Risk assessment method for blocking and sedimentation in small mountainous watersheds

Abstract

The present invention discloses a risk assessment method for blocking and sedimentation in mountainous small watersheds. Based on the concept of resilient disaster reduction, the method constructs a four-stage macro-analysis model of preparation - resistance and absorption - response and adaptation - resilient recovery from the perspective of the entire life cycle of blocking measures. The latter three risky stages are projected into three observable evaluation windows, namely the pre-full-silting period - the full-silting surrounding period - the post-full-silting period, and the slope erosion and sedimentation hazard degree G , the vulnerability of the blocking engineering body V , and the resilience of the treatment measures R are used to evaluate the risk levels of the three window periods respectively, and then the comprehensive risk level of the entire life cycle is evaluated by the risk degree I indicator in the circulation area. The method uses the meso-analysis of the material characteristics of the pre-dam pile and the disaster-causing evolution, and establishes evaluation schemes applicable to two types of treatment measures, namely geotechnical type and ecological geotechnical type, around the pre-full-silting prediction time model of the dam. The method places the treatment in the circulation area in the perspective of the entire service life cycle of the project, and proposes and solves the problem of disaster risk assessment in the dynamic space-time under the blocking treatment measures.

Description

Technical Field

[0001] The present invention relates to a technology for preventing and controlling disasters in small mountainous watersheds, and particularly to an evaluation method for environmental governance using retaining engineering measures in small watersheds, belonging to the technical fields of water conservancy engineering, data processing technology specifically applicable to environmental management supervision or prediction purposes, and mountain disaster environmental governance technology. Background Art

[0002] The concept of "blocking" is one of the most important technical concepts in the mountain environment governance for debris flow disaster prevention and control, specifically including two measures: blocking and blocking-draining. In the flow area of small mountainous watersheds, various retaining projects mainly in the form of retaining dams and sand retaining dams are the most intuitive manifestations of this technical concept. The retaining project can not only retain some solid-phase substances of the debris flow, reduce the scale of the debris flow, but also stabilize the gully slope, control gully erosion, inhibit the development of the debris flow, and have an obvious effect on the environmental governance of small watersheds. However, since the debris flow disaster is the result of the comprehensive action of various deep environmental factors in the formation area of the small watershed, during the operation and service of the retaining project, affected by the regular flow of the gully and the soil erosion on both sides of the slope, sediment deposition will occur in the area in front of the gully dam. While gradually changing environmental conditions such as the shape of the gully bed, the volume of the reservoir in front of the dam, and the hydrodynamic characteristics of the fluid in front of the dam, a huge sediment deposition body will be formed, becoming a new hidden danger factor for disasters.

[0003] The prior art CN2023105477054 discloses an evaluation method for the disaster risk caused by sediment deposition behind a debris flow dam. Based on the results of the full-process analysis and calculation of the debris flow with different degrees of sediment burial behind the dam by smoothed particle hydrodynamics, taking the scouring and silting scale, flow depth, and flow rate of the debris flow as the key indicators for disaster reduction evaluation, it realizes the quantitative and dynamic evaluation of the disaster risk caused by sediment deposition behind the dam. This technology only solves the problem of evaluating the disaster risk caused by sediment deposition behind the dam. The prior art CN2020105678882 discloses a method for calculating the danger of the sediment deposition body in front of a debris flow dam during the demolition of the retaining dam and its application, which is a scheme for quantitatively evaluating the sediment deposition body in front of the debris flow dam as a potential hazard. However, this technology only solves the danger of the sediment deposition body in front of the dam in the specific project of demolishing the retaining dam, and does not solve the problem of evaluating the danger of the sediment deposition body over a longer time and space period.

[0004] Among different types of retaining measures for environmental governance in small mountainous watersheds, the sediment deposition body in front of the dam formed by the gradual sediment deposition of sediment and stones continuously transported by the gully and both sides of the slope, especially during the relatively long time window of its formation and evolution, what types and levels of disaster-causing properties it has for the retaining project and the flow area of the small watershed are new technical problems that the prior art has not paid attention to and has not solved. Summary of the Invention

[0005] The object of the present invention is to provide a risk assessment technology for retaining and blocking treatment in small mountainous watersheds in view of the deficiencies of the existing technologies. Different types of retaining engineering measures in the circulation area of small mountainous watersheds are provided with assessment indicators at different levels for the disaster-causing design of small watersheds due to the appearance and evolution of sediment deposition in front of the dam during their long-term service operation time, and a quantitative assessment scheme is constructed.

[0006] To achieve the above object, the present invention provides a method for assessing the risk of retaining and siltation in small mountainous watersheds, and its technical solution is as follows.

