Method for determining the dredging volume of tidal river sections based on the principle of river management

Abstract

The present invention relates to a method for determining the dredging volume of a tidal river section based on the principle of river management. The method comprises screening dredged areas based on the changes in scouring and silting in the river section and formulating a dredging range and dredging elevation; establishing a two-dimensional water-sediment mathematical model of the river section, simulating the changes in the diversion ratio of multiple dredged areas after dredging and the amount of back-silting after N years of dredging, and calculating the back-silting rate; when the diversion ratio change or back-silting rate does not meet the requirements, the dredged areas with the unsatisfactory back-silting rate are first eliminated, and the diversion ratio change of the remaining dredged areas is re-simulated. The dredging range or dredging elevation of the adjustment area is adjusted using the ratio of the diversion ratio change threshold and the current diversion ratio change as the adjustment step size, to obtain dredging parameters for each dredged area that makes the diversion ratio meet the requirements. The method provided by the present invention can scientifically and rationally determine the range and elevation of dredged sand in a tidal river section, providing a quick and simple method for determining the amount of dredged sand in a tidal river section.

Description

Technical Field

[0001] The present invention relates to the technical field of river flow control and guidance, and in particular to a method for determining the dredging volume of a tidal current river section based on the principle of river management. Background Art

[0002] The impacts of natural conditions and human activities can cause changes in the spatiotemporal distribution and transport of river sediment. On the one hand, reduced upstream sediment transport leads to increased scour downstream and occasional bank collapses. On the other hand, dredging projects required to maintain port and waterway depths remain. Systematically elucidating the combined impacts of changing water and sediment conditions and human engineering on river regime evolution is fundamental to further research on the regional utilization of dredged sediment and ensuring river regime stability and flood control safety. For river sections with complex and variable water and sediment conditions, large-scale dredging projects have a profound impact on river regime evolution. A comprehensive dredged sediment utilization plan, including maintenance dredging, is urgently needed to maintain river regime stability and ensure flood control and navigation safety.

[0003] For a long time, scholars at home and abroad have conducted extensive research on the resource utilization technology of dredged sand. Most existing studies have focused on the implementation and management models of dredged sand resource utilization, as well as on the use of physical and chemical methods to modify the properties of dredged sand to make it more suitable for shore use. However, the acquisition of dredged sand sources has not been deeply studied. Based on the principle of river management, dredged sand obtained from dredging projects can be used to construct protective devices. In this case, the determination of the dredging area and dredging volume are the main factors affecting the acquisition of dredged sand. Previous results have mainly determined the scale of waterway dredging by the impact of the diversion ratio of the branch channel after dredging. This method is only effective for waterway dredging and is difficult to apply to the overall dredging area of ​​tidal river sections. In actual engineering, a river section often includes multiple dredged areas, and each dredged area has a different impact on the diversion ratio, making it impossible to adjust the diversion ratio through separate simulations. In addition, existing methods only consider the impact of water flow on river flow, but do not consider the impact of erosion and deposition changes on river flow. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for determining the dredging volume of a tidal river section based on the idea of ​​managing the river according to the river.

[0005] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0006] Obtain changes in scouring and sedimentation in a river section, screen dredgeable areas based on sediment thickness and sediment area, and propose a dredging range and dredging elevation for each dredgeable area; establish a two-dimensional water-sediment mathematical model for the river section, and, combining the dredging range and dredging elevation, simultaneously simulate the changes in the diversion ratio of the distributary channels after dredging in multiple dredgeable areas, as well as the amount of back-sedimentation after dredging for N years, and calculate the back-sedimentation rate;

[0007] Obtain the dredging range and dredging elevation that make the diversion ratio change of the branch channel and the back-silting rate of each dredgeable area meet the preset range, and determine the dredging volume of each dredgeable area; when the diversion ratio change or back-silting rate does not meet the requirements, handle it based on the following methods:

[0008] Eliminate dredgeable areas with unsatisfactory siltation rates and simulate the changes in branch channel diversion ratios after dredging for the remaining dredgeable areas.

[0009] For the branch channel whose diversion ratio change does not meet the requirements, the dredging area contained in the river section from the upstream of the branch channel to the branch channel diversion or confluence is selected as the adjustment area, and the ratio of the diversion ratio change threshold and the current diversion ratio change is used as the adjustment step. The dredging range or dredging elevation of the adjustment area is adjusted, and the branch channel diversion ratio change after dredging is re-simulated for the dredged area after parameter adjustment, and repeated until the branch channel diversion ratio change meets the requirements.

[0010] In some embodiments of the present invention, the method further includes eliminating dredging areas that overlap with the engineering area and the ecological protection area, and carrying out dredging projects in the remaining dredging areas.

[0011] In some embodiments of the present invention, the method of screening dredging areas based on siltation thickness and siltation area includes: obtaining the scouring and silting changes of a river section within a certain period of time, and selecting areas with siltation thickness greater than a first preset value and area greater than a second preset value within the period as the initial dredging areas. The first preset value and the second preset value are determined based on the calculated scouring and silting period. For example, in a three-year period, the area with siltation thickness greater than 1m and area greater than 10,000m within three years is selected. 2 areas as the primary dredging areas.

[0012] In some embodiments of the present invention, when constructing the two-dimensional water-sand mathematical model, the rectangular coordinate system is transformed into a body-fitted coordinate system;

[0013] The two-dimensional water-sediment mathematical model includes a water flow module and a sediment module. The control equation of the water flow module is the incompressible flow Navier-Stokes equation based on the shallow water assumption and the Boussines assumption; the control equation of the sediment module is the bedload transport equation under the combined action of waves and currents.

[0014] In some embodiments of the present invention, during model simulation, representative tide generalization is used for the downstream boundary, including:

[0015] Get the main tidal component of the river section;

[0016] Calculate the residual sediment transport ratio of the tidal cycle for different tidal combinations;

[0017] The residual sediment transport ratio is accumulated from large to small until the ratio is greater than 90%, and the corresponding tidal type is selected as the representative tide for simulation; when simulating the model, the calculation time is an integer multiple of half a month.

[0018] In some embodiments of the present invention, during model simulation, representative runoff generalization is used for the upstream boundary, including:

[0019] The annual flow process is divided into flood flow process and other processes: the actual flow process simulation is used for flood flow process, and the model acceleration factor is set to 1;

[0020] For non-flood flow processes, the average value of a time period is taken as the generalized flow value of the corresponding time period, and the acceleration factor is taken as the number of days in the corresponding time period. The number of days included in the time period is determined based on the flow change.

[0021] In some embodiments of the present invention, when simulating the change of the branch channel diversion ratio, the diversion ratio changes under low water flow, flood flow and amortized flow are simulated respectively, and whether it meets the preset range is determined based on the maximum diversion ratio change.

[0022] In some embodiments of the present invention, the dredging range or dredging elevation of the adjustment area is adjusted as follows:

[0023] In each round of parameter adjustment, the dredging range is first selected for adjustment. If the dredging range still does not meet the requirements after adjustment, the dredging elevation is adjusted based on the initial state of this round of parameter adjustment. If the dredging elevation still does not meet the requirements after adjustment, the result close to the diversion ratio threshold value is selected from the diversion ratio change results after this round of dredging range adjustment and dredging elevation adjustment as the initial state for the next round of parameter adjustment.

[0024] In some embodiments of the present invention, the planned dredging volume that makes the absolute value of the change in the diversion ratio of the dredgeable area less than 0.5% and the siltation rate greater than 50% is determined as the dredging volume of the dredgeable area.

[0025] In some embodiments of the present invention, the method further includes determining the scope of the bank collapse risk section of the river section to which the dredging area belongs, selecting an area within the bank collapse risk section and with an average scouring depth exceeding a third preset value as an area to be protected, and using the sediment obtained by dredging the dredging area to construct a protective device in the corresponding area to be protected.

