Calculation method for seabed erosion amount under vertical groundwater discharge in underwater delta

Abstract

The present invention provides a method for calculating the seabed erosion amount under the vertical discharge of groundwater in an underwater delta, including data collection and preliminary analysis, calculating the influence of groundwater discharge on seabed pressure transmission, calculating the critical shear stress corrected by groundwater discharge, calculating the seabed erosion amount caused by submarine groundwater discharge, simulation results and applications, verification methods, and optimization schemes. Through the technical solution of the present invention, the problem that the existing calculation model of seabed erosion amount lacks consideration of the factor of submarine groundwater discharge, resulting in the calculation result may not conform to the actual situation, is solved. By calculating the changes in seabed pressure distribution, the effective stress of seabed soil, correcting the critical shear stress for the occurrence of seabed erosion, and calculating the seabed erosion depth under the influence of groundwater discharge, it has an important role in areas with well-developed groundwater discharge such as underwater deltas.

Description

Technical Field

[0001] The present invention relates to the field of marine geological engineering, and more particularly, to a method for calculating the seabed erosion amount under the vertical discharge of groundwater in an underwater delta. Background Art

[0002] The existing technology mainly focuses on calculating the seabed erosion amount caused by hydrodynamic conditions such as waves, tides, and water currents, ignoring the seabed erosion amount caused by the discharge of groundwater within the seabed soil mass. And quite a lot of research shows that in areas such as large river estuaries and underwater deltas, the submarine groundwater discharge is very developed, with large discharge rates and fluxes, and when the groundwater vertically discharges from the land aquifer into the seawater, it will affect the stress state of the seabed soil mass, promote seabed erosion, and affect the seabed stability. Therefore, a calculation model of seabed erosion amount that lacks consideration of groundwater discharge factors may lead to a large error between the calculation result and the actual situation, and it is impossible to accurately evaluate the seabed erosion amount in the complex hydrodynamic environment of the underwater delta. Summary of the Invention

[0003] In order to make up for the deficiencies of the existing technology, the present invention provides a method for calculating the seabed erosion amount under the vertical discharge of groundwater in an underwater delta. It solves the problem that the existing calculation model of seabed erosion amount lacks consideration of submarine groundwater discharge factors, resulting in the calculation result possibly not conforming to the actual situation. By calculating the change in seabed pressure distribution, the change in effective stress of seabed soil mass, modifying the critical shear stress for seabed erosion occurrence, and calculating the seabed erosion depth under the influence of groundwater discharge, it has an important role in areas with relatively developed groundwater discharge such as underwater deltas.

[0004] The present invention is realized through the following technical solutions: A method for calculating the seabed erosion amount under the vertical discharge of groundwater in an underwater delta, specifically including the following steps:

[0005] Step S1, Data collection and preliminary analysis:

[0006] Step S1-1, Groundwater discharge data collection:

[0007] Collect the groundwater discharge flux , the discharge rate , and the groundwater pressure ;

[0008] Step S1-2, Analysis of seabed sediment characteristics:

[0009] Obtain the median particle size D (cm), porosity , and permeability coefficient of the seabed sediment;

[0010] Step S1-3, Preliminary hydrogeological modeling:

[0011] Calculate the groundwater flow and pressure field, simulate the groundwater flow equation in one-dimensional case, without considering inhomogeneous media and porosity, and express it with hydraulic gradient as:

[0012]

[0013] Where, is the permeability coefficient of seabed sediments, S is the source term of groundwater (natural recharge, human activities, etc., which can be calculated by the groundwater discharge flux Q), t is time, x is the spatial coordinate in the horizontal direction, and h is the hydraulic gradient;

[0014] Step S2. Calculate the influence of groundwater discharge on seabed pressure transmission:

[0015] Step S2-1. Pressure transmission model:

[0016] The pressure change Δ caused by groundwater discharge affects the effective stress of the soil mass:

[0017] Δ

[0018] Where, is the density of groundwater, g is the acceleration of gravity, is the final hydraulic gradient, is the initial hydraulic gradient;