[0007] A method for assessing the risk of retaining and siltation in small mountainous watersheds,

[0008] Conduct on-site investigations in the circulation area of the small watershed to obtain basic data for assessment. The on-site investigation area includes the construction planning area of debris flow prevention and control projects in the circulation area;

[0009] Use the vulnerability model to calculate the vulnerability of the retaining engineering body V , V The larger the value, the higher the vulnerability degree of the retaining engineering body structure caused by the sediment deposition body in front of the dam during the full-siltation period, and the greater the risk of dam break; the vulnerability model is expressed by Equation 1,

[0010] Equation 1

[0011] In the formula, σ tot -The total lateral pressure of the sediment deposition body in front of the dam of the retaining engineering body, unit kN / m 2 , determined by the basic data for assessment,

[0012] σ des -The maximum designed bearing pressure of the retaining engineering body, unit kN / m 2 , determined by the basic data for assessment.

[0013] In the above method for assessing the risk of retaining and siltation in small mountainous watersheds, the vulnerability of the retaining engineering body V index is used to evaluate the disaster risk level in the circulation area during the full-siltation period. During the full-siltation period, the lateral pressure of the sediment deposition body in front of the dam may gradually exceed the bearing limit value of the retaining dam structure, thus triggering a dam break disaster. Therefore, during this stage, the vulnerability degree of the retaining engineering body is the key potential hidden danger for the occurrence of disasters in the circulation area.

[0014] In the assessment, the risk level interval of the vulnerability of the retaining engineering body V can be delimited as: 0 < V ≤0.2 is low vulnerability, 0.2 < V ≤0.4 is relatively low vulnerability, 0.4 < V ≤0.6 is medium vulnerability, 0.6 < V ≤0.8 is relatively high vulnerability, 0.8 <V ≤1 indicates a high vulnerability level.

[0015] Due to σ tot the large amount of field measurement work and the fact that the measurement operation will increase the disturbance to the dam structure and the sediment deposit in front of the dam, therefore, the present invention provides σ tot a specific calculation model, expressed by Equation 2.

[0016] Equation 2

[0017] In the formula, μ - correction factor, K 0 - coefficient of earth pressure at rest, σ v - vertical stress of the sediment deposit (unit: kN / m 2 );

[0018] The parameters determined according to the evaluation basic data in the formula include: γ w - unit weight of water (unit: kN / m 3 ), h w - water depth in the sediment deposit (unit: m), C g - percentage of the volume of coarse particles in the sediment deposit (%), D 50 , D 90 - reference particle size of the solid particles in the sediment deposit (unit: mm), φ - internal friction angle of the sediment deposit (unit: º), α - volume weight factor, γ d — unit weight of the debris flow deposit (unit: kN / m³), γ s - unit weight of the sediment deposit of the slope surface runoff erosion soil (unit: kN / m³), H - dam height of the retaining engineering structure (unit: m).

[0019] An optimized scheme of the above-mentioned risk assessment method for retaining and siltation in mountainous small watersheds is that it also includes the disaster risk assessment in the pre-full-siltation period. Specifically: The hazard model is used to calculate the hazard degree of slope erosion and siltation G , G The larger the value, the greater the contribution of the slope erosion soil to the evolution process of the sediment deposit in front of the dam and the greater the full-siltation risk in the pre-full-siltation period; the hazard model is expressed by Equation 3.

[0020] Equation 3

[0021] In the formula,t 0 – The sediment deposition time in front of the dam, unit: d, determined according to the basic data for evaluation

[0022] t – The predicted time for full sedimentation in front of the dam under the blocking and treatment, unit: d, determined according to t the measurement plan.

[0023] The above t The measurement plan includes two sets of plans, applicable to different types of blocking and treatment measures respectively. If the blocking and treatment measure is a geotechnical type of treatment measure, it is applicable to t Measurement Plan 1, if it is an ecological geotechnical type of treatment measure, it is applicable to t Measurement Plan 2.

[0024] t Measurement Plan 1: t Take the value t 1, t 1 is calculated according to Equations 4 and 5.

[0025] Equation 4

[0026] Equation 5

[0027] t Measurement Plan 2: t Take the value t 2, t 2 is calculated according to Equations 5 and 6.

[0028] Equation 6

[0029] In the formula, t 1, t 2 – are respectively the t under the action of geotechnical measures and ecological geotechnical measures; V r – The volume of the full sedimentation body (unit: m 3 ), w – The average sedimentation rate of sediment in front of the dam (unit: m / d), w 1, w 2 – are respectively the w under the action of geotechnical measures and ecological geotechnical measures, q i – The soil runoff loss of the soil erosion slope during the i th rainfall (unit: kg), λ – The vegetation correction coefficient;