[0026] This invention attempts to manage the river based on the idea of ​​river management. First, on the basis of analyzing the river flow evolution characteristics of the tidal river section under the new water and sediment conditions, it clarifies the comprehensive impact of water and sediment changes and dredging projects on the river flow evolution, clarifies the river flow response mechanism of large-scale dredging projects, further determines the key areas for dredged sand utilization, and performs scheme optimization calculations for different dredging ranges and dredging elevations, so as to determine the dredging volume of the river section.

[0027] The present invention has the following beneficial effects:

[0028] (1) The method provided by the present invention can scientifically and reasonably determine the range and elevation of dredged sand in tidal river sections, and provides a quick and simple method for determining the amount of dredged sand in tidal river sections.

[0029] (2) The boundary generalization method proposed in this paper can effectively improve the calculation efficiency of the model. Combined with the variable acceleration factor, the model can improve efficiency without affecting the calculation accuracy, providing a relatively simple calculation method for analyzing the impact of dredged sand on river flow evolution.

[0030] (3) The method for determining the amount of dredged sand provided by the present invention can effectively reduce the adverse effects of dredging projects on river flow, ensure the comprehensive utilization of dredged sand while not changing the stability of the existing river flow, and thus achieve the purpose of managing the river according to its principles. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the dredging area selected in the river section Z in Example 1.

[0032] Figure 2 It is the flow measurement section mark of river section Z in Example 1; the black lines at A, B, C, E, F, and G are the positions of the flow measurement sections in the water level process verification; and D is the water level station included in river section Z.

[0033] Figure 3 This is a diagram showing the generalized results of upstream flow in Example 1. DETAILED DESCRIPTION

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] Example 1

[0036] The embodiment takes river section Z as an example to illustrate a method for determining the dredging amount of the river section. The method includes the following steps:

[0037] 1. Determine the scouring and silting conditions of river section Z based on measured topographic data and define the dredging area.

[0038] The scouring and silting changes were obtained by comparing the measured topographic data of a branch channel section of river section Z from 2018 to 2021. In this embodiment, based on the measured data (measured every three years), the siltation thickness is greater than 1m and the area is greater than 10,000m in three years. 2Areas with a thickness or area too small to be dredged are initially designated as dredging areas. These areas are excluded. Furthermore, if the dredging area is located within a revetment or bottom protection project, this area is excluded to avoid damaging existing water conservancy structures. If the dredging area is located within an ecological protection area, such as a finless porpoise reserve, this area is also excluded. Dredging projects must not damage the river's ecological environment.

[0039] According to the above screening conditions, 11 dredging areas, namely D1, D2, D3, D4, D5, D6, D7, D8, D9, D10 and D11, were initially delineated. Figure 1 .

[0040] For each dredging area, the siltation area (the area where the siltation thickness is greater than 1m) is used as the initial dredging area, and the dredging volume is determined in combination with the siltation thickness, as shown in Table 1.

[0041] Table 1 Proposed dredging volume for each dredging area

[0042]

[0043] 2. Establish a two-dimensional water and sediment mathematical model for river section Z

[0044] ①Coordinate system transformation

[0045] An arbitrary shape region in a plane rectangular coordinate system (xy) can be transformed into a new coordinate system ( - ). The new and old coordinates are: , The functional relationship is transformed, and the transformation relationship must satisfy the two-dimensional Poisson equation and Dirichlet boundary conditions.

[0046] ②Water flow module equation

[0047] The governing equations of the hydrodynamic model are the Navier-Stokes equations for incompressible flow based on the shallow water and Boussines assumptions. The vertical momentum equation, based on the shallow water assumption, can be simplified to the static pressure equation because vertical acceleration can be neglected. Appropriate boundary conditions and the governing equations form a definite solution condition. The equations are discretized spatially using the finite difference method and temporally using the Euler forward difference method. The solution variables are staggered in the grid, and the ADI alternating implicit method is used for the solution.

