Design method for preventing siltation of tailrace outlet of downstream reservoir

Abstract

The application discloses a tailwater outlet anti-silting safety design method for the upstream power station of a downstream reservoir tail river section, and the method is characterized in that: through comprehensive long-period numerical simulation results and physical model test achievements, the sediment silting response law of the tailwater outlet adjacent section under different incoming water and sediment conditions and different operation modes is identified, the occurrence probability of the tailwater outlet control section sediment silting higher than the target control elevation is counted, the grading safety guarantee criterion for different states such as normal operation, risk early warning and extreme response is established, and thus an integrated design method considering tailwater anti-silting safety, risk prevention and control and operation benefit improvement is formed. The application performs systematic analysis on the sediment disaster background of the tailwater outlet river section, the long-term equilibrium silting state, the flood dynamic silting process and the extreme working condition reconstruction bed response, constructs a tailwater outlet 'three states and three levels' anti-silting safety guarantee system, and is used for the grading anti-silting safety design of the tailwater outlet of a complex high-sediment mountainous river cascade hydropower project.

Description

Technical Field

[0001] This invention belongs to the field of sediment and reservoir scheduling technology in water conservancy and hydropower engineering, and has a design and evaluation method for identifying sediment deposition risk in the tailrace outlet area under the condition of cascade hydropower development, verifying siltation prevention safety, implementing hierarchical management and control of operation, and improving comprehensive benefits. Background Technology

[0002] In high mountain and canyon rivers, the combined effects of strong tectonic uplift, deeply incised valley topography, and monsoon rainfall typically result in river channels characterized by steep gradients, narrow and deep valleys, rapid currents, and strong sediment transport capacity, making them potential for large-scale cascade hydropower development. However, frequent geological disasters such as landslides, debris flows, collapses, and dam failures lead to abundant sediment sources, sudden sediment inflows, and complex particle composition. Therefore, for cascade hydropower projects on such rivers, once the downstream cascade reservoirs form backwater, the river section where the upstream power plant's tailwater outlet is located may transform from a naturally rapid-flowing section into a variable backwater section controlled by the reservoir's backwater, significantly altering the flow structure, sediment transport methods, and riverbed scouring and deposition processes. In these areas, sediment tends to accumulate at the reservoir tailwater and in adjacent river sections, and undergoes rapid redistribution under conditions of floods, unsteady inflows, or abnormal sediment inflows, thereby raising the local riverbed elevation, compressing the flow cross-section, and affecting tailwater outflow conditions. In China's cascade development of rivers, existing engineering studies have shown that the operation of downstream reservoirs can have a significant backwater effect on the downstream section of upstream power stations or tailrace sections, altering the original design of the downstream water level-discharge relationship and the connection conditions between upstream and downstream.

[0003] Current research on sediment problems in hydropower projects mainly focuses on overall reservoir siltation, sediment control in front of the dam, layout of sediment discharge structures, silt prevention at the intake, and reservoir capacity maintenance. However, there is insufficient research on silt prevention and safety design methods for the special scenario of "the tailrace outlet of an upstream power station located in the downstream section of a reservoir". Compared with conventional tailrace outlets, this type of tailrace outlet is simultaneously affected by multiple factors such as the rise in downstream reservoir water level, increased upstream sediment inflow, non-constant changes in flood processes, and strong constraints from local topography. Its safety issues are not simply manifested as general riverbed siltation, but have stronger dynamics, probabilistic nature, and risks: under normal operating conditions, a certain dynamic scouring and siltation balance may be maintained near the tailrace outlet; under high sediment load floods or catastrophic sediment inflow conditions, short-term rapid siltation, sand waves, or channel reorganization may occur, causing local blockage of the tailrace outlet, poor outflow, deterioration of energy dissipation conditions, and even weakening unit output and system operation safety. International research and engineering practice on reservoir sediment management generally agree that sedimentation has become a global problem affecting the lifespan, operational safety, and overall benefits of reservoirs. Furthermore, under conditions of cascaded development, backwater connection, and complex operation and scheduling, sediment issues need to be considered in conjunction with engineering operation rules.

[0004] For downstream cascade engineering projects in high mountain and canyon rivers, the problems are further manifested in the following aspects: First, when the reservoir sediment ratio is low, the reservoir often experiences significant siltation in a short period of time, and the tail section of the reservoir is prone to become the most sensitive area for sediment deposition and scouring adjustment; Second, the long duration and large peak value of flood events, coupled with the influx of hazardous sediment, can significantly increase the probability of siltation and the magnitude of riverbed uplift in the adjacent river sections at the tail of the reservoir; Third, the tailwater of upstream power plants often has characteristics such as large flow, lateral inflow or concentrated outflow, which, after coupling with the inflow from the natural main channel and the backwater at the tail of the reservoir, results in a complex local three-dimensional flow structure, which can easily induce the alternating evolution of backflow, separation flow, local scouring and siltation; Fourth, traditional designs usually focus on a single design condition or static verification, and do not adequately consider the probability distribution, risk classification response and operation-engineering coordinated control of sediment deposition elevation at the tailwater outlet section under long-term operating conditions, thus making it difficult to simultaneously achieve multiple objectives such as tailwater siltation prevention safety, risk control and power generation efficiency improvement.

[0005] Therefore, there is an urgent need to propose a safety design method for preventing siltation at the tailrace outlet of a power station located in the downstream section of a reservoir. Summary of the Invention