[0019] Step S2-2. The effective stress corrected by submarine groundwater discharge:

[0020] The effective stress of the soil mass considering submarine groundwater discharge Calculation formula:

[0021]

[0022] Where, is the total stress, is the hydrostatic pressure, Δ is the pressure change caused by groundwater discharge;

[0023] Step S3. Calculate the critical shear stress corrected by groundwater discharge:

[0024] Step S3-1. Considering groundwater discharge, the corrected critical Shields parameter

[0025]

[0026] Where, h is the hydraulic gradient, is the critical hydraulic gradient, is the classical Shields parameter;

[0027] Step S3-2: Critical shear stress formula considering groundwater discharge:

[0028] Critical shear stress Calculation formula:

[0029]

[0030] where, is the corrected critical Shields parameter considering groundwater discharge, is the density of sediment particles, is the density of groundwater, g is the acceleration due to gravity, D is the median grain size of seabed sediments, is the effective stress ratio factor, is the groundwater discharge rate;

[0031] Step S4: Calculate the seabed erosion amount caused by submarine groundwater discharge:

[0032] Step S4-1: Calculate the seabed erosion rate:

[0033]

[0034] where, is the erosion depth, is the actual bed shear stress, is the critical shear stress considering the influence of groundwater discharge, and are empirical coefficients, is the effective stress ratio factor.

[0035] Step S4-2: Calculate the seabed erosion depth:

[0036] Substitute the formula for the variation of critical shear stress with time into the erosion depth calculation model and integrate with respect to time to obtain the variation of erosion depth with time:

[0037]

[0038] where, is the initial erosion depth;

[0039] Step S5: Simulation results and applications:

[0040] Step S5-1: Preparation before simulation:

[0041] Step S5-1-1: Set initial conditions:

[0042] Initial groundwater pressure 、Initial hydraulic gradient , Initial groundwater discharge rate , Particle density of seabed sediment , Permeability coefficient of seabed sediment , Porosity of seabed sediment , Median grain size D of seabed sediment, relative density s of seabed sediment and its variation range, time step ;

[0043] Step S5-1-2, Simulation area and grid division:

[0044] According to the research area, grid division is carried out in the horizontal and vertical directions;

[0045] Step S5-2, Simulation process:

[0046] Use the following steps to calculate the seabed erosion depth at different times in the simulation:

[0047] Step S5-2-1, Calculate the change of groundwater pressure with time

[0048] According to the groundwater flow equation and the aforementioned groundwater pressure change formula:

[0049]

[0050] Gradually calculate the groundwater pressure at each time step ; Use the finite difference method for spatial and temporal discretization, and calculate with the following formula:

[0051]

[0052] Among them, is the groundwater pressure at position x at the next time step ; is the groundwater pressure at position x at the current time step ;

[0053] Step S5-2-2, Calculate the effective stress:

[0054] The change of effective stress is due to the change of pressure difference caused by groundwater discharge; The effective stress within each time step can be calculated by the following formula:

[0055]

[0056] Step S5-2-3, Calculate the critical shear stress:

[0057] Calculate the hydraulic gradient at different positions x and different times t respectively according to the above formula , and then calculate the Shields parameter corrected by groundwater discharge , finally calculate the critical shear stress value.

[0058] Step S5-2-4, update of seabed erosion depth:

[0059] The calculation formula for the seabed erosion rate considering groundwater discharge is

[0060]

[0061] The corresponding calculation formulas for the seabed erosion depth at different positions and times are ; Similarly, use the finite difference method to discretize the differential equation for calculating the seabed erosion depth in time, and obtain:

[0062]

[0063] where is the erosion depth at the next time step , is the erosion depth at the current time step , is the time step, is the bed shear stress at the current time step , is the critical shear stress at the current time step ;

[0064] Step S5-2-5, simulation result output and display:

[0065] Through the calculated obtain the seabed erosion depth at different time points;

[0066] Step S6, verification method and optimization plan:

[0067] Step S6-1, verification method:

[0068] Step S6-1-1, data collection:

[0069] Collect groundwater discharge flux , discharge rate , pressure , and seabed erosion depth ; Measure the bed shear stress of the seabed on-site ;

[0070] Step S6-1-2, compare the simulation calculation results with the measured data:

[0071] Take the average of the simulation results in space to obtain

[0072]

[0073] In the formula, X is the total number of spatial grids, and x is the length of each spatial grid. is the seabed erosion depth at different spatial positions and different times.

