Method for preventing salt water invasion of bottom aquifer of estuary river channel by building impermeable layer

Abstract

The invention discloses a method for preventing and controlling salt water invasion of an aquifer at the bottom of an estuary river channel by building an impermeable layer, which comprises the following steps of: acquiring hydrogeological data of a prevention and control area, and determining the size of the area to be prevented and controlled according to an engineering expected effect; performing curing treatment on the foundation bed in the prevention and treatment area by using a curing agent material; leveling treatment is conducted on the curing area; laying an impermeable layer on the surface of the solidified foundation bed; smoothing the connecting area of the impermeable layer and the surrounding foundation bed; and laying a protective layer on the surface of the impermeable layer. Compared with an existing seawater invasion prevention and control method, the method is relatively simple in construction arrangement, low in investment cost and low in later maintenance cost, salt water can be effectively prevented from invading the coastal underground aquifer from the bottom of the river channel, and the method has a good application prospect.

Description

Technical Field

[0001] The invention belongs to the field of prevention and control of seawater intrusion in aquifers, and in particular relates to a method for preventing and controlling saltwater intrusion in aquifers at the bottom of estuaries and rivers by constructing an impermeable layer. Background Art

[0002] With the rapid economic development and intensified human activities in coastal and estuarine areas, people's demand for water resources has grown rapidly, leading to over-exploitation of groundwater resources. At the same time, the intensification of extreme climate and frequent droughts in recent years have increased the frequency, intensity and duration of saltwater upstream events. Upstream salt water infiltrates into coastal underground aquifers through interfaces such as riverbeds and river banks, causing an increase in the salinity of underground freshwater, affecting drinking water safety, threatening the quality of agricultural irrigation water, destroying the balance of wetland ecosystems, and causing a series of problems such as soil salinization. Existing seawater intrusion prevention and control technologies are mainly aimed at salt water invading nearby underground aquifers from estuaries, river banks or coastal zones, but there is a lack of prevention and control measures for salt water infiltrating aquifers from the bottom of the river. Summary of the invention

[0003] Purpose of the invention: In order to overcome the shortcomings of the background technology, the present invention discloses a method for preventing and controlling salt water intrusion into the aquifer at the bottom of an estuary river channel by constructing an impermeable layer.

[0004] Technical solution: The method disclosed in the present invention for preventing and controlling saltwater intrusion into the aquifer at the bottom of an estuary river channel by constructing an impermeable layer comprises the following steps: S1. Obtain the hydrogeological data of the control area and determine the size of the area to be controlled based on the expected effect of the project; S2. Use curing agent materials to cure the base bed in the prevention and control area; S3, leveling the solidification area; S4. Laying an anti-seepage layer on the surface of the solidified base bed; S5. Smoothing the connection area between the anti-seepage layer and the surrounding base beds; S6. Lay a protective layer on the surface of the anti-seepage layer.

[0005] Among them, the hydrogeological data in S1 include the permeability coefficient and porosity of the aquifer, the average water level of the river channel, the upstream distance of the salt tide, the toe position of the salt wedge of seawater intrusion, the geological and geomorphological characteristics of the riverbed, and the type of sediments at the bottom of the riverbed.

[0006] Furthermore, in S2, the curing agent material is selected according to the type of sediment at the bottom of the riverbed in the prevention and control area.

[0007] Furthermore, in S2, a suitable solidification treatment construction process is selected according to the geological and geomorphological characteristics of the riverbed and the type of sediment at the bottom of the riverbed.

[0008] Furthermore, the material of the anti-seepage layer in S4 is selected from geomembrane, bentonite waterproof blanket or underwater concrete.

[0009] Furthermore, in S5, the anti-seepage layer is specifically extended obliquely to the inside of the surrounding base bed, and horizontally extended for a certain distance inside the base bed.

[0010] Furthermore, the protective layer in S6 is an underwater concrete or fluidized solidified soil protective layer.

[0011] Furthermore, in S6, the coverage area of ​​the protective layer is slightly larger than that of the anti-seepage layer, and the edges of the protective layer are in a gentle slope shape with a slope ratio of 1:10-15. The front corners of the gentle slope overlap the base bed surface at a small angle.

[0012] Beneficial effects: Compared with the existing seawater intrusion prevention and control methods, the method proposed by the present invention has relatively simple construction layout, low investment cost, and low later maintenance cost. It can effectively prevent salt water from invading the coastal underground aquifer from the bottom of the river, and has a good application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is the construction flow chart of the present invention; Figure 2 It is a schematic diagram of the structure of the anti-seepage layer of the present invention; Figure 3 This is a comparison chart of the prevention and control effects before and after the construction of the anti-seepage layer. DETAILED DESCRIPTION

[0014] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings. like Figure 1 and Figure 2 As shown, the method of preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer comprises the following steps: (1) Obtain hydrogeological data of the control area, including permeability and porosity of the aquifer, average water level of the river channel, upstream distance of the saltwater tide, toe position of the saltwater wedge of seawater intrusion, geological and geomorphological characteristics of the riverbed, and sediment type at the bottom of the riverbed, and determine the size of the area to be controlled based on the expected effect of the project.

[0015] (2) According to the type of riverbed sediment in the prevention and control area 2, select appropriate curing agent materials, such as silt curing agent for silty riverbed and sediment curing agent for sandy riverbed, to solidify the sediment of a certain thickness on the surface of the riverbed in the prevention and control area 2; according to the geological and geomorphological characteristics of the riverbed and the type of riverbed sediment, etc., select appropriate curing treatment construction technology, such as the selection of agitator, agitator speed, lifting speed, stirring power, etc.

[0016] (3) The solidified base bed in the prevention and control area 2 is leveled to reduce the stress concentration caused by local settlement and uneven support that damages the anti-seepage layer 3, thereby causing the anti-seepage function to fail.

[0017] (4) An anti-seepage layer 3 is laid on the surface of the solidified base bed to prevent salt water from the river from invading the aquifer below the riverbed and polluting the coastal underground fresh water. The material of the anti-seepage layer 3 can be selected according to the actual situation of the project, such as geomembrane, bentonite waterproof blanket, underwater concrete, etc.

[0018] (5) Transition treatment is performed on the connection area between the anti-seepage layer 3 material and the surrounding subgrade. The specific method is: the anti-seepage layer 3 is extended obliquely to the inside of the surrounding subgrade 1, and horizontally extended for a certain distance inside the subgrade 1. The purