Method for analyzing and determining optimal desilting section of bifurcated river

Abstract

The invention provides a method for analyzing and determining a preferential desilting section of a bifurcated river, and belongs to the technical field of water conservancy and hydropower engineering. The invention provides a bifurcated river dredging project desilting section optimization method based on two-dimensional hydrodynamic numerical simulation calculation analysis, and the method comprises the steps: carrying out the simulation calculation of the flood discharge, the split ratio, the moisture absorption amount and the like of a desilting port after different desilting section schemes under the same total desilting amount are implemented; according to the method, the correlation among the flood discharge, the split ratio, the moisture absorption amount and the width-depth ratio of the desilting section is established, then the maximum value of the flood discharge, the maximum value of the split ratio and the maximum value of the moisture absorption amount are solved, the optimal width-depth ratio of the desilting section is determined, and therefore the optimal desilting section is determined.

Description

Technical Field

[0001] The invention relates to a method for analyzing and determining an optimal dredging section of a bifurcated river, and belongs to the technical field of water conservancy and hydropower engineering. Background Art

[0002] The type of dredging section is related to the dredging workload, total project investment and dredging effect. Reasonable selection of dredging section can save dredging workload and reduce total project investment; in other words, when the dredging workload and total project investment are certain, reasonable selection of dredging section can maximize the dredging effect.

[0003] Related research shows that when the water-passing section is the optimal hydraulic section, the flood discharge of the same area is the largest, which also means that when the same discharge is discharged, the closer the water-passing section is to the optimal hydraulic section, the smaller the cross-sectional area. In other words, in theory, if the dredging section used in the dredging project is closer to the optimal hydraulic section, the amount of regulation excavation will be smaller. The optimal hydraulic section is the open channel (channel) section with a certain flow rate and the smallest wetted perimeter, and it is the hydraulic section with the smallest resistance coefficient. However, since the natural river channel is not an ideal channel, and the dredging section should meet the requirements of slope stability, the hydraulic optimal section in the strict sense is often difficult to meet in actual dredging projects. For this reason, a wide and shallow trapezoidal section should be sought, so that its water depth and bottom width have a wider selection range to meet the needs of various situations, and within this range, it can basically meet the requirements of the hydraulic optimal section, that is, choose a practical and economic section.

[0004] The Code for Design of River Regulation (GB50707-2011) states: "The cross-section of the dredging project should be designed as a trapezoid, and the river channel with multi-functional use can also be designed as a compound section." "When digging a trench or excavating a new artificial river in the river channel, the excavation depth and bottom width should be determined by hydraulic and sediment transport calculations according to the requirements of flood discharge, drainage, navigation, water intake or water delivery." However, the current code does not involve how to determine the optimal dredging section for dredging projects. In addition, due to the different boundaries of the natural river channel such as river width, embankment type, beach channel shape, bed elevation, and distribution of river-related buildings, different dredging sections will bring different dredging effects. Therefore, it is often difficult to simply determine the optimal dredging section through hydraulic calculations.