[0048] Integrating the Navier-Stokes equations along the water depth and taking the average value can obtain the two-dimensional continuity equation and momentum equation, namely the shallow water equation. Using this as the control equation, the body-fitted coordinate technology is used, and the orthogonal curvilinear coordinate system can be obtained through coordinate transformation ( ) under the Navier-Stokes equations.

[0049] (1)

[0050] Where: It represents the source or sink flow per unit area. 、 、 、 Represent the conversion coefficients between the Cartesian coordinate system and the orthogonal curvilinear coordinate system in the horizontal and vertical directions, 、 are the flow velocity components in the horizontal and vertical directions, respectively. is the water depth (relative to the reference plane), is the height of the free surface.

[0051] and The momentum equations for the directions are:

[0052] (2)

[0053] (3)

[0054] Where: Indicates the conversion coefficient between the Cartesian coordinate system and the orthogonal curvilinear coordinate system; 、 are the flow velocity components in the horizontal and vertical directions respectively; is the parameter related to the Coriolis force, ; and is the pressure gradient; and represents the imbalance of horizontal Reynolds stress; and Represents the additional momentum caused by external sources.

[0055] ③ Sediment module equation

[0056] In the dynamic geomorphological model, sediment is divided into cohesive sand and non-cohesive sand.

[0057] In the model, the calculation of cohesive sand is solved by introducing source and sink terms into the advection-diffusion equation:

[0058] (4)

[0059] Where, is the water depth (m); It is Suspended sediment concentration of each sediment component (kg / m 3 ); and They are and Directional vertical velocity component (m / s); and is the eddy diffusion coefficient of the sediment component (m 2 / s); and They are Suspension and settling fluxes of sediment components (kg / m 2 / s).

[0060] The sediment settling velocity of cohesive sand is calculated based on a simplified flocculation model:

[0061]

[0062] Where, is the sediment settling velocity after taking salinity into account; is the sediment settling velocity at maximum salinity; is salinity; It is the maximum salinity corresponding to the maximum sinking velocity.

[0063] Bottom reference height of non-cohesive sand:

[0064] (6)

[0065] Where, is a custom scale factor, is the roughness height associated with the water flow, is the height of the sand ripples caused by waves (0.025m), It's the water depth.

[0066] Sediment settling velocity Calculate the suspended sediment particle size using the formula:

[0067]

[0068] Where, is the relative density of sediment, that is , is the sediment density, is the density of water; is the acceleration due to gravity; is the median particle size of sediment; is the kinematic viscosity of water (m 2 / s).

[0069] The formula for bed load transport under the combined action of waves and currents is as follows:

[0070] (8)

[0071] Where, is the bed load transport rate (km / m / s); is the median particle size of sediment; is the relative density of sediment; is the sediment settling velocity; It is the velocity value calculated based on the vertical average velocity of the significant wave height and the velocity of the near-bottom calculation layer; is the critical vertical mean velocity of incohesive sediment based on the Scherz curve; is the acceleration due to gravity.

[0072] 3. Establishment of the two-dimensional water-sediment mathematical model of river section Z

[0073] (1) Mesh generation

[0074] The upper and lower boundaries of the calculation area were determined based on the extent of river section Z. The calculated river section is approximately 66 km long. The calculation area was gridded, with 732 grid points arranged along the tidal current and 146 grid points arranged approximately perpendicular to the tidal current. The resulting calculation area consists of 732 by 146 grid points. Orthogonal curve calculations were performed to form an orthogonal grid. Except for a few points along the shore, the resulting grid has an intersection angle of 89° to 92°. The longitudinal grid spacing (along the flow direction) was mostly between 8 and 102 meters, and the transverse grid spacing (along the river width) was mostly between 5 and 20 meters. Partial grid spacing was implemented within the project area.

[0075] (2) Generalization of boundary conditions of tidal stream sections

[0076] Using real-time models for long-term morphological simulations incurs a significant computational burden, hence the introduction of an acceleration factor. This factor extends the timescale of hydrodynamic processes, extending the geomorphological duration of a single tidal cycle to days or even months, a phenomenon clearly inconsistent with physical reality. Therefore, to complement geomorphological acceleration techniques, the model utilizes tides, representing offshore geomorphology, and upstream runoff, representing sediment transport, to capture the core process driving geomorphological evolution.