[0030] The parameters determined according to the basic data for evaluation in the formula include: A – The upper surface area of the full sedimentation body (unit: m 2 ),n - Manning's roughness coefficient for the bank slope (dimensionless) n 1. n 2 - are respectively under the action of geotechnical measures and ecological geotechnical measures n , B - Average width of the full-silt deposition body (unit: m) ΔJ - Variation of the longitudinal slope of the channel with full silt (%) s - Average sediment concentration per unit cross-sectional area of the normal flow in front of the dam (unit: kg / m 3 ), s 1. s 2 - are respectively under the action of geotechnical measures and ecological geotechnical measures s , r - Average hydraulic radius of the slope surface of the soil and water erosion bank slope in the investigation area (unit: m) J - Longitudinal slope of the slope surface of the soil and water erosion bank slope in the investigation area (%) U - Average hydraulic radius in front of the dam (unit: m) D - Effective grain size of the sediment in the normal flow in front of the dam (unit: mm) ρ - Average bulk density of the sediment in the normal flow in front of the dam (unit: kg / m 3 ), N - Number of rainfall events from empty reservoir to evaluation period p t , p h 、p s - Percentage of the distribution area of arbors, herbs, and shrubs in the investigation area (%) L - Orthogonal area of the slope surface of the soil and water erosion bank slope (unit: m 2 ), P i -[[]]END]] N In the i th rainfall event within the

[0031] In the early stage of full siltation, the main hidden danger of the disaster comes from the deposition speed of the sediment in front of the dam. The above evaluation method uses the G index of slope erosion and siltation risk to evaluate the contribution degree of the sediment deposition body transported by surface runoff to the evolution process of the sediment deposition body in front of the dam after soil erosion on the slope surface. The higher the contribution degree, the higher the proportion of the sediment transport volume in the surface runoff in the sediment deposition body in front of the dam, that is, the deposition speed in front of the dam is more affected by the daily flow. Even excluding the influence of sudden events of large-scale material transportation (such as debris flow, hyperconcentrated flow, collapse, landslide, etc.), the risk of full siltation in front of the dam is still relatively high.

[0032] In the evaluation, the risk level interval of the G index of slope erosion and siltation risk can be defined as: 0 < G ≤ 0.2 is a low risk level, 0.2 <G ≤ 0.4 represents a lower risk level, 0.4 < G ≤ 0.6 represents a medium risk level, 0.6 < G ≤ 0.8 represents a relatively high risk level, 0.8 < G ≤ 1 represents a high risk level.

[0033] The optimization plan for the above-mentioned risk assessment method of small mountainous watersheds' retaining and siltation includes the disaster risk assessment in the late stage of full siltation. Specifically: for ecological geotechnical treatment measures, the toughness model is used to calculate the toughness of the treatment measures R , R The larger the value, the higher the ability of the debris flow prevention and control measures to absorb the destructive energy of debris flow and solidify soil and reduce sand in the late stage of full siltation, and the smaller the disaster risk in the flow area; the toughness model is expressed by Equation 7.

[0034] Equation 7

[0035] In the formula, τ c − The normalized critical shear strength at the beginning of soil erosion in the surveyed area (unit: Pa);

[0036] The parameters determined according to the evaluation basic data in the formula include: NDVI − The normalized vegetation index in the flow area (dimensionless), VC − The vegetation coverage in the surveyed area (%), τ c,RMD − The critical shear strength of the soil in the surveyed area under the root mass density RMD (unit: Pa), τ c,0 − The critical shear strength of bare soil in the surveyed area (unit: Pa).

[0037] The ability to improve and repair the environment by making full use of ecological means is the reason why the ecological geotechnical treatment concept has been introduced into the field of small watershed disaster prevention and control technology and has been increasingly valued. In the late stage of full siltation, without artificial disaster reduction interventions such as dredging, the disaster risk level in the flow area is mainly affected by its own and even the regulation ability of the small watershed ecosystem. In the late stage of full siltation, the channel bed in the flow area rises and widens more severely, gradually evolving into a main channel - floodplain binary structure. In the areas where ecological geotechnical treatment is implemented, the binary structure environment becomes a favorable environment for the model community in the process of ecological restoration. Relying on the updated power of the species community and ecosystem constructed in the ecological treatment means, over time, the effects of soil solidification, sand reduction, siltation stopping, rainfall interception, and splash erosion reduction of ecological measures are enhanced, becoming the toughness recovery force for the flow area to digest disaster destructive energy, and can alleviate the soil and water disaster risks in the small watershed, especially in the downstream area.

[0038] In the assessment, the risk level range of the toughness of the treatment measures R can be delimited as: 0 < R≤0.2 represents low toughness, 0.2 < R ≤0.4 represents relatively low toughness, 0.4 < R ≤0.6 represents medium toughness, 0.6 < R ≤0.8 represents relatively high toughness, 0.8 < R ≤1 represents high toughness.

[0039] The optimization plan of the above-mentioned risk assessment method for the small mountainous watershed's retaining and siltation is that it also includes the comprehensive risk assessment of the flow-through area during the entire life cycle of the retaining and prevention engineering measures. Specifically: The risk degree of the flow-through area is calculated using the risk degree model I , I The larger the value, the greater the comprehensive disaster risk of the flow-through area during the entire life cycle of the retaining and prevention engineering measures; the risk degree model is expressed by Equation 8; for the geotechnical treatment measures, R = 0.

[0040] Equation 8

[0041] The risk degree of the flow-through area I represents the comprehensive disaster risk level of the flow-through area during the entire life cycle perspective of the complete pre-preparation - resistance and absorption - response and adaptation - resilience recovery service process under the retaining treatment.

[0042] In the assessment, the risk level range of the risk degree of the flow-through area I can be demarcated as: for the ecological geotechnical treatment measures, 0 < I ≤0.024 represents low risk, 0.024 < I ≤0.064 represents relatively low risk, 0.064 < I ≤0.128 represents medium risk, 0.128 < I ≤0.216 represents relatively high risk, I > 0.216 represents high risk; for the geotechnical treatment measures, I ≤0.04 represents low risk, 0.04 < I ≤0.16 represents relatively low risk, 0.16 < I ≤0.36 represents medium risk, 0.36 < I ≤0.64 represents relatively high risk, I > 0.64 represents high risk.

[0043] The inventive concept of the above-mentioned risk assessment technical solution for the small mountainous watershed's retaining and siltation of the present invention includes the following two sets of key concepts constructed based on theoretical analysis.

[0044] Concept Group 1: Pre-dam deposit body, pre-dam accumulation body, pre-dam siltation body. The pre-dam mud, sand, and stone block heap in the retaining dam is a mixture of mud, sand, and stone blocks that is transported to the pre-dam area by the water flow in the circulation area and then stagnates and accumulates. Depending on the type of water flow, the material sources and their properties are different. During different periods of the operation of the retaining dam, with the change of the circulation area environment, the scale and material composition of the pre-dam heap are different, and the structure and dynamic characteristics of the heap change. In order to accurately describe the stage differences in the properties and evolution characteristics of the pre-dam heap and accurately measure its dynamic characteristics, the present invention constructs a group of concepts for describing the pre-dam heap, including pre-dam deposit body, pre-dam accumulation body, and pre-dam siltation body. Among them, the pre-dam accumulation body refers to the heap formed by the mud, sand, and stone block materials carried by the debris flow and transported to the pre-dam area; the pre-dam siltation body refers to the heap formed by the soil erosion soil materials on the slope transported to the pre-dam area by surface runoff; the pre-dam deposit body refers to the superimposed combination of the former two.

[0045] Concept Group 2: Risk time-course stage and evaluation period. When the small watershed system is affected by external impacts, it enters the resilience response process and successively experiences four stages: "preparation - resistance and absorption - response and adaptation - resilience recovery". Among them, the risk in the first stage can be ignored, and there are risks in the latter three stages and they have time-course change characteristics. For the implementation of risk assessment, the latter three stages in the theoretical framework are projected into three evaluation windows at the observational level, namely the pre-full-siltation period - the full-siltation period - the post-full-siltation period. The full-siltation period refers to the observational period before and after the full-siltation state of the retaining dam in practice; the pre-full-siltation period refers to the observational period from the occurrence of the pre-dam heap siltation and deposition phenomenon to the entry into the full-siltation period; the post-full-siltation period refers to the observational period after the full-siltation period.

[0046] The on-site investigation referred to in this technology includes various geological surveys, reconnaissances, surveys, and measurements of the mountainous small watershed where the engineering measures are located, as well as existing simulation experiments, test experiments, observational experiments, and analysis experiments in the field, as well as the acquisition of historical disaster records, relevant technical specifications, and experience methods and data acquisition with reference and reference functions. The data obtained from the on-site investigation are collectively referred to as the evaluation basic data of this technical solution.

[0047] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Planning and constructing retaining engineering measures in the circulation area of small mountainous watersheds is a transformation of the disaster environment of small watersheds, aiming to achieve disaster prevention and environmental governance, which complement each other. During the operation and service of the retaining project, the environmental conditions in the circulation area change, and the level of environmental disaster risk also changes accordingly. The present invention provides a long-term assessment technical concept for the disaster risk in the circulation area of small watersheds with the operation and service of retaining prevention and control engineering measures as the window period, so as to evaluate the disaster risk under the retaining treatment measures in the circulation area of small watersheds, which is a new technical concept different from the short-term disaster-causing assessment of the prior art. For the two common types of measures in the current debris flow prevention and control engineering measures, namely pure geotechnical type and ecological geotechnical combination type, the evaluation method of the present invention is based on the characteristics of their different service stages, and specifically provides two different evaluation schemes under the same technical concept. (2) The method of the present invention is based on the concept of resilient disaster reduction. First, a complete macro-analysis model of the four service stages is constructed from the perspective of the entire life cycle of the retaining prevention and control engineering measures, and then combined with the meso-analysis of the material source characteristics and disaster-causing evolution of the sediment and rock block piles in front of the dam, a risk assessment scheme for the second to fourth stage windows is established. Based on the previous analysis results of the influence of the sediment deposition in front of the dam on the channel morphology and hydrodynamic conditions, the assessment scheme takes the full siltation in front of the dam under the retaining treatment measures as the core perspective, and determines three assessment windows: the pre-full siltation period, the full siltation period, and the post-full siltation period at the observation level. The assessment method introduces the G hazard degree of slope erosion and sedimentation V vulnerability of the retaining engineering body R and resilience degree of the treatment measures I to evaluate the disaster attributes and levels in the circulation area during the pre-full siltation period, full siltation period, and post-full siltation period respectively. On this basis, the risk degree index of the circulation area is introduced to evaluate the comprehensive risk level from the perspective of the entire life cycle. (3) The technical solution of the present invention takes the planning and construction area of the retaining project in the circulation area as the evaluation environment, including the "ternary pattern area" composed of the channels, floodplains, and slopes where the debris flow prevention and control measures are distributed, and measures the environmental governance benefits of the debris flow retaining prevention and control engineering measures in the entire life cycle of the project operation and service. It is the first to propose the problem of disaster risk assessment in the dynamic time and space under the retaining treatment measures in the circulation area of small mountainous watersheds and solve this problem. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of the risk assessment framework for retaining sedimentation in small mountainous watersheds. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] The following is a further description of the preferred embodiments of the present invention with reference to the drawings.

