Calculation method and system for marginal beach scouring and silting conversion critical flow

Abstract

The invention provides a calculation method and system for beach scouring and silting conversion critical flow, and the method comprises the following steps: preliminarily judging the number of critical flow of a target beach based on the type of the beach; gridding a research area, establishing a planar two-dimensional water-sediment mathematical model, and performing parameter calibration and verification; sequentially determining a plurality of groups of typical flows from the minimum flow to the maximum flow; the water flow starting capacity and the water flow sand carrying capacity at the target beach under each typical flow are calculated, and the flow interval of strong and weak conversion of the hydrodynamic force at the beach is judged; dividing more detailed water and sediment calculation conditions based on the determined flow interval; and calculating the erosion and deposition amount of the target beach under each water and sand condition, and finally determining a critical flow interval in which the target beach is subjected to erosion and deposition conversion. According to the method, the internal mechanism of scouring and silting conversion of the marginal beach is disclosed from the angle of water and sediment power, the recognition of the critical flow interval based on the dynamic mechanism is more accurate, the required hydrotopographic data is less than that of the existing method, and the practicability is higher.

Description

Technical Field

[0001] The invention belongs to the technical field of river dynamics, and in particular relates to a calculation method and system for a critical flow rate of bank scouring and silting conversion. Background Art

[0002] Beaches are common geomorphic units in alluvial rivers, their formation closely related to river sedimentation. Three main types of beach are found in alluvial rivers: staggered beach in straight rivers, convex bank beach in curved rivers, and diversion zone beach in bifurcated rivers. These changes in beach morphology, such as incision, displacement, and siltation caused by flow variability, can alter local river systems, impacting flood flow, water intake, and navigation safety.

[0003] Under certain water and sediment saturation conditions, hydrodynamic changes caused by flow variability are the primary determinant of shoal morphological changes. Cross-shoals often occur in straight rivers with relatively small width-to-depth ratios. When upstream flow is low, the mainstream bends under the pressure of the shoals, resulting in low flow velocities and making it difficult to scour the shoals. As flow increases, the mainstream swings less due to the limited width of the river, increasing overall flow velocity and causing sediment movement throughout the channel, allowing the cross-shoals to migrate freely downward. Under moderate low-flow conditions, the influence of circulation causes siltation on the convex bank shoals. Under flood flow, the mainstream flow deviates toward the convex bank shoals due to inertia, making them susceptible to slicing and forming gullies. The main stream of the bifurcated river type shifts under different flow rates. When the upstream flow increases from low-water flow to flood flow, the main stream gradually shifts from the low-water tending branch channel to the flood-tending branch channel. The water flow dynamics at the beach of the diversion area upstream of the flood-tending branch channel continues to increase, and the beach develops from siltation to scouring. Conversely, the water flow dynamics at the beach of the diversion area upstream of the low-water tending branch channel first increases and then decreases, and its evolution characteristics also correspondingly change from non-scouring to scouring, and then to siltation.

[0004] As the flow increases, the scouring and silting properties of different types of beaches change. The free staggered beaches develop from non-scouring to scouring, the convex bank beaches and the diversion area beaches in the upstream area of ​​flood-prone distributary channels develop from silting to scouring, and the diversion area beaches in the upstream area of ​​low-flow-prone distributary channels are more complicated, showing a development from silting to scouring and then to silting. The flow that causes the scouring and silting properties of the beach to change can be considered as the critical flow. Existing studies have mostly used statistical methods to prove that the deformation of a single or single type of beach is positively correlated with the duration of a certain characteristic level of flow. For example, after the Three Gorges Reservoir was filled with water, the flat beach flow (Q=20000~25000m 3 / s) is the primary cause of scouring along the convex banks of the Yangtze River's lower Jingjiang River. However, statistical methods lack theoretical analysis of the dynamic mechanisms of these banks. Furthermore, statistically based methods for determining critical flow must be based on extensive field data, significantly limiting their application. Summary of the Invention

[0005] In response to the above-mentioned technical conditions and limitations, the present invention provides a method for calculating the critical flow rate for scouring and silting conversion of side beaches. According to the scouring and silting change laws of different types of side beaches under different flow rates, combined with the two-dimensional water and sediment mathematical model of the river plane, the critical flow rate for scouring and silting conversion of side beaches can be accurately calculated.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions: A method for calculating the critical flow rate of bank scouring and silting conversion includes the following steps: Step 1. Based on the location of the target beach and the upstream and downstream river conditions, classify the beach type and preliminarily determine the number of critical flows of the target beach; Step 2. Grid the study area including the target beach, establish a two-dimensional water-sediment mathematical model, and calibrate and verify the parameters; Step 3. Based on the multi-year flow process of the study area, determine multiple groups of typical flows from the lowest flow to the highest flow; Step 4. Select the initial topography and use a two-dimensional water-sediment mathematical model to calculate the flow initiation capacity and sediment-carrying capacity at the target beach under various typical flow rates. Graphs of the two factors as they change with flow rate are plotted to determine the flow range where the hydrodynamic force transitions from strong to weak. Step 5. Based on the flow interval where the hydrodynamic force changes from strong to weak, which is determined in step 4, divide the water and sediment calculation conditions into more detailed ones; Step 6. Select the initial terrain and use a two-dimensional water-sediment mathematical model to calculate the erosion and deposition volume at the target beach under various water-sediment conditions. Plot a graph of the erosion and deposition volume versus flow rate to verify the accuracy of the critical flow rate number initially determined in Step 1. Ultimately, determine the critical flow rate interval that causes erosion and deposition to occur at the target beach.