[0006] The purpose of this invention is to address the problem of insufficient consideration in the design of tailrace outlet silt prevention safety for upstream power stations located in the downstream tailrace section of a reservoir in existing technologies. In particular, existing technologies cannot take into account multiple operating conditions such as long-term sedimentation equilibrium, dynamic sedimentation during floods, and extreme bed regeneration under extreme natural disasters. This results in a lack of hierarchical control basis for tailrace outlet silt prevention design, making it difficult to achieve a balance between silt prevention safety, risk control, and improved operational efficiency. This invention provides a method for silt prevention safety design of tailrace outlets of upstream power stations in the downstream tailrace section of a reservoir. Through systematic analysis of the sediment disaster background, long-term equilibrium sedimentation state, dynamic sedimentation process during floods, and bed regeneration response under extreme operating conditions in the tailrace section and tailrace outlet, a "three-state, three-level" silt prevention safety guarantee system is constructed under the combined influence of downstream backwater and flood sediment transport. This solves the problem of hierarchical silt prevention safety design and efficiency improvement for tailrace outlets of complex, high-sand-content mountainous river cascade hydropower projects.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: This invention, by integrating long-term numerical simulation results and physical model test results, identifies the sediment deposition response patterns of the tailrace outlet adjacent to the power station under different inflow and sediment conditions and different operating modes. It also statistically analyzes the probability of sediment deposition at the tailrace outlet control section exceeding the target control elevation and establishes a graded safety assurance criterion for three states of reservoir sediment deposition: long-term equilibrium state, dynamic uncertain deposition state, and extreme disaster abnormal state. This results in an integrated design method that takes into account tailrace sedimentation safety, risk control, and operational efficiency improvement, thus making up for the shortcomings of existing technologies in the design of tailrace outlet sedimentation safety in the variable backwater area of ​​the reservoir tail.

[0008] The silt prevention and safety design method for the tailrace outlet of the upstream power station in the downstream reservoir tail section provided by this invention is as follows: Step 1: Identification of sediment hazards and assessment of siltation risk in the tailrace outlet section of the power station The sediment disaster at the tailrace outlet of the power station was identified and the siltation risk was analyzed. Based on the identification and analysis results, the siltation risk at the tailrace outlet was divided into long-term equilibrium siltation risk, dynamic siltation risk during floods, and bed regeneration risk under extreme natural disasters. The corresponding sediment response states were divided into three states: basic equilibrium state of reservoir sedimentation, dynamic uncertain siltation state, and abnormal state under extreme disasters. The key control factors affecting the siltation safety at the tailrace outlet were determined.

[0009] Step Two: Long-term equilibrium sedimentation analysis of the reservoir area and determination of the bottom elevation of the low-lying tunnel at the tailrace outlet of the power station S201. Based on the inflow and sediment series of the studied river section, the operation mode of the downstream reservoir, and the boundary conditions of the reservoir area, an empirical formula method based on the equilibrium sedimentation gradient relationship of the reservoir area, the sediment retention rate relationship of the reservoir, and the longitudinal sedimentation morphology relationship is used to make a preliminary estimate of the development trend of sedimentation in the reservoir area, the longitudinal profile morphology of sedimentation, and the range of long-term equilibrium sedimentation elevation that may be formed in the tail section of the reservoir.

[0010] S202. Establish a one-dimensional mathematical model of water and sediment that reflects the longitudinal evolution of water and sediment in the reservoir area. Input multi-year series of runoff processes, sediment transport processes, boundary water level processes and reservoir scheduling processes to simulate the sediment deposition evolution under long-term operation conditions in the reservoir area and obtain the long-term scouring and deposition process of the tail section and the section adjacent to the tailwater outlet.

[0011] S203. Establish a physical model similar to the prototype to verify or supplement the analysis of the sedimentation development process in the reservoir area, and obtain the sedimentation morphology, sedimentation elevation, and scour and sedimentation distribution characteristics of the main control sections in the area adjacent to the tailwater outlet under long-term operating conditions.

[0012] S204. By comprehensively comparing and selecting the experimental results of empirical formula method, one-dimensional water and sediment mathematical model and physical model, the basic equilibrium sedimentation elevation of the tailwater outlet control section under a series of annual water and sediment conditions is determined.

[0013] S205. Based on the tailwater outlet layout, tailwater outflow direction, tunnel opening size, and relationship with the natural mainstream and backwater flow patterns, analyze the siltation control conditions and flow conditions in front of the tunnel corresponding to different tailwater outlet locations.

[0014] S206. Based on the basic equilibrium siltation elevation, superimpose the design superelevation to determine the bottom elevation of the tailwater outlet low hole. The design superelevation is used to meet the structural safety, flow safety and operational margin requirements of the tailwater outlet under the basic equilibrium siltation conditions.

[0015] The elevation of the bottom plate of the low-level tunnel serves as the first level of silt control elevation for the tailwater outlet under equilibrium conditions.

[0016] S207. Based on the determined elevation of the bottom plate of the low-level tunnel, propose basic prevention and control measures to address the basic equilibrium siltation. The preferred basic prevention and control measures are to set up sand-blocking barriers at the leading edge or adjacent area of ​​the low-level tunnel to reduce the adverse effects of equilibrium siltation on the outflow of the low-level tunnel.

[0017] Step 3: Dynamic Uncertainty Sedimentation Analysis and Determination of Top Elevation of Low-Level Overlapping Beam Gate at Power Station Tailwater Outlet under Frequency Flood Conditions S301. Using hydrological analysis methods, determine the design flood process curves of the study river section under the conditions of minor floods (less than once in 5 years), medium floods (once in 5 to 20 years), major floods (once in 20 to 50 years), and catastrophic floods (once in 50 years). Combined with sediment content processes, bedload recharge processes, and flood duration, construct the design water and sediment processes under different frequency flood conditions.

[0018] S302. Based on the design water and sediment process obtained in step S301, the formula method is used to make a preliminary estimate of the sediment transport capacity, scouring and deposition trend and riverbed response amplitude of the river section near the tailwater outlet under the flood frequency conditions.

[0019] S303. Establish a one-dimensional water and sediment mathematical model applicable to the flood process scouring and deposition response analysis of the river section adjacent to the tailwater outlet of the power station. Simulate the dynamic changes in riverbed elevation under different combinations of inflow and sediment, different downstream backflow conditions, and different tailwater outflow conditions during the flood, and obtain the range of dynamic uncertain sedimentation elevation changes at the tailwater outlet control section.

[0020] S304. Through physical model tests, analyze the local flow patterns, the convergence patterns of tailwater and natural inflow during the flood process, the local scouring and deposition evolution, and the changes in riverbed elevation near the tailwater outlet section to identify the maximum dynamic siltation height and its duration during the flood process.

[0021] S305. Based on the results of combined steps S302 to S304, determine the dynamic elevation variation range of the tailrace outlet control section of the power station under different frequency flood conditions, and select the control elevation under unfavorable dynamic siltation conditions as the dynamic uncertain siltation elevation.

[0022] S306. Based on the dynamic and uncertain siltation elevation, superimpose the design superelevation to determine the top control elevation of the tailwater outlet low-level tunnel beam gate (i.e., the top elevation after superimposing the beam gate on the bottom plate of the low-level tunnel).

[0023] The top elevation of the low-level tunnel's stacked beam gate serves as the second anti-siltation control elevation for the tailwater outlet under dynamic and uncertain siltation conditions. The top elevation of the low-level tunnel's stacked beam gate and the top elevation of the low-level tunnel itself jointly determine the effective flow section range of the low-level tunnel.

[0024] S307. Based on the top control elevation of the tailrace outlet low-level tunnel beam gate, an adjustable height beam gate structure is installed at the low-level tunnel location to raise the low-level tunnel outflow elevation according to different flood sediment-carrying processes, adjust the effective flow section elevation of the low-level tunnel, realize dynamic adaptation of the top control elevation of the low-level tunnel beam gate, avoid sediment blockage of the low tailrace outlet while maintaining a lower outflow elevation as much as possible.

[0025] S308. Based on the dynamic elevation change range corresponding to floods of different frequencies, establish the opening and closing conditions and operation rules of the low-cavity stacked beam gate, so that the tailwater outlet can maintain its outflow capacity and silt prevention safety under dynamic siltation conditions.

[0026] Step 4: Analysis of Regenerated Bed under Extreme Operating Conditions and Determination of Control Elevation of Bottom Plate of High Tunnel at Power Plant Tailwater Outlet S401. Identify the types of extreme natural disasters that may affect the tailrace outlet section of the river.

[0027] S402. Collect historical event data, geomorphological evidence, remote sensing monitoring data, and regional geological background data on extreme natural disasters. Combine the disaster scale, frequency, impact range, and material source conditions to determine the design magnitude for various extreme working conditions.

[0028] S403. Construct the water and sediment inflow processes under extreme working conditions, including extreme peak flow processes, high sediment load flood processes, sudden sediment replenishment processes, and duration of disastrous sediment inflow processes.

[0029] S404. A one-dimensional water and sediment mathematical model is used to analyze the sediment deposition process in the reservoir area and the river section near the tailwater outlet under extreme working conditions, and to obtain the longitudinal sedimentation evolution and the extreme sedimentation height of the control section during the bed regeneration process.

[0030] S405. Physical model tests were used to analyze the rapid uplift of the riverbed, sand wave propagation, beach-channel reorganization and re-creation morphology in the area near the tailwater outlet under extreme working conditions, and to determine the unfavorable sedimentation elevation of the tailwater outlet control section under extreme natural disaster conditions.

[0031] S406. Combining the results of steps S404 and S405, determine the extreme sedimentation elevation of the regenerated bed under extreme operating conditions at the tailwater outlet control section.

[0032] S407. Based on the extreme siltation elevation, superimpose the design superelevation to determine the control elevation of the tailwater outlet tunnel bottom plate.

[0033] The high-hole bottom plate control elevation serves as the third anti-siltation control elevation for the tailwater outlet under extreme natural disaster conditions. When the low-hole cannot meet the safe outflow requirements, the outflow is switched to the high-hole.

[0034] In the above technical solution of the present invention, step one, which involves identifying sediment hazards and conducting siltation risk analysis in the tailrace outlet section of the power station, includes the following: S101. Collect basic data on the river section where the tailwater outlet of the power station is located and its upstream and downstream basins. The basic data includes topographic data, hydrological and sediment data, geological disaster data, historical flood data, remote sensing image data, existing survey data and similar engineering data.

[0035] S102. Based on historical measured water and sediment data and disaster event data, analyze existing or under-construction projects that are similar to the target river section in terms of valley morphology, water and sediment sources, geological disaster types, cascade development conditions and reservoir operation modes. Summarize the main problems and prevention and control methods of tailwater outlet in the reservoir tail section under conditions of sediment deposition, backwater backwater, flood carrying sediment and extreme disaster sediment inflow.

[0036] S103. Using on-site survey methods, investigate the riverbed composition, bank stability, gully distribution, landslide deposits, debris flow gully distribution, local bottleneck topography, and scour and sedimentation traces in the river section adjacent to the tailwater outlet, and identify sediment source areas, transport channels, and river sections prone to sedimentation.

[0037] S104. Using remote sensing interpretation methods, landslides, collapses, debris flow fans, blockage bodies, glacial lakes, and historical course changes in the study river section and its basin are identified, and their spatial distribution, scale, and potential impact on the tailrace outlet section are analyzed.

[0038] Furthermore, in step one, the key control factors include, but are not limited to: the backwater backwater range of the downstream reservoir, the siltation development trend of the reservoir tail section, the water and sediment process of each flood, the sudden sediment inflow process induced by geological disasters, the plane location of the tailwater outlet, the tailwater outflow mode, and the local topographic constraints of the river section.

[0039] Furthermore, in step two, the preferred basic prevention and control measure is to set up a sand-blocking embankment at or near the leading edge of the low-level tunnel to reduce the adverse effects of equilibrium siltation on the outflow of the low-level tunnel.

[0040] Furthermore, the design superelevation in steps two, three, and four can be comprehensively determined based on the safety requirements of the tailrace outlet structure, construction errors, model uncertainties, sediment fluctuation amplitude, and operational margin requirements.

[0041] Furthermore, in step three, the dynamic uncertain siltation elevation is determined by the envelope value of the maximum unfavorable siltation elevation at the tailwater outlet control section under different frequency flood conditions.

[0042] Furthermore, the extreme sedimentation elevation in step four is determined by the envelope value of the maximum unfavorable sedimentation elevation at the tailwater outlet control section under various extreme disaster conditions.

[0043] Furthermore, the extreme natural disaster types mentioned in step four include one or more of the following: flash floods and mudslides, landslides blocking rivers, mudslides flowing into confluences, floods caused by the outflow of landslide-dammed lakes, and floods caused by the outflow of glacial lakes.

[0044] Furthermore, step four also includes establishing criteria for switching the tailwater outlet from the low-level tunnel to the high-level tunnel. These criteria include, but are not limited to: the siltation elevation at the front edge or cross-section of the low-level tunnel reaching or exceeding the top elevation of the low-level tunnel's stacked beam gate; the stacked beam gate failing to meet safety outflow requirements after adjustment; and extreme operating condition warnings reaching a preset level.

