collection to laboratory testing, a; T is 210 Pb half-life, 22.26a; The area activity is calculated according to formula (3): A=C×BD i ×D i ×1000(3) Where A is the soil sample 210 Pb ex Area activity, Bq m -2 ; C is the nuclide mass activity of the soil sample at the time of sampling, Bqkg -1 BD is soil bulk density (g cm2) -3 ; D is the thickness of the surface soil, cm; 1000 is the unit correction factor.

7. The method according to claim 5, characterized in that The method for collecting soil samples from the surface layer of soil at the background point is as follows: The soil of N background points was collected separately using the plum blossom five-point sampling method or the S-shaped sampling method, and then mixed into one sample; The calculation formula of N is: Where t′ is the t value of the 95% confidence interval; C v is the coefficient of variation; AE is the allowable error.

8. The method according to claim 2, characterized in that When the vegetation coverage after debris flow accumulation in the collection area is higher than 75%, the soil erosion amount in the collection area is 0, and α is 210 The natural decay factor of Pb is 0.031 year -1 .

9. The method according to claim 2, characterized in that The vegetation coverage after debris flow deposition in the collection area is less than 75%, and the soil erosion amount in the collection area is not 0. The α is: λ+h / H, where h is the soil erosion amount, kg m -2 ; H is the mass of surface soil, kg m -2 .

10. The method according to claim 9, characterized in that The calculation formula of h is: h=R×K×LS×C m ×P / 10(5) Where h is the amount of soil erosion, kg / (m 2 ·a); R is the rainfall erosivity factor, (MJ·mm) / (hm 2 ·h·a); K is the soil erodibility factor, (t·hm 2 ·h) / (MJ·mm·hm 2 ) ; LS is the slope length and slope factor, dimensionless; C m is the coverage and management factor, dimensionless; P is the maintenance measure factor, dimensionless.

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