[0005] The present invention aims to solve the technical problem that in dredging projects, under the same dredging workload, there are multiple different dredging sections (i.e., multiple different combinations of dredging widths and dredging depths), and it is difficult to determine the optimal dredging section. A method for analyzing and determining the optimal dredging section for bifurcated rivers is proposed, thereby ensuring that the dredging section can be reasonably selected to maximize the dredging effect. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a method for analyzing and determining an optimal dredging section for a bifurcated river, comprising the following steps: (1) Analyze the evolution of the river channel and the current status of siltation in the branched rivers and determine the river sections to be desilted Collect basic data on river topography, geology, hydrology, sediment, river sand mining, river-related construction activities, river (waterway) regulation, and previous relevant research results. Analyze the evolution characteristics of the river from the aspects of changes in river bank boundaries, changes in plane scouring and silting patterns, changes in cross-sectional topography, changes in the longitudinal profile of the deep line, and changes in riverbed scouring and silting volume. Combined with on-site river surveys, visit river management units and residents on both sides of the river to understand the siltation of river branches and determine the river sections to be desilted. (2) Establishment of a two-dimensional hydrodynamic mathematical model for bifurcated rivers Collect data on the river's topography, geology, hydrology, riverbed, shoreline, river-related structures, operation and management of major water projects, and related research results, and establish a two-dimensional hydrodynamic mathematical model for bifurcated rivers; (3) Conduct calibration and verification of mathematical models The model is verified using measured hydrological data, and the verification content mainly includes the flow and diversion ratio of each port, water (tidal) level, flow velocity, and flow direction. The grid file, model roughness, and initial water level field boundary are adjusted to ensure that the difference between the calculated value and the measured value of each hydrological station meets the error requirements. The error between the model calculated value and the measured value should meet the relevant requirements of the Technical Specifications for Simulation Tests of Water Transport Engineering (JTS-T 231-2021); (4) Numerical simulation analysis of river hydrodynamics before dredging and remediation Using the above-mentioned verified two-dimensional river hydrodynamic mathematical model, carry out hydrodynamic numerical simulation calculations for different runoff flood conditions and tidal conditions (if relevant), and analyze the hydrodynamic characteristics of each branch such as flow, diversion ratio, water level, and flow velocity; (5) Propose the desilting section type, propose the desilting centerline, determine the desilting bottom width B and desilting depth H range, and propose the desilting section selection plan According to the Code for Design of River Regulation (GB50707-2011), a trapezoidal desilting section is adopted, and the slope ratio i on both sides of the excavated river channel is determined according to the geological conditions of the river channel and the requirements for slope stability, i≤i 0 ,i 0 The maximum slope ratio to meet the slope stability requirements; The desilting centerline is determined according to the hydrodynamic axis of common floods, which is determined by the hydrodynamic numerical simulation analysis in step 4. The common flood standard can be taken as once every 2 to 5 years. The bottom width B of the dredging section is initially determined based on the width B0 of the main river channel of the dredged harbor where no obvious siltation has occurred. The bottom width B of the dredging section should not be less than the minimum value (B0min) of the width B0 of the main river channel of the dredged harbor where no obvious siltation has occurred. In addition, the excavation section should be controlled so as not to affect the stability of the embankment foot on both sides, that is, to meet the relationship shown in formula 4 and formula 5; (6) Based on the initial proposed desilting section width and depth range, calculate and determine the desilting section width and depth combination plan under the premise of the same total desilting volume, and define the average desilting section width Average depth of desilting section The ratio of is λ, and the λ values ​​of different dredging width and depth schemes are calculated respectively; (7) Using the above-mentioned verified two-dimensional river hydrodynamic mathematical model, carry out numerical simulation calculations of hydrodynamics under different runoff and flood conditions and various dredging cross-section schemes, and calculate the flow rate Q and diversion ratio η of the dredged harbors under different schemes. For tidal estuaries, it is also necessary to carry out numerical simulation calculations of tidal conditions and calculate the tidal volume W of the dredged harbors under different schemes; (8) Establish a relationship diagram between the flow rate Q, diversion ratio η, tidal volume W and the width-to-depth ratio λ of the dredged branch for different dredging section schemes, fit the trend lines of Q~λ, η~λ and W~λ, and establish the correlation equations of Q~λ, η~λ and W~λ; according to the relationship diagram and relationship equation, calculate the maximum value of the flow rate Q, diversion ratio η and tidal volume W of the dredged branch after dredging. The dredging section corresponding to the scheme with the maximum flow rate Q, diversion ratio η and tidal volume W of the dredged branch is the preferred dredging section.

[0007] Preferably, the mathematical model based on Mike21 software in step (2) is based on the Navier-Stokes equations with incompressibility and uniform distribution of Reynolds values, and is subject to the Boussinesq assumption and the assumption of hydrostatic pressure; The two-dimensional non-steady shallow water equations are: Formula 1: ; Formula 2: ; Formula 3: ; Where: For time; is the Cartesian coordinate system coordinate; is the water level; is the still water depth; is the total water depth; They are The velocity component in the direction; , are the average flow velocities in the x and y directions based on the water depth; Pa is the local atmospheric pressure; is the density of water at standard temperature and pressure; , Along Shear stress in the direction; , Along Bottom shear stress in the direction; is the Coriolis force coefficient, , is the Earth's rotation angular velocity, is the local latitude; is the acceleration due to gravity; is the density of water; are the radiation stress components respectively; is the horizontal viscous stress term, is the source-sink term, The source-sink flow is Flow velocity in direction.