[0045] Furthermore, the high and low tailwater outlets are arranged in a layered manner within the same tailwater outlet structure.

[0046] The above-mentioned technical solution of this invention constructs a "three-state, three-level" siltation prevention and safety assurance system for the tailwater outlet. "Three-state, three-level" refers to: based on the basic equilibrium siltation elevation determined in step two, the dynamic uncertain siltation elevation determined in step three, and the extreme siltation elevation determined in step four, the siltation prevention and safety status of the tailwater outlet is divided into three states: a basic equilibrium state of sediment deposition, a dynamic uncertain siltation state, and an extreme disaster abnormal state. Three levels of siltation prevention measures are proposed for these three states, namely, the first basic siltation prevention measure, the second dynamic adjustment measure, and the third extreme protection measure, corresponding sequentially to the three states, thus forming a multi-level defense system for the tailwater outlet facade. Specifically, as follows: To address the basic equilibrium state of sediment deposition, sand-trapping barriers are used to control long-term equilibrium deposition and ensure basic outflow conditions above the bottom of the low-lying tunnel.

[0047] To address the dynamic and uncertain siltation state, a stacked beam gate is used to regulate the effective flow elevation of the low-level tunnel, adapting to the dynamic rise and fall of the riverbed during flood processes.

[0048] In response to extreme disasters and abnormal situations, high-hole outflow is adopted to avoid the safety risks caused by the rapid blockage of low-holes or the deterioration of flow conditions.

[0049] The tailwater outlet facade is constructed by the elevation of the bottom plate of the low-level tunnel, the top control elevation of the low-level tunnel's stacked beam gate, and the bottom plate control elevation of the high-level tunnel. This forms a "three-state, three-level" silt prevention and safety guarantee system, consisting of balanced foundation interception, dynamic graded adjustment, and extreme high-level protection.

[0050] Compared with the prior art, the present invention has at least the following beneficial effects: This invention expands the tailwater outlet sediment prevention design from the traditional single design condition and static verification method to a graded design method for long-term equilibrium state, flood dynamic state and extreme state, which can more realistically reflect the long-term, dynamic and sudden nature of sediment response at the tailwater outlet of the reservoir tail section.

[0051] This invention integrates various methods, including similar engineering surveys, on-site investigations, remote sensing interpretation, formulaic methods, physical models, and mathematical models, to achieve a systematic integration of sediment deposition elevation identification and risk classification response in the tailrace outlet section, thereby improving the reliability and applicability of the design results.

[0052] This invention constructs a multi-level defense system for the tailwater outlet facade by determining the elevation of the bottom plate of the low-level tunnel, the top elevation of the low-level tunnel, and the elevation of the high-level tunnel. This system can effectively improve the outflow safety, silt prevention capability, and operational adaptability of the tailwater outlet under complex high sediment conditions.

[0053] This invention can take into account tailwater siltation prevention safety, risk control, and improved operational efficiency, providing an implementable, tiered, and switchable technical approach for the siltation prevention safety design of power station tailwater outlets located in the downstream reservoir tail section. Attached Figure Description

[0054] Figure 1 This is a probability diagram showing that the sediment deposition at the tailrace outlet section of the power plant is higher than a certain elevation, according to an embodiment of the present invention. Detailed Implementation

[0055] The present invention will be further illustrated below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above-described invention, and these improvements and adjustments still fall within the scope of protection of the invention.

[0056] Example 1 This embodiment addresses the sediment deposition problem in the tailrace outlet section of a third-level hydropower station located downstream of a fourth-level reservoir. It employs a combination of a one-dimensional water-sediment mathematical model and a comprehensive abnormal sediment model of the reservoir area to conduct simulation studies on sediment deposition in the tailrace section under conditions of 50-year and 100-year reservoir operation and different frequencies of flood-carried sediment. The study analyzes the riverbed response patterns in the tailrace outlet section of the power station under the combined effects of extremely low reservoir-sediment ratio sediment deposition development, backwater inflow at the reservoir tail, and high-intensity sediment carryover from catastrophic floods. Based on this, a "three-state, three-level" flood control and sediment prevention safety assurance engineering design scheme for the tailrace outlet, integrating "tailrace safety, risk prevention and control, and benefit improvement," is proposed.

[0057] This embodiment constructs a "three-state, three-level" siltation prevention and safety assurance system for the tailrace outlet. The "three-state, three-level" refers to: based on the basic equilibrium siltation elevation determined in step two, the dynamic uncertain siltation elevation determined in step three, and the extreme siltation elevation determined in step four, the siltation prevention and safety status of the tailrace outlet is divided into three states: a basic equilibrium state of sediment deposition, a dynamic uncertain siltation state, and an extreme disaster abnormal state. Three levels of siltation prevention measures are proposed for these three states, corresponding sequentially to the first basic siltation prevention measure, the second dynamic adjustment measure, and the third extreme protection measure, thus forming a multi-level defense system for the tailrace outlet facade.

[0058] I. Basic Data Acquisition and Research Methods First, basic data on the tailrace section's topography, hydrology, sediment, operational scheduling, historical floods, disaster events, measured cross-sections, reservoir topography, and riverbed composition were collected and studied to establish a fundamental database for water and sediment analysis of the tailrace section and reservoir area. Second, a one-dimensional water and sediment mathematical model applicable to the four downstream reservoirs was established to analyze the longitudinal scouring and deposition evolution of the reservoir area during long-term reservoir operation, the changes in sedimentation elevation at the tailrace outlet control section, and the basic equilibrium state of sediment deposition under different operating years. Furthermore, a comprehensive abnormal sediment model of the reservoir area was established to simulate the local flow regime, local scouring and deposition patterns, riverbed elevation changes, and sediment response near the outlet in the tailrace section under reservoir operation and flood sediment-carrying processes. Through mutual verification between numerical simulation and physical model experiments, a comprehensive analysis of the sedimentation characteristics of the tailrace section under long-term, dynamic, and extreme conditions was conducted.