[0074] The actual monitoring data is , and compare the two; calculate the error:

[0075]

[0076] Or use the root mean square error RMSE to evaluate the error:

[0077]

[0078] where N is the total number of data points;

[0079] Step S6-1-3, Error analysis:

[0080] According to the error calculation, adjust the model parameters to make the simulation results closer to the measured data;

[0081] Step S6-2, Optimization plan:

[0082] Step S6-2-1, Adjust the model parameters:

[0083] According to the verification results, adjust the following key parameters:

[0084] The permeability coefficient of seabed sediment :

[0085] Adjust the permeability coefficient of seabed sediment so that the simulated groundwater discharge rate is more in line with the measured value :

[0086]

[0087] where A is the cross-sectional area of the groundwater discharge area, is the groundwater pressure difference.

[0088] Empirical coefficient and :

[0089] Adjust the empirical coefficient to improve the erosion rate formula:

[0090]

[0091] Step S6-2-2, Improve the simulation accuracy:

[0092] Time step:

[0093] Adjust the time step to improve the accuracy of the simulation:

[0094]

[0095] wherein, is the total number of simulation steps;

[0096] Spatial step:

[0097] Optimize the spatial step to improve the spatial resolution and ensure the accurate simulation of relevant parameters of groundwater discharge.

[0098] As a preferred solution, in step S1-1, the groundwater discharge flux Q is obtained by direct measurement with a piezometer, calculation by constructing a hydrological model, or calculation by isotope tracing method

[0099] As a preferred solution, the calculation formula of h in step S1-3 is , wherein, is the groundwater pressure, is the density of groundwater, g is the acceleration due to gravity, and z is the ground height).

[0100] As a preferred solution, in step S3-2, is the effective stress after correction of submarine groundwater discharge, is the initial effective stress without submarine groundwater discharge, .

[0101] Due to the adoption of the above technical solutions, the present invention has the following beneficial effects compared with the prior art:

[0102] 1. By introducing multiple dynamic parameters such as groundwater discharge rate, flux, and pressure, the present application establishes a more accurate seabed erosion amount calculation model, which can dynamically calculate the changes in seabed pressure, effective stress, and seabed erosion depth caused by vertical groundwater discharge.

[0103] 2. Compared with the current seabed erosion calculation models that mainly consider factors such as waves, tides, and water currents, the method proposed in the present application introduces the factor of submarine groundwater discharge, corrects and improves the existing models, and can improve the accuracy of seabed erosion depth prediction.

[0104] 3. The present application can be applied to different scenarios, not only applicable to areas with large groundwater discharge rate and discharge volume such as underwater deltas, but also able to provide solutions for the calculation of seabed erosion amount and seabed stability analysis under other hydrodynamic environments. By adjusting parameters according to the groundwater discharge characteristics of different regions, it can be applied to different environments.

[0105] 4. The present application provides a result verification and optimization solution. By comparing with in-situ observation data, calculating the calculation error of the model, and further adjusting the calculation result by modifying the model parameters, the accuracy of simulation can be effectively improved.

[0106] In summary, the present application is applicable to the underwater delta area where submarine groundwater discharge develops and can be applied to other scenarios by adjusting parameters. By comparing with in-situ observation data, the calculation result is corrected and the prediction accuracy is improved.

[0107] The additional aspects and advantages of the present invention will become apparent in the following description section or be learned through the practice of the present invention. Brief Description of the Drawings

[0108] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, wherein:

[0109] Figure 1 is a schematic flowchart of the present invention;

[0110] Figure 2 is a graph of the simulation calculation results of seabed erosion (groundwater pressure, corrected effective stress, erosion depth). Detailed Description of the Embodiments

[0111] In order to more clearly understand the above objects, features, and advantages of the present invention, the present invention will be further described in detail below in conjunction with the drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other.

