Anti-seepage method in karst depression catchment reservoir-forming project

Abstract

The invention discloses an anti-seepage method in a karst depression catchment reservoir-forming project, which adopts a grouting curtain combined underground river plugging measure, and comprises the following steps: 1) collecting hydraulic parameters; 2) underground river slope calculation: based on the hydraulic parameters and the difference between the underground river outlet flow velocity and the underground river inlet flow velocity, adopting different methods to calculate the underground river overall slope; (3) determining the position of an upper layer grouting tunnel; 4) determining an underground river plugging position based on the slope; 5) calculating the height of the grouting curtain based on the slope; according to the anti-seepage method in the karst depression catchment reservoir forming project, determination of key parameters in construction is improved, the anti-seepage construction precision and the construction efficiency can be improved, and the construction cost is saved.

Description

Technical Field

[0001] The present invention relates to the field of water conservancy projects, and in particular to an anti-seepage curtain and underground river blocking method in the process of water collection and reservoir formation in karst depressions, providing a new approach to solving engineering water shortage problems in karst areas, and specifically to an anti-seepage method in karst depression water collection and reservoir formation projects. Background Art

[0002] Karst regions contain large amounts of soluble rocks such as calcium carbonate, which, under the action of flowing water, form dissolution fissures, caves, and underground rivers. The regional water cycle exhibits a distinct above-ground and underground dual structure. The representative karst regions of southwestern my country are rich in rainwater resources, but large amounts of rainfall are rapidly lost through the well-developed groundwater system, leading to severe engineering water shortages in regional production and daily life. This requires engineering measures to effectively utilize rainwater resources.

[0003] Damming rivers to form lakes and reservoirs is the primary solution to water shortages, but these established water conservancy solutions are ineffective in addressing water shortages in karst areas. The topography of karst areas typically develops into mountainous areas dominated by intersecting peaks and forests, with very few plains. Water for production and life is mostly concentrated in karst depressions and surrounding areas. Rivers in these areas are primarily composed of small and medium-sized tributaries, and traditional dam-based interception methods face obstacles such as high costs, poor effectiveness, high energy consumption, and low returns. Long-term erosion has carved deep valleys between mountains into riverbanks, forming steep, high-drop riverbanks. Constructed reservoirs face high water extraction costs in addressing water shortages, and their role in addressing regional navigation and power generation needs is greater than meeting water needs for production and life. Therefore, in response to the contradiction between water demand for life and production in karst depressions, the use of engineering anti-seepage measures to achieve rainwater resource storage and utilization has become a key direction of current water conservancy projects in karst areas. In particular, the engineering plugging of geological defects in the karst seepage zone on the plateau surface of the slopes of deep river valleys and the use of the natural concave terrain of karst depressions to form damless reservoirs have become a research hotspot. Reasonable site selection can not only solve the water supply problem in the depression area, but also effectively radiate to the surrounding areas. Compared with traditional river damming and reservoir construction, it has significant advantages of low cost and high regional adaptability. However, the engineering plugging of geological defects in the karst seepage zone during the construction of karst depression reservoirs is an important technical node currently under research.

[0004] Currently, curtain grouting, horizontal anti-seepage measures, and underground river blocking are the main anti-seepage technologies used in karst depression water collection and reservoir construction. Grouting curtains and underground river blocking are key technologies for addressing leakage in the karst geological layers surrounding karst depressions. Grouting curtains seal and prevent seepage through the vertical injection of mortar and other anti-seepage materials into the karst geological massif surrounding the depression. Underground river blocking, on the other hand, achieves this by engineering measures to prevent seepage through the cross-section of underground underground rivers connected to the depression. These two technologies can repair the main leakage channels below the design water level of the depression. Combined with horizontal anti-seepage measures on the surface of the depression, the goal of water collection and reservoir construction can be achieved. However, current grouting curtain and underground river plugging technologies still face numerous challenges in their construction. While preliminary geological surveys can identify the primary distribution zones of karst geological layers, the spatial distribution and size of cavities, and establish the main construction areas, the primary issues that need to be addressed in actual construction are determining the height of the grouting curtain, the spacing between grouting cavities, and the location of underground river plugging points. These parameters are crucial to the success of plugging and cost control. While high-density geological surveys combined with construction experience can improve the accuracy and success rate of construction, this undoubtedly increases project costs and poses the problem of low accuracy. Finding a reliable method has become a key focus of current technological research and development. Underground rivers are often inaccessible during construction, making underground conditions and related parameters extremely difficult to measure. The required data can only be calculated by measuring relevant parameters in the surrounding areas of the depression and at the underground river entrances and exits.