[0059] In this embodiment, the long-term sedimentation evolution analysis uses a series of 8-year water and sediment inflow processes as input to simulate the sedimentation process of the reservoir over 30, 50, and 100 years of operation. For frequency flood conditions, based on measured flood data from control stations or reference stations, the design peak flow and design flood volume for floods with different return periods are determined using frequency analysis. Typical flood processes are selected, and the design flood processes for 5-year, 20-year, 30-year, 50-year, 100-year, 1000-year, and 5000-year return periods are derived using the same frequency amplification method or the same ratio amplification method. The extreme disaster condition analysis considers sudden high-intensity water and sediment inflow processes caused by landslides, debris flows, and flash floods. This river section mainly considers the large-scale landslide dam failure condition.

[0060] II. First State, First Level: Determining the Basic Equilibrium State of Reservoir Sedimentation and the Elevation of the Bottom Slab of the Power Station Tailwater Outlet (Elevation of the Foundation Cutting for Silt Prevention at the Bottom Slab of the Low Outlet) Studies show that the upstream three-stage hydropower plant is located within the fluctuating backwater area of ​​the downstream four-stage reservoir, and the downstream reservoir has an extremely low reservoir-to-sludge ratio. In the early stages of reservoir operation, both suspended and bedload sediments deposited heavily in the reservoir area, resulting in rapid sediment accumulation. Suspended sediment reached equilibrium earlier, followed by bedload sediment deposition, which gradually became dominant, causing the reservoir channel to evolve into a gravel channel. Suspended sediment deposition continued even during the non-flood season. Based on a one-dimensional hydro-sediment mathematical model and experimental results from a reservoir-wide abnormal sediment model, after approximately 30 years of reservoir operation, the bedload sediment outflow rate exceeded 90%, and bedload sediment deposition in the reservoir area reached a basic equilibrium state. Subsequently, the reservoir entered a stage of alternating scouring and deposition with slow sedimentation. Therefore, the sediment deposition elevation of the tailrace control section after 30 years of reservoir operation can be considered as the basic equilibrium control elevation of the tailrace outlet under long-term operating conditions.

[0061] Specifically, the sedimentation elevation at the tailwater outlet control section, which was in basic equilibrium after 30 years of reservoir operation, was statistically determined. H 1m. Considering the uncertainties in the actual inflow of water and sediment into the reservoir, and the rapid sedimentation of both suspended and bedload sediments during the initial stage of reservoir operation, to ensure sufficient anti-siltation margin at the low outlet of the tailrace under basic equilibrium conditions, the sedimentation elevation at the basic equilibrium state is [not specified]. H Super high design on top of 1m Δh 1m, determine the elevation of the bottom plate of the tailwater outlet low tunnel. H b m is ( H 1+ Δh 1) m.

[0062] This level of measures, as the "first level of the first state" in the "three states and three levels", is mainly used to deal with the long-term siltation impact under the basic equilibrium state of reservoir siltation. By setting up a siltation prevention foundation sill (low hole bottom elevation) at the front edge of the tailwater low outlet, the tailwater outlet maintains low hole outflow during most of the normal operating periods, and ensures siltation prevention safety and power generation efficiency under low tailwater operating conditions.

[0063] III. Second State, Second Level: Uncertain Dynamics of Reservoir Sedimentation and Determination of the Top Elevation of the Low-Level Overlapping Beam Gate at the Power Station Tailwater Outlet After the reservoir reaches a basic equilibrium in sediment deposition, slow sedimentation will continue in the reservoir area. Furthermore, under the influence of variations in water and sediment inflows from different years and the impact of various flood events, the tailrace riverbed elevation will exhibit a dynamic equilibrium of alternating scouring and deposition. According to simulation results, the sediment deposition elevation in the tailrace river section after 50 years of reservoir operation will be... H 2m, the sediment deposition elevation of the tailrace section after 100 years of reservoir operation is H 3m.

[0064] Furthermore, based on the 50-year sediment deposition topography, the sediment-carrying processes of 5-year, 20-year, 30-year, 50-year, 100-year, 1000-year, and 5000-year floods were superimposed to simulate the dynamic elevation of the riverbed at the tailrace outlet control section. The resulting dynamic elevations of sediment near the tailrace outlet under different flood frequencies were: Hf 5m Hf 20 m、 Hf 30 m、 Hf 50 m、 Hf 100 m、 Hf 1000 m and Hf 5000 m, where the most unfavorable dynamic sedimentation control elevation is H d m represents the dynamic, uncertain sedimentation elevation of the tailrace under different frequency flood conditions.

[0065] Based on the dynamic and uncertain sedimentation elevation of the tailrace section under different flood frequencies, the changes in tailrace outlet elevation caused by sedimentation evolution over 30, 50, and 100 years of reservoir operation are comprehensively considered. ΔH 30 m、 ΔH 50 m、 ΔH 100 m, determine the top control elevation of the tailrace outlet low-level tunnel beam gate, which serves as the second anti-siltation control elevation for the tailrace outlet. H t To balance siltation prevention safety and power generation head benefits under different operational stages and flood conditions, this embodiment employs a dynamic stacked beam gate as a second-level siltation prevention measure, ensuring that the effective flow elevation at the tailrace outlet can be [m]. H t,min m to H t,max Dynamic adjustment between m.

[0066] In actual operation, when the sediment deposition elevation of the tailrace section is lower than the control elevation of the bottom plate of the low tunnel, the power station maintains the operation of the low tunnel and the low sill. When the local dynamic sedimentation elevation at the tailrace outlet exceeds the bottom plate elevation of the low tunnel but has not yet reached the extreme state control standard, the stacked beam gate is activated to raise the outflow elevation of the low tunnel to avoid sediment blockage of the low tailrace outlet. At the same time, the power station maintains the operation at a lower outflow elevation as much as possible, thereby increasing the power generation head and improving the power generation efficiency.

[0067] This embodiment also demonstrates that directly using the envelope elevation of siltation accumulated over 30, 50, and 100 years of reservoir operation, superimposed with flood processes of different frequencies, as the second-level control elevation is feasible from a siltation prevention and safety perspective. However, given that the main bedload transport zone of the tailrace section is located in the area opposite the tailrace outlet, under the combined effects of partial cumulative siltation and flood processes, although the siltation near the tailrace outlet may temporarily exceed the elevation of the low-level tunnel floor, the scouring effect generated by the short-term operation of the high tailrace can reduce the siltation elevation near the tailrace outlet to below the low-level tunnel floor elevation, thereby restoring the low-level tunnel and low-sill operation state. Therefore, adopting the dynamic stacked beam gate scheme can improve operational flexibility and long-term comprehensive benefits while meeting safety requirements.

[0068] IV. Third State, Third Level: Determination of the control elevation of the high tailrace outlet slab of the hydropower station's tailrace flood with high-intensity sediment-laden, abnormal siltation, and formulation of the high tailrace outlet elevation. Studies have shown that the tailrace section is located downstream of multiple potential geological hazard risk sources and may be affected by sudden high-intensity water and sediment flows such as landslides and debris flows under extreme circumstances. Before such disasters occur, the peak flow, sediment load, and duration are usually highly uncertain. However, after the reservoir sedimentation has reached a basic equilibrium, a sudden surge in extreme disaster floods could "replenish and lift" a large amount of sediment, which would be rapidly transported to the tailrace section, causing rapid siltation of the low tailrace outlet.

[0069] Therefore, under extreme disaster and abnormal conditions, the low tailrace outlet should be closed, and the tailrace of the power station should be raised to an elevation where it is not silted up and coarse sediment is unlikely to enter the tunnel, i.e., the high tailrace outlet (high tunnel) should be activated. A physical model and a one-dimensional hydro-sediment mathematical model were used to jointly analyze the tailrace re-bed formation process under extreme conditions, yielding the extreme sedimentation elevation as follows: H e m. Based on this, super-high design is superimposed. Δh After 3m, the preliminary control elevation of the bottom plate of the tailrace outlet tunnel is ( H e + Δh 3) m, i.e. H g m.

[0070] The determination of the control elevation of the high tailwater outlet high tunnel bottom plate must simultaneously meet the following functional requirements: First, during extreme flood disasters, if the power station is still in the state of water diversion and power generation, the high tailwater outlet should ensure that the tailwater is not blocked and maintain the necessary outflow capacity; Second, after the extreme flood recedes, even if the low tailwater tunnel is partially or completely blocked by silt, driftwood and debris, the high tailwater outlet should still be able to provide sufficient outflow drop and potential energy to convert it into the kinetic energy required to flush the silt at the front edge of the low tunnel and remove obstacles at the tunnel entrance, so as to restore the low tailwater outlet operation mode.

[0071] In this embodiment, the preliminary control elevation of the high-cavity bottom slab is set as follows: H g m can be further optimized by combining model test results, structural design conditions, engineering investment and hydropower economic indicators.

[0072] V. Layout and Operation Mode of the "Three-State and Three-Level" Project In summary, this embodiment adopts a "three-state, three-level" engineering layout scheme for the tailrace outlet, integrating "tailrace water siltation prevention and safety, risk control, and benefit improvement." Specifically: In the first stage of the first state, the basic equilibrium state of reservoir sediment deposition is used as the control basis to determine the elevation of the bottom plate of the low-level tunnel, that is, the elevation of the foundation sill of the low tailwater outlet. H b m is used to address long-term equilibrium siltation; In the second stage of the second state, the dynamic elevation changes of the tailrace section caused by long-term slow siltation of the reservoir and flood processes of different frequencies are used as the control basis. A dynamic adjustment system of the stacked beam gate is set up to effectively control the elevation of the low tailrace outlet, that is, the top elevation of the low-hole stacked beam gate is at... H t,min m to H t,max The variation between m is used to address dynamic siltation; In the third stage of the third state, the elevation of the abnormal sedimentation and bed regeneration caused by extreme natural disasters is used as the control basis, and a high-level tunnel (high tailwater outlet) is constructed, with its bottom plate control elevation being [missing information]. H g m is used to switch outflow when the low hole cannot meet the safe outflow conditions.

[0073] In operation, priority is given to ensuring that the tailrace maintains a low tailrace outlet for the vast majority of the time to improve the power station's head and operational efficiency. When the siltation elevation of the tailrace section exceeds the first control elevation, the stacked beam gate is activated for dynamic adjustment. When the tailrace section enters an extreme disaster or abnormal state, or when the low-level tunnel has a significant risk of siltation or can no longer meet the safe outflow requirements, the low tailrace outlet is closed and operation is switched to the high tailrace outlet. Through the coordinated operation of high and low tailrace, the system achieves a balance between operational safety, risk control, and improved overall benefits, while ensuring the safety of the tailrace system in terms of flood control and siltation prevention.

[0074] VI. Implementation Results By employing the method described in this embodiment, the probability of sediment deposition exceeding the target elevation at the tailrace outlet control section of a power station can be statistically analyzed based on long-term numerical simulation and physical model test results. This analysis allows for the establishment of a tiered anti-siltation control standard for the power station's tailrace outlet. This method integrates long-term equilibrium sedimentation, dynamic flood sedimentation, and abnormal sedimentation due to extreme disasters into a unified design framework, avoiding the insufficient adaptability caused by static design based solely on a single operating condition. By setting low tailrace foundation sills, dynamic stacked beam gates, and high tailrace outlets, the anti-siltation capability, operational regulation capability, and long-term comprehensive benefits of the power station's tailrace outlet located in the reservoir tail section under complex water and sediment conditions can be effectively improved.

