Method for ecological restoration of large shallow lakes

Abstract

This invention discloses a method for ecological restoration of large shallow lakes, including establishing a hydrodynamic model to simulate the lake flow field, including water depth, flow velocity, and flow direction; dividing ecological restoration areas using enclosures, including: analyzing the forces between the water body and the enclosures, calculating the maximum flow velocity that the enclosures can withstand when their resistance to water flow is greater than or equal to the impact force of the water flow; determining the location of the ecological restoration areas based on the maximum flow velocity and the simulation results of the flow field; determining the area and shape of the ecological restoration areas divided by the enclosures based on the vegetation coverage and water system connectivity required for lake water environment restoration; and finally evaluating the rationality of the ecological restoration area division based on the hydrodynamic model. This method facilitates the phased, unit-based, and standardized implementation of ecological restoration of large shallow lakes, ensuring the safety and stability of ecological zoning engineering measures, and improving the feasibility of implementing ecological restoration of large shallow lakes.

Description

Technical Field

[0001] This invention belongs to the field of ecological restoration technology, specifically relating to a method for ecological restoration of large shallow lakes. Background Technology

[0002] Shallow lakes are characterized by a depth of less than 6 meters, the absence of a thermocline, and intense exchange of substances between the lake water and sediment. Aquatic plants play a vital role in shallow lake ecosystems, and their degradation is a key cause of cyanobacterial blooms and water quality deterioration. Therefore, restoring submerged plants is a crucial step in the reconstruction and restoration of lake ecosystems. Restoring a clear-water herbaceous ecosystem has become the mainstream approach for water management in shallow lakes. This involves constructing submerged plant communities at the lake bottom, while simultaneously introducing a certain amount of plankton, fish, microorganisms, and other organisms according to the requirements of the project area, forming an underwater ecosystem centered on submerged plants.

[0003] Currently, large shallow lakes suffer from severe eutrophication, low transparency, and significant underwater desertification, making them unsuitable for the restoration of submerged plants. Furthermore, these lakes also serve flood control and drainage functions, resulting in frequent water exchange during the flood season, making comprehensive ecological restoration of the entire lake area difficult. While setting up zonal enclosures to block the exchange of water and organisms between the ecological restoration area and the main lake area can ensure the effectiveness of the restoration, in practical engineering applications, the lack of scientifically sound calculation methods for enclosure design often leads to the enclosures failing to meet design requirements in terms of both barrier effectiveness and lifespan. This makes it difficult to guarantee the long-term stability of the ecological restoration area and significantly increases the cost of ecosystem maintenance. The lifespan of zonal enclosures is influenced by several factors, primarily the durability of the enclosure materials, the rationality of the enclosure structure design, the water volume and flow field changes inside and outside the enclosure, and enclosure maintenance. While the enclosure materials and structure are fixed after installation, the water volume and flow field changes inside and outside the enclosure are often affected by factors such as wind, water flow, and flood control and drainage. Therefore, scientifically designing the size, dimensions, and layout of the enclosures is crucial for maximizing their effectiveness. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects of the existing technology and provide a method for ecological restoration of large shallow lakes.

[0005] To achieve the above technical objectives, the present invention adopts the following solution:

[0006] A method for ecological restoration of large shallow lakes includes:

[0007] Acquire hydrological, topographic, and meteorological data for the study area, establish a hydrodynamic model, and simulate the lake flow field, including water depth, flow velocity, and flow direction;

[0008] The ecological restoration area is delineated using enclosures, including: analyzing the forces between the water body and the enclosures, calculating the maximum flow velocity that the enclosures can withstand when their resistance to water flow is greater than or equal to the impact force of the water flow; and determining the location of the ecological restoration area based on the maximum flow velocity and the simulation results of the flow field.

[0009] The area and shape of the ecological restoration area are determined based on the vegetation coverage and water system connectivity required for lake water environment restoration.

[0010] The location and geometry of the ecological restoration area determined by the enclosure are generalized into the hydrodynamic model. The lake flow field after the enclosure is added is analyzed to evaluate the rationality of the ecological restoration area division.

[0011] As a preferred implementation method, the lake flow field is simulated based on a two-dimensional hydrodynamic model.

[0012] As a preferred embodiment, the establishment of a hydrodynamic model to simulate the lake flow field includes:

[0013] After establishing the model working area and importing the land boundary of the study area, the computational domain is defined.