[0005] In recent years, there have been many studies on the anti-seepage technology of karst depression water collection and reservoir engineering based on curtain grouting and river blocking. For example, the Chinese patent with publication number CN202210163631.X proposed a curtain grouting construction method for reservoirs in karst areas, and improved the hole processing steps, curtain grouting method, and grouting pipe structure; the Chinese patent with publication number CN202123449289.X proposed a grouting curtain structure in karst areas, and improved the grouting pipe setting, steel pipe pile setting, and slurry injection process; another Chinese patent with publication number CN201610434986.2 proposed a curtain grouting construction method for strongly karst developed strata, which improved the grouting effect and reduced unnecessary slurry loss. While these technologies have, to a certain extent, addressed the construction process for anti-seepage curtains and underground river sealing in karst depressions, refining the grouting curtain construction process, they remain problematic in determining and setting key parameters for these projects. This inefficiency and accuracy in determining these key parameters leads to high construction costs and a significant impact on quality. Summary of the Invention

[0006] The purpose of this invention is to propose an anti-seepage method for karst depression water collection and reservoir construction, improve the determination of key parameters during construction, thereby improving construction accuracy and efficiency and saving construction costs. The purpose of this invention is achieved through the following technical solutions: A method for preventing seepage in a karst depression water collection and reservoir project uses a grouting curtain combined with underground river blocking measures. The three-dimensional schematic diagram of the karst depression is shown in the figure below. Figure 1 , the present invention comprises the following steps: Step 1: Hydraulic parameter collection: Through field work, investigate the entrance and exit of the underground river in the planned construction depression and the direction of the underground river, and measure and obtain hydraulic parameters; Step 2: Calculation of underground river slope: Based on the hydraulic parameters obtained in the first step, such as Figure 2 As shown, when the ratio of the inlet flow velocity to the outlet flow velocity of the underground river is greater than or equal to the set threshold, the overall slope of the underground river is calculated by formula (1); when the ratio of the inlet flow velocity to the outlet flow velocity is less than the set threshold, the overall slope of the underground river is calculated by formula (2); the set threshold is 0.4~0.6; (1) Where: S It is the overall slope of the underground river; v 1 is the inlet water velocity, m / s; v 2 is the outlet water velocity, m / s; n 1 is the entrance roughness; n 2 is the export roughness ratio; b 1 is the inlet river width, m; b 2 is the outlet river width, m; h 1 is the inlet river water depth, m; h 2 is the outlet river water depth, m; (2) Where: z1 is the entrance elevation, m; z2 is the exit elevation, m; l is the straight-line distance from the exit to the entrance of the underground river, m; g is the acceleration due to gravity, m / s 2 ; Step 3: Determine the location of the upper grouting tunnel: Figure 3 As shown, according to the designed water storage level and the measured karst geological distribution range, the upper grouting tunnel is selected above the designed water storage level, and the elevation of the upper grouting tunnel is determined; Step 4: Determine the location of the underground river blocking: The underground river blocking location is determined by the elevation of the blocking point. HAnd the distance from the blocking point to the underground river exit L The calculation formula is as follows: (3) (4) Where: H is the elevation of the underground river blocking point, m; z 3 is the elevation of the upper grouting tunnel, m; S It is the overall slope of the underground river; d t is the diameter of the upper grouting tunnel, m; L is the distance from the blocking point to the underground river outlet, m; Step 5: Calculation of grouting curtain height: The calculation formula is as follows:

[0007] (5) Where: d is the height of the grouting curtain, m; Step 6: Figure 3 、 4 As shown, according to the upper grouting tunnel position and the underground river blocking position determined in steps three and four, the grouting tunnel construction and underground river blocking are carried out: the upper grouting tunnel is constructed above the designed water storage level line; the lower grouting tunnel is constructed below the designed water storage level line in parallel with the upper grouting tunnel, and the lower grouting tunnel is perpendicular to the underground river in the horizontal plane, and the intersection with the underground river is the underground river blocking position; The seventh step is to carry out grouting curtain construction according to the grouting curtain height obtained in step five: drill holes from the bottom of the upper grouting tunnel and vertically pour mortar into the lower karst geological layer until the top of the underground river.

[0008] Further optimization solutions, step 7 also includes: Curtain spacing calculation: based on the grouting pump pressure measured during the previous grouting curtain construction P 1 , formation pressure P 2 , grouting time t , mud viscosity µ , calculate the distance between the next curtain and the previous curtain using the following formula; (6) Where: L’ is the distance between the two curtains, m; k is the equivalent permeability, m 2 ; P 1 is the perfusion pump pressure, Pa; P 2 is the formation pressure, Pa; tis the grouting time, s; µ is the slurry viscosity, Pa∙s; Ø is the effective porosity.

[0009] The construction of each grouting curtain is carried out in sequence based on the calculated curtain spacing.

[0010] For further optimization, in step 2, the threshold is set to 0.5.

[0011] Further optimization, the specific operations of step one include: measuring the water flow velocity at the entrance and exit of the underground river v , roughness n 、River width b , water depth h , elevation z and longitude and latitude; determine the karst geological distribution range of the underground river outlet in the depression through geological exploration, and collect cores in the karst geological distribution range to determine the equivalent permeability of rock fractures k and effective porosity Ø ; Obtain the spatial straight-line distance between the exit and entrance of the underground river through latitude and longitude and drone orthophotos l .

[0012] The following is the derivation process of the main formula of the present invention: 1. The derivation process of the slope calculation formula (1) in the present invention is: Formula (1) is based on the Manning formula and is derived by reversely solving the slope drop. The derivation process is as follows.

[0013] (7) In the formula V is the river velocity, R is the hydraulic radius, S For the slope, n For the roughness.

[0014] In the above formula, the wetted perimeter R It can be further calculated by the following formula (8): (8) In the formula R is the hydraulic radius, A is the cross-sectional area of ​​the river, P For wet week, b For the width of the river, h For water depth.

[0015] Substituting formula (8) into formula (7), the Manning formula obtained is as follows: (9) The slope in the above formula (9) is S When used as the solved term, the formula further becomes the following form: (10) The flow rate at the entrance and exit of the underground river v 1 、v 2 , roughness n 1 、n 2 、River width b 1 、b 2 and water depth h 1 、h 2 Substituting into formula (10) we get the final slope calculation formula as follows: (1) 2. The slope calculation formula (2) takes into account the possibility of a steep drop in the slope of the underground river, so the slope of the underground river ( S ) consists of two parts: energy slope and friction slope (11), where the friction slope ( S f ) is calculated by formula (1), and the energy slope ( S e ) is calculated using the energy slope formula.

[0016] (11) The energy slope ( S e ) is calculated by formula (12).

[0017] (12) Where, S e is the energy slope, ∆E is the change in water energy during the flow (m), and l is the spatial distance from the entrance to the exit of the underground river (m).

[0018] ∆E is further calculated by the following formula (13). (13) Where z1 and z2 are the elevations of the underground river entrance and exit respectively (m), v1 and v2 are the flow velocities of the underground river entrance and exit respectively (m / s), h1 and h2 are the water depths of the underground river entrance and exit respectively (m), and g is the acceleration of gravity (m / s 2 ).

[0019] Substituting formula (13) into (12) yields the energy slope ( S e )Calculation formula (14).

[0020] (14) Further bring formula (14) and formula (1) into formula (11) to get the slope drop ( S ) is calculated using the formula (2).