[0007] Furthermore, the implementation of step 1 is as follows: The river channel plan morphology of the study area, including the target beach, was collected, and the target beach types were classified into four categories. The number of critical flows of the target beach was preliminarily determined, including: (1) Straight river type staggered bank: As the flow increases, there is a critical flow that causes it to develop from no scouring to scouring; (2) Convex bank beach of curved river: As the flow increases, there is a critical flow that causes the process to change from siltation to erosion; (3) The flood of bifurcated river tends to be on the bank of the upstream area of ​​the bifurcated river: as the flow increases, there is a critical flow that causes the flow to develop from siltation to erosion; (4) The upstream bank of the low-flow branch channel of the bifurcated river: In the process of flow increase, there are two critical flow rates that cause it to develop from no erosion to erosion and then to siltation.

[0008] Furthermore, the implementation of step 2 is as follows: Step 21. Generate a two-dimensional body-fitting orthogonal curve grid containing the target beach area based on the study area boundary and the left and right bank boundaries; Step 22. Collect measured topographic data and corresponding hydrological data of the study area during the same time period to determine the inlet flow, sediment content, and outlet water level of the two-dimensional water-sediment mathematical model; Step 23. Initialize the roughness value of the study area. Calculate the water level and cross-sectional flow velocity along the study area under this roughness value using a two-dimensional water-sediment mathematical model. Compare these values ​​with the measured values. If they do not match, adjust the roughness value from downstream to upstream and repeat the calculation until the calculated water level and cross-sectional flow velocity along the study area match the measured values. Step 24. Initialize the sediment parameters of the study area, and calculate the erosion and deposition distribution and erosion and deposition volume of the study area under the two-dimensional water-sediment mathematical model. Compare them with the measured values. If they do not match, adjust the sediment parameters and repeat the calculation until the calculated erosion and deposition distribution and erosion and deposition volume match the measured values.

[0009] Furthermore, the implementation of step 3 is as follows: Step 31. Collect the daily average water level and flow data measured by the nearest hydrological station in the study area and draw the water level-flow relationship curve; Step 32. Divide the flow range from the lowest flow to the highest flow in the study area into multiple typical flows: low flow, multi-year average flow, flat land flow, multi-year average flood flow, and flood flow; Step 33. For runoff river sections, calculate the water level at each hydrological station under each typical flow rate using the water level-discharge relationship curve, deduce the outlet water level according to the water surface gradient, and determine multiple sets of typical flow rate calculation conditions. For tidal river sections, select the 85% cumulative frequency tidal range corresponding to each typical flow rate as the outlet water level process, and determine multiple sets of typical flow rate calculation conditions.

[0010] Furthermore, the implementation of step 4 is as follows: Step 41. Use a two-dimensional water-sediment mathematical model to calculate the flow velocity at the target beach under various typical flow conditions. and water depth , calculate the water flow starting capacity at the target beach and the force of water flow carrying sediment ; Step 42. Draw the graph of water flow starting capacity and water flow sediment entrainment with flow rate. The flow interval from less than 1 to greater than 1 is considered to be the flow interval where the hydrodynamic force at the beach changes from weak to strong. The flow interval from increasing to decreasing is regarded as the flow interval where the hydrodynamic force at the bank changes from strong to weak; Furthermore, in step 4, the water flow starting capacity is , is the starting flow rate.

[0011] Furthermore, the implementation of step 5 is as follows: Step 51. Collect the daily average flow and sediment concentration data measured by the nearest hydrological station in the study area and draw a flow-sediment concentration relationship curve; Step 52. Based on the flow intervals where the hydrodynamics at the beach change from weak to strong and from strong to weak as determined in step 42, divide the flow levels into more detailed levels, determine the typical flow calculation conditions in the same manner as in step 33, calculate the sediment content of the hydrological station under each typical flow through the flow-sediment content relationship curve described in step 51, and determine multiple sets of water-sediment calculation conditions.

[0012] Furthermore, the implementation of step 6 is as follows: Step 61. Calculate the erosion and deposition volume at the target beach under various water and sediment conditions using a two-dimensional water and sediment mathematical model. The erosion and deposition volume of a cell grid is the product of the erosion and deposition change value of the cell grid and its area. The erosion and deposition volume at the target beach is the cumulative erosion and deposition volume of the cell grids within the beach area. Step 62. Draw a graph of the change in erosion and deposition volume versus flow rate to verify the accuracy of the critical flow rate number determined in step 1. The flow rate interval where the erosion and deposition volume changes from positive to negative can be regarded as the critical flow rate interval for the transition from siltation to scouring of the beach. The flow rate interval where the erosion and deposition volume changes from negative to positive can be regarded as the critical flow rate interval for the transition from scouring to siltation of the beach.