[0112] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0113] The following combines Figures 1 to 2 to specifically describe the calculation method of the seabed erosion amount under the vertical discharge of groundwater in the underwater delta of the embodiments of the present invention.

[0114] As Figure 1 , Figure 2 shows, the present invention proposes a calculation method of the seabed erosion amount under the vertical discharge of groundwater in the underwater delta, which is characterized by specifically including the following steps:

[0115] Step S1, Data collection and preliminary analysis:

[0116] Step S1-1, Groundwater discharge data collection:

[0117] Collect the groundwater discharge flux , discharge rate , groundwater pressure ;

[0118] For example, the groundwater discharge flux Q can be directly measured by a piezometer, calculated by constructing a hydrological model, or calculated by the isotope tracer method.

[0119] Step S1-2, analysis of seabed sediment characteristics:

[0120] Obtain the median grain size D (cm) and porosity of the seabed sediment , coefficient of permeability ;

[0121] Step S1-3, preliminary hydrogeological modeling:

[0122] Calculate the groundwater flow and pressure field, simulate the groundwater flow equation in one dimension, without considering non-uniform media and porosity, and express it with the hydraulic gradient as:

[0123]

[0124] Among them, is the coefficient of permeability of the seabed sediment, S is the source term of groundwater (natural recharge, human activities, etc., which can be calculated by the groundwater discharge flux Q), t is the time, x is the spatial coordinate in the horizontal direction, and h is the hydraulic gradient; the calculation formula of h is , among which, is the groundwater pressure, is the density of groundwater, g is the acceleration of gravity, and z is the ground height).

[0125] Step S2, calculate the influence of groundwater discharge on seabed pressure transmission:

[0126] Step S2-1, pressure transmission model:

[0127] The pressure change Δ caused by groundwater discharge affects the effective stress of the soil mass:

[0128] Δ

[0129] Among them, is the density of groundwater, g is the acceleration of gravity, is the final hydraulic gradient, is the initial hydraulic gradient;

[0130] Step S2-2, effective stress corrected by submarine groundwater discharge:

[0131] The effective stress of the soil mass considering submarine groundwater discharge Calculation formula:

[0132]

[0133] Among them, is the total stress, is the hydrostatic pressure, Δ is the pressure change caused by groundwater discharge;

[0134] Step S3: Calculate the critical shear stress corrected for groundwater discharge:

[0135] Step S3-1: Considering groundwater discharge, the corrected critical Shields parameter

[0136]

[0137] Among them, h is the hydraulic gradient, is the critical hydraulic gradient, is the classical Shields parameter (obtained through the classical Shields curve or formula);

[0138] Step S3-2: Critical shear stress formula considering groundwater discharge:

[0139] Critical shear stress Calculation formula:

[0140]

[0141] Among them, is the corrected critical Shields parameter considering groundwater discharge, is the density of sediment particles, is the density of groundwater, g is the acceleration due to gravity, D is the median grain size of seabed sediments, is the effective stress ratio factor, is the effective stress corrected for submarine groundwater discharge, is the initial effective stress without submarine groundwater discharge, . is the groundwater discharge rate;

[0142] Step S4: Calculate the seabed erosion amount caused by submarine groundwater discharge:

[0143] Step S4-1: Calculate the seabed erosion rate:

[0144]

[0145] Among them, is the erosion depth, is the actual bed shear stress, is the critical shear stress considering the influence of groundwater discharge, and is an empirical coefficient, and is the effective stress ratio factor.

[0146] Step S4-2: Calculate the seabed erosion depth:

[0147] Substitute the formula for the critical shear stress varying with time into the erosion depth calculation model and integrate with respect to time to obtain the variation of the erosion depth with time:

[0148]

[0149] where, is the initial erosion depth;

[0150] Step S5: Simulation results and applications:

[0151] Step S5-1: Preparation before simulation:

[0152] Step S5-1-1: Before simulation, the following preparatory work needs to be carried out:

[0153] Initial condition setting: Initial groundwater pressure , initial hydraulic gradient , initial groundwater discharge rate , particle density of seabed sediment , permeability coefficient of seabed sediment , porosity of seabed sediment , median grain size D of seabed sediment, relative density s of seabed sediment and its variation range, time step : To conduct dynamic simulation, set an appropriate time step , such as one day or one week, so as to update the pressure and erosion amount in each step;

[0154] Step S5-1-2: Simulation area and grid division:

[0155] According to the research area (such as the Yellow River Estuary), conduct grid division in the horizontal and vertical directions; the size of the grid cells is determined according to the seabed characteristics and the complexity of groundwater flow.

[0156] Step S5-2: Simulation process:

[0157] Use the following steps to calculate the seabed erosion depth at different times in the simulation:

[0158] Step S5-2-1: Calculate the variation of groundwater pressure with time

[0159] According to the groundwater flow equation and the aforementioned formula for the variation of groundwater pressure:

[0160]

[0161] Calculate the groundwater pressure at each time step step by step; use the finite difference method (FDM) for spatial and temporal discretization, and calculate it with the following formula: The groundwater pressure at the next time step

[0162]

[0163] where, is the groundwater pressure at position x at the next time step ; is the groundwater pressure at position x at the current time step ;

[0164] Step S5-2-2, calculate the effective stress:

[0165] The change in effective stress is due to the change in pressure difference caused by groundwater discharge; the effective stress within each time step can be calculated by the following formula:

[0166]

[0167] Step S5-2-3, calculate the critical shear stress:

[0168] Calculate the hydraulic gradient at different positions x and different times t according to the above formula , and then calculate the Shields parameter corrected by groundwater discharge, and finally calculate the value of the critical shear stress .

[0169] Step S5-2-4, update the seabed erosion depth:

[0170] The formula for calculating the seabed erosion rate considering groundwater discharge is

[0171]

[0172] The formula for the seabed erosion depth at different positions and different times is ; also use the finite difference method (FDM) to discretize the differential equation for calculating the seabed erosion depth in time, and obtain:

[0173]

[0174] where, is the erosion depth at the next time step , is the erosion depth at the current time step , is the time step, is the bed shear stress at the current time step ; is the critical shear stress at the current time step ;

[0175] Step S5-2-5, simulation result output and display:

[0176] The seabed erosion depth at different time points is obtained through calculation ;

[0177] Step S6, verification method and optimization plan:

[0178] Step S6-1, verification method:

[0179] Step S6-1-1, data collection:

[0180] Collect the groundwater discharge flux in the Yellow River Estuary area , discharge rate , pressure , and seabed erosion depth ; Measure the bed shear stress of the seabed on-site ;

[0181] Step S6-1-2, compare the simulation calculation results with the measured data:

[0182] The average value of the simulation results in space is obtained

[0183]

[0184] where X is the total number of spatial grids, x is the length of each spatial grid, is the seabed erosion depth at different spatial positions and different times

[0185] The actual monitoring data is , compare the two; Calculate the error:

[0186]

[0187] Or use the root mean square error RMSE to evaluate the error:

[0188]

[0189] where N is the total number of data points;

[0190] Step S6-1-3, error analysis:

[0191] According to the error calculation, adjust the model parameters to make the simulation results closer to the measured data;

[0192] Step S6-2, Optimization Plan:

[0193] Step S6-2-1, Adjust Model Parameters:

[0194] According to the verification results, adjust the following key parameters:

[0195] The permeability coefficient of seabed sediments :

[0196] Adjust the permeability coefficient of seabed sediments so that the simulated groundwater discharge rate is more consistent with the measured value :

[0197]

[0198] where A is the cross-sectional area of the groundwater discharge area, is the groundwater pressure difference.