[0050] Embodiment 1

[0051] Conduct a risk assessment on the environmental governance of the circulation area of a small watershed in a mountainous area.

[0052] A debris flow retaining dam project has been built in the circulation area of a small watershed in a mountainous area to treat the environment. During the operation of the project, due to the long-term sediment transportation caused by the daily sand-bearing water flow in the gully and the soil erosion on both sides of the slope, siltation and deposition occur in front of the dam. The method of the present invention is used to evaluate the disaster risk level of the circulation area of the small watershed.

[0053] 1. Obtain basic data

[0054] Carry out on-site investigations in the small watershed to obtain basic data for evaluation. The on-site investigation area is the construction planning area of the debris flow prevention and control project in the circulation area, including the gully, the floodplain, the bank slope, etc.

[0055] After investigation and confirmation, the debris flow prevention and control measures in the circulation area are comprehensive prevention and control projects with retaining dams as the main body. The prevention and control measures take the geotechnical engineering body of the retaining dam in the gully as the core, and there are tree, shrub and grass vegetation for environmental restoration and transformation planted on both sides of the floodplain and the bank slope, forming ecological engineering measures. The overall debris flow prevention and control measures in the circulation area belong to ecological geotechnical type governance measures.

[0056] The basic data for evaluation obtained are shown in Tables 1 to 3. The classification of the data in the tables is not a strict type division, only to make the meaning of the listed data clear and easy to read. The same applies hereinafter.

[0057] 2. Evaluate the debris flow disaster risk level in the small watershed

[0058] In order to comprehensively evaluate the disaster risk level at each stage of the full cycle from the start of siltation and deposition in front of the dam to the termination of the environmental ecological self-repair that can be achieved by the ecological geotechnical type governance measures during the operation of the above-mentioned retaining dam in the small watershed, 4 indicators are introduced in this example for full-cycle risk assessment.

[0059] 2.1 Use the slope erosion and siltation hazard G Index evaluation

[0060] Use t Calculation scheme two to calculate the predicted time of full siltation in front of the dam under the retaining treatment t 2. The basic data are shown in Table 1.

[0061] Table 1 Basic data for evaluation in Example 1

[0062]

[0063] The parameters in Table 1 are determined by conventional investigation methods. Among them, the roughness coefficient n 2 Query Table C.2.1-1 in the "Code for Investigation and Prevention and Control Design of Debris Flows in Hydropower Projects NB / T 10139-2019", sDetermined according to the in-situ measurement results, the number of rainfall events N and the rainfall amount P i are determined according to the observation records λ The determination of p t first obtains the proportion of the distribution areas of arbors, herbs, and shrubs in the surveyed area p h 、p s , and then calculates according to the 4th formula of Equation 6

[0064] According to the parameters r and J and n 2 s 2 U and D data, calculate the average sedimentation rate of the sediment deposition in front of the dam under the ecological geotechnical measures according to Equation 5 and the 3rd formula of Equation 6 w 2 = 0.03 m / d; According to the parameters λ and L and J and P i (taking a certain P i = 80 mm as an example) data, calculate and determine the soil runoff loss amount of the soil erosion slope during the i th rainfall according to the 2nd formula of Equation 6 q i = 3.30×10 5 Kg; According to the parameters N, q i and ρ, A, w 2 data, calculate and determine t 2 = 0 d to 293 d. In the Equation 6 model, the value method of the weight factor α is: if there is no debris flow occurrence during the evaluation period, α＝ 1, if the reservoir is full with one debris flow during the evaluation period, α＝ 0, for other cases, it is determined according to the ratio of the volume of the sediment deposit in front of the dam during the evaluation period to the reservoir capacity e (%), α＝ 1 - e .