[0013] Furthermore, the two-dimensional water-sand mathematical model is: (1) Continuity equation of water flow:

[0014] (2) Water flow equation Directional momentum equation:

[0015]

[0016] Directional momentum equation:

[0017]

[0018] Where: are two orthogonal curvilinear coordinates in the orthogonal curvilinear coordinate system; For time; They are Directional flow velocity; is the water level; For water depth; is the acceleration due to gravity; is the Xiecai coefficient; They are Lame coefficient of direction; are turbulent stresses in different directions, the first subscript indicates the direction of the outer normal of the stress surface, and the second subscript indicates the direction of the stress; (3) Unbalanced sediment transport equation for heterogeneous suspended sediment:

[0019]

[0020] Where, 、 、 、 For the The sediment content of the suspended load is composed of the recovery saturation coefficient, sedimentation velocity, sediment entrainment capacity of the water flow and volumetric sediment content of the suspended load; represents the sediment diffusion coefficient; represents the Prandtl-Schmidt number; (4) Unbalanced sediment transport equation for heterogeneous bed load:

[0021]

[0022] Where: 、 For the Combination bed load recovery saturation coefficient, sediment settling velocity; 、 They are bed load sediment transport rate and bed load saturated sediment transport rate respectively; (5) Riverbed deformation equation:

[0023] Where: The number of particle size groups of bed load and suspended load respectively; is the bed sand dry density; is the bed surface elevation.

[0024] In another aspect, the present invention provides a system for calculating the critical flow rate for beach scouring and silting conversion, comprising: The beach type classification module is used to classify the beach type based on the location of the target beach and the upstream and downstream river conditions, and preliminarily determine the number of critical flows of the target beach; Mathematical model building module. It is used to grid the study area including the target beach, establish a two-dimensional water-sediment mathematical model, and calibrate and verify the parameters; Typical flow determination module. It is used to determine multiple groups of typical flows from the lowest flow to the highest flow based on the multi-year flow process of the study area; The initial interval determination module is used to select the initial terrain, use a two-dimensional water-sediment mathematical model to calculate the water flow starting capacity and water flow sediment entrainment at the target beach under various typical flow rates, plot the changes of these two factors with flow rate, and determine the flow range where the hydrodynamic force changes from strong to weak; Water and sediment calculation condition division module. It is used to divide water and sediment calculation conditions into more detailed conditions based on the determined flow range; The final critical flow interval determination module is used to select the initial terrain, use a two-dimensional water-sediment mathematical model to calculate the erosion and deposition volume at the target beach under various water-sediment conditions, draw a graph of the erosion and deposition volume versus flow rate, verify the accuracy of the critical flow number initially determined, and ultimately determine the critical flow interval that causes erosion and deposition conversion at the target beach.

[0025] Compared with existing technologies, this method offers the following advantages: The modeling process closely replicates the actual landforms and scouring and deposition processes of the coastal beaches. It also reveals the underlying mechanisms of scouring and deposition transitions from the perspective of hydrodynamics, specifically the varying trends in hydrodynamics and sediment inflow with increasing flow. The dynamic mechanism-based identification of critical flow intervals is more accurate, and the required hydrological and topographic data are easier to collect than with existing methods, making this method more practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 The present invention is a flowchart of the method.

[0028] Figure 2 This is a river flow diagram of the Sanyi Bridge Beach in an embodiment of the present invention.

[0029] Figure 3This is a graph showing the change in diversion ratio of the left branch of Luocheng Island with flow rate.

[0030] Figure 4 The two-dimensional water-sediment mathematical model verification results are shown in Figure 1: (a) and (b) are the water level verifications of the left and right branches of Luochengzhou; (c) and (d) are the flow velocity verifications of the left and right branches of Luochengzhou; and (e) and (f) are the riverbed erosion and deposition distribution verifications.

[0031] Figure 5 This is the tide level process at the model inlet under flat beach flow.

[0032] Figure 6 This is a trend diagram of the water flow starting capacity (a) and water flow sediment entrainment (b) at the Sanyi Bridge beach as a function of flow rate.

[0033] Figure 7 This is the flow-sediment content relationship curve of Datong Hydrological Station.

[0034] Figure 8 This is a graph showing the changing trend of erosion and sedimentation at the Sanyi Bridge beach as the flow rate changes. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0036] Example 1 like Figure 1 As shown, the present invention provides a method for calculating the critical flow rate of bank scouring and silting conversion, comprising the following steps: Step 1. Based on the location of the target beach and the upstream and downstream river conditions, classify the beach type and preliminarily determine the number of critical flows of the target beach; Step 2. Grid the study area including the target beach, establish a two-dimensional water-sediment mathematical model, and calibrate and verify the parameters; Step 3. Based on the multi-year flow process of the study area, determine multiple groups of typical flows from the lowest flow to the highest flow; Step 4. Select the initial topography and use a two-dimensional water-sediment mathematical model to calculate the flow initiation capacity and sediment-carrying capacity at the target beach under various typical flow rates. Graphs of the two factors as they change with flow rate are plotted to determine the flow range where the hydrodynamic force transitions from strong to weak. Step 5. Based on the flow range determined in step 4, divide the water and sediment calculation conditions into more detailed ones; Step 6. Select the initial terrain and use a two-dimensional water-sediment mathematical model to calculate the erosion and deposition volume at the target beach under various water-sediment conditions. Plot a graph of the erosion and deposition volume versus flow rate to verify the accuracy of the critical flow rate determined in Step 1. Ultimately, determine the critical flow rate interval that causes erosion and deposition to occur at the target beach.

[0037] The implementation of step 1 is as follows: The river channel plan morphology of the study area, including the target beach, was collected, and the target beach types were classified into the following four categories. The number of critical flows that the target beach may have was preliminarily determined.

[0038] Straight river type staggered bank: As the flow increases, there is a critical flow Make it develop from no flushing to flushing; Curved river convex bank beach: As the flow increases, there is a critical flow It develops from siltation to erosion; The flood of bifurcated river tends to flow to the upstream bank of the bifurcated river: in the process of increasing flow, there is a critical flow. It develops from siltation to erosion; The low-flow tendency of the bifurcated river is on the upstream side of the bifurcated river channel: in the process of flow increase, there are two critical flow rates. and It develops from non-erosion to erosion and then to siltation.

[0039] The implementation of step 2 is as follows: Step 21. Generate a two-dimensional body-fitting orthogonal curve grid containing the target beach area based on the study area boundary and the left and right bank boundaries; Step 22. Collect measured topographic data and corresponding hydrological data of the study area during the same time period to determine the inlet flow, sediment content, and outlet water level of the two-dimensional water-sediment mathematical model; Step 23. Initialize the roughness value of the study area. Calculate the water level and cross-sectional flow velocity along the study area under this roughness value using a two-dimensional water-sediment mathematical model. Compare these values ​​with the measured values. If they do not match, adjust the roughness value from downstream to upstream and repeat the calculation until the calculated water level and cross-sectional flow velocity along the study area match the measured values. Step 24. Initialize the sediment parameters of the study area, and calculate the erosion and deposition distribution and erosion and deposition volume of the study area under the sediment parameters using a two-dimensional water-sediment mathematical model. Compare them with the measured values. If they do not match, adjust the sediment parameters and repeat the calculation until the calculated erosion and deposition distribution and erosion and deposition volume match the actual values.

[0040] The implementation of step 3 is as follows: Step 31. Collect the daily average water level and flow data measured by the nearest hydrological station in the study area and draw the water level-flow relationship curve; Step 32. Divide the flow range from the lowest flow to the highest flow in the study area into multiple typical flows, such as low water flow, multi-year average flow, flat land flow, multi-year average flood flow, and flood flow; Step 33. For runoff river sections, calculate the water level at each hydrological station under each typical flow rate using the water level-discharge relationship curve, deduce the outlet water level according to the water surface gradient, and determine multiple sets of typical flow rate calculation conditions. For tidal river sections, select the 85% cumulative frequency tidal range and high tide type corresponding to each typical flow rate as the model outlet water level process, and determine multiple sets of typical flow rate calculation conditions.

[0041] The implementation of step 4 is as follows: Step 41. Use a two-dimensional water-sediment mathematical model to calculate the flow velocity at the target beach under various typical flow conditions. and water depth , calculate the water flow starting capacity at the target beach and the force of water flow carrying sediment ; The water flow starting capacity is defined as , is the starting flow rate, which is calculated as follows:

[0042] Where: is the sediment particle size, is the water density, is the sediment density. For runoff river sections, the sediment carrying capacity of water flow is For tidal river sections, the sediment carrying capacity of the water flow is .

[0043] Step 42. Draw the graph of water flow starting capacity and water flow sediment entrainment with flow rate. The flow interval from less than 1 to greater than 1 is considered to be the flow interval where the hydrodynamic force at the beach changes from weak to strong. The flow interval from increasing to decreasing is regarded as the flow interval where the hydrodynamics at the beach changes from strong to weak.

[0044] The implementation of step 5 is as follows: Step 51. Collect the daily average flow and sediment concentration data measured by the nearest hydrological station in the study area and draw a flow-sediment concentration relationship curve; Step 52. Based on the flow intervals where the hydrodynamics at the beach change from weak to strong and from strong to weak as determined in step 42, divide the flow levels into more detailed levels, determine the typical flow calculation conditions in the same manner as in step 33, calculate the sediment content of the hydrological station under each typical flow through the flow-sediment content relationship curve described in step 51, and determine multiple sets of water-sediment calculation conditions.

[0045] The implementation of step 6 is as follows: Step 61. Use a two-dimensional water-sediment mathematical model to calculate the erosion and sedimentation volume at the target beach under various water and sediment conditions. The erosion and sedimentation volume of a unit grid is the product of the erosion and sedimentation change value of the unit grid and its area. The erosion and sedimentation volume at the target beach is the cumulative erosion and sedimentation volume of the unit grids included in the beach area. Step 62. Draw a graph of the change in erosion and deposition volume versus flow rate to verify the accuracy of the number of critical flows initially determined in Step 1. The flow interval where the erosion and deposition volume changes from positive to negative can be considered the critical flow interval for the transition from siltation to scouring at the beach, and the flow interval where the erosion and deposition volume changes from negative to positive can be considered the critical flow interval for the transition from scouring to siltation at the beach.