[0077] ① Downstream boundary representative tide generalization method

[0078] A multi-year tidal harmonic analysis was conducted on the measured data of offshore tidal levels and station tidal levels in the study area. According to the amplitude, the main tidal components were obtained as M2, S2, O1, K1, M4, MO3, Mk3, and MSf.

[0079] In estuarine systems dominated by tidal forces, tidal asymmetry is the main driving force for residual sediment transport. :

[0080] = (9)

[0081] Where: and are the suspended sediment transport rate and the bed load transport rate, is the flow rate, is the Xie Cai coefficient, is the bulk density of sediment, is the median particle size of sediment.

[0082] The flow velocity at any measuring point can be decomposed into the average flow velocity and the superposition of the flow velocities of different tidal currents:

[0083] (10)

[0084] in, Refers to the average flow velocity, with a subscript of the tidal 、 、 They refer to the flow velocity, frequency and phase of the tide respectively.

[0085] Based on the measured flow velocities at the measuring points, a harmonic analysis was performed to calculate the residual sediment transport of different components, and the proportion of residual sediment transport during the tidal cycle caused by different tidal combinations was obtained. The calculation results show that the interaction between the average tidal flow velocity and the tidal components M2, S2, K1, and O1 leads to the largest residual sediment transport (see Table 2), accounting for more than 90%. Therefore, M2, S2, K1, and O1 were selected as representative tides for the open sea. Because the generalized representative tides still have large and small tides, a complete tidal cycle is every half month. Therefore, when performing landform evolution simulation calculations, the calculation time must be an integer multiple of half a month.

[0086] Table 2

[0087]

[0088] ② Upstream boundary representative runoff generalization method

[0089] Tidal and sediment calculations utilize a coupled algorithm. Each tidal calculation step is followed by calculations of the sediment concentration field for each particle size, bedload transport, and riverbed deformation. This calculation is then used to calculate the riverbed deformation caused by a full tidal process. An acceleration factor is introduced during the calculation to accelerate the topographic calculation and improve computational efficiency. However, inlet boundary runoff is a continuously changing process. To maintain guaranteed accuracy after acceleration, the inlet flow must be generalized, and the generalized flow must be consistent with actual flow.

[0090] The generalization method provided by the present invention is: the annual flow process is divided into two parts: flood flow process and other processes: for flood flow process (maximum flow greater than 50,000 m 3 / s), due to the drastic changes in terrain, it is easy to be distorted after acceleration. Therefore, the actual flow process is used in the flood process, and the model acceleration factor is set to 1 (no acceleration); for the non-flood flow process, the average flow rate every 10 to 30 days is taken as a generalized flow value (when the flow change is small, the number of days in each section can be appropriately increased, and when the flow change is large, the value is smaller). At this time, the acceleration factor is taken as the number of days corresponding to each generalized flow, that is, a variable acceleration factor is used.

[0091] 4. Verification of local water-sediment dynamic geomorphological model

[0092] ① Water flow verification

[0093] The model was validated using flow data from river section Z. During the test period, upstream inflows varied from low water to flood water and then to dry water. The upper and lower boundaries were controlled by inlet and outlet flow rates and water level processes, respectively. After repeated adjustments, the water level processes at the main gauges, the flow velocity and direction of each vertical line, the cross-sectional flow rate, and the diversion ratios of the main distributaries during the test period remained essentially consistent with the prototype.

[0094] The flow measurement section arranged in this embodiment is as follows Figure 2 The flow measurement section was selected at the junction of each branch channel. Water gauges were placed on the left and right banks of the section to calculate the water level. Two time periods were selected for verification. The maximum flow in time period 1 was about 20,000 m 3 / s, which is low water flow; the maximum flow in time period 2 is about 77100m 3 / s, which is flood flow.