[0014] Import terrain data into the model for spatial interpolation, set the grid type and side length, divide the grid, and generate a terrain file;

[0015] Import the terrain file into the hydrodynamic module, set the boundary conditions and initial parameters, simulate and generate two-dimensional flow field results, and extract lake flow field information.

[0016] As a preferred implementation, the boundary conditions of the hydrodynamic model include flow boundary, rainfall, and wind field; the selection of boundary conditions is based on historical monitoring data.

[0017] In a preferred embodiment, the method further includes first determining a pre-selected site area based on the simulated water depth and the recommended planting water depth for the plants to be planted, and then selecting the location of the ecological restoration area within the pre-selected site area.

[0018] As a preferred implementation method, based on the recommended planting water depth for the plants to be planted, an area with a water depth less than the upper limit of the recommended planting water depth is selected as the pre-selected site area.

[0019] The enclosure structure is as follows: the upper part of the partition is fixed with submerged piles, and the bottom is fixed to the bottom of the water with weights.

[0020] The resistance of the enclosure to water flow is calculated based on the following formula:

[0021] F 抗 =m1g+T+f 浮 +f 摩

[0022] f 摩 =um2g

[0023] In the formula, m1 is the mass of the enclosure; g is the acceleration due to gravity; T is the tension of the weight at the bottom; f 浮 f is the buoyancy force experienced by the enclosure and the weight in the water; 摩 denoted as ρ, where ρ is the frictional force between the weight and the lakebed; u is the coefficient of friction between the weight and the lakebed; and m2 is the mass of the weight at the bottom.

[0024] As a preferred embodiment, the water flow impact force is calculated based on the following formula:

[0025] F 冲 =ρAv 2 cosθ

[0026] In the formula, ρ is the density of the water; A is the area of ​​the enclosure; v is the velocity of the water flow; and θ is the impact angle, which is the angle between the water flow and the normal to the surface of the object.

[0027] As a preferred embodiment, determining the location of the ecological restoration area based on the simulation results of the maximum water flow velocity and the flow field includes:

[0028] The area where the simulated flow velocity is less than the maximum water flow velocity is selected as the site location, and the direction of the enclosure design and installation is parallel to the simulated water flow direction.

[0029] As a preferred embodiment, determining the area and shape of the ecological restoration area based on the required vegetation coverage and water system connectivity for lake water environment restoration includes:

[0030] The area of ​​the ecological restoration zone is determined based on the lake area and vegetation coverage;

[0031] The scope of zoning and enclosure is constrained by the principle of minimizing the selection of water flow channels.

[0032] This invention addresses the current lack of ecological restoration zoning methods that comprehensively consider factors such as underwater topography, flow field, and aquatic environment restoration goals for large, shallow lakes. Based on two-dimensional hydrodynamic model simulations, it obtains water depth, flow velocity, and flow direction, and determines the lake's ecological restoration zoning method in conjunction with aquatic environment restoration goals. Through this invention, by comprehensively considering factors such as topography, wind field, flow boundaries, and aquatic environment goals, a more comprehensive and detailed understanding of the hydrology, topography, and aquatic environment of lakes can be achieved. This allows for the rational selection of ecological restoration areas and the implementation of zoning engineering measures such as enclosure. This facilitates the phased, unit-based, and standardized implementation of ecological restoration for large, shallow lakes, ensuring the safety and stability of ecological zoning engineering measures and improving the feasibility of ecological restoration implementation for large, shallow lakes. Attached Figure Description

[0033] Figure 1 It is a wind rose diagram.

[0034] Figure 2 It is a characteristic of lake flow field.

[0035] Figure 3 It is a zoned and enclosed pre-selected site area.

[0036] Figure 4 This is a map showing the flow path of the lake.

[0037] Figure 5 It is a design scheme for zoned and enclosed site selection.

[0038] Figure 6 This is the flow field diagram after the partition enclosure is installed. Detailed Implementation

[0039] The purpose of this invention is to comprehensively consider the flow field characteristics and ecological restoration requirements of large lakes, select the preferred zoning areas based on the flow field simulated by a two-dimensional hydrodynamic model, and construct an analysis method for suitable aquatic ecological restoration zones in combination with aquatic restoration needs.

[0040] The embodiment uses a lake as an example to provide a design method for zoned enclosure and restoration, which specifically includes the following steps:

[0041] (1) Lake flow field analysis

[0042] Many factors influence the flow velocity of large lakes, including the lake's shape, size, water level, as well as wind, rainfall, and inflow. Constructing hydrodynamic models helps us better understand the characteristics of the lake flow field, thereby better predicting the flow velocity and direction. This is crucial for assessing the lifespan of zonal enclosures and the stability of the aquatic environment within restoration zones. Large, shallow lakes typically have a depth of no more than 6 meters. Under the influence of wind, waves, and currents, the water mixes sufficiently, allowing for simplified treatment without considering water stratification. Two-dimensional simulations can meet the requirements for accuracy and computational efficiency.

[0043] This embodiment uses the MIKE 21 hydrodynamic module (HD) model, which is based on the Navier-Stoker equations of triaxial incompressibility and uniform Reynodls value distribution, and obeys the Boussinesq assumption and the assumption of hydrostatic pressure. The main modeling process of the two-dimensional hydrodynamic model is as follows: importing the land boundary and underwater topography, mesh generation, defining boundary conditions, setting calculation parameters (simulation time, time step, Manning coefficient, etc.), and result analysis.

[0044] 1) Importing underwater topography and shore-land boundaries and mesh generation

[0045] First, establish the model working area, import the land boundary of the study area, then define the computational domain, import the elevation data into the model for spatial interpolation, then set the grid type and side length according to the requirements of accuracy and computational efficiency, perform grid division, and then generate the terrain file.

[0046] 2) Boundary conditions

[0047] The model primarily considers flow boundary, rainfall, and wind field as boundary conditions. It takes into account factors such as the duration of enclosure use and construction costs, as well as the extreme values ​​of flow, rainfall, and wind field. Flow boundary data for each gate and outlet can be obtained from cross-sectional monitoring or historical data; rainfall data uses data from the rainy season; the dominant wind field is determined by the average wind speed and direction at a 2-meter height above ground level at the local meteorological station. Figure 1 This shows the wind field data used in the model.

[0048] 3) Parameter settings

[0049] The purpose of this case simulation is to calculate the changes in the flow field of a lake, extracting elements such as water depth, flow velocity, and flow direction. Therefore, empirical values ​​are used for calculation parameters that have no significant impact on the results, including eddy viscosity coefficient and drag coefficient.

[0050] 4) Result Analysis: Based on the simulation results, a two-dimensional flow field diagram is generated, such as... Figure 2 .

[0051] (2) Based on the lake flow field, the lake ecological restoration area is divided by enclosure;

[0052] The location for fencing deployment needs to be determined, including:

[0053] A. Determine the pre-selected site area based on underwater topographic information and the recommended planting water depth of the plants to be planted;

[0054] In this case, the main purpose of constructing zoned enclosures is to maintain the stability of the aquatic environment within the enclosure area and promote the restoration of aquatic vegetation. According to the "Technical Guidelines for the Protection and Restoration of River and Lake Ecological Buffer Zones," the recommended planting depth for submerged plants is 0.5m to 2.5m. Therefore, in this invention, a water depth of 0-2.5m is selected as the pre-selected site for the zoned enclosures. This is combined with the water depth results from numerical simulations, such as... Figure 3 As shown, the light blue area represents the pre-selected site area within the partitioned area.

[0055] B. Determine the site selection based on the simulation results of the maximum water flow velocity and the flow field;

[0056] The maximum flow velocity refers to the maximum flow velocity that the enclosure can withstand when its resistance to water flow exceeds the impact force of the water flow. It is determined by analyzing the interaction forces between the water body and the enclosure.

[0057] The forces acting on a partitioned enclosure in a body of water include the following:

[0058] 1) Water pressure: The vertical force exerted by water on the surface of the enclosure, which is proportional to the water depth;

[0059] 2) Flow resistance: The tangential force exerted by the water flow on the enclosure surface, which is proportional to the flow velocity;

[0060] 3) Buoyancy: When the enclosure is submerged in water, the pressure difference between its bottom and top generates an upward force;

[0061] 4) Gravity: The downward force generated by the mass of the enclosure itself.

[0062] 5) Friction: The force generated by the relative motion between the weight at the bottom of the enclosure and the bottom mud.

[0063] In this invention, water pressure is related to water depth. Since the water level changes are not significant on either side of the partitioned enclosure, the water pressure on both sides can be considered equal and cancel each other out; therefore, the influence of hydrostatic pressure is not considered. Buoyancy and gravity are related to the chosen material and are inherent physical properties of the object. Flow resistance is related to the enclosure's size, shape, and location, and is a key factor affecting the operational stability of the partitioned enclosure. In engineering applications, the stability of the enclosure can be improved by reducing the tangential force of the water flow on its surface. In this invention, stability is assessed by the resultant force of the water flow's impact on the enclosure and the enclosure's resistance to the water flow.

[0064] The magnitude of the force exerted by water flow on an object depends on the impact area and the impact angle. The impact force is greatest when the impact angle is perpendicular to the horizontal surface of the enclosure. The formula for the water flow impact force is as follows:

[0065] F 冲 =ρAv 2 cosθ (1)

[0066] Where ρ: water density (kg / m³) 3 A: Enclosure area (m²) 2 v: water flow velocity (m / s); θ: impact angle, the angle between the water flow and the normal to the surface of the object (°).

[0067] Resistance is caused by the pressure and friction of the fluid on the surface of an object, and the direction of the force is opposite to the direction of the water flow. When the impact force of the water flow is greater than the resistance of the enclosure, the enclosure structure deforms to counteract this force. Currently, most partitioned enclosures are installed by using submerged piles to fix the upper part of the enclosure and binding weights to the bottom for fixation. This design has the problem that when the impact force of the water flow is greater than the resistance of the partitioned enclosure, the water flow will cause the enclosure to shift, leading to structural instability. In this case, we assume that the enclosure does not deform and that it is perpendicular to the lakebed after installation, with no tilt angle. According to Newton's third law, the formula for calculating the resistance of the enclosure to the water flow is:

[0068] F 抗 =m1g+T+f 浮 +f 摩 (2)

[0069] f 摩 =um2g (3)

[0070] In the formula, m1: enclosing mass (kg); g: gravitational acceleration (m / s²). 2 T: Tension of the weight at the bottom (N); f 浮 : The buoyant force on the enclosure and the weight in the water; f 摩 : Frictional force between the weight and the lakebed; u: Coefficient of friction between the weight and the lakebed, with a value of 0.4; m2: Mass of the weight at the bottom (kg).

[0071] In this invention, the enclosure structure is designed according to the specifications of the patent "A Zoning Device for Ecological Restoration of Shallow Lakes" (Patent No.: ZL 202222375881.8). According to the above formula, if the structure of the zoning enclosure needs to remain unchanged, the enclosure's resistance must be greater than the water flow impact force. In this embodiment, for ease of calculation, it is assumed that the water flow velocity and water depth are uniform, and the resultant force of the enclosure's own weight and buoyancy is ignored. Based on the above formula, the maximum vertical water flow velocity that the enclosure can withstand is 0.1283 m / s.

[0072] F 冲 =1000*2.5*5*v 2 *cosθ<15*9.8+0.4*15*9.8, v=0.1283 (θ is 90°)

[0073] The location of the ecological restoration zone is determined based on the simulation results of the maximum water flow velocity and flow field, such as... Figure 2 As shown, it can be seen that:

[0074] ① The green, yellow, and red areas in the figure represent areas where the flow velocity exceeds the maximum flow velocity that the enclosure can withstand (v = 0.12 m / s). These areas should be avoided when considering enclosure site selection.

[0075] ②At the same time, the direction of the enclosure design and installation should be as parallel as possible to the direction of water flow to reduce the vertical component of the water flow on the enclosure.

[0076] (3) Determining the scale of the ecological restoration area

[0077] The shape and area of ​​ecological restoration zones are not fixed standards and need to be comprehensively considered based on the specific lake management situation. Generally speaking, the larger the area of ​​an ecological restoration zone, the greater the pollution it can withstand and the more complex the ecosystem. In this invention, the area and shape of ecological zones are determined by considering the requirements of water ecological restoration goals. Specifically, the scope of the restoration zone is further constrained by vegetation coverage and water system connectivity indicators, as follows:

[0078] The first step is to calculate the required vegetation cover (n) based on the lake's water environment conditions, and then determine the total area of ​​the restoration zone. Studies have shown that when the submerged plant cover is greater than 30%, submerged plants can suppress phytoplankton ecology, allowing the water body to maintain a clear state for a long period.

[0079] The calculation formula is as follows:

[0080] A = a * n (4)

[0081] Where: A: area of ​​ecological restoration zone; a: lake area; n: vegetation coverage.

[0082] The second step is to analyze the current water flow path of the lake, such as... Figure 4 As shown in the diagram, the lake in this case has a relatively obvious water flow channel. The water flow path can be seen in the diagram. The water mainly flows from the West Lake to the East Lake, connected by two intermediate channels. In order to maintain the original connectivity of the water body, the area selected for the enclosure installation should be minimized as much as possible to ensure the continuity of the water system when the lake is used for flood storage and drainage.

[0083] (4) Assess the rationality of the ecological restoration area division: generalize the location and geometry of the ecological restoration area determined by the enclosure into the hydrodynamic model, analyze the lake flow field after the enclosure is added, and assess the rationality of the ecological restoration area division.

[0084] Specifically, based on the results of the above three steps, and taking into account both the simulation results of the lake's water flow field and the needs of water environment restoration goals, a preliminary design scheme for zoned enclosure and installation has been drafted, such as... Figure 5 As shown. Further evaluation of the design rationality and potential risks using a hydrodynamic model can ultimately determine the size, shape, and installation location of the partitioned enclosure. This is based on the flow field simulation results ( Figure 6 It can be seen that after the partition enclosure is installed, under the premise of meeting the ecological restoration goal, the enclosure section is basically parallel to the water flow direction, and at the same time, it has little impact on the original water flow path.

Claims

1. A method for ecological restoration of large shallow lakes, characterized by, include: Acquire hydrological, topographic, and meteorological data for the study area, establish a hydrodynamic model, and simulate the lake flow field, including water depth, flow velocity, and flow direction; The ecological restoration area is delineated using enclosures, including: analyzing the forces between the water body and the enclosures, calculating the maximum flow velocity that the enclosures can withstand when their resistance to water flow is greater than or equal to the impact force of the water flow; and determining the location of the ecological restoration area based on the maximum flow velocity and the simulation results of the flow field. The area and shape of the ecological restoration area are determined based on the vegetation coverage and water system connectivity required for lake water environment restoration. The location and geometry of the ecological restoration area determined by the enclosure are generalized into the hydrodynamic model. The lake flow field after the enclosure is added is analyzed to evaluate the rationality of the ecological restoration area division. The upper part of the partition is fixed with submerged piles, and the bottom is fixed to the bottom of the water with weights. The resistance of the enclosure to water flow is calculated based on the following formula: In the formula, g is the mass of the enclosure; g is the acceleration due to gravity; T is the tension of the weight at the bottom. This refers to the buoyancy force experienced by the enclosure and the weight in the water. denoted as , where is the frictional force between the weight and the lakebed; u is the coefficient of friction between the weight and the lakebed. The mass of the object at the bottom; The water flow impact force is calculated based on the following formula: In the formula, ρ is the density of the water; A is the enclosing area; v is the water velocity; θ is the impact angle, which is the angle between the water flow and the normal to the surface of the object. The determination of the area and shape of the ecological restoration area based on the required vegetation coverage and water system connectivity for lake water environment restoration includes: The area of ​​the ecological restoration zone is determined based on the lake area and vegetation coverage; The scope of zoning and enclosure is constrained by the principle of minimizing the selection of water flow channels.

2. The method according to claim 1, characterized in that, Simulation of lake flow field based on two-dimensional hydrodynamic model.

3. The method of claim 1, wherein, The establishment of the hydrodynamic model to simulate the lake flow field includes: After establishing the model working area and importing the land boundary of the study area, the computational domain is defined. Import terrain data into the model for spatial interpolation, set the grid type and side length, divide the grid, and generate a terrain file; Import the terrain file into the hydrodynamic module, set the boundary conditions and initial parameters, simulate and generate two-dimensional flow field results, and extract lake flow field information.

4. The method of claim 3, wherein, The boundary conditions of the hydrodynamic model include flow boundary, rainfall, and wind field; the selection of boundary conditions is based on historical monitoring data.

5. The method of claim 1, wherein, It also includes determining the pre-selected site area based on the simulated water depth and the recommended planting water depth for the plants to be planted, and then selecting the location of the ecological restoration area within the pre-selected site area.

6. The method of claim 5, wherein, Based on the recommended planting water depth for the plants to be planted, areas with water depths less than the upper limit of the recommended planting water depth are selected as pre-selected sites.

7. The method according to claim 1, characterized in that, The determination of the location of the ecological restoration area based on the simulation results of the maximum water flow velocity and flow field includes: The area where the simulated flow velocity is less than the maximum water flow velocity is selected as the site location, and the direction of the enclosure design and installation is parallel to the simulated water flow direction.

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