[0046] The basic equation of the two-dimensional water-sand mathematical model is as follows: (1) Continuity equation of water flow:

[0047] (2) Water flow equation Directional momentum equation:

[0048]

[0049] Directional momentum equation:

[0050]

[0051] Where: are two orthogonal curvilinear coordinates in the orthogonal curvilinear coordinate system; For time; for Directional flow velocity; is the water level; For water depth; is the acceleration due to gravity; is the Xiecai coefficient; They are Lame coefficient of direction; They are turbulent stresses in different directions. The first subscript indicates the direction of the external normal of the stress surface; the second subscript indicates the direction of the stress.

[0052]

[0053]

[0054] and represents the horizontal Cartesian coordinate, represents the turbulent viscosity coefficient, , represents the coefficient; represents turbulent kinetic energy; represents the turbulent kinetic energy dissipation rate.

[0055] (3) Unbalanced sediment transport equation for heterogeneous suspended sediment:

[0056]

[0057] Where, 、 、 、 For the The sediment content of the suspended sediment is restored to saturation coefficient, sedimentation velocity, sediment entrainment capacity of the water flow and volumetric sediment content of the suspended sediment. represents the sediment diffusion coefficient; represents the Prandtl-Schmidt number.

[0058] Water flow and sand carrying capacity and Shen Su Calculated by Zhang Ruijin's formula:

[0059]

[0060] Where: If runoff is the main driving force in the study area, , if the study area is driven by tidal currents, ; is the sediment carrying capacity coefficient; For the Average particle size of sediment, For water depth, are water flow and sediment density, respectively. are water flow and sediment density, is the viscosity coefficient of water flow.

[0061] The concept of water flow sediment entrainment is applicable to the total suspended matter and each particle size group. The calculation adopts the method proposed by Hu Haiming and Li Yitian, taking into account the influence of water flow conditions and bed sand composition, that is, the bed sand gradation and the grouped sand entrainment gradation are linked by the following formula:

[0062]

[0063] Where, 、 They are respectively the sand holding capacity gradation and the bed sand gradation. is the Karman constant, which is set to 0.4 in the model. is the total number of particle size groups, is the vertical turbulence intensity, usually taken as , is a parameter related to the turbulence intensity and the normal distribution function.

[0064] Therefore, when the bed sand composition is known, the total sand holding capacity and the grouped sand holding capacity gradation are calculated respectively. Available through Calculated.

[0065] (4) Unbalanced sediment transport equation for heterogeneous bed load:

[0066]

[0067] Where: 、 For the Combination bed load recovery saturation coefficient, sediment settling velocity, 、 They are bed load sediment transport rate and bed load saturated sediment transport rate converted to concentrations at full water depth, which can be converted using the following formula:

[0068] The saturated sediment transport rate of bed load can be calculated by Dou Guoren's formula:

[0069] Where: is the empirical coefficient, are water flow and sediment density, respectively; is the sediment starting velocity, which can be calculated by formula (3-1). Available through Calculated.

[0070] (5) Riverbed deformation equation:

[0071] Where: The number of particle size groups of bed load and suspended load respectively; is the bed sand dry density; is the bed surface elevation.

[0072] The Luochengzhou section of the Yangtze River is located in the tidal boundary of the dry season, with a length of about 23 km. The Luochengzhou section divides the river into two branches, the left branch is the main branch, and the right branch is the branch. The Sanyi Bridge Beach is located on the left bank of the diversion area of ​​the Luochengzhou section of the river. Figure 2 Datong Hydrological Station is the closest hydrological station to the Luochengzhou section of the river. No major tributaries flow into it, so the water and sediment conditions at Datong Hydrological Station can be used to replace the water and sediment conditions of the Luochengzhou section. The specific implementation steps of the present invention will now be described using the Sanyi Bridge Beach as an example.

[0073] Step 1: The hydrodynamic characteristics of curved rivers are that small flows bend and large flows tend to be straight, which indicates that the left branch of the Luochengzhou section is a low-flow branch. In addition, the diversion ratio of the left branch of Luochengzhou decreases with the increase of flow (see Appendix). Figure 3 ), therefore, the Sanyi Bridge beach is a low-flow-trending branch channel diversion area beach. It can be preliminarily judged that there are two critical flow rates in the process of flow increase. and , causing the Sanyi Bridge beach to develop from non-erosion to erosion and then to siltation.

[0074] Step 2: The Luochengzhou River section is located in a tidal river. The inlet of the two-dimensional water-sediment mathematical model was chosen at Guazhou Town in the Hechangzhou waterway, a section of the river largely unaffected by tidal currents. The outlet was chosen at the Jiangyin station, which has long-term tidal level records. The model uses a body-fitting orthogonal curved grid with a total of 1370×160 computational grid nodes. The grid spacing is 40m-90m in the direction of flow and 10m-50m perpendicular to the flow. The model uses topographic and water level data from February 2021 to calibrate and validate the roughness value. Sediment parameters were calibrated and validated from April 2019 to February 2021. The particle size distribution of suspended sediment and bed sediment is divided into six groups: 0.002mm, 0.008mm, 0.016mm, 0.031mm, 0.125mm, and 0.25mm. The proportions of each size component are weighted averages of measured data from April 2019. The verification results of water level, flow velocity and riverbed erosion and deposition distribution are attached. Figure 4 Overall, the water level, flow velocity, and river channel change trends calculated by the two-dimensional water-sediment model are in good agreement with the measured results, and can reasonably simulate the recent geomorphological evolution of the Sanyi Bridge beach.