[0095] Table 3 compares the water levels calculated by the mathematical model with the measured values ​​for each water gauge during two measurements during the low-flow period. Table 4 compares the calculated and measured diversion ratios for the left branch of Island S and Island H during the corresponding periods. Tables 5 and 6 present the corresponding data for the flood-flow period.

[0096] Table 3 Comparison between the mathematical model calculation and the measured values ​​of the water levels at each gauge during the two measurements during the dry season

[0097]

[0098] Table 4 Comparison of calculated and measured diversion ratios of the left branch of S Island and the left branch of H Island

[0099]

[0100] The model's calculated water levels at each measurement are generally close to the prototype's measured water levels, with a typical error of ±0.01 to ±0.04 and a maximum error of ±0.04. The diversion ratio between the left branch of S Island and the right branch of H Island deviates from the prototype by 0.31% to 0.79%. During the dry season, the model's water surface contour and diversion ratio are highly similar to those of the prototype.

[0101] Table 5 Comparison between the mathematical model calculation and the measured values ​​of the water levels at each gauge during the two measurements during the flood flow period

[0102]

[0103] Table 6 Comparison of calculated and measured diversion ratios of the left branch of S Island and the left branch of H Island

[0104]

[0105] It can be seen that the model-calculated water levels at each measurement during the flood flow period are basically close to the prototype measured water levels, with most deviations less than 0.1m; the deviation of the diversion ratio between the left branch of S Island and the right branch of H Island and the prototype is between 0.27% and 0.61%, and the model water surface line and diversion ratio are quite similar to the prototype.

[0106] ② Riverbed deformation verification

[0107] The riverbed deformation verification was conducted for 2.5 years, starting from the measured terrain in April 2019 and continuing until December 2021. The upstream boundary flow was generalized using the generalization method described in step 3. The generalization process is as follows: Figure 3 shown.

[0108] Table 7 shows a comparison of the calculated and measured values ​​for scour and silt volume and average scour and silt thickness. The calculated scour and silt locations from April 2019 to December 2021 are generally consistent with the measured scour and silt locations, with the exception of a few areas. The calculated values ​​are also quite close to the measured values, indicating that this model can be used to predict the evolution of riverbed scour and silt after the project.

[0109] Table 7 Comparison of calculated and measured scouring and deposition in river section Z from April 2019 to December 2021

[0110]

[0111] 5. Feasibility calculation and analysis of dredging area

[0112] A verified mathematical model is used to calculate the impact of dredging on river flow in each dredging area, and the feasibility analysis of the dredging area is carried out according to the following steps.

[0113] (1) Calculation of siltation volume;

[0114] Since the present invention is to provide dredged areas and dredged volumes within a river channel, the purpose is to stably obtain sand sources within the river channel for comprehensive utilization, but the premise is that it does not affect the stability of the river flow. Therefore, it is required that the cumulative siltation rate of the dredged area should be greater than 50% within the next two years after dredging, indicating that the river flow will automatically adjust in a positive manner after dredging. In addition, the impact of dredging projects on the diversion ratio is relatively small, and the adjustment of dredging parameters based on the diversion ratio has a small effect on the siltation rate. Therefore, the present invention uses the siltation rate as the first evaluation indicator for screening dredged areas, and simulates the siltation volume in the dredged area one and two years after the implementation of the proposed dredging parameter simulation scheme based on Table 1.

[0115] Tables 8 and 9 give the statistics of the amount of sediment returned in the dredged area one year and two years after the implementation of the proposed dredging plan, respectively.

[0116] Table 8 Calculation of water and sediment conditions in each dredging project area in the first year

[0117]

[0118] Table 9 Calculation of water and sediment conditions in each dredging project area after 2 years

[0119]

[0120] According to the calculation results, only D1, D2, D3, D6, D9 and D11 meet the requirement of siltation rate>50%, and dredging in other areas will have an adverse effect on river flow.

