A hydrological model-based gully debris flow early warning method

Abstract

The application discloses a kind of steep gully debris flow early warning method based on hydrological model, belong to debris flow prevention engineering technical field, it is characterized in that, comprising the following steps: a, determine the area of potential debris flow basin and the longitudinal slope of debris flow basin formation area gully bed of debris flow basin formation area;B, real-time monitoring or forecasting the rainfall process of the location where debris flow basin formation area is located;C, field investigation measures the channel width of debris flow basin formation area, source permeability coefficient, source thickness, source saturation density and source particle angle of repose;D, calculate the overall instability condition of debris flow basin formation area gully bed;E, according to HEC-HMS model, calculate the flood peak flow caused by rainfall;F, calculate the occurrence index P of debris flow, and judge the occurrence of debris flow.The application is aimed at small watershed steep gully debris flow influencing factor research, make quantitative accurate judgment, suitable for steep gully debris flow early warning, can effectively improve the precision of early warning.

Description

Technical Field

[0001] This invention relates to the field of debris flow prevention and control engineering technology, and in particular to a method for early warning of debris flows in steep gullies based on a hydrological model. Background Technology

[0002] Following a strong earthquake, numerous landslides and collapses are triggered in the severely affected area, providing a large source of solid material for subsequent debris flows. Domestic and international research indicates that multiple debris flows, sometimes even clustered, often occur during post-earthquake rainfall in areas affected by strong earthquakes. Watersheds that were not originally debris flow gullies may evolve into debris flow basins under the influence of strong earthquakes; conversely, watersheds that were originally debris flow gullies may evolve into high-frequency debris flow basins, with larger-scale debris flows and lower induction conditions. Regarding the induction conditions for debris flows, some watersheds that were not debris flow gullies before a strong earthquake were unlikely to induce debris flows, or had very high induction conditions; some watersheds that were already debris flow gullies before a strong earthquake had very high induction conditions. However, after a strong earthquake, landslides and collapses may occur in debris flow basins, as well as potential landslides and collapses, significantly changing the original induction conditions for debris flows and drastically reducing the critical conditions. The critical conditions for mudslides to occur decrease sharply after a strong earthquake, but increase year by year during the subsequent rainy season until they return to pre-earthquake levels.

[0003] For debris flow basins in strong earthquake zones, the solid material source conditions of the basin have changed significantly due to the impact of strong earthquakes. The critical induction conditions for debris flows in a single gully vary greatly. Even in adjacent debris flow basins, the critical induction conditions may be completely different due to the different landslide and collapse situations and debris flow source conditions. However, accurate quantitative early warning of debris flow occurrence in a single gully is very important and is one of the main prevention and mitigation measures.

[0004] Chinese patent document CN109448325A, published on March 8, 2019, discloses a refined early warning method for debris flows based on one-hour rainfall. The method comprises the following steps: a) determining the area A, shape coefficient F, gully length L, and longitudinal slope J of the debris flow basin; b) obtaining the annual average rainfall R0 and the 10-minute rainfall variation coefficient Cv, and real-time monitoring of the preceding rainfall B and the rainfall I one hour before triggering the debris flow; c) determining the average gully width W and particle size D; d) calculating the topographic factor T; e) calculating the geological factor G; f) calculating the rainfall factor R; g) calculating the debris flow occurrence index P; and h) determining the occurrence of the debris flow.

[0005] The patent document discloses a refined early warning method for debris flows based on one-hour rainfall. By studying the topographic and geological features and rainfall characteristics of the debris flow formation area, it uses the rainfall in the hour preceding the event as the key indicator, avoiding potential misjudgments when there is no preceding rainfall in the case of shorter rainfall durations, thus achieving refined early warning. However, this method mainly considers the mechanism and process of debris flow formation in general gullies, where the bed sediment is eroded by water flow, starting from surface coarse particles, gradually initiating the coarsening layer, and then peeling away the bed. For some small watersheds, with a drainage area of ​​5 km² or even less than 2 km², due to the very steep gullies and a longitudinal gradient of 25% or more, debris flows may occur even with relatively small water flow rates, causing the entire bed material to become unstable and thus forming a debris flow. The occurrence of such phenomena means that the rainfall in some small watersheds that lead to debris flows is very small, making it impossible to use the above-mentioned methods for early warning. Since the method in this patent document does not take into account the actual conditions of debris flow formation due to the overall instability of the gully bed material source in steep gullies of small watersheds, it is not suitable for early warning of debris flows in steep gullies, and the early warning accuracy is poor. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a method for early warning of debris flows in steep gullies based on a hydrological model. This invention studies the influencing factors of debris flows in steep gullies in small watersheds, makes quantitative and accurate judgments, is suitable for early warning of debris flows in steep gullies, and can effectively improve the accuracy of early warning.