[0075] Step 3: The flow range of Datong Hydrological Station is from low water flow (16500m 3 / s) to flood flow (85400m 3 / s), therefore, 6 groups of typical flow rates are set: Q1 low water flow (16500m 3 / s), Q2 multi-year average flow (28500m 3 / s), Q3 flat beach flow (45000m 3 / s), Q4 multi-year average flood flow (57500m 3 / s), Q5 flood flow (70000m 3 / s) and Q6 flood flow (85400m 3 / s). The inlet flow of the two-dimensional water-sediment mathematical model can adopt a constant flow, and the outlet water level process adopts the 85% cumulative frequency tidal range and high tide type under each typical flow at the Jiangyin Hydrological Station. The daily tidal range of the Jiangyin Hydrological Station from 2014 to 2021 was calculated and frequency processed to obtain the corresponding tidal level processes under the above 6 groups of typical flow rates. Figure 5 The flat beach flow (45000m 3 / s) corresponding to the downstream tidal process.

[0076] Step 4: Use a two-dimensional water-sediment mathematical model to calculate the flow initiation capacity at the Sanyi Bridge beach under various typical flow rates. Sediment carrying force of water flow , attached Figure 6 The trend of the two changes with flow rate was plotted. As the flow rate increases, the water flow starting capacity and the water flow sediment carrying capacity at the Sanyi Bridge beach both show a trend of increasing first and then decreasing. 3 / s, the water flow starting capacity is less than 1, and the sediment at the Sanyi Bridge beach cannot be started. When the flow is 28500m 3 / s, the water flow starting capacity is greater than 1, and the sediment at the Sanyi Bridge beach begins to start moving. It can be seen that the flow range where the water dynamics at the Sanyi Bridge beach changes from weak to strong is between 16500 and 28500 m 3 / s. When the flow rate is from 28500m 3 / s increased to 57000m 3 / s, the water flow starting capacity and water flow sediment entrainment capacity continue to increase with the flow rate. 3 / s increased to 85400m 3 / s, the sediment-carrying force of the water flow gradually decreases, and the turning point is at 57500m 3 / s and 70000m 3 / s, it can be seen that the flow range where the water dynamics at the Sanyi Bridge beach changes from strong to weak is between 57500 and 70000m 3 / s. Therefore, it is preliminarily believed that with the increase of flow rate, there are two flow intervals of strong and weak conversion of hydrodynamics at the Sanyi Bridge beach: the flow interval from weak to strong is 16500~28500m 3 / s, the flow range from strong to weak is 57500~70000m 3 / s.

[0077] Step 5: Based on the flow range obtained in step 4, the flow range is between 16500 and 70000m 3 / s flow range has set 9 groups of more detailed typical flow: QS1 (16500m 3 / s)、QS2(20000m 3 / s)、QS3(25000m 3 / s)、QS4(30000m 3 / s)、QS5(40000m 3 / s)、QS6(45000m 3 / s)、QS7(50000m 3 / s)、QS8(60000m 3 / s)、QS9(650000m 3 Based on the average daily flow and sediment content data of Datong from 2019 to 2021, a flow-sediment content relationship curve was drawn. See the attached Figure 7 , and based on the flow-sediment content relationship curve, the sediment content under each typical flow rate is determined: QS1 (0.041kg / m 3 )、QS2(0.051kg / m 3 )、QS3(0.066kg / m 3 )、QS4(0.082kg / m 3 )、QS5(0.115kg / m 3 )、QS6(0.132kg / m 3 )、QS7(0.149kg / m 3 )、QS8(0.184kg / m 3 )、QS9(0.221kg / m 3 ).

[0078] Step 6: Use a two-dimensional water-sediment mathematical model to calculate the erosion and sedimentation volume at the Sanyi Bridge beach under typical water-sediment conditions, and draw a trend chart of erosion and sedimentation volume versus flow rate, see attached. Figure 8 As the flow rate increases, the Sanyi Bridge beach presents a process of no erosion and no silting, erosion, and then erosion and silting. 3 / s, the beach basically does not change in shape. When the upstream flow is between 25000~45000m 3 / s, the Sanyi Bridge beach generally presents a scouring trend, and the scouring intensity first increases and then decreases. 3 / s, the scouring intensity is most significant, and the scouring sediment is 76,000 m 3 When the upstream flow increases to 50000m 3 / s, a large amount of siltation occurred in the middle and tail of the Sanyi Bridge beach, and the beach turned from erosion to siltation. As the flow rate further increased to 65000m 3 / s, the amount of sedimentation on the Sanyi Bridge beach increased significantly, with the amount of sedimentation reaching about 608,000 m 3 , which is 10 to 20 times the amount of sediment washed away at low flow. Step 6 further confirms the judgment of the two critical flow rates obtained in step 1, and finally clarifies the range of the two critical flow rates. Located between 20,000 and 25,000 meters 3 / s, and Located between 45,000 and 50,000 meters 3 / s.

