Calculation Method and Application of the Maximum Scouring Depth under the Window Dam

Abstract

The present invention discloses a method for calculating the maximum scour depth under a window dam and its application. Aiming at the defect that the influence of the opening form of the window dam on the scour depth under the dam is not considered in the prior art, the present invention provides a method for calculating the maximum scour depth under the window dam. The method calculates the maximum scour depth under the window dam according to the basic data obtained from the on-site investigation of the window dam by a scientific calculation formula. The calculation method of the invention takes the opening form of the window dam as an influencing factor. Essentially, it takes the differences in the dynamic process characteristics of debris flow over-topping and scour caused by different opening forms as the constraint conditions for calculating the scour depth, ensuring that the structural significance of the "large opening" feature of the window dam is fully reflected in the calculation principle of the maximum scour depth. The calculation process is simple and can reflect the influence law of each parameter on the calculation result, providing a more reliable basis for the design of the window dam. The present invention also provides application schemes of the calculation method in the design of the window dam, the prevention and control engineering system of the window dam, and safety assessment.

Description

Technical Field

[0001] The invention relates to debris flow prevention and control engineering construction technology, in particular to a design method for a debris flow sand retaining dam and its application, belonging to the debris flow disaster prevention and control engineering and construction engineering design technology field. Background Art

[0002] Sand-blocking dams are key prevention and control projects for regulating debris flow movement. With its functional characteristics of sand-blocking and throttling, reducing debris flow power, raising the erosion surface of local gully beds, controlling gully gravity erosion, and increasing gully bed stability, as well as its technical characteristics such as economical cost, low construction technology requirements, and rapid engineering results, sand-blocking dams are widely used in debris flow disaster prevention and control, and have become the most important debris flow prevention and control engineering means. According to the overflow section structure of sand-blocking dams, they can be divided into two categories: permeable (open-type) and non-permeable (closed-type) (also known as solid dams). Window dams are a type of permeable sand-blocking dams, and their basic feature is the "large opening" open structure of the overflow section of the dam body.

[0003] The essence of scour under the dam is the scouring and impact of the mudslide over the dam and the large rocks it carries on the ditch bed, which is a natural phenomenon that cannot be eliminated. Scour under the dam forms scour pits, and at the same time, the erosion of the dam foundation soil is aggravated by the upstream erosion, which can easily cause the dam foundation to be suspended, resulting in the instability and overturning of the main body of the sand-blocking dam or the failure of scour. Therefore, it has become an extended problem that needs to be solved in the sand-blocking dam project. Relevant research shows that 65% of the instability of sand-blocking dams is caused by the continuous development of scour behind the dam, which leads to the instability and destruction of the dam body. In the sand-blocking dam project, it is generally adopted to add a tank, auxiliary dam, submerged dam and other anti-scour and energy dissipation facilities under the dam to protect the ditch bed and delay the occurrence of the above problems. However, as the operation time of the sand-blocking dam increases, the energy dissipation facilities will gradually be damaged and fail, and cannot prevent the formation of scour dams under the dam. Therefore, in the design of window dams, the accurate estimation of the scour depth under the dam is an important reference or basis for the design of the dam foundation depth.

[0004] The prior art generally refers to the "Code for Design of Debris Flow Control Engineering (Trial) T / CAGHP021-2018" and comprehensively uses the Fugu-Ichi formula, the impact depth of falling rocks, and the local scour depth of mountain torrents and debris flows to determine the scour depth under the dam (Shi Shengwei et al., Research on Technical Methods for Repairing and Reinforcing the Foundation Soil of Sand Retaining Dams with Small-Diameter Steel Pipe Piles, Engineering Science and Technology, Vol. 51 No. 5, September 2019). This method comprehensively considers the influence of the unit-width flow of debris flow, the flow velocity of debris flow, the standard particle size of bedload sand, the mud depth at the overflow outlet, the mud level difference above and below the dam, etc. on the scour depth under the dam, but does not consider the influence of the opening (hole) form of the window dam on the scour depth under the dam. The main feature of the window dam is "large opening", and its most core function is to use different forms of large openings (holes) to achieve a series of regulation functions for peak shaving and flow reduction of debris flow and sediment retention. Due to the large differences in the over-dam process and dynamic characteristics of debris flow for window dams with different openings (holes) / conditions, on the one hand, there are differences in the debris flow regulation functions of window dams with different opening forms (holes) / conditions, and on the other hand, there are also large differences in the scour phenomena under the dams of window dams with different opening forms (holes) / conditions. This means that when calculating the scour depth under the dam, if the opening (hole) form / conditions are not considered, the calculation principle may be inconsistent with the actual regulation principle of the dam, which limits the practical reference value of the calculation results in engineering. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for calculating the maximum scour depth under a window dam and its application in the engineering design of sand retaining dams in view of the deficiencies of the prior art.

[0006] To achieve the above purpose, the present invention first provides a method for calculating the maximum scour depth under a window dam, and its technical solution is as follows:

[0007] A method for calculating the maximum scour depth under a window dam, characterized in that: first, select the location of the window dam and conduct on-site investigations to obtain basic data; secondly, calculate the maximum scour depth h under the window dam according to Formula 1 and Formula 2 d ,

[0008] h d / H = 0.076I + 1.975i - 0.209C v -0.096R + 0.234 Formula 1

[0009] I = b / d max Formula 2

[0010] In the formula, h d / H - dimensionless scour depth,

[0011] H - effective dam height of the window dam, unit m, determined by basic data,

[0012] I—Relative opening of the window dam,

[0013] b—Opening width of the window dam, unit: m, determined by basic data,

[0014] d max —Maximum particle size of solid materials in debris flow, unit: m, determined by basic data,

[0015] i—Bed slope ratio at the location of the window dam, %, determined by basic data,

[0016] C v —Volume concentration of sediment in debris flow, dimensionless quantity, determined by basic data,

[0017] R—Opening ratio of the window dam, determined by basic data.

[0018] The maximum scour depth h below the window dam measured by the above measurement method d , and a more complete expression is the maximum possible depth of the scour pit formed by the over-dam debris flow of the window dam on the scour surface of the downstream channel bed. The measurement method is to establish an equation based on a large amount of experimental test data and field observation data, and complete it through dimensional analysis. The significance of the measurement method is that under the condition of comprehensively considering the opening form of the window dam, a new dimensionless calculation method for the scour depth below the dam is proposed. This method is not only simple in calculation, but also can reflect the influence law of each parameter. In this method, the newly introduced I is a measurement index reflecting the size of a single window opening; the newly introduced R is a measurement index reflecting the opening characteristics of the entire window dam. These two indexes are the core parameters that distinguish the window dam from the solid dam. The introduction of I and R makes the measurement method closer to the actual operation process of the window dam, and the measurement results can better guide the design of the window dam.

[0019] To ensure the effectiveness of the measurement results, under optimal conditions, the above measurement method for the maximum scour depth below the window dam is applicable to window dams with an opening ratio R = 0.08 - 0.32, and is applicable to the general characteristics of the bottom sediment of natural debris flow channels, that is, the particle size distribution of the bottom sediment is similar to that of the solid materials in the debris flow.

[0020] The maximum scour depth h below the window dam d is an important reference index for completing the design of the buried depth parameter of the window dam foundation in the window dam design. Therefore, the present invention also provides:

[0021] The application of the above measurement method for the maximum scour depth below the window dam in the window dam design.

[0022] The maximum scour depth h below the window dam d is an important reference index for the design of the thickness parameters of anti-scour and energy dissipation facilities such as apron, auxiliary dam, and submerged dam in the window dam prevention and control engineering system. Therefore, the present invention also provides:

[0023] The above-mentioned maximum depth calculation method of scour under the window dam is applied in the design of the window dam prevention and control engineering system. More specifically, it is applied in the design of anti-scour and energy dissipation facilities under the dam.

[0024] The maximum scouring depth under the window dam of the present invention refers to the index of the ditch bed gradient i. After the window dam is put into operation, with the increase of the operating years, the accumulation of silt filling, back siltation and other phenomena, the ditch bed morphology will change, and the direct result is the change of the ditch bed gradient i index. Therefore, for the window dam in operation with the change of the ditch bed gradient i, the maximum scouring depth h can be calculated by the calculation method of the present invention. d And compared with the design parameters of the window dam foundation depth, the operational safety and stability of the window dam can be roughly evaluated. Therefore, the present invention also provides:

[0025] The above-mentioned method for calculating the maximum depth of scour under the window dam is applied in the safety assessment of the operation of the window dam / window dam prevention and control project system.

[0026] According to the experimental design constructed by the method for calculating the maximum depth of scour under the window dam of the present invention, the above two technical solutions of the present invention involving the application of the method for calculating the maximum depth of scour under the window dam have the priority restriction condition that they are applied to window dams with an opening ratio of R=0.08~0.32.

[0027] In each technical solution of the present invention, the field investigation implemented includes various mapping, measurement, simulation experiment tests, acquisition of historical disaster records, and acquisition of empirical data for reference and reference at the site of the mountain torrent mud-rock flow channel where the project is located.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Compared with the prior art method of measuring the scour depth under the dam by referring to the formula of water conservancy engineering and supplemented by the designer's experience, the present invention is a scientific calculation method based on standardized survey data and a complete scientific solution. (2) The main defect of the prior art is that the window dam opening form is not included in the consideration of depth measurement. The present invention takes the window dam opening form as an influencing factor and decomposes it into two measurement variables: the window dam relative opening I and the window dam opening ratio R. In essence, the difference in the dynamic process characteristics of debris flow over the dam and scour caused by different opening forms is used as a constraint condition for measuring the scour depth, ensuring that the structural significance of the "large opening" feature of the window dam is fully reflected in the measurement principle of the maximum scour depth, making the measurement method principle more in line with natural laws. (3) The calculation process of the present invention is simple, the values of each parameter are simple, and it can reflect the influence of each parameter on the calculation result, which can provide a more reliable basis for the design of the window dam. (3) The present invention also provides an application scheme of the measurement method in window dam design, window dam prevention and control engineering system, and safety assessment. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention will be further described below.

[0030] Embodiment 1

[0031] In the design of a certain window dam, the maximum scour depth h under the dam of the window dam is measured by the method of the present invention d .

[0032] A certain debris flow gully is located in Wenchuan County, Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province, and is a first-level tributary on the right bank of the Minjiang River. The drainage area of the gully is 54.26 km 2 , the main gully is 15.74 km long, and the average longitudinal gradient of the main gully bed is 201‰. The plane shape of the drainage area is in the shape of a "leaf". The upper reaches of the gully are narrow and mostly in a "V" shape, while the middle and lower reaches are wide and gentle in a "U" shape, with local narrow and wide intervals. The mountains in the gully area are high and the slopes are steep, and the longitudinal slope of the gully is large, which is conducive to the collection of rainfall and provides a basis for the outbreak of debris flows. National Highway G213 and the Duwen Expressway project pass through the mouth of the gully, and the 017 township road passes through the middle and lower reaches of the drainage area. The Duwen Expressway passes through the mouth of the gully in the form of a bridge. In addition, dozens of residential areas are distributed at the mouth of the gully and a hydropower station is built. The gully has burst out debris flows many times, posing a serious threat to the hydropower station, residential areas and traffic projects at the mouth of the gully. In order to meet the needs of disaster prevention and mitigation, it is planned to build a window dam project in the gully, and the design standard is P = 2% (once in 50 years).

[0033] 1. Site selection and on-site investigation

[0034] According to the design and construction specifications of debris flow prevention and control projects, the site selection of the window dam is completed in the gully, and on-site investigations are carried out to obtain various basic data. Including:

[0035] When P = 5% (once in 20 years): the density ρ of the debris flow body in the gully c = 1.81 t / m 3 , the density σ of the solid substances in the debris flow = 2.65 t / m 3 , the density ρ of the clear water in the debris flow = 1.0 t / m 3 , the maximum particle size d of the solid substances in the debris flow max = 1.0 m, and the bed slope i of the proposed window dam location is 100‰.

[0036] The designed average width B of the window dam is 40 m, the effective dam height H is 7 m, the opening width b of the window dam is 0.8 m, the opening height h is 1.0 m, and the number of openings n is 20.

[0037] According to the existing technology (Equations 3 and 4), the sediment volume concentration C of the debris flow is calculated and determined v = 0.49 (unit), and the opening ratio R of the window dam is 5.71%.

[0038]

[0039]

[0040] In the above formula (3), ρ c is the density of the debris flow (t / m 3 ), ρ is the density of clear water in the debris flow (t / m 3 ), σ is the density of solid materials in the debris flow (t / m 3 ). In formula (4), n is the number of openings, b is the opening width (m), h is the opening height (m), B is the average width of the window dam (m); H is the effective dam height of the window dam (m).

[0041] 2. Measuring the maximum scour depth h below the dam d

[0042] According to the above parameters, the relative opening I of the window dam is calculated according to formulas (1) and (2) to be I = 1.0, the dimensionless scour depth h d / H = 0.384, and the maximum scour depth h below the dam d = 2.69 m.

[0043] Example 2

[0044] Using the method of the present invention to check the maximum scour depth h below a certain window dam d .

[0045] A certain debris flow gully is located in Group 4, Xiaba Village, Puji Town, Puge County, Liangshan Prefecture, Sichuan Province, and is a first-level tributary on the right bank of the Heishui River. The drainage area of this gully is 23.30 km 2 , the main gully is 10.47 km long, and the average longitudinal gradient of the main gully bed is 195‰. The overall shape of the basin is in the shape of a "leaf". The mountains in the gully area are high and the slopes are steep. The longitudinal slope of the gully is relatively large. The gully area is rich in debris flow deposits. There are many landslide and loose slope deposits on both sides of the gully, providing source conditions for the outbreak of debris flows. The gully mouth is a typical ethnic minority village. Large debris flows have occurred in this gully. The debris flow washed out the highway at the gully mouth and caused river blockage, posing a serious threat to the village at the gully mouth. To meet the needs of disaster prevention and reduction, three sand retention dams have been built in the gully over the decades, all of which are window dams. At present, Dam 1 is operating well; Dam 2 has toppled and been damaged; Dam 3 has been operating for decades and is close to the damaged state. The scour below the dam is the most serious. The on-site measurement results of the scour surface and scour pit show that the scour depth at the maximum scour position reaches 1.9 m. Referring to relevant materials, the design standard of the three dams is P = 2% (once in 50 years).

[0046] 1. On-site investigation

[0047] According to the on-site investigation of Dam 3, various basic data are obtained. Including:

[0048] When P = 5% (once in 20 years): The density ρ of the debris flow in the gullyc = 1.84 t / m 3 、The density of solid debris flow materials σ = 2.65 t / m 3 、The density of clear water in the debris flow ρ = 1.0 t / m 3 , the maximum particle size d of the solid debris flow materials max = 1.0 m, the bed slope i at the location of the No. 3 window dam is 89‰.

[0049] The average width B of the No. 3 window dam is 30 m, the effective dam height H is 5.5 m, the opening width b of the window dam is 0.8 m, the opening height h is 1.0 m, and the number of openings n is 8.

[0050] The existing technology same as that in Example 1 is adopted to calculate and determine the sediment volume concentration C of the debris flow v = 0.48 (unit), and the opening ratio R of the window dam is 3.88%.

[0051] 2. Measure the maximum scour depth h below the dam d

[0052] According to the above parameters, calculated by Equation 1 and Equation 2, the relative opening I of the window dam is 1.0, the dimensionless scour depth h d / H = 0.365, and the maximum scour depth h below the dam d = 2.01 m.

[0053] The on-site investigation results show that the No. 3 dam is close to the damaged degree, and multiple scour pits have been formed on the scour surface below the dam. The average water depth in the pits is about 1.4 m. If a debris flow occurs again, even if the dam can continue to operate, the debris flow over the dam will mainly cause the phenomenon of filling the scour pits, and it is difficult to further increase the scour depth. Therefore, the scour depth data at the maximum scour location below the dam obtained from the on-site investigation can be regarded as the maximum scour depth below the No. 3 window dam. The measured result of 2.01 m by using the method of the present invention is close to the measured value of 1.9 m.

Claims

1. Method for calculating the maximum scour depth under the window dam, characterized in that: First, complete the site selection of the window dam and conduct on-site investigations to obtain basic data; secondly, calculate the maximum scour depth h under the window dam according to Equations (1) and (2). d , h d / H = 0.076I + 1.975i - 0.209C v -0.096R + 0.234 Equation 1 I = b / d max Equation 2 where h d / H - dimensionless scour depth, Effective dam height of the H-window dam, unit: m, determined by basic data Relative opening of the I-window dam Opening width of the b-window dam, unit: m, determined by basic data d max - Maximum particle size of debris flow solid material, unit m, determined by basic data Bed slope ratio of the position of the i-window dam, %, determined by basic data C v - Debris flow sediment volume concentration, dimensionless quantity, determined by basic data Opening ratio of the R-window dam, determined by basic data 2. The measurement method according to claim 1, wherein: The opening ratio R of the said window dam is determined by calculation according to Equation 4 In the formula, n - number of openings Opening height of the h-window dam, unit: m, determined by basic data Average width of the B-window dam, unit: m, determined by basic data 3. The degree measurement method according to claim 1, characterized in that: Applicable to window dams with an opening ratio R = 0.08 - 0.32 4. The degree measurement method according to claim 1, wherein: Applicable to natural debris flow channels where the bed material gradation is similar to the solid material gradation of debris flows 5. Application of the method for calculating the maximum scour depth under the window dam according to any one of Claims 1 to 3 in the design of window dams 6. Application of the method for calculating the maximum scour depth under the window dam according to any one of Claims 1 to 3 in the design of the window dam prevention and control engineering system 7. The application according to claim 6, characterized in that: It is the application in the design of the anti-scour and energy dissipation facilities under the dam 8. Application of the method for calculating the maximum scour depth under the window dam according to any one of Claims 1 to 3 in the safety assessment of the operation of window dams / window dam prevention and control engineering systems

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