[0007] This invention is achieved through the following technical solution:

[0008] A method for early warning of debris flows in steep gullies based on a hydrological model, characterized by the following steps:

[0009] a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps;

[0010] b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event;

[0011] c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area;

[0012] d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area;

[0013] e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model;

[0014] f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow.

[0015] In step a, the debris flow basin formation area refers to the area above the debris flow flow area and the debris flow deposition area. When there are several tributary ditches in the basin, the tributary ditches basin is the debris flow basin formation area of ​​the tributary ditches.

[0016] In step d, the overall instability condition of the gully bed in the debris flow basin formation area is calculated using Equation 1;

[0017] J=tanα≥(1-c)tanθ Equation 1

[0018] In the formula, J is the longitudinal slope of the gully bed in the debris flow basin formation area, α is the gully slope in the debris flow basin formation area, c is a coefficient, and θ is the angle of repose of the source particles.

[0019] The coefficient c is calculated using Equation 2;

[0020] c = 1 / ρ s Formula 2

[0021] In the formula, ρ s The density of the source material is saturated with water.

[0022] In step f, the occurrence index P of debris flow is calculated using Equation 3;

[0023] P = Q / (SWH) Equation 3

[0024] In the formula, Q is the peak flow of flash flood, S is the source permeability coefficient, W is the channel width, and H is the source thickness.

[0025] In step f, determining whether a debris flow has occurred specifically means that when P < 0.9, the probability of a debris flow occurring is low; when 1.0 > P ≥ 0.9, the probability of a debris flow occurring is moderate; and when P ≥ 1, the probability of a debris flow occurring is high.

[0026] The HEC-HMS model described in this invention is a computer program developed by the U.S. Army Corps of Engineers Hydrographic Center to simulate the evolution of watershed rainfall-runoff and flood processes.

[0027] The basic principle of this invention is as follows:

[0028] Debris flows in steep gullies of small watersheds are formed when flash floods caused by rainfall saturate the gully bed with water. After the solid material in the gully bed becomes saturated, under conditions of steep slope and the inherent properties of the bed material, such as the angle of repose of the source particles and the density of the saturated material, overall instability may occur, leading to a debris flow. If the probability of occurrence is too low, such as P < 0.9, the flash flood caused by rainfall will only cause localized saturation within the gully formation area, making it difficult to initiate a full-scale movement, and the probability of occurrence is small; conversely, if P ≥ 1, the probability of occurrence is high. Reasons for a low probability of debris flow occurrence include:

[0029] 1. The slope of the formation zone channel is too small, which prevents the overall initiation of the source material in the channel bed; the angle of repose of the source material particles is too large, which does not meet the conditions of Equation 1; the saturated density of the source material is too large, which does not meet the conditions of Equation 1; the longitudinal gradient of the formation zone channel bed is too small, which does not meet the conditions of Equation 1.

[0030] 2. Insufficient rainfall: The infiltration coefficient of the sediment source is too high, and the flash flood formed by the rainfall cannot rise to a sufficient water level before flowing away, resulting in a low P value (P < 0.9); The gully width is too high, and the flash flood formed by the rainfall spreads in a wide gully, failing to raise the water level sufficiently, resulting in a low P value (P < 0.9); The sediment source thickness is too high, and the flash flood formed by the rainfall cannot raise the water level sufficiently, resulting in a low P value (P < 0.9); The rainfall amount is insufficient, and the peak flow of the flash flood formed by the rainfall is insufficient to raise the water level sufficiently before flowing away, resulting in a low P value (P < 0.9).

[0031] The beneficial effects of this invention are mainly reflected in the following aspects:

[0032] I. This invention studies the influencing factors of debris flows in steep gullies of small watersheds, makes accurate quantitative judgments, is suitable for early warning of debris flows in steep gullies, and can effectively improve the accuracy of early warning.