[0065] Substitute t 2 t 0 data into Equation 3, and calculate to get the hazard degree G = 0.61

[0066] Evaluation results G= 0.61 indicates that, since the spontaneous deposition in front of the dam in the flow-through area, during the early stage of full siltation, the deposition in front of the dam formed by the amount of sediment transported by soil erosion on the slope contributes highly to the evolution process of the sediment body in front of the dam. Even if there are no sudden large-scale sediment deposition events in the small watershed, the risk of full siltation in front of the dam is still relatively high.

[0067] 2.2 Using the vulnerability of the retaining engineering structure V Index evaluation

[0068] Due to the large workload of actual on-site measurement of the total lateral pressure of the sediment body in front of the retaining engineering structure σ tot In this example, the model is used to calculate this parameter. All the parameter indexes in Table 2 are obtained by conventional measurement methods.

[0069] Table 2 Basic data for evaluation of Example 1, Part II

[0070]

[0071] According to the parameters α = 1, γ d , H , γ s , φ , C g , D 50 , D 90 , γ w , h w Data, calculated according to Equation 2 of the model, the correction factor μ = 1.01, the coefficient of earth pressure at rest K 0 = 0.5, the vertical stress of the sediment body in front of the dam σ v = 225 kN / m 2 , σ tot = 152.87 Pa. Further, according to σ tot , σ des Calculated according to Equation 1 of the model to determine the vulnerability V = 0.61.

[0072] Evaluation results V = 0.61 indicates that during the full siltation period in front of the retaining dam, that is, the time period before and after the full siltation state of the reservoir capacity, the possibility of structural damage caused by the sediment body in front of the dam is relatively high, and the risk of dam break in the flow-through area is relatively high.

[0073] 2.3 Adopt the toughness degree of governance measures R Index evaluation

[0074] All the parameter indexes in Table 3 are obtained by conventional measurement methods. Among them, τ c,RMD 、 τ c,0 are all determined by in-situ test results.

[0075] Table 3 Evaluation basic data three

[0076]

[0077] According to the parameters τ c,RMD 、 τ c,0 、 NDVI 、 VC data, calculated according to Equation 7 model, there is τ c = 0.57 Pa, toughness degree R = 0.62.

[0078] Evaluation results R = 0.62 indicates that in the late stage of full siltation, the debris flow prevention and control measures in the circulation area have a high ability to absorb disaster damage energy and solidify soil and reduce sand, and can relieve the debris flow disaster risk in the small watershed to a certain extent.

[0079] 2.4 Adopt the risk degree of the circulation area I Index evaluation

[0080] Substitute the V 、 G 、 R index scores into Equation 8 model, and calculate the risk degree I = 0.14. It shows that in this example, the circulation area of the small watershed faces a relatively high comprehensive disaster risk during the whole cycle of the retaining and siltation prevention engineering measures.

[0081] Figure 1 is a schematic diagram of the risk assessment framework for retaining and siltation in mountainous small watersheds. In the figure, the horizontal axis represents the service operation time of the retaining and siltation prevention engineering measures; the row of "risk time course of toughness response" below the horizontal axis represents the four stages of the risk time course of the toughness response process of the small watershed system in the theoretical framework from the perspective of the whole cycle; the row of "window period of observation level" above the horizontal axis represents the window period of the four stages of the risk time course projected onto the observation level in the theoretical framework, and the latter three are evaluation windows; the corresponding relationship between the four evaluation indexes in the evaluation method of the present invention and the evaluation windows is shown above the row of "window period of observation level".

[0082] Example 2

[0083] Conduct a risk assessment on the environmental governance of the circulation area of a small mountainous watershed.

[0084] A debris flow retaining dam project has been built in the circulation area of a small mountainous watershed to control the environment. During the operation of the project, sediment deposition in front of the dam occurred. The method of the present invention is used to evaluate the disaster risk level of the circulation area of the small watershed.

[0085] 1. Obtain basic data

[0086] Adopt the same on-site investigation method as in Example 1 to obtain the basic data for evaluation. Confirm that the debris flow prevention and control measures in the circulation area in this example belong to geotechnical treatment measures. The basic data obtained are shown in Tables 4 to 5.

[0087] 2. Evaluate the debris flow disaster risk level in the small watershed

[0088] To comprehensively evaluate the risk level, three indicators are introduced in this example for full-cycle risk assessment.

[0089] 2.1 Use the slope erosion and sedimentation hazard G Index evaluation

[0090] Use t Calculation plan 1 to calculate the predicted time of full sedimentation in front of the dam under the retaining treatment t 1. The basic data are shown in Table 4.

[0091] Table 4 Basic data for evaluation in Example 2 - 1

[0092]

[0093] The parameters in Table 4 are determined by conventional investigation methods. Among them, the roughness coefficient n 1 Query Table C.2.1 - 1 in the "Code for Investigation and Prevention Design of Debris Flows in Hydropower Projects NB / T 10139 - 2019", s 1 Determine according to in-situ measurement results. ΔJ is the change in longitudinal slope ratio of the channel before and after full sedimentation.