[0121] (2) Calculation of split ratio;

[0122] According to the calculation results of (1), only D1, D2, D3, D6, D9, and D11 are retained as dredging areas for the next step of diversion ratio calculation.

[0123] For bifurcated river sections, maintaining stable river flow requires that the diversion ratio remain constant. This study simulates the diversion ratio for each dredging area before and after dredging, taking the average value of each flow level into account. Dredging schemes with diversion ratio changes exceeding 0.5% are considered to have a significant impact on river flow. In these cases, the impact of dredging on river flow can be mitigated by reducing the dredging range (reducing the area) or increasing the dredging elevation (i.e., reducing the average dredging depth).

[0124] Considering that the influence of dredging elevation adjustment on the siltation rate is greater than that of dredging range adjustment, the present invention gives priority to adjustment based on the dredging range. In addition, the influence of dredging elevation and dredging range on the diversion ratio may be different in different ranges. Therefore, in each round of adjustment, the parameters are adjusted in the order of dredging range first and dredging elevation second. When adjusting the parameters, uniform parameter adjustment is performed on all simulated dredged areas. The ratio of the diversion ratio threshold to the current diversion ratio is used as the factor for step adjustment, which can optimize the dredging parameters more quickly.

[0125] Specifically, assuming that the diversion ratio after dredging changes to a=1%>0.5%, the following adjustment is adopted:

[0126] Option 1: The average dredging thickness remains unchanged, and the dredging range is A=0.5% / a×B=0.5B, where B is the dredging range of the original dredging plan. Option 2: The dredging range remains unchanged, and the average dredging thickness is h=0.5% / a×b=0.5b, where b is the average dredging thickness of the original dredging plan.

[0127] ① First, adjust the dredging scope based on Scheme 1 and calculate the river flow evolution of the branch channel after the dredging scope is adjusted. If a is still greater than 0.5% after the adjustment of Scheme 1, then adjust again using Scheme 2 based on the initial state, that is, use B and 0.5b as the dredging parameters. If a is still greater than 0.5% after the adjustment of Scheme 2, then select the scheme with the better diversion ratio adjustment effect (0.5B+b or B+0.5b) from Schemes 1 and 2 as the current adjustment scheme, that is, the initial parameters for the next round of adjustments;

[0128] ② Repeat step ① until the branch channel diversion ratio changes to meet the requirements (a < 0.5%), and determine the dredging volume of each dredging area based on the dredging range and dredging elevation of each dredging area at this time.

[0129] Table 10 shows the diversion ratios of the left and right branches of S State and H State under low water, medium water and flat beach flow before and after the implementation of the dredging project.

[0130] Table 10 Changes in diversion ratio of branch channels before and after dredging project implementation

[0131]

[0132] It can be seen that after the implementation of the dredging project, the diversion ratio of the right branch of S State changed very little, with a slight increase of only about 0.10%, less than 0.5%. This was mainly due to the excavation of a certain anchorage area in the right branch and a dam field in S State. The corresponding diversion ratio of the left branch decreased slightly. The diversion ratio of the left branch of H State increased slightly by 0.02~0.05%, and the change was less than 0.5%. This was mainly due to the dredging of the submerged dam group in the left branch and the local sedimentation body in the beach return flow area. The corresponding diversion ratio of the right branch decreased slightly. The diversion ratio change decreased after the upstream flow increased.

[0133] It can be seen that the impact of the implementation plan of the maintenance dredging and sand mining project on the diversion ratio of the left and right branches of S State and H State is less than 0.5%, which meets the requirements.

Claims

1. A method for determining the dredging volume of a river section based on the principle of river management, characterized by: include: Obtaining changes in scouring and sedimentation in river sections, screening dredging areas based on siltation thickness and siltation area, and formulating dredging scope and dredging elevation for each dredging area; A two-dimensional water and sediment mathematical model is established for the river section. Combined with the dredging range and dredging elevation, the changes in the diversion ratio of the distributary channels after dredging in multiple dredging areas and the amount of siltation after dredging for N years are simulated to calculate the siltation rate. Obtain the dredging range and dredging elevation that make the diversion ratio change of the branch channel and the back-silting rate of each dredgeable area meet the preset range, and determine the dredging volume of each dredgeable area; when the diversion ratio change or back-silting rate does not meet the requirements, handle it based on the following methods: Eliminate dredgeable areas with unsatisfactory siltation rates and simulate the changes in branch channel diversion ratios after dredging for the remaining dredgeable areas. For the branch channel whose diversion ratio change does not meet the requirements, the dredging area contained in the river section from the upstream of the branch channel to the branch channel diversion or confluence is selected as the adjustment area, and the ratio of the diversion ratio change threshold and the current diversion ratio change is used as the adjustment step size. The dredging range or dredging elevation of the adjustment area is adjusted, and the branch channel diversion ratio change after dredging is re-simulated for the dredged dredgeable area after parameter adjustment, and repeated until the branch channel diversion ratio change meets the requirements; the dredging range or dredging elevation of the adjustment area is adjusted as follows: in each round of parameter adjustment, the dredging range is first selected for adjustment. If the dredging range still does not meet the requirements after adjustment, the dredging elevation is adjusted based on the initial state of this round of parameter adjustment. If the dredging elevation still does not meet the requirements after adjustment, the result close to the diversion ratio threshold is selected from the diversion ratio change results after this round of dredging range adjustment and dredging elevation adjustment as the initial state for the next round of parameter adjustment; During model simulation, a representative runoff generalization is used for the upstream boundary, including: The annual flow process is divided into flood flow process and other processes: the actual flow process simulation is used for flood flow process, and the model acceleration factor is set to 1; For non-flood flow processes, the average value of a time period is taken as the generalized flow value of the corresponding time period, and the acceleration factor is taken as the number of days in the corresponding time period. The number of days included in the time period is determined based on the flow change.

2. The method according to claim 1, characterized in that The method further includes eliminating dredging areas that overlap with the project area and the ecological protection area, and carrying out dredging projects in the remaining dredging areas.

3. The method according to claim 1, characterized in that The screening of dredging areas based on siltation thickness and siltation area includes: obtaining the scouring and silting changes of a river section within a certain period of time, and selecting areas with siltation thickness greater than a first preset value and area greater than a second preset value within the period of time as preliminary dredging areas.

4. The method according to claim 1, wherein When constructing the two-dimensional water-sand mathematical model, the rectangular coordinate system is converted into a body-fitting coordinate system; The two-dimensional water-sediment mathematical model includes a water flow module and a sediment module. The control equation of the water flow module is the incompressible flow Navier-Stokes equation based on the shallow water assumption and the Boussines assumption; the control equation of the sediment module is the bedload transport equation under the combined action of waves and currents.

5. The method according to claim 1 or 4, characterized in that During model simulation, a representative tide generalization is used for the downstream boundary, including: Obtain the main tidal components of the river section; Calculate the residual sediment transport ratio of the tidal cycle for different tidal combinations; The residual sediment transport ratio is accumulated from large to small until the ratio is greater than 90%, and the corresponding tidal type is selected as the representative tide for simulation; when simulating the model, the calculation time is an integer multiple of half a month.

6. The method according to claim 1, characterized in that When simulating the change of diversion ratio of the branch channel, the diversion ratio changes under low water flow, flood flow and average flow are simulated respectively, and whether it meets the preset range is determined based on the maximum diversion ratio change.

7. The method according to claim 1, characterized in that The planned dredging volume that makes the absolute value of the change in the diversion ratio of the dredged area less than 0.5% and the siltation rate greater than 50% is determined as the dredging volume of the dredged area.

8. The method according to claim 1, characterized in that It also includes determining the scope of the bank collapse risk section of the river section to which the dredging area belongs, selecting the area within the bank collapse risk section and with an average scouring depth exceeding a third preset value as the area to be protected, and using the sediment obtained by dredging the dredging area to construct a protective device in the corresponding area to be protected.

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