[0033] Second, this invention studies the influence of the conditions for debris flow occurrence in steep gullies of small watersheds on the formation of debris flows, and provides accurate calculation indicators, especially the angle of repose of the source particles and the source water saturation density, so as to make the early warning of debris flow in steep gullies of small watersheds more accurate.

[0034] Third, this invention studies the influence of indicators for judging the probability of debris flow in steep gullies of small watersheds on the formation of debris flow, especially the source permeability coefficient, gully width and source thickness, so as to make the early warning of debris flow more accurate.

[0035] IV. This invention, by studying the overall instability initiation and formation mechanism of debris flows in steep gullies of small watersheds, and combining it with the HEC-HMS hydrological model, proposes a quantitative method and index for calculating the probability of debris flows occurring in steep gullies of small watersheds, which has good applicability.

[0036] V. This invention studies the conditions for debris flows in steep gullies of small watersheds and the influence of probability assessment indicators on debris flow formation. Combined with the HEC-HMS hydrological calculation model, it provides flow calculation and assessment indicators under rainfall conditions, thereby improving the accuracy of debris flow early warning. Detailed Implementation

[0037] Example 1

[0038] A method for early warning of debris flows in steep gullies based on a hydrological model includes the following steps:

[0039] a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps;

[0040] b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event;

[0041] c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area;

[0042] d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area;

[0043] e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model;

[0044] f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow.

[0045] This embodiment is the most basic implementation method. It studies the influencing factors of debris flows in steep gullies in small watersheds, makes accurate quantitative judgments, is suitable for early warning of debris flows in steep gullies, and can effectively improve the accuracy of early warning.

[0046] Example 2

[0047] A method for early warning of debris flows in steep gullies based on a hydrological model includes the following steps:

[0048] a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps;

[0049] b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event;

[0050] c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area;

[0051] d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area;

[0052] e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model;

[0053] f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow.

[0054] In step a, the debris flow basin formation area refers to the area above the debris flow flow area and the debris flow deposition area. When there are several tributary ditches in the basin, the tributary ditches basin is the debris flow basin formation area of ​​the tributary ditches.

[0055] This embodiment is a preferred implementation method. By studying the influence of the conditions for the occurrence of debris flows in steep gullies of small watersheds on the formation of debris flows, accurate calculation indicators are given, especially the angle of repose of the source particles and the source water saturation density, so that the early warning of debris flows in steep gullies of small watersheds is more accurate.

[0056] Example 3

[0057] A method for early warning of debris flows in steep gullies based on a hydrological model includes the following steps:

[0058] a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps;

[0059] b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event;

[0060] c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area;

[0061] d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area;

[0062] e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model;

[0063] f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow.

[0064] In step a, the debris flow basin formation area refers to the area above the debris flow flow area and the debris flow deposition area. When there are several tributary ditches in the basin, the tributary ditches basin is the debris flow basin formation area of ​​the tributary ditches.

[0065] In step d, the overall instability condition of the gully bed in the debris flow basin formation area is calculated using Equation 1;

[0066] J=tanα≥(1-c)tanθ Equation 1

[0067] In the formula, J is the longitudinal slope of the gully bed in the debris flow basin formation area, α is the gully slope in the debris flow basin formation area, c is a coefficient, and θ is the angle of repose of the source particles.

[0068] This embodiment is another preferred implementation method. By studying the influence of the probability judgment index of debris flow in steep gullies of small watersheds on the formation of debris flow, especially the source permeability coefficient, gully width and source thickness, the early warning of debris flow is made more accurate.

[0069] Example 4

[0070] A method for early warning of debris flows in steep gullies based on a hydrological model includes the following steps:

[0071] a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps;

[0072] b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event;

[0073] c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area;

[0074] d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area;

[0075] e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model;

[0076] f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow.

[0077] In step a, the debris flow basin formation area refers to the area above the debris flow flow area and the debris flow deposition area. When there are several tributary ditches in the basin, the tributary ditches basin is the debris flow basin formation area of ​​the tributary ditches.