[0008] Preferably, the hydrodynamic model in step (2) ranges from the section of the main and tributary control hydrological stations or the section below the dam to the ocean tide station section at the estuary; the hydrodynamic model adopts an unstructured triangular grid, and the grid size is comprehensively determined based on the accuracy of the modeled topographic map, the degree of terrain undulation, the size of important river-related structures, etc. The grid size should be equivalent to the interval between the elevation measurement points of the modeled topographic map. The local grids in areas with drastic terrain undulations and important river-related structures should be encrypted, and the scale of the encrypted grid should be equivalent to the scale of the river-related structures.

[0009] Preferably, the roughness of the hydrodynamic model in step (2) can be preliminarily determined based on experience, in combination with the river channel type, riverbed morphology, riverbed composition, distribution of river-related structures, etc., and finally determined through model verification and adjustment.

[0010] Preferably, the upper boundary of the hydrodynamic model in step (2) is set at the starting section of the main stream and tributary, and the upper boundary adopts the flow rate; the lower boundary of the model is set at the section of the ocean tide station at the estuary, and the lower boundary adopts the tide level.

[0011] Preferably, Formula 4 and Formula 5 in step (5) are specifically as follows: Formula 4: B+2L+2△b≤b Formula 5:

[0012] Where: B is the bottom width of the desilted section; L is the horizontal projection length of the slope of the desilted section; △b is the safe distance of the embankment foot, which can be determined according to the "Code for Design of Embankment Engineering" (GB50286-2013), depending on the level of the embankment engineering, and referring to the width of the backwater side embankment (Table 13.2.2 of "Code for Design of Embankment Engineering" (GB50286-2013)); b is the horizontal distance between the embankment foot lines on both sides of the river; is the average desilting depth; i is the slope ratio of the excavated slope of the desilting section.

[0013] Preferably, the initial proposed dredging depth in step (5) is mainly analogous to the channel depth of the unsilted section of the dredged harbor and the branch-type river in which it is located that is relatively similar to the dredged harbor, and is determined in combination with aspects such as ecological water depth. First, the elevation of the riverbed after dredging is controlled to be basically lower than the measured lowest water (tide) level of the river section to ensure continuous flow during low water season; second, the channel depth after dredging is controlled to smoothly connect with the channel depth of the unsilted section of the dredged harbor; third, the ecological water depth requirements of the river are taken into consideration so that the riverbed after dredging can meet certain water depth requirements, ensure that the riverbed can be submerged for a long time during the dry season, inhibit the growth of Spartina alterniflora or other invasive alien plants, and protect the water ecological environment.

[0014] Preferably, the total dredging volume V in step (6) can be calculated by the average cross-sectional area of ​​the dredging sections in each scheme: Multiply by the length of the dredged river section L 0 Determine, that is, V = ×L 0 ,in , It can be simplified as the difference between the average elevation of the original riverbed within the dredging range and the bottom elevation of the dredging section.

[0015] The present invention proposes a method for optimizing the desilting section of a bifurcated river dredging project based on two-dimensional hydrodynamic numerical simulation calculation and analysis. By simulating and calculating the flood flow, diversion ratio, tidal volume and the like of the desilting branch after implementing different desilting section schemes under the same total desilting amount, a correlation between the flood flow, diversion ratio, tidal volume and the width-to-depth ratio of the desilting section is established. Then, by calculating the maximum values ​​of the flood flow, diversion ratio and tidal volume, the optimal width-to-depth ratio of the desilting section is determined, thereby determining the optimal desilting section. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the relationship between the dredging excavation section and the location of the embankment footings on both sides of the river; Figure 2 This is a schematic diagram of a bifurcated river; Figure 3 This is the relationship between the branch flow ratio η and the width-to-depth ratio λ of the dredging section after dredging for each dredging scheme under the flood condition with a return period of 10 years; Figure 4This is the relationship between the branch flow diversion ratio η and the width-to-depth ratio λ of the dredging section after dredging for each dredging scheme under the flood condition with a return period of 50 years; Figure 5 This is the relationship between the flood flow rate Q of the harbor after dredging and the width-to-depth ratio λ of the dredging section under various dredging schemes under the 10-year flood condition; Figure 6 This is the relationship between the flood flow rate Q of the harbor after dredging and the width-to-depth ratio λ of the dredging section under various dredging schemes under the flood condition with a return period of 50 years; Figure 7 This is the relationship between the tidal volume W of the harbor after dredging and the width-to-depth ratio λ of the dredging section under various dredging schemes during the low water season. DETAILED DESCRIPTION

[0017] The present invention is described in detail below in conjunction with the accompanying drawings.