[0021] (2) 3. Calculation formula for the location of underground river blocking points The calculation and derivation process of the elevation of the underground river blocking point: Figure 5 As shown, the exit point of the underground river is set to the coordinate origin (0,0,0), and the coordinates of the entrance point of the underground river are set to (0,y1,z1), where y1 is the Y coordinate of the entrance, z1 is the entrance elevation, and the river slope is S. The upper grouting tunnel is in the XZ plane (i.e., y = 0), and the tunnel elevation is a fixed value z3. Its coordinates are expressed as (x,0,z3). The underground river is in the YZ plane, connecting the entrance (0,y1,z1) and the exit (0,0,0). Its parametric equation is (15): (15) Calculate the shortest distance between two straight lines. A point on the underground river is (0, y, Sy) and a point on the tunnel line is (x, 0, z3). The square of the distance between the two points is (16): (16) Taking the derivative of x and y to find the extreme value, we get: x = 0 (minimum point), which is substituted into formula (16), and taking into account the diameter of the upper grouting tunnel ( d t ), the distance between the blocking point and the upper grouting tunnel is calculated using the formula (5): (5) The elevation of the blocking point is obtained by subtracting the straight-line distance between the grouting tunnel elevation and the blocking point (3).

[0022] (3) Where, H is the elevation of the underground river blocking point (m), z 3 is the elevation of the upper grouting tunnel (m), S It is a dark river slope. d t is the diameter of the upper grouting tunnel (m).

[0023] The derivation process of calculating the distance to the underground river blocking point: The distance to the underground river blocking point refers to the projected straight-line distance from the blocking point to the underground river outlet, which is calculated based on the slope and the elevation of the blocking point (4).

[0024] (4) Where, L is the projected straight-line distance from the blocking point to the underground river outlet (m), H is the elevation of the blocking point (m), and S is the river slope.

[0025] 4. Derivation of grouting curtain height The height of the grouting curtain refers to the vertical distance from the upper grouting tunnel to the underground river. The derivation process is shown in the derivation process of the elevation calculation of the underground river blocking point. The distance between the underground river blocking point and the upper grouting tunnel ( d )equal: (5) Where, d is the grouting curtain height (m), z 3 is the elevation of the upper grouting tunnel (m), S It is a dark river slope. d t is the diameter of the upper grouting tunnel (m).

[0026] 5. Calculation and derivation of grouting hole spacing The spacing between grouting holes refers to the distance between multiple downward-pointing grouting holes in the upper grouting tunnel. These grouting holes are distributed in the XZ plane and are primarily used to fill mortar to seal cracks in the stratum, forming a grouting curtain. The distance between grouting holes is closely related to the lateral movement distance of the mortar in a single grouting hole. Assuming that the lateral movement distance of the mortar in a single grouting hole is l', the spacing L' between two adjacent grouting holes should be calculated according to the following formula (17): (17) The lateral movement distance (l') of the mortar in a single grouting hole can be derived based on Darcy's law (18): (18) Where, v is the Darcy velocity (m / s), Δ P is the difference between injection pressure and formation pressure (Pa), l ' is the flow path length (m), k is the equivalent permeability (m 2 ), µ is the slurry viscosity (Pa·s).

[0027] However, the pore occupancy needs to be considered in the actual flow rate, and formula (18) is further modified to (19): (19) Build moving distance ( l ') and time ( t ), the moving distance ( l ') over time (t ) changes satisfy: (20) Further organized into differential equations: (twenty one) Integrate formula (21) and solve it. Integrate both sides, and the initial condition is l'=0 when t=0: (twenty two) get: (twenty three) The final solution is: (twenty four) However, according to the law of conservation of mass, the injection volume is equal to the filling volume, and the coefficient is corrected to 1, which gives: (25) Substituting formula (25) into (17) yields formula (26) for calculating the distance L' between two adjacent filling holes.

[0028] (26) Where, Δ P is the difference between injection pressure and formation pressure ( Pa ), l ' is the flow path length (m), k is the equivalent permeability (m 2 ), µ is the slurry viscosity (Pa·s), Ø is the effective porosity, and t is the injection time (s). Thus, formula (6) is obtained: The advantages and beneficial effects of the present invention are: The present invention's anti-seepage method for karst depression water collection and reservoir construction proposes a novel method for determining key parameters such as grouting curtain height, grouting spacing, and the location of underground river blocking points, and applies this method to the construction process. This method eliminates the need for intensive geological surveys and extensive empirical estimates, calculating the required data based solely on measurements of relevant parameters in the depression's surrounding area and at the underground river's entrances and exits. Practical application demonstrates that the present anti-seepage method improves construction efficiency while maintaining quality and reducing project costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below with reference to the accompanying drawings and examples.