[0094] Use the same method as in Example 1 to determine the weight factor α value; according to the parameters r 、 J 、 n 1、 s 1、 U 、 D 、 ΔJ 、H, B, W, α = 1 data, calculate according to Equation 5 and Equation 4 models, w 1 = 0.08 m / d, V r = 1.19 m / s, t1 = 204d; Substitute t 1, t the 0 data into Equation 3, and calculate that the risk degree G = 0.88.

[0095] 2.2 Adopt the vulnerability of the retaining engineering structure V index for evaluation

[0096] Table 5 Basic data for evaluation in Example 2 - Part 2

[0097]

[0098] Adopt the same method as in Example 1, and calculate and determine the vulnerability according to the vulnerability model V = 0.61.

[0099] 2.3 Adopt the risk degree of the circulation area I index for evaluation

[0100] Substitute V , G , R the 0 index score into Equation 8 model, and calculate that the risk degree I = 0.54. It shows that in this small watershed during the whole cycle of the retaining prevention and control engineering measures, the comprehensive disaster risk faced is relatively high.

Claims

1. A risk assessment method for silting up small watersheds in mountainous areas, characterized by: Conduct on-site investigations in the small watershed circulation area to obtain basic data for assessment. The on-site investigation area includes the planned construction area of ​​debris flow prevention and control projects in the circulation area; Using vulnerability model to calculate the vulnerability of blocking engineering body V , V The larger the value, the higher the vulnerability of the retaining structure caused by the sediment in front of the dam during the full siltation period, and the greater the risk of dam failure. The vulnerability model is expressed according to formula 1: Formula 1 Formula 2 In the formula, σ tot - Total lateral pressure of sedimentary body in front of the dam, unit kN / m 2 , determined according to formula 2, σ des - Maximum design bearing pressure of the retaining project, unit: kN / m 2 , determined based on the basic data of the assessment; μ - Correction factor, K 0 - static earth pressure coefficient, σ v - Vertical stress of sediment, unit kN / m 2 , γ w - Water density, in kN / m 3 , determined based on the basic data of the assessment, h w - Water depth in the sediment, in meters, determined based on the basic data for the assessment, C g - Volume percentage of coarse particles in sediments, %, determined based on basic assessment data, D 50 , D 90 - Reference particle size of solid particles in sediment, in mm, determined based on the basic data of the assessment, φ - Friction angle of sediment, unit: °, determined according to the basic data of the assessment, α - Volume weight factor, determined based on the basic data of the assessment, γ d —Debris flow accumulation weight, unit kN / m³, determined based on the basic data of the assessment, γ s - Weight of soil deposits from runoff erosion on the bank slope, in kN / m³, determined based on the basic data of the assessment. H - Height of retaining dam, in meters, to be determined based on basic assessment data.

2. According to the method for risk assessment of siltation in small watersheds in mountainous areas according to claim 1, it is characterized by: Volume weight factor α The value is obtained by: If no debris flow occurs during the assessment period, α＝ 1. If the reservoir is full due to a debris flow during the assessment period, α＝ 0, and in other cases, the percentage of the sediment volume in front of the dam to the reservoir capacity during the assessment period e Sure, α＝ 1- e .

3. The method for assessing the risk of silting up in a small watershed in a mountainous area according to claim 1 or 2, characterized in that: 0< V ≤0.2 is low vulnerability, 0.2< V ≤0.4 is a lower vulnerability, 0.4< V ≤0.6 is medium fragility, 0.6< V ≤0.8 is a higher vulnerability, 0.8< V ≤1 indicates high vulnerability.