[0018] Example 1 The present invention establishes a two-dimensional hydrodynamic mathematical model of a bifurcated river, conducts simulation analysis on different dredging depth and dredging bottom width combination schemes under the premise of a fixed total dredging amount, and introduces the dredging section width-to-depth ratio to establish the changing relationship between the dredged branch flow, diversion ratio, tidal volume and the dredging section width-to-depth ratio of different dredging schemes, thereby determining the optimal dredging section.

[0019] The method for analyzing and determining the optimal dredging section of a bifurcated river according to the present invention comprises the following steps: (1) Analyze the evolution of the river channel and the current status of siltation in the branched rivers and determine the river sections to be desilted.

[0020] Collect basic data on river topography, geology, hydrology, sediment, river sand mining, river-related construction activities, river (waterway) regulation, previous relevant research results, etc. Analyze the evolution characteristics of the river from the aspects of changes in river bank boundaries, plane scouring and silting patterns, cross-sectional topography, longitudinal profile of the deep-water line, and riverbed scouring and silting volume. Combined with on-site river surveys, visit river management units and residents on both sides of the river to understand the siltation situation of river branches and determine the river sections to be dredged.

[0021] (2) Establish a two-dimensional hydrodynamic mathematical model for bifurcated rivers.

[0022] Collect data on the river's topography, geology, hydrology, riverbed, shoreline, river-related structures, operation and management of major water projects, and related research results, and establish a two-dimensional hydrodynamic mathematical model for bifurcated rivers.

[0023] The mathematical model based on Mike21 software is based on the Navier-Stokes equations with incompressibility and uniform distribution of Reynolds values, and is subject to the Boussinesq assumption and the assumption of hydrostatic pressure.

[0024] The two-dimensional non-steady shallow water equations are: (Formula 1) (Formula 2) (Formula 3) Where: For time; is the Cartesian coordinate system coordinate; is the water level; is the still water depth; is the total water depth; They are The velocity component in the direction; , are the average flow velocities in the x and y directions based on the water depth; Pa is the local atmospheric pressure; is the density of water at standard temperature and pressure; , Along Shear stress in the direction; , Along Bottom shear stress in the direction; is the Coriolis force coefficient, , is the Earth's rotation angular velocity, is the local latitude; is the acceleration due to gravity; is the density of water; are the radiation stress components respectively; is the horizontal viscous stress term, is the source-sink term, The source-sink flow is Flow velocity in direction.

[0025] The hydrodynamic model ranges from the section of the controlling hydrological station of the main stream and tributary or below the dam to the section of the ocean tide station at the estuary.

[0026] The hydrodynamic model adopts unstructured triangular grids, and the grid size is determined comprehensively based on the accuracy of the modeled topographic map, the degree of terrain undulation, the size of important river-related structures, etc. The grid size should be equivalent to the interval between elevation measurement points of the modeled topographic mapping. The local grids in areas with drastic terrain undulations and important river-related structures should be encrypted, and the scale of the encrypted grid should be equivalent to the scale of the river-related structures.

[0027] The roughness of the hydrodynamic model can be preliminarily determined based on experience, combined with the river channel type, riverbed morphology, riverbed composition, distribution of river-related structures, etc., and finally determined through model verification and adjustment.

[0028] The upper boundary of the hydrodynamic model is set at the starting section of the main and tributary rivers, and the upper boundary adopts the flow rate. The lower boundary of the model is set at the section of the ocean tide station at the estuary, and the lower boundary adopts the tide level.

[0029] (3) Carry out calibration and verification of mathematical models.

[0030] The model is verified using measured hydrological data, and the verification content mainly includes the flow and diversion ratio of each port, water (tide) level, flow velocity, and flow direction. The difference between the calculated value and the measured value of each hydrological station meets the error requirements mainly by adjusting the grid file, model roughness, and initial water level field boundary. The error between the calculated value and the measured value of the model should meet the relevant requirements of the Technical Specifications for Simulation Tests of Water Transport Engineering (JTS-T 231-2021).

[0031] (4) Conduct numerical simulation analysis of river hydrodynamics before dredging and remediation.

[0032] Using the above-mentioned verified two-dimensional river plane hydrodynamic mathematical model, hydrodynamic numerical simulation calculations of different runoff flood conditions and tidal conditions (if involved) are carried out to analyze the hydrodynamic characteristics of each branch, such as flow, diversion ratio, water level, and flow velocity.