[0078] In step d, the overall instability condition of the gully bed in the debris flow basin formation area is calculated using Equation 1;

[0079] J=tanα≥(1-c)tanθ Equation 1

[0080] In the formula, J is the longitudinal slope of the gully bed in the debris flow basin formation area, α is the gully slope in the debris flow basin formation area, c is a coefficient, and θ is the angle of repose of the source particles.

[0081] The coefficient c is calculated using Equation 2;

[0082] c = 1 / ρ s Formula 2

[0083] In the formula, ρ s The density of the source material is saturated with water.

[0084] In step f, the occurrence index P of debris flow is calculated using Equation 3;

[0085] P = Q / (SWH) Equation 3

[0086] In the formula, Q is the peak flow of flash flood, S is the source permeability coefficient, W is the channel width, and H is the source thickness.

[0087] This embodiment is another preferred implementation method. By studying the overall instability initiation and formation mechanism of debris flows in steep gullies of small watersheds, and combining the HEC-HMS hydrological model, a quantitative method and index for calculating the probability of debris flows occurring in steep gullies of small watersheds are proposed, which has good applicability.

[0088] Example 5

[0089] A method for early warning of debris flows in steep gullies based on a hydrological model includes the following steps:

[0090] a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps;

[0091] b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event;

[0092] c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area;

[0093] d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area;

[0094] e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model;

[0095] f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow.

[0096] In step a, the debris flow basin formation area refers to the area above the debris flow flow area and the debris flow deposition area. When there are several tributary ditches in the basin, the tributary ditches basin is the debris flow basin formation area of ​​the tributary ditches.

[0097] In step d, the overall instability condition of the gully bed in the debris flow basin formation area is calculated using Equation 1;

[0098] J=tanα≥(1-c)tanθ Equation 1

[0099] In the formula, J is the longitudinal slope of the gully bed in the debris flow basin formation area, α is the gully slope in the debris flow basin formation area, c is a coefficient, and θ is the angle of repose of the source particles.

[0100] The coefficient c is calculated using Equation 2;

[0101] c = 1 / ρ s Formula 2

[0102] In the formula, ρ s The density of the source material is saturated with water.

[0103] In step f, the occurrence index P of debris flow is calculated using Equation 3;

[0104] P = Q / (SWH) Equation 3

[0105] In the formula, Q is the peak flow of flash flood, S is the source permeability coefficient, W is the channel width, and H is the source thickness.

[0106] In step f, determining whether a debris flow has occurred specifically means that when P < 0.9, the probability of a debris flow occurring is low; when 1.0 > P ≥ 0.9, the probability of a debris flow occurring is moderate; and when P ≥ 1, the probability of a debris flow occurring is high.

[0107] When a watershed has a main gully and one or more tributaries, the highest P-value is used as the criterion to determine the likelihood of debris flow.

[0108] This embodiment represents the optimal implementation method. By studying the conditions for the occurrence of debris flows in steep gullies of small watersheds and the influence of probability assessment indicators on debris flow formation, and combining the HEC-HMS hydrological calculation model, it provides flow calculation and assessment indicators under rainfall conditions, thereby improving the accuracy of debris flow early warning.

[0109] The embodiments of the present invention will be described in detail below with reference to specific examples:

[0110] On August 8, 2017, a powerful earthquake struck Jiuzhaigou. Following the earthquake, the Tiantanggou and Xiajihaizigou valleys in the affected area experienced multiple mudslides. Prior to the earthquake, these two valleys had almost no mudslides, and their frequency was low. The drainage basins of these two mudslide-prone areas cover an area of ​​approximately 2 km². 2 Within this area, the longitudinal slope of the gully bed in the debris flow basin formation zone is more than 40%, which is a typical small watershed with steep gully debris flow.

[0111] Following the "8.8" earthquake in Jiuzhaigou, numerous landslides and collapses occurred in both watersheds, providing abundant loose material sources for debris flows and creating favorable conditions for their occurrence: the loose solid material sources had high porosity and low saturated density; the solid material sources had small particle sizes in the short period after the strong earthquake, and the smaller particle sizes also had smaller angles of repose. All of these factors provided the basic conditions for the overall instability and initiation of debris flows in the steep gullies of the small watersheds in the Jiuzhaigou earthquake zone.