[0030] Figure 1 The figure is a three-dimensional schematic diagram of a karst depression according to the method of the present invention.

[0031] Figure 2This is a schematic diagram of the vertical section of the karst depression underground river according to the method of the present invention.

[0032] Figure 3 This is a schematic diagram of the vertical cross-section construction of a karst depression underground river according to the method of the present invention.

[0033] Figure 4 This is a schematic diagram of the vertical section construction of the anti-seepage curtain according to the method of the present invention.

[0034] Figure 5 Schematic diagram of the derivation process of key formulas.

[0035] Figure 6 This is a comparison diagram before and after the reservoir was built in the embodiment. Figure 6 a is before construction, Figure 6 b is after construction.

[0036] Figure 7 Schematic diagram of the longitudinal section of the middle river channel. DETAILED DESCRIPTION

[0037] Example 1: A method for preventing seepage in a karst depression water collection and reservoir construction project is disclosed. In this embodiment, the Lujiaoba karst depression construction project is carried out in Kaiyang County, Guizhou Province to realize water collection and reservoir construction. Figure 6 This is a comparison picture before and after the construction of the reservoir.

[0038] The present invention comprises the following steps: Step 1: Hydraulic parameter collection: Through field work, investigate the entrance and exit of the underground river in the proposed construction depression and the direction of the underground river, and measure the water flow velocity at the entrance and exit of the underground river. v , roughness n 、River width b , water depth h and longitude and latitude; determine the karst geological distribution range of the underground river outlet in the depression through geological exploration, and collect cores in the karst geological distribution range to determine the equivalent permeability of rock fractures k and effective porosity Ø ; Obtain the spatial straight-line distance between the exit and entrance of the underground river through latitude and longitude and drone orthophotos l , and elevation z .

[0039] In this embodiment, before the construction of the project, an underground river was investigated within the planned reservoir area. It was the Lianchun River. The upstream entrance of the underground river was in Lujiaoba Village, Kaiyang County, and the downstream outlet was in Lacaitang Group, Kaiyang County. The downstream of the river merged into the Yujing River. The depression reservoir was selected in the Lujiaoba area. The downstream rivers were all underground and needed to be blocked in the downstream mountains. Through on-site monitoring, the river width, water depth, flow rate and other information are as follows: Step 2: Calculation of underground river slope: Based on the hydraulic parameters obtained in the first step, when the ratio of the inlet flow velocity to the outlet flow velocity of the underground river is greater than or equal to the set threshold, the overall slope of the underground river is calculated using formula (1); when the ratio of the inlet flow velocity to the outlet flow velocity is less than the set threshold, the overall slope of the underground river is calculated using formula (2); the set threshold is 0.5; (1) Where: S It is the overall slope of the underground river; v 1 is the inlet water velocity, m / s; v 2 is the outlet water velocity, m / s; n 1 is the entrance roughness; n 2 is the export roughness ratio; b 1 is the inlet river width, m; b 2 is the outlet river width, m; h 1 is the inlet river water depth, m; h 2 is the outlet river water depth, m; (2) Where: z1 is the entrance elevation, m; z2 is the exit elevation, m; l is the projected distance from the exit to the entrance of the underground river, m; g is the acceleration due to gravity, m / s 2 ; Since the inlet and outlet flow rates are approximately the same, the slope formula (1) in this technology is used to calculate the slope. The measured parameters are substituted into the formula to calculate the slope of the Lianchun River from the downstream entrance of Lujiaoba to the exit of Lacaitang, which is 13 / 1000. This section is the main construction area for the grouting curtain during the underground river construction process and is also the main area for the underground river blocking point. The schematic diagram of the longitudinal section of the Qijiaojing-Lianchun River is shown in the figure below. Figure 7 shown.

[0040] Step 3: Determine the location of the upper grouting tunnel: Based on the designed water level and the karst geological distribution range obtained in the first step, select the upper grouting tunnel above the designed water level and determine the elevation of the upper grouting tunnel. Step 4: Determine the location of the underground river blocking: The underground river blocking location is determined by the elevation of the blocking point. H Distance from the blocking point to the underground river exit L The calculation formula is as follows: (3) (4) Where: H is the elevation of the underground river blocking point, m; z 3 is the elevation of the upper grouting tunnel, m; S It is the overall slope of the underground river; d t is the diameter of the upper grouting tunnel, m; L is the distance from the blocking point to the underground river outlet, m; Step 5: Calculation of grouting curtain height: As mentioned above, the calculation formula for grouting curtain height is as follows: (5) Where: d is the height of the grouting curtain, m; Step 6: Carry out grouting tunnel construction and underground river sealing according to the upper grouting tunnel position and underground river sealing position determined in steps three and four: Construct the upper grouting tunnel above the designed water storage level line; construct the lower grouting tunnel below the designed water storage level line in parallel with the upper grouting tunnel. The lower grouting tunnel is perpendicular to the underground river in the horizontal plane, and the intersection with the underground river is the underground river sealing position.

[0041] During construction: construct the upper grouting tunnel above the designed water storage level line. In principle, the upper grouting tunnel should be more than 10 meters above the designed water storage level line. Choose to excavate the upper grouting tunnel above the direction of the underground river and at the vertical intersection of the underground river, parallel to the plane where the underground river is located. The lower grouting tunnel is the underground river blocking tunnel, which is parallel to the upper grouting tunnel in the vertical plane. The front end of the lower tunnel excavation is excavated downward with a certain slope. When the tunnel height is the calculated blocking point elevation, excavate horizontally forward. During excavation, calculate the horizontal projection distance between the excavation point and the underground river outlet at any time to ensure the correct excavation direction.

[0042] Step 7: Carry out grouting curtain construction according to the grouting curtain height obtained in step 5: Drill a hole from the bottom of the upper grouting tunnel vertically to the lower karst geological layer and inject mortar until the top of the underground river. Use a geological drill to drill a hole vertically downward at the bottom of the upper grouting tunnel with a diameter between 56mm and 76mm. The drilling height should be consistent with the grouting curtain height. Monitor the grouting pump pressure during the first drilling construction project. P 1 , formation pressure P 2 , grouting time t , mud viscosity µ , providing data for curtain spacing calculation in subsequent construction. Curtain spacing calculation: based on the grouting pump pressure measured in the previous grouting curtain construction P 1 , formation pressure P 2 , grouting time t, mud viscosity µ , calculate the distance between the next curtain and the previous curtain using the following formula; (6) Where: L’ is the distance between the two curtains, m; k is the equivalent permeability, m 2 ; P 1 is the perfusion pump pressure, Pa; P 2 is the ground stress, Pa; t is the grouting time, s; µ is the slurry viscosity, Pa∙s; Ø is the effective porosity; The construction of each grouting curtain is carried out in sequence based on the calculated curtain spacing.

[0043] In this example, after obtaining the slope of the underground river, the upper grouting tunnel elevation was designed to be 1302.96 meters, the underground river outlet elevation to be 1243.36, and the grouting tunnel diameter to be 4 meters. Substituting the elevation data into formula (3), the underground river blocking point elevation was calculated to be 1265.08 meters, 37.88 meters away from the upper grouting tunnel, which is also the grouting curtain height. Formula (4) calculated that the underground river blocking point is 163.08 meters away from the outlet. Further, through field experiments, the equivalent permeability (k) and effective porosity (Ø) of rock fractures measured by core collection were obtained, with values ​​of 0.0001 and 0.9 respectively. The slurry viscosity experimental value was 20, the injection pressure difference was 780.4 Pa, and the injection time was 16200 s. Substituting these into the formula, the spacing between multiple grouting curtains was calculated to be 16.76 meters.

[0044] Finally, it should be noted that the above is only used to illustrate the technical solution of the present invention and is not limiting. Although the present invention is described in detail with reference to the preferred arrangement scheme, ordinary technicians in this field should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preventing seepage in a karst depression water collection and reservoir project, characterized by: The grouting curtain is combined with underground river blocking measures, including the following steps: Step 1: Hydraulic parameter collection: Through field work, investigate the entrance and exit of the underground river in the planned construction depression and the direction of the underground river, and measure and obtain hydraulic parameters; Step 2: Calculation of underground river slope: Based on the hydraulic parameters obtained in the first step, when the ratio of the inlet flow velocity to the outlet flow velocity of the underground river is greater than or equal to the set threshold, the overall slope of the underground river is calculated using formula (1); when the ratio of the inlet flow velocity to the outlet flow velocity is less than the set threshold, the overall slope of the underground river is calculated using formula (2); the set threshold is 0.4~0.6; （1） Where: S It is the overall slope of the underground river; v 1 is the inlet water velocity, m / s; v 2 is the outlet water velocity, m / s; n 1 is the entrance roughness; n 2 is the export roughness ratio; b 1 is the inlet river width, m; b 2 is the outlet river width, m; h 1 is the inlet river water depth, m; h 2 is the outlet river water depth, m; （2） Where: z1 is the entrance elevation, m; z2 is the exit elevation, m; l is the straight-line distance from the exit to the entrance of the underground river, m; g is the acceleration due to gravity, m / s 2 ; Step 3: Determine the location of the upper grouting tunnel: Based on the designed water level and the measured karst geological distribution range, select the upper grouting tunnel above the designed water level and determine the elevation of the upper grouting tunnel; Step 4: Determine the location of the underground river blocking: The underground river blocking location is determined by the elevation of the blocking point. H And the distance from the blocking point to the underground river exit L The calculation formula is as follows: （3） （4） Where: H is the elevation of the underground river blocking point, m; z 3 is the elevation of the upper grouting tunnel, m; S It is the overall slope of the underground river; d t is the diameter of the upper grouting tunnel, m; L is the distance from the blocking point to the underground river outlet, m; Step 5: Calculation of grouting curtain height: The calculation formula is as follows: （5） Where: d is the height of the grouting curtain, m; Step 6: Carry out grouting tunnel construction and underground river blocking according to the upper grouting tunnel position and underground river blocking position determined in steps 3 and 4: construct the upper grouting tunnel above the designed water storage level; construct the lower grouting tunnel below the designed water storage level in parallel with the upper grouting tunnel. The lower grouting tunnel is perpendicular to the underground river in the horizontal plane, and the intersection with the underground river is the underground river blocking position; Step 7: Carry out grouting curtain construction according to the grouting curtain height obtained in step 5: drill holes from the bottom of the upper grouting tunnel and vertically pour mortar into the lower karst geological layer until the top of the underground river.

2. The anti-seepage method in a karst depression water collection and reservoir engineering according to claim 1, characterized in that: Step 7 also includes: Curtain spacing calculation: based on the grouting pump pressure measured during the previous grouting curtain construction P 1 , formation pressure P 2 , grouting time t , mud viscosity µ , calculate the distance between the next curtain and the previous curtain using the following formula; （6） Where: L’ is the distance between the two curtains, m; k is the equivalent permeability, m 2 ; P 1 is the perfusion pump pressure, Pa; P 2 is the formation pressure, Pa; t is the grouting time, s; µ is the slurry viscosity, Pa∙s; Ø is the effective porosity; The construction of each grouting curtain is carried out in sequence based on the calculated curtain spacing.

3. The anti-seepage method in a karst depression water collection and reservoir engineering according to claim 1, characterized in that: In step 2, the threshold is set to 0.

5.

4. The anti-seepage method in a karst depression water collection and reservoir engineering according to claim 1, characterized in that: The specific operations of step one include: measuring the water flow velocity at the entrance and exit of the underground river v , roughness n 、River width b , water depth h , elevation z and longitude and latitude; determine the karst geological distribution range of the underground river outlet in the depression through geological exploration, and collect cores in the karst geological distribution range to determine the equivalent permeability of rock fractures k and effective porosity Ø ; Obtain the spatial straight-line distance between the exit and entrance of the underground river through latitude and longitude and drone orthophotos l .

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