[0033] (5) Propose the desilting section type and desilting centerline, determine the desilting bottom width B and desilting depth H, and formulate a desilting section selection plan.

[0034] According to the "River Regulation Design Code" (GB50707-2011), a trapezoidal desilting section is adopted. The slope ratio i on both sides of the excavated river channel is determined according to the geological conditions of the river channel and the requirements for slope stability, i≤i 0 ,i 0 The maximum slope ratio that meets the slope stability requirements.

[0035] The desilting centerline is determined according to the hydrodynamic axis of common floods, which is determined by the hydrodynamic numerical simulation analysis in step 4. The common flood standard can be taken as once every 2 to 5 years.

[0036] The bottom width B of the dredging section is initially determined based on the width B0 of the main river channel in the dredged harbor where no obvious siltation has occurred. The bottom width B of the dredging section should not be less than the minimum value (B0min) of the width B0 of the main river channel in the dredged harbor where no obvious siltation has occurred. In addition, the excavation section should be controlled so as not to affect the stability of the embankment foot on both sides, that is, to meet Figure 1 And the relationship shown in Equation 4 and Equation 5.

[0037] B+2L+2△b≤b (Formula 4) (Formula 5) Where: B is the bottom width of the desilted section; L is the horizontal projection length of the slope of the desilted section; △b is the safe distance of the embankment foot, which can be determined according to the "Code for Design of Embankment Engineering" (GB50286-2013), depending on the level of the embankment engineering, and referring to the width of the backwater side embankment (Table 13.2.2 of "Code for Design of Embankment Engineering" (GB50286-2013)); b is the horizontal distance between the embankment foot lines on both sides of the river; is the average desilting depth; i is the slope ratio of the excavated slope of the desilting section.

[0038] The initial proposed dredging depth is mainly analogous to the unsilted sections of the dredged harbors and the river depth of the branches in the bifurcated rivers that are similar to the dredged harbors, and is determined in combination with ecological water depth and other aspects. First, the elevation of the riverbed after dredging is basically lower than the measured lowest water (tide) level of the river section to ensure continuous flow during low water; second, the river depth after dredging is controlled to smoothly connect with the river depth of the unsilted sections of the dredged harbors; third, the ecological water depth requirements of the river are considered, so that the river after dredging can meet certain water depth requirements, ensure that the riverbed can be submerged for a long time during the dry season, inhibit the growth of Spartina alterniflora or other invasive alien plants, and protect the water ecological environment.

[0039] (6) Based on the initial proposed desilting section width and depth range, calculate and determine the desilting section width and depth combination plan under the premise of the same total desilting volume, and define the average desilting section width Average depth of desilting section The ratio of is λ, and the λ values ​​of different dredging width and depth schemes are calculated respectively. The total dredging volume V can be calculated by the average cross-sectional area of ​​the dredging section of each scheme. Multiply it by the length of the dredged river section L0, that is, V= ×L0, where , It can be simplified as the difference between the average elevation of the original riverbed within the dredging range and the bottom elevation of the dredging section.

[0040] (7) Using the above-mentioned verified two-dimensional river hydrodynamic mathematical model, carry out numerical simulation calculations of the hydrodynamics of various dredging section schemes under different runoff and flood conditions, and calculate the flow rate Q and diversion ratio η of the dredged harbors under different schemes. For tidal estuaries, it is also necessary to carry out numerical simulation calculations of tidal conditions and calculate the tidal volume W of the dredged harbors under different schemes.

[0041] (8) Establish a relationship diagram between the flow rate Q, diversion ratio η, tidal volume W and the width-to-depth ratio λ of the dredged branch for different dredging section schemes, fit the trend lines of Q~λ, η~λ and W~λ, and establish the correlation equations of Q~λ, η~λ and W~λ. According to the relationship diagram and relationship, calculate the maximum value of the flow rate Q, diversion ratio η and tidal volume W of the dredged branch after dredging. The dredging section corresponding to the scheme with the maximum flow rate Q, diversion ratio η and tidal volume W of the dredged branch is the preferred dredging section.

[0042] by Figure 2 A bifurcated river is shown as an example for explanation, and the bifurcated river is located at the estuary. In the figure, after the rivers 1 and 2 merge at the estuary delta, they are divided into branch 1, branch 2, and branch 3, and branch 1, branch 2, and branch 3 are connected to each other by connecting ports.