[0112] During five rainfall events in the area—September 14, 25, 2017, June 25, 2018, and July 10, 2018—five debris flows occurred in these two gullies. However, the rainfall amounts for these five events were 17.1, 15.4, 16.9, 20.2, and 26.2 mm respectively, and the maximum hourly rainfall amounts were 7.6, 6.9, 12.4, 7.4, and 11.2 mm respectively. These amounts were not particularly high, and some were quite small, generally unlikely to trigger debris flows.

[0113] The method of this invention was used to make early warning judgments for the above five debris flows:

[0114] First, the area A of the debris flow formation zone, the longitudinal slope J of the debris flow formation zone, and the width W of the channel, the angle of repose θ of the source particles, and the saturated density ρ of the source particles were measured using topographic maps. s The source permeability coefficient S and source thickness H are used to determine whether overall instability and debris flow formation will occur, according to Equation 1. The results are shown in Table 1, which lists the survey and measurement parameters and overall instability initiation conditions for the debris flow basin formation area. If such a debris flow can form, hourly rainfall data at the location of the debris flow formation area obtained from actual monitoring is used to calculate the flash flood peak discharge Q using the HEC-HMS hydrological calculation model. Combined with Equation 3, the probability of debris flow occurrence is determined.

[0115] The parameters of the five debris flows, the calculated debris flow occurrence index P, and the actual occurrence of the debris flows are shown in Table 2. Table 2 is a table of survey and measurement parameters and early warning values ​​for the debris flow formation area.

[0116] Table 1

[0117] Lower Season Haizi Gully 1.08 0.49 0.73 1.85 0.54 Yes Jiuzhai Tiantang Gully 0.71 0.43 0.74 1.77 0.56 Yes

[0118] Table 2

[0119]

[0120]

[0121] To determine the occurrence of debris flows: when P < 0.9, the probability of a debris flow is low; when 1.0 > P ≥ 0.9, the probability of a debris flow is moderate; and when P ≥ 1, the probability of a debris flow is high.

[0122] All of the cases in Table 2 were judged to have a high probability of mudslides, and all of them did indeed experience mudslides.

[0123] In summary, the method described in this invention has a high accuracy rate in providing early warning of debris flows in steep gullies within small watersheds.

Claims

1. A method for early warning of debris flows in steep gullies based on a hydrological model, characterized in that, Includes the following steps: a. Determine the area of ​​the debris flow formation zone and the longitudinal slope of the gully bed in the potential debris flow basin using topographic maps; b. Real-time monitoring or forecasting of rainfall events in the debris flow basin formation area, including previous rainfall and rainfall in the hour before triggering the event; c. Conduct on-site investigations and measurements of channel width, source permeability coefficient, source thickness, source saturation density, and source particle repose in the debris flow basin formation area; d. Calculate the overall instability conditions of the gully bed in the debris flow basin formation area; e. Calculate the peak flow of flash floods caused by rainfall based on the HEC-HMS model; f. Calculate the occurrence index P of debris flow and determine the occurrence of debris flow; In step f, the occurrence index P of debris flow is calculated using Equation 3; P=Q / (SWH) Equation 3 In the formula, Q is the peak flow of flash flood, S is the source permeability coefficient, W is the channel width, and H is the source thickness; In step f, determining whether a debris flow has occurred specifically means that when P < 0.9, the probability of a debris flow occurring is low; when 1.0 > P ≥ 0.9, the probability of a debris flow occurring is moderate; and when P ≥ 1, the probability of a debris flow occurring is high.

2. The method for early warning of debris flows in steep gullies based on a hydrological model according to claim 1, characterized in that: In step a, the debris flow basin formation area refers to the area above the debris flow flow area and the debris flow deposition area. When there are several tributary ditches in the basin, the tributary ditches basin is the debris flow basin formation area of ​​the tributary ditches.

3. The method for early warning of debris flow in steep gullies based on a hydrological model according to claim 1, characterized in that: In step d, the overall instability condition of the gully bed in the debris flow basin formation area is calculated using Equation 1; Formula 1 In the formula, The longitudinal gradient of the gully bed in the debris flow basin formation area, The slope of the gully in the debris flow basin formation area, For coefficients, The angle of repose of the source particles.

4. The method for early warning of debris flows in steep gullies based on a hydrological model according to claim 3, characterized in that: The coefficient c is calculated using Equation 2; Formula 2 In the formula, The density of the source material is saturated with water.

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