[0075] Figure 1 This invention relates to the probability that sediment deposition at the tailrace outlet section of a power station exceeds a certain elevation. Based on a one-dimensional hydro-sediment mathematical model, and using the method described in this embodiment, an 8-year series of 100-year hydro-sediment process simulations were conducted on the river section of the reservoir area of ​​a large-gradient, narrow-deep canyon hydropower station. The model recorded the riverbed sedimentation elevation in 10-day increments, generating a total of 3600 sets of time-series data. Based on this, through statistical extension and probability fitting of the data, a long-term probability distribution function of the riverbed sedimentation elevation was constructed, obtaining the probability of "sediment deposition exceeding this elevation" for each elevation, as shown below. Figure 1 Based on the probability distribution characteristics of sedimentation elevation in the MT3 tailrace section, a sedimentation prevention and control mechanism coupled with "probabilistic risk-engineering classification" was constructed. Risk probability thresholds corresponding to different sedimentation levels were identified, and a three-tiered control elevation system (low, medium, and high) was established. H b —Elevation of the low-cavity floor corresponding to Example 1, H t —Control elevation of the top of the low-cavity stacked beam gate corresponding to Embodiment 1, H g —Corresponding to the control elevation of the high-level tunnel bottom slab in Implementation Example 1), these correspond to three types of engineering measures: fixed sand-trapping embankment, dynamic stacked beam gate, and high tailwater outlet. The blue area represents the probability density of long-term siltation elevation, indicating that riverbed siltation elevation is concentrated in most cases... H b Therefore, fixed sand-retaining embankments are used as the basic defense line to intercept siltation under normal conditions; as the siltation elevation exceeds... H b The probability of such events gradually decreased, and the extreme siltation events with a medium to high probability were... H t The dynamic overlapping of the elevation gates, through adjustable engineering boundaries, matches the dynamic rise and fall of the sedimentation elevation during floods; while for extremely rare cases of excessive sedimentation exceeding standards, the elevation of the tailrace outlet is determined by a high-level arrangement. H gThis provides a safety net to prevent extreme siltation from obstructing tailwater outflow. Its core mechanism lies in: by identifying the quantile distribution of siltation elevation probability, matching siltation risks of different frequencies and magnitudes with the corresponding engineering control elevations, achieving tiered defense from normal siltation to extreme disasters, thereby ensuring the long-term safety and stability of tailwater outflow in a probabilistic sense.

Claims

1. A safety design method for preventing siltation at the tailrace outlet of an upstream power station in the downstream section of a reservoir, characterized in that: Includes the following steps: Step 1: Identification of sediment hazards and assessment of siltation risk in the tailrace outlet section of the power station The sediment disasters at the tailwater outlet of the power station were identified and the siltation risk was analyzed. Based on the identification and analysis results, the siltation risk at the tailwater outlet was divided into long-term equilibrium siltation risk, dynamic siltation risk during floods, and bed regeneration risk under extreme natural disasters. The corresponding sediment response states were divided into three states: basic equilibrium state of reservoir sedimentation, dynamic uncertain siltation state, and abnormal state under extreme disasters. At the same time, the key control factors affecting the siltation safety at the tailwater outlet were identified. Step Two: Long-term equilibrium sedimentation analysis of the reservoir area and determination of the bottom elevation of the low-lying tunnel at the tailrace outlet of the power station S201. Based on the inflow and sediment series of the studied river section, the operation mode of the downstream reservoir and the boundary conditions of the reservoir area, the empirical formula method based on the equilibrium sedimentation gradient relationship of the reservoir area, the sediment interception rate relationship of the reservoir and the longitudinal sedimentation morphology relationship is used to make a preliminary estimate of the sedimentation development trend, sedimentation longitudinal profile morphology and the long-term equilibrium sedimentation elevation range that may be formed in the tail section of the reservoir. S202. Establish a one-dimensional water and sediment mathematical model that reflects the longitudinal water and sediment evolution process in the reservoir area. Input multi-year series of runoff processes, sediment transport processes, boundary water level processes and reservoir scheduling processes to simulate the sediment deposition evolution under long-term operation conditions in the reservoir area and obtain the long-term scouring and deposition process of the tail section and the section adjacent to the tailwater outlet. S203. Establish a physical model to verify or supplement the analysis of the sedimentation development process in the reservoir area, and obtain the sedimentation morphology, sedimentation elevation and scour and sedimentation distribution characteristics of the main control sections in the area adjacent to the tailwater outlet under long-term operating conditions. S204. By combining the experimental results of empirical formulas, one-dimensional water and sediment mathematical models and physical models, the basic equilibrium sedimentation elevation of the tailwater outlet control section under a series of annual water and sediment conditions is determined. S205. Based on the tailwater outlet layout, tailwater outflow direction, tunnel opening size, and relationship with the natural main flow and backwater flow, analyze the tunnel front sedimentation control conditions and flow conditions corresponding to different tailwater outlet locations. S206. Based on the basic equilibrium siltation elevation, superimpose the design superelevation to determine the bottom elevation of the tailwater outlet low hole. The design superelevation is used to meet the structural safety, flow safety and operational margin requirements of the tailwater outlet under the basic equilibrium siltation conditions. The elevation of the bottom plate of the low-level tunnel serves as the first anti-siltation control elevation for the tailwater outlet under equilibrium conditions. S207. Based on the determined elevation of the bottom plate of the low-lying tunnel, propose basic prevention and control measures to address the siltation in a state of basic equilibrium. Step 3: Dynamic Uncertainty Sedimentation Analysis and Determination of Top Elevation of Low-Level Overlapping Beam Gate at Power Station Tailwater Outlet under Frequency Flood Conditions S301. Using hydrological analysis methods, determine the design flood process line of the study river section under the conditions of minor flood, medium flood, large flood and catastrophic flood, and construct the design water and sediment process under different frequency flood conditions by combining the sediment content process, bedload recharge process and flood duration. S302. Based on the design water and sediment process obtained in step S301, the formula method is used to make a preliminary estimate of the sediment transport capacity, scouring and deposition trend and riverbed response amplitude of the river section near the tailwater outlet under the flood frequency conditions. S303. Establish a one-dimensional water and sediment mathematical model applicable to the flood process scouring and deposition response analysis of the river section adjacent to the tailwater outlet of the power station. Simulate the dynamic changes in riverbed elevation under different combinations of inflow and sediment, different downstream backflow conditions and different tailwater outflow conditions during the flood, and obtain the dynamic uncertain sedimentation elevation change range of the tailwater outlet control section. S304. Through physical model tests, analyze the local flow patterns, the convergence pattern of tailwater and natural inflow during the flood process, the local scouring and deposition evolution, and the changes in riverbed elevation near the tailwater outlet section to identify the maximum dynamic deposition height and its duration during the flood process. S305. Based on the results of steps S302 to S304, determine the dynamic elevation variation range of the tailrace outlet control section of the power station under different frequency flood conditions, and select the control elevation under unfavorable dynamic siltation conditions as the dynamic uncertain siltation elevation. S306. Based on the dynamic uncertain siltation elevation, superimpose the design superelevation to determine the control elevation of the top of the tailwater outlet low-hole stacked beam gate; The top elevation of the low-level tunnel's stacked beam gate serves as the second anti-siltation control elevation for the tailwater outlet under dynamic and uncertain siltation conditions. The top elevation of the low-level tunnel's stacked beam gate and the top elevation of the low-level tunnel together determine the effective flow section range of the low-level tunnel. S307. Based on the control elevation of the top of the stacked beam gate at the tailwater outlet, an adjustable height stacked beam gate structure is set at the location of the tailwater outlet to raise the outflow elevation of the tailwater outlet according to different flood sediment carrying processes, adjust the effective flow section elevation of the tailwater outlet, realize the dynamic adaptation of the control elevation of the top of the stacked beam gate, avoid sediment blockage of the tailwater outlet while maintaining a lower outflow elevation as much as possible. S308. Based on the dynamic elevation change range corresponding to floods of different frequencies, establish the opening and closing conditions and operation rules of the low-cavity stacked beam gate so that the tailwater outlet can maintain its outflow capacity and anti-siltation safety under dynamic siltation conditions. Step 4: Analysis of Regenerated Bed under Extreme Operating Conditions and Determination of Control Elevation of Bottom Plate of High Tunnel at Power Plant Tailwater Outlet S401. Identify the types of extreme natural disasters that may affect the tailrace outlet section; S402. Collect historical event data, geomorphological evidence, remote sensing monitoring data and regional geological background data of extreme natural disasters, and determine the design magnitude of various extreme working conditions based on the disaster scale, frequency, impact range and material source conditions. S403. Construct the water and sediment inflow processes under extreme working conditions, including extreme peak flow processes, high sediment load flood processes, sudden depositary mass replenishment processes, and duration of disastrous sediment inflow processes; S404. A one-dimensional water and sediment mathematical model is used to analyze the sediment deposition process in the reservoir area and the river section near the tailwater outlet under extreme working conditions, and to obtain the longitudinal sediment deposition evolution and the extreme sedimentation height of the control section during the bed regeneration process. S405. Physical model tests were used to analyze the rapid uplift of the riverbed, sand wave propagation, beach-channel reorganization and re-creation morphology in the area near the tailwater outlet under extreme working conditions, and to determine the unfavorable sedimentation elevation of the tailwater outlet control section under extreme natural disaster conditions. S406. Based on the results of combined steps S404 and S405, determine the extreme state sedimentation elevation of the regenerated bed at the tailwater outlet control section under extreme working conditions. S407. Based on the extreme siltation elevation, superimpose the design superelevation to determine the control elevation of the tailwater outlet tunnel bottom plate; The high-cavity bottom plate control elevation serves as the third anti-siltation control elevation for the tailwater outlet under extreme natural disaster conditions. When the low-cavity outlet cannot meet the safe outflow requirements, the outflow is switched to the high-cavity outlet.