[0043] The analysis of river channel evolution and on-site investigation and survey show that in recent years, due to the combined influence of water and sand movement changes and human activities, the river section about 5 km long below the entrance of Gangcha 2 (the shaded area in the figure) is seriously silted up and almost cut off during the dry season, which seriously affects flood discharge and tidal intake, has an adverse impact on the safety and health of the river, and restricts the high-quality development of the economy and society. Therefore, it is necessary to dredge and regulate the river section about 5 km long below the entrance of Gangcha 2.

[0044] The verified hydrodynamic mathematical model was used to perform numerical simulations of different runoff flood and tidal conditions. The desilting centerline was initially determined based on the hydrodynamic axis of the 2-year return period calculated by the numerical model, and the main channel at the concave bank where the mainstream deviated from the river bank was appropriately straightened to avoid the dredging section from affecting the stability of the concave bank slope.

[0045] The proposed dredging section is trapezoidal, and the excavation slope ratio i on both sides is 1:5 based on the geological conditions and stability requirements of the dredged river section and taking into account a certain stability margin.

[0046] From the perspective of river channel morphology, the runoff floods that flow through the head of Island 2 and enter Harbor 1 and the connecting port between Harbor 1 and Harbor 2 are roughly the same as the angle between the upstream water flow direction; Harbor 1 and Harbor 2 are similar in length and width, and the average elevation of the river channel deep line and main channel of the lower half of Harbor 2, which has not yet been seriously silted up, is not much different from that of the lower half of Harbor 1. In addition, data show that the diversion ratio of Harbor 3, Harbor 2, and Harbor 1 during the 1960 flood was 0.50:0.26:0.24. At that time, the river channel was less affected by human intervention, and the diversion reflected the natural situation better. It can be inferred that under historical natural conditions, Harbor 2 and Harbor 1 have similar flow capacity, and the dredging width and depth of the river channel in the silted section of Harbor 2 can refer to Harbor 1. Therefore, the dredging depth range of Harbor 2 is similar to that of Harbor 1 and Harbor 2 which have not yet been silted up, and is determined in combination with requirements of ecological water depth and other aspects.

[0047] According to the hydrological data of the diversion channel hydrological station for many years, the lowest historical tide level of Harbor 2 is about -2m. Therefore, the bottom elevation of Harbor 2 after dredging should not be higher than -2m. The average elevation of the main channel of Harbor 2 before siltation is -3.15m, and the average elevation of the main channel of Harbor 1 is about -4m. Therefore, the bottom elevation selection range of Harbor 2 after dredging is determined to be -2~-4m. According to the statistical data of the main channel width of Harbor 1 and Harbor 2 before siltation, the minimum width of the main channel below -2m elevation is about 70~100m, and the minimum width of the main channel corresponding to -4m elevation is about 30~50m. Therefore, the bottom width selection range of Harbor 2 after dredging is determined to be 30~100m.

[0048] According to the above preliminary proposed dredging bottom width range of 30~100m and dredging bottom elevation range of -2~-4m, based on the designed dredging total volume of approximately 1.47 million m³, the dredging section selection schemes are determined as shown in Table 1.

[0049] Table 1 Desilting section selection scheme under certain desilting total amount conditions

[0050] According to Formula 4 and Formula 5, the relationship between the dredging section and the embankment foot of each dredging plan is further verified. Figure 2 The embankment level of the dredged river section shown is level 2, and the safe distance △b of the embankment foot is taken as 20m. The calculated range of each scheme B+2L+2△b is 120.5m~151m, which all meet the minimum distance b of the embankment foot on both sides of the dredged river section. The minimum distance b of the embankment foot is 200m.

[0051] Using the verified two-dimensional hydrodynamic mathematical model, different runoff and flood conditions were carried out, and the hydrodynamic numerical simulation calculations of each dredging section scheme were carried out, and the flow rate Q, diversion ratio η, and tidal volume W of the dredged harbor branch under different schemes were statistically analyzed. The relationship diagram between the flow rate Q, diversion ratio η, tidal volume W of the dredged harbor branch and the width-to-depth ratio λ of the dredged section under different dredging section schemes was established, as shown in the figure. Figure 3~Figure 7 Fit the Q~λ, η~λ and W~λ trend lines, and establish the correlation equations of Q~λ, η~λ and W~λ, see equations 6 to 10. Figure 3~Figure 7 From equations 6 to 10, it can be seen that when the width-to-depth ratio λ of the dredging section is approximately 17.20, the flood flow Q, diversion ratio η, and tidal volume W of the dredged harbor after dredging all reach the maximum value. Therefore, it can be seen that the dredging section corresponding to Scheme 5 is the optimal dredging section.