[0199] Empirical coefficients and :

[0200] Adjust the empirical coefficients to improve the erosion rate formula:

[0201]

[0202] Step S6-2-2, Improve Simulation Accuracy:

[0203] Time step:

[0204] Adjust the time step to improve the simulation accuracy:

[0205]

[0206] where, is the total number of simulation steps;

[0207] Spatial step:

[0208] Optimize the spatial step to improve the spatial resolution and ensure the accurate simulation of relevant parameters of groundwater discharge;

[0209] Step S6-2-3, Cross-regional Application

[0210] Apply the optimized model to other underwater delta regions (such as the Pearl River Estuary or the Yangtze River Estuary), and adjust other regional parameters such as the local groundwater discharge flux Q and discharge rate v to verify the universality of the model. Example

[0211] Suppose the calculation process of the impact of submarine groundwater vertical discharge proposed in this application is applied to the Yellow River subaqueous delta environment for simulation to evaluate the impact of submarine groundwater discharge on the seabed.

[0212] To conduct the simulation, it is necessary to set the seabed sediment properties and submarine groundwater discharge related parameters in the Yellow River subaqueous delta. The relevant parameters are taken from the in-situ observation data and relevant literature of the Yellow River subaqueous delta. The simulation time is 10 days, set as normal periods (1 - 2 days, 7 - 10 days) and storm periods (3 - 6 days), and the corresponding parameters are modified. Specifically as follows:

[0213] Fixed parameters:

[0214] 1. Groundwater discharge parameters

[0215] Groundwater density 、 (During storms).

[0216] Hydraulic gradient 。

[0217] Critical hydraulic gradient 。

[0218] Seabed sediment characteristics

[0219] Sediment particle density 。

[0220] Porosity 、 (During storms).

[0221] Initial stress conditions

[0222] Total stress 。

[0223] Hydrostatic pressure 。

[0224] Initial effective stress 。

[0225] Erosion model parameters

[0226] Classical Shields parameter = 0.03.

[0227] Empirical coefficient 。

[0228] Bed shear stress 、 (During storms).

[0229] Time step and simulation duration

[0230] Time step day

[0231] Simulation duration day

[0232] Assignment of spatial coordinate x

[0233] Assume the length of the simulation area is L = 100m, and the grid is divided into 。

[0234] Assume the simulation area is uniformly distributed in space, and the spatial distribution does not need to be considered.

[0235] Data collection:

[0236] 1. Groundwater discharge data collection

[0237] Groundwater discharge rate 、 (During the storm)

[0238] The groundwater discharge flux Q is calculated according to the discharge rate of unit volume of groundwater.

[0239] Groundwater pressure (Uniform distribution)

[0240] During normal periods, the groundwater pressure increases by ;

[0241] During the storm period, the groundwater pressure increases by 。

[0242] Analysis of seabed sediment characteristics

[0243] Median grain size of seabed sediment 。

[0244] Permeability coefficient 、 (During the storm).

[0245] Initial erosion depth

[0246] Initial erosion depth 。

[0247] According to the calculation steps in this article, calculate the groundwater pressure, effective stress, critical shear stress and erosion depth day by day. The following is the calculation process (taking the first day as an example):

[0248] Step1 Update groundwater pressure

[0249]

[0250] Step2 Calculate the change in effective stress caused by submarine groundwater discharge

[0251]

[0252] Step3 Calculate the critical shear stress considering submarine groundwater discharge

[0253] Pa

[0254]

[0255] Step4 Calculate the erosion rate and erosion depth under the influence of submarine groundwater discharge

[0256]

[0257]

[0258] The above is the overall calculation process of the analysis method for seabed erosion caused by vertical discharge of submarine groundwater proposed in this application. The flowchart is shown in Figure 1 , taking the simulation calculation of the seabed erosion volume on the first day as an example. Substitute the updated groundwater pressure, effective stress, etc. into the calculation of the seabed erosion volume from the second day to the tenth day, and the ten-day dynamic simulation results are shown in the following table

[0259] Table 1 Simulation operation results

[0260]