[0079] Example 2 This embodiment provides a system for calculating the critical flow rate for beach scouring and silting conversion, including: The beach type classification module is used to classify the beach type based on the location of the target beach and the upstream and downstream river conditions, and preliminarily determine the number of critical flows of the target beach; Mathematical model building module. It is used to grid the study area including the target beach, establish a two-dimensional water-sediment mathematical model, and calibrate and verify the parameters; Typical flow determination module. It is used to determine multiple groups of typical flows from the lowest flow to the highest flow based on the multi-year flow process of the study area; The initial interval determination module is used to select the initial terrain, use a two-dimensional water-sediment mathematical model to calculate the water flow starting capacity and water flow sediment entrainment at the target beach under various typical flow rates, plot the changes of these two factors with flow rate, and determine the flow range where the hydrodynamic force changes from strong to weak; Water and sediment calculation condition division module. It is used to divide water and sediment calculation conditions into more detailed conditions based on the determined flow range; The final critical flow interval determination module is used to select the initial terrain, use a two-dimensional water-sediment mathematical model to calculate the erosion and deposition volume at the target beach under various water-sediment conditions, draw a graph of the erosion and deposition volume versus flow rate, verify the accuracy of the critical flow number initially determined, and ultimately determine the critical flow interval that causes erosion and deposition conversion at the target beach.

[0080] It should be understood that parts not elaborated in detail in this specification belong to the prior art.

[0081] It should be understood that the above description of the preferred embodiments is relatively detailed and cannot be considered as limiting the scope of protection of the present invention. It is not necessary and impossible to list all embodiments here. Under the guidance of the present invention, ordinary technicians in this field can also make substitutions or modifications without departing from the scope of protection of the claims of the present invention, which fall within the scope of protection of the present invention. The scope of protection of the present invention shall be based on the attached claims.

Claims

1. A method for calculating the critical flow rate of beach erosion and siltation conversion, characterized in that: The following steps are involved: Step 1. Based on the location of the target beach and the upstream and downstream river conditions, classify the beach type and preliminarily determine the number of critical flows of the target beach; Step 2. Grid the study area including the target beach, establish a two-dimensional water-sediment mathematical model, and calibrate and verify the parameters; Step 3. Based on the multi-year flow process of the study area, determine multiple groups of typical flows from the lowest flow to the highest flow; Step 4. Select the initial topography and use a two-dimensional water-sediment mathematical model to calculate the flow initiation capacity and sediment-carrying capacity at the target beach under various typical flow rates. Graphs of the two factors as they change with flow rate are plotted to determine the flow range where the hydrodynamic force transitions from strong to weak. Step 5. Based on the flow interval where the hydrodynamic force changes from strong to weak, which is determined in step 4, divide the water and sediment calculation conditions into more detailed ones; Step 6. Select the initial terrain and use a two-dimensional water-sediment mathematical model to calculate the erosion and deposition volume at the target beach under various water-sediment conditions. Plot a graph of the erosion and deposition volume versus flow rate to verify the accuracy of the critical flow rate number initially determined in Step 1. Ultimately, determine the critical flow rate interval that causes erosion and deposition to occur at the target beach.

2. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 1 is characterized in that: The implementation of step 1 is as follows: The river channel plan morphology of the study area, including the target beach, was collected, and the target beach types were classified into four categories. The number of critical flows of the target beach was preliminarily determined, including: (1) Straight river type staggered bank: As the flow increases, there is a critical flow that causes it to develop from no scouring to scouring; (2) Convex bank beach of curved river: As the flow increases, there is a critical flow that causes the process to change from siltation to scouring; (3) The flood of bifurcated river tends to be on the bank of the upstream area of ​​the bifurcated river: as the flow increases, there is a critical flow that causes the flow to develop from siltation to erosion; (4) The upstream bank of the low-flow branch channel of the bifurcated river: In the process of flow increase, there are two critical flow rates that cause it to develop from no erosion to erosion and then to siltation.

3. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 2 is characterized in that: The implementation of step 2 is as follows: Step 21. Generate a two-dimensional body-fitting orthogonal curve grid containing the target beach area based on the study area boundary and the left and right bank boundaries; Step 22. Collect measured topographic data and corresponding hydrological data of the study area during the same time period to determine the inlet flow, sediment content, and outlet water level of the two-dimensional water-sediment mathematical model; Step 23. Initialize the roughness value of the study area. Calculate the water level and cross-sectional flow velocity along the study area under this roughness value using a two-dimensional water-sediment mathematical model. Compare these values ​​with the measured values. If they do not match, adjust the roughness value from downstream to upstream and repeat the calculation until the calculated water level and cross-sectional flow velocity along the study area match the measured values. Step 24. Initialize the sediment parameters of the study area, and calculate the erosion and deposition distribution and erosion and deposition volume of the study area under the two-dimensional water-sediment mathematical model. Compare them with the measured values. If they do not match, adjust the sediment parameters and repeat the calculation until the calculated erosion and deposition distribution and erosion and deposition volume match the measured values.

4. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 3 is characterized in that: The implementation of step 3 is as follows: Step 31. Collect the daily average water level and flow data measured by the nearest hydrological station in the study area and draw the water level-flow relationship curve; Step 32. Divide the flow range from the lowest flow to the highest flow in the study area into multiple typical flows: low flow, multi-year average flow, flat land flow, multi-year average flood flow, and flood flow; Step 33. For runoff river sections, calculate the water level at each hydrological station under each typical flow rate using the water level-discharge relationship curve, deduce the outlet water level according to the water surface gradient, and determine multiple sets of typical flow rate calculation conditions. For tidal river sections, select the 85% cumulative frequency tidal range corresponding to each typical flow rate as the outlet water level process, and determine multiple sets of typical flow rate calculation conditions.

5. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 4 is characterized in that: The implementation of step 4 is as follows: Step 41. Use a two-dimensional water-sediment mathematical model to calculate the flow velocity at the target beach under various typical flow conditions. and water depth , calculate the water flow starting capacity at the target beach and the force of water flow carrying sediment ; Step 42. Draw the graph of water flow starting capacity and water flow sediment entrainment with flow rate. The flow interval from less than 1 to greater than 1 is considered to be the flow interval where the hydrodynamic force at the beach changes from weak to strong. The flow interval from increasing to decreasing is regarded as the flow interval where the hydrodynamics at the beach changes from strong to weak.

6. The method for calculating the critical flow rate of beach scouring and silting conversion according to claim 5 is characterized in that: In step 4, the water flow starting capacity is , is the starting flow rate.

7. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 6 is characterized in that: The implementation of step 5 is as follows: Step 51. Collect the daily average flow and sediment concentration data measured by the nearest hydrological station in the study area and draw a flow-sediment concentration relationship curve; Step 52. Based on the flow intervals where the hydrodynamics at the beach change from weak to strong and from strong to weak as determined in step 42, divide the flow levels into more detailed levels, determine the typical flow calculation conditions in the same manner as in step 33, calculate the sediment content of the hydrological station under each typical flow through the flow-sediment content relationship curve described in step 51, and determine multiple sets of water-sediment calculation conditions.

8. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 7 is characterized in that: The implementation of step 6 is as follows: Step 61. Calculate the erosion and deposition volume at the target beach under various water and sediment conditions using a two-dimensional water and sediment mathematical model. The erosion and deposition volume of a cell grid is the product of the erosion and deposition change value of the cell grid and its area. The erosion and deposition volume at the target beach is the cumulative erosion and deposition volume of the cell grids within the beach area. Step 62. Draw a graph of the change in erosion and deposition volume versus flow rate to verify the accuracy of the critical flow rate number determined in step 1. The flow rate interval where the erosion and deposition volume changes from positive to negative can be regarded as the critical flow rate interval for the transition from siltation to scouring of the beach. The flow rate interval where the erosion and deposition volume changes from negative to positive can be regarded as the critical flow rate interval for the transition from scouring to siltation of the beach.

9. The method for calculating the critical flow rate of shoal erosion and siltation conversion according to claim 1 is characterized in that: The two-dimensional water-sand mathematical model is: (1) Continuity equation of water flow: (2) Water flow equation Directional momentum equation: Directional momentum equation: Where: are two orthogonal curvilinear coordinates in the orthogonal curvilinear coordinate system; For time; They are Directional flow velocity; is the water level; For water depth; is the acceleration due to gravity; is the Xiecai coefficient; They are Lame coefficient of direction; are turbulent stresses in different directions, the first subscript indicates the direction of the outer normal of the stress surface, and the second subscript indicates the direction of the stress; (3) Unbalanced sediment transport equation for heterogeneous suspended sediment: Where, 、 、 、 For the The sediment content of the suspended sediment is restored to saturation coefficient, settling velocity, sediment entrainment capacity of the water flow and volumetric sediment content of the suspended sediment; represents the sediment diffusion coefficient; represents the Prandtl-Schmidt number; (4) Unbalanced sediment transport equation for heterogeneous bed load: Where: 、 For the Combination bed load recovery saturation coefficient, sediment settling velocity; 、 They are bed load sediment transport rate and bed load saturated sediment transport rate respectively; (5) Riverbed deformation equation: Where: The number of particle size groups of bed load and suspended load, respectively; is the bed sand dry density; is the bed surface elevation.

10. A calculation system for critical flow rate of bank scouring and silting conversion, characterized in that: include: The beach type classification module is used to classify the beach type based on the location of the target beach and the upstream and downstream river conditions, and preliminarily determine the number of critical flows of the target beach; Mathematical model building module. It is used to grid the study area including the target beach, establish a two-dimensional water-sediment mathematical model, and calibrate and verify the parameters; Typical flow determination module. It is used to determine multiple groups of typical flows from the lowest flow to the highest flow based on the multi-year flow process of the study area; The initial interval determination module is used to select the initial terrain, use a two-dimensional water-sediment mathematical model to calculate the water flow starting capacity and water flow sediment entrainment at the target beach under various typical flow rates, plot the changes of these two factors with flow rate, and determine the flow range where the hydrodynamic force changes from strong to weak; Water and sediment calculation condition division module. It is used to divide water and sediment calculation conditions into more detailed conditions based on the determined flow range; The final critical flow interval determination module is used to select the initial terrain, use a two-dimensional water-sediment mathematical model to calculate the erosion and deposition volume at the target beach under various water and sediment conditions, plot the erosion and deposition volume versus flow rate, verify the accuracy of the initially determined critical flow number, and ultimately determine the critical flow interval that causes erosion and deposition to occur at the target beach; The calculation system for the critical flow rate of the conversion of scouring and silting of the bank is used to execute the steps of the calculation method for the critical flow rate of the conversion of scouring and silting of the bank as claimed in any one of the claims.

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