[0052] (Formula 6) (Formula 7) (Formula 8) (Formula 9) (Formula 10) The above-described embodiments only express the preferred embodiments of the present invention, and the description thereof is relatively specific and detailed, but the present invention is not limited to these embodiments. It should be noted that it is obvious to those skilled in the art. Without departing from the purpose of the present invention, any improvement made shall fall within the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for analyzing and determining an optimal dredging section for a bifurcated river, characterized in that: The following steps are involved: (1) Analyze the evolution of the river channel and the current status of siltation in the branched rivers and determine the river sections to be desilted Collect basic data on river topography, geology, hydrology, sediment, river sand mining, river-related construction activities, river regulation, and previous relevant research results; analyze the evolution characteristics of the river from the aspects of changes in river bank boundaries, changes in planar scouring and silting patterns, changes in cross-sectional topography, changes in the longitudinal profile of the deep line, and changes in riverbed scouring and silting; combine on-site river surveys, visit river management units and residents on both sides of the river, understand the siltation of river branches, and determine the river sections to be desilted; (2) Establishment of a two-dimensional hydrodynamic mathematical model for bifurcated rivers Collect data on the river's topography, geology, hydrology, riverbed, shoreline, river-related structures, operation and management of major water projects, and related research results, and establish a two-dimensional hydrodynamic mathematical model for bifurcated rivers; (3) Conduct calibration and verification of mathematical models The model is verified using measured hydrological data, and the verification content mainly includes the flow and diversion ratio of each branch, water level, flow velocity, and flow direction; the difference between the calculated value and the measured value of each hydrological station meets the error requirements mainly by adjusting the grid file, model roughness, and initial water level field boundary. The error between the calculated value and the measured value of the model should meet the relevant requirements of the Technical Specifications for Simulation Tests of Water Transport Engineering JTS-T 231-2021; (4) Numerical simulation analysis of river hydrodynamics before dredging and remediation Using the above-mentioned verified two-dimensional river hydrodynamic mathematical model, we carried out hydrodynamic numerical simulation calculations under different runoff flood conditions and tidal conditions, and analyzed the hydrodynamic characteristics of each branch flow, diversion ratio, water level, and flow velocity; (5) Propose the desilting section type, propose the desilting centerline, determine the desilting bottom width B and desilting depth H range, and propose the desilting section selection plan According to the "River Regulation Design Code" GB50707-2011, a trapezoidal desilting section is used, and the slope ratio i on both sides of the excavated river channel is determined according to the river geological conditions and slope stability requirements, i≤i0, i0 is the maximum slope ratio that meets the slope stability requirements; The desilting centerline is determined according to the hydrodynamic axis of common floods, which is determined by the hydrodynamic numerical simulation analysis in step 4. The common flood standard can be taken as once every 2 to 5 years. The bottom width B of the dredging section is initially determined based on the width B0 of the main river channel of the dredged harbor where no obvious siltation has occurred. The bottom width B of the dredging section should not be less than the minimum value of the width B0 of the main river channel of the dredged harbor where no obvious siltation has occurred. In addition, the excavation section should be controlled so as not to affect the stability of the embankment foot on both sides, that is, to meet the relationship shown in formula 4 and formula 5; (6) Based on the initial proposed desilting section width and depth range, calculate and determine the desilting section width and depth combination plan under the premise of the same total desilting volume, and define the average desilting section width Average depth of desilting section The ratio of is λ, and the λ values ​​of different dredging width and depth schemes are calculated respectively; (7) Using the above-mentioned verified two-dimensional river hydrodynamic mathematical model, carry out numerical simulation calculations of hydrodynamics under different runoff and flood conditions and various dredging cross-section schemes, and calculate the flow rate Q and diversion ratio η of the dredged harbors under different schemes. For tidal estuaries, it is also necessary to carry out numerical simulation calculations of tidal conditions and calculate the tidal volume W of the dredged harbors under different schemes; (8) Establish the relationship diagram between the dredged harbor flow Q, diversion ratio η, tidal volume W and the dredged section width-to-depth ratio λ for different dredging section schemes, fit the Q~λ, η~λ and W~λ trend lines, and establish the correlation equation between Q~λ, η~λ and W~λ; According to the relationship diagram and relationship, the maximum values ​​of flow rate Q, diversion ratio η and tidal volume W of the dredged harbor after dredging are calculated. The dredging section corresponding to the scheme with the maximum flow rate Q, diversion ratio η and tidal volume W of the dredged harbor is the preferred dredging section.

2. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: The mathematical model based on Mike21 software in step (2) is based on the Navier-Stokes equations with incompressibility and uniform distribution of Reynolds values, and is subject to the Boussinesq assumption and the assumption of hydrostatic pressure; The two-dimensional non-steady shallow water equations are: Formula 1: ; Formula 2: ; Formula 3: ; Where: For time; is the Cartesian coordinate system coordinate; is the water level; is the still water depth; is the total water depth; They are The velocity component in the direction; , are the average flow velocities in the x and y directions based on the water depth; Pa is the local atmospheric pressure; is the density of water at standard temperature and pressure; , Along Shear stress in the direction; , Along Bottom shear stress in direction; is the Coriolis force coefficient, , is the Earth's rotation angular velocity, is the local latitude; is the acceleration due to gravity; is the density of water; are the radiation stress components respectively; is the horizontal viscous stress term, is the source-sink term, The source-sink flow is Flow velocity in direction.

3. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: The hydrodynamic model in step (2) ranges from the section of the main river and tributary control hydrological station or the section below the dam to the ocean tide station at the estuary; the hydrodynamic model adopts an unstructured triangular grid, and the grid size is determined comprehensively based on the accuracy of the modeled topographic map, the degree of terrain undulation, and the size of important river-related structures. The grid size should be equivalent to the interval between the elevation measurement points of the modeled topographic map. The local grid in the area with drastic terrain undulation and important river-related structures should be encrypted, and the scale of the encrypted grid should be equivalent to the scale of the river-related structures.

4. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: The roughness of the hydrodynamic model in step (2) can be determined based on experience, and can be preliminarily determined in combination with the river channel type, riverbed morphology, riverbed composition, and distribution of river-related structures, and can be finally determined through model verification and adjustment.

5. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: In step (2), the upper boundary of the hydrodynamic model is set at the starting section of the main stream and tributary, and the upper boundary adopts the flow rate. The lower boundary of the model is set at the section of the ocean tide station at the estuary, and the lower boundary adopts the tide level.

6. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: Formula 4 and Formula 5 in step (5) are specifically as follows: Formula 4: B+2L+2△b≤b Formula 5: ； Where: B is the bottom width of the desilted section; L is the horizontal projection length of the desilted section slope; △b is the safe distance of the embankment foot, which can be determined according to the "Design Code for Embankment Engineering" GB50286-2013, depending on the level of the embankment project, and referring to the width of the backwater side embankment "Design Code for Embankment Engineering" GB50286-2013 Table 13.2.2; b is the horizontal distance between the embankment foot lines on both sides of the river; is the average desilting depth; i is the slope ratio of the excavated slope of the desilting section.

7. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: The initial proposed dredging depth in step (5) is mainly analogous to the channel depth of the unsilted section of the dredged harbor and the branch-type river where the dredged harbor is located and is relatively similar to the dredged harbor. It is determined in combination with the ecological water depth. First, the elevation of the riverbed after dredging is controlled to be basically lower than the measured lowest water level of the river section to ensure continuous flow during low water season; second, the channel depth after dredging is controlled to smoothly connect with the channel depth of the unsilted section of the dredged harbor; third, the ecological water depth requirements of the river are considered, so that the riverbed after dredging can meet certain water depth requirements, ensure that the riverbed can be submerged for a long time during the dry season, inhibit the growth of Spartina alterniflora or other invasive alien plants, and protect the water ecological environment.

8. The method for analyzing and determining the optimal dredging section of a bifurcated river according to claim 1, characterized in that: The total amount of dredging V in step (6) can be calculated by the average cross-sectional area of ​​the dredging sections in each scheme: Multiply it by the length of the dredged river section L0, that is, V= ×L0, where , It can be simplified as the difference between the average elevation of the original riverbed within the dredging range and the bottom elevation of the dredging section.

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