[0261] Figure 2 is the time series diagram of the seabed erosion depth under the influence of submarine groundwater discharge during calm sea conditions and storm periods obtained through simulation calculation. It can be intuitively seen that the seabed erosion volume caused by submarine groundwater discharge during calm sea conditions is small, while during storm periods, it promotes groundwater discharge, increases groundwater pressure, further reduces the effective stress, and exacerbates seabed erosion

[0262] Step5 Result verification and optimization plan

[0263] Result verification:

[0264] Measure relevant parameters such as seabed sediments and submarine groundwater discharge in the simulation calculation area through in-situ observation and other methods, such as groundwater discharge flux and groundwater discharge rate etc. By comparing the simulation results with the measured data, calculate the simulation error

[0265]

[0266] Optimization plan:

[0267] Adjust the permeability coefficient , the classical Shields empirical coefficient and empirical coefficients 、 to optimize the simulation results.

[0268] By adjusting the time step and spatial grid division to improve the simulation accuracy.

[0269] In the description of the present invention, the term "a plurality" means two or more, unless otherwise clearly defined. The orientation or positional relationship indicated by terms such as "upper", "lower", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention; terms such as "connection", "installation", "fixation", etc. should all be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0270] In the description of this specification, the description of terms such as "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or instance. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0271] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A calculation method for the seabed erosion amount under the vertical discharge of groundwater in an underwater delta, characterized in that , specifically including the following steps: Step S1, Data collection and preliminary analysis: Step S1-1, Groundwater discharge data collection: Collect the groundwater discharge flux , the discharge rate , the groundwater pressure ; Step S1-2, Seabed sediment property analysis: Obtain the median grain size D (cm) and porosity of seabed sediments , permeability coefficient ; Step S1-3, Preliminary hydrogeological modeling: Calculate the groundwater flow and pressure field, simulate the groundwater flow equation in one-dimensional case, without considering non-uniform media and porosity, and express it with hydraulic gradient as: wherein, is the permeability coefficient of seabed sediment, S is the source term of groundwater (natural recharge, human activities, etc., which can be calculated by the groundwater discharge flux Q), t is the time, x is the spatial coordinate in the horizontal direction, and h is the hydraulic gradient; Step S2, Calculate the influence of groundwater discharge on seabed pressure transmission: Step S2-1, Pressure transmission model: The pressure change Δ caused by groundwater discharge affects the effective stress of the soil mass: Δ wherein, is the density of groundwater, g is the acceleration of gravity, is the final hydraulic gradient, is the initial hydraulic gradient; Step S2-2, Effective stress corrected by submarine groundwater discharge: Effective stress of soil considering submarine groundwater discharge Calculation formula: Among them, is the total stress, is the hydrostatic pressure, and Δ is the pressure change caused by groundwater discharge; Step S3, Calculate the critical shear stress corrected by groundwater discharge: Step S3-1, Corrected critical Shields parameter considering groundwater discharge where h is the hydraulic gradient, is the critical hydraulic gradient, is the classical Shields parameter; Step S3-2, Critical shear stress formula considering groundwater discharge: Critical shear stress Calculation formula: Among them, is the corrected critical Shields parameter considering groundwater discharge, is the density of sediment particles, is the density of groundwater, g is the acceleration of gravity, D is the median grain size of seabed sediment, is the effective stress ratio factor, is the groundwater discharge rate; Step S4, Calculate the seabed erosion amount caused by submarine groundwater discharge: Step S4-1, Calculate the seabed erosion rate: Among them, is the erosion depth, is the actual bed shear stress, is the critical shear stress considering the influence of groundwater discharge, and are empirical coefficients, is the effective stress ratio factor; Step S4-2, Calculate the seabed erosion depth: Substituting the formula for the critical shear stress varying with time into the erosion depth calculation model and integrating with respect to time, the erosion depth variation with time can be obtained: Among them, is the initial erosion depth; Step S5, Simulation results and applications: Step S5-1, Preparation before simulation: Step S5-1-1, Initial condition setting: Initial groundwater pressure 、Initial hydraulic gradient 、Initial groundwater discharge