2. The method according to claim 1, characterized in that, Step one involves identifying sediment hazards and conducting a siltation risk analysis of the tailrace outlet section of the power station, including the following steps: S101. Collect basic data on the river section where the tailwater outlet of the power station is located and its upstream and downstream basins. The basic data includes topographic data, hydrological and sediment data, geological disaster data, historical flood data, remote sensing image data, existing survey data and similar engineering data. S102. Based on historical measured water and sediment data and disaster event data, analyze existing or under-construction projects that are similar to the target river section in terms of valley morphology, water and sediment sources, geological disaster types, cascade development conditions and reservoir operation mode. Summarize the main problems and prevention and control methods of tailwater outlet in the reservoir tail section under conditions of sediment deposition, backwater backwater, flood sediment transport and extreme disaster sediment inflow. S103. Using on-site survey methods, investigate the riverbed composition, bank stability, gully distribution, landslide deposits, debris flow gully distribution, local bottleneck topography, and scour and sedimentation traces in the river section adjacent to the tailwater outlet, and identify sediment source areas, transport channels, and river sections prone to sedimentation. S104. Using remote sensing interpretation methods, landslides, collapses, debris flow fans, blockage bodies, glacial lakes, and historical course changes in the study river section and its basin are identified, and their spatial distribution, scale, and potential impact on the tailrace outlet section are analyzed.

3. The method according to claim 1, characterized in that, In step one, the key control factors affecting the safety of tailwater outlet siltation prevention include: the backwater backwater range of the downstream reservoir, the siltation development trend of the reservoir tail section, the water and sediment process of each flood, the sudden sediment inflow process induced by geological disasters, the plane location of the tailwater outlet, the tailwater outflow mode, and the local topographic constraints of the river section.

4. The method according to claim 1, characterized in that, In step two, the preferred basic prevention and control measure is to set up a sand-blocking embankment at or near the leading edge of the low-level hole to reduce the adverse effects of equilibrium siltation on the outflow of the low-level hole.

5. The method according to claim 1, characterized in that, The design superelevation in steps two, three, and four can be determined comprehensively based on the safety requirements of the tailrace outlet structure, construction errors, model uncertainties, sediment fluctuation amplitude, and operational margin requirements.

6. The method according to claim 1, characterized in that, The dynamic uncertain siltation elevation in step three is determined by the envelope value of the maximum unfavorable siltation elevation at the tailwater outlet control section under different frequency flood conditions.

7. The method according to claim 1, characterized in that, The extreme siltation elevation in step four is determined by the envelope value of the maximum unfavorable siltation elevation at the tailwater outlet control section under various extreme disaster conditions.

8. The method according to claim 1, characterized in that, The extreme natural disaster types mentioned in step four include one or more of the following: flash floods and mudslides, landslides blocking rivers, mudslides flowing into confluences, landslide-dammed lake outburst floods, and glacial lake outburst floods.

9. The method according to claim 1, characterized in that, Step four also includes establishing criteria for switching the tailwater outlet from the low-level tunnel to the high-level tunnel. These criteria include: the siltation elevation at the leading edge or cross-section of the low-level tunnel reaches or exceeds the top elevation of the low-level tunnel's stacked beam gate; the stacked beam gate cannot meet the safe outflow requirements after adjustment; and the extreme operating condition warning reaches the preset level.

10. The method according to claim 1, characterized in that, The high and low tailwater outlets are arranged in a layered manner within the same tailwater outlet structure.

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