4. The method for assessing the risk of silting up in a small watershed in a mountainous area according to claim 1 or 2, characterized in that: Using the hazard model to calculate the hazard level of slope erosion and sedimentation G , G The larger the value, the greater the contribution of slope erosion soil to the evolution of sedimentary bodies in front of the dam in the early stage of full siltation, and the greater the risk of full siltation. The hazard model is expressed according to formula 3: Formula 3 In the formula, t 0 - dam front siltation and sedimentation time, unit: d, determined based on the basic data of the assessment. t - Prediction time of full siltation in front of the dam under interception and management, unit: d, based on t The calculation plan is determined; if the blocking and control measures are geotechnical control measures, t Calculation scheme 1: if it is an ecological geotechnical treatment measure, adopt t Calculation scheme 2; t Calculation plan 1: t Value t 1, t 1 Calculated according to formula 4 and formula 5, Formula 4 Formula 5 t Calculation scheme 2: t Value t 2, t 2 Calculated according to formula 5 and formula 6, Formula 6 In the formula, t 1. t 2- respectively under the effects of geotechnical measures and ecological geotechnical measures t , V r - Volume of fully silted sediment, in m 3 , A - The upper surface area of ​​the fully silted sediment, in m 2 , determined based on the basic data of the assessment, w - Average sedimentation rate of mud and sand in front of the dam, unit: m / d, w 1. w 2- respectively under the effects of geotechnical measures and ecological geotechnical measures w , n - The roughness coefficient of the bank slope is determined based on the basic data of the assessment. n 1. n 2- respectively under the effects of geotechnical measures and ecological geotechnical measures n , α - Volume weighting factor, H - Height of the retaining dam, in meters, determined based on the basic data of the assessment. B - Average width of the fully silted sediment body, in meters, determined based on the basic data of the assessment. ΔJ - Change in channel longitudinal gradient due to siltation, %, determined based on the basic data of the assessment. s - Average sediment content per unit cross-section of the water flowing in front of the dam, unit: kg / m 3 , determined based on the basic data of the assessment, s 1. s 2- respectively under the effects of geotechnical measures and ecological geotechnical measures s , r - Average hydraulic radius of the slope surface of the water and soil erosion in the survey area, in meters, determined based on the basic data of the assessment, J - Vertical slope gradient of the bank slope of water and soil erosion in the survey area, %, determined based on the basic data of the assessment, U - Average hydraulic radius in front of the dam, in meters, determined based on the basic data of the assessment, D - Effective particle size of cement sand flowing in front of the dam, in mm, determined based on the basic data of the assessment, ρ - Average bulk density of cement sand flowing in front of the dam, unit: kg / m 3 , determined based on the basic data of the assessment, N - The number of rainfalls from empty reservoir to the assessment period is determined based on the basic assessment data. q i -No. i The amount of soil runoff loss from the slope due to soil erosion during the rainfall, in kg, λ - Vegetation correction factor, p t , p h 、p s - The percentage of the distribution area of ​​trees, herbs and shrubs in the survey area, %, is determined based on the basic data of the assessment. L - Orthotropic area of ​​the slope of the soil erosion bank, unit: m 2 , determined based on the basic data of the assessment, P i - N Rainfall Nedi i The amount of rainfall per rainfall event, in mm, is determined based on the basic data for assessment.

5. The risk assessment method for silting up of small watersheds in mountainous areas according to claim 4 is characterized by: 0< G ≤0.2 is low risk, 0.2< G ≤0.4 is a lower risk, 0.4< G ≤0.6 is moderate risk, 0.6< G ≤0.8 is a higher risk, 0.8< G ≤1 indicates high risk.

6. The risk assessment method for silting up of small watersheds in mountainous areas according to claim 4 is characterized by: For ecological geotechnical treatment measures, the toughness model is used to calculate the toughness of the treatment measures. R , R The larger the value, the higher the ability of debris flow prevention measures to absorb debris flow destructive energy and consolidate soil and reduce sand in the late stage of full siltation, and the smaller the disaster risk in the flow area; the toughness model is expressed according to formula 7, Formula 7 In the formula, NDVI - Normalized difference vegetation index in the circulation area, dimensionless, determined based on the basic data of the assessment, VC - Vegetation coverage of the survey area, %, determined based on the basic data of the assessment, τ c - Normalized critical shear strength of soil erosion at the beginning of the survey area, in Pa, τ c,RMD - Critical shear strength of soil in the survey area under root mass density RMD, in Pa, determined based on the basic data of the assessment, τ c,0 - Critical shear strength of bare soil in the survey area, in Pa, determined based on basic assessment data.

7. The method for risk assessment of siltation in small watersheds in mountainous areas according to claim 6 is characterized by: 0< R ≤0.2 means low toughness, 0.2< R ≤0.4 is lower toughness, 0.4< R ≤0.6 is medium toughness, 0.6< R ≤0.8 indicates higher toughness, 0.8< R ≤1 means high toughness.

8. The method for assessing the risk of silting up in a small watershed in a mountainous area according to claim 6 or 7, characterized in that: Using risk model to calculate the risk of circulation area I , I The larger the value, the greater the comprehensive disaster risk of the circulation area in the whole cycle of the blocking and prevention engineering measures; The risk model is expressed as Equation 8: Formula 8 In the formula, G - Slope erosion and sedimentation risk, V - The vulnerability of the blocking works, R - Toughness of control measures, for geotechnical control measures, R =0.

9. The risk assessment method for silting up of small watersheds in mountainous areas according to claim 8 is characterized by: For ecological geotechnical treatment measures, 0< I ≤0.024 is low risk, 0.024< I ≤0.064 is a lower risk, 0.064< I ≤0.128 is medium risk, 0.128< I ≤0.216 indicates a higher risk. I >0.216 is high risk; for geotechnical control measures, I ≤0.04 is low risk, 0.04< I ≤0.16 is a lower risk, 0.16< I ≤0.36 is medium risk, 0.36< I ≤0.64 indicates a higher risk. I >0.64 indicates high risk.

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