rate 、Particle density of seabed sediment 、Permeability coefficient of seabed sediment 、Porosity of seabed sediment 、Median grain size D of seabed sediment, relative density s of seabed sediment and its variation range, time step ; Step S5-1-2, Simulation area and grid division: According to the research area, conduct grid division in the horizontal and vertical directions; Step S5-2, Simulation process: Use the following steps to calculate the seabed erosion depth at different times in the simulation: Step S5-2-1, Calculate the change of groundwater pressure with time According to the groundwater flow equation and the aforementioned groundwater pressure change formula: Calculate the groundwater pressure step by step for each time step The spatial and temporal discretization is carried out using the finite difference method and calculated by the following formula: wherein, is the groundwater pressure at position x at the next time step moment; is the groundwater pressure at position x at the current time step time. Step S5-2-2, Calculate the effective stress: The change of effective stress is due to the change of pressure difference caused by groundwater discharge; the effective stress within each time step can be calculated by the following formula: Step S5-2-3, Calculate the critical shear stress: Calculate the hydraulic gradient at different positions \(x\) and different times \(t\) according to the above formula , and then calculate the Shields parameter after groundwater discharge correction , and finally calculate the value of the critical shear stress ; Step S5-2-4, Update of seabed erosion depth: The calculation formula of seabed erosion rate considering groundwater discharge is The calculation formula corresponding to the seabed erosion depth at different positions and different times is ; Similarly, the finite difference method is used to discretize the differential equation for calculating the seabed erosion depth in time, and we get: wherein, is the erosion depth at the next time step, is the erosion depth at the current time step, is the time step, is the bed shear stress at the current time step, is the critical shear stress at the current time step; Step S5-2-5, Output and display of simulation results: Obtained by calculation Obtain the seabed erosion depth at different time points; Step S6, Verification method and optimization scheme: Step S6-1, Verification method: Step S6-1-1, Data collection: Collect groundwater discharge , discharge rate , pressure , and seabed erosion depth ; On-site measurement of the bed shear stress of the seabed ; Step S6-1-2, Compare the simulation calculation results with the measured data: The average of the simulation results is obtained spatially where X is the total number of spatial grids, and x is the length of each spatial grid, is the seabed erosion depth at different spatial positions and different times; the actual monitoring data is , compare the two; calculate the error: Or use the root mean square error RMSE to evaluate the error: where, N is the total number of data points; Step S6-1-3, Error analysis: According to the error calculation, adjust the model parameters to make the simulation results closer to the measured data; Step S6-2, Optimization scheme: Step S6-2-1, Adjust model parameters: According to the verification results, adjust the following key parameters: Permeability Coefficient of Seabed Sediments :[[-END]] Adjust the permeability coefficient of seabed sediments to make the simulated groundwater discharge rate more consistent with the measured value : where A is the cross-sectional area of the groundwater discharge area, is the groundwater pressure difference; Empirical coefficient and : Adjust the empirical coefficient to improve the erosion rate formula: Step S6-2-2, Improve simulation accuracy: Time step: Adjust the time step to improve the accuracy of the simulation: Among them, is the total number of simulated steps; Spatial step: Optimize the spatial step To improve the spatial resolution and ensure the accurate simulation of relevant parameters for groundwater discharge.

2. The calculation method of seabed erosion amount under the vertical discharge of groundwater in the underwater delta according to claim 1, characterized in that , in step S1-1, the groundwater discharge flux Q is obtained by direct measurement with a piezometer, calculation by constructing a hydrogeological model, or calculation by isotope tracer method.

3. The calculation method of seabed erosion amount under the vertical discharge of groundwater in the underwater delta according to claim 1, characterized in that , in the steps S1-3, the calculation formula of h is , where is the groundwater pressure, is the density of groundwater, g is the acceleration of gravity, and z is the ground height.

4. The calculation method of seabed erosion amount under the vertical discharge of groundwater in the underwater delta according to claim 1, characterized in that , the effective stress corrected for submarine groundwater discharge in step S3-2, and the initial effective stress without submarine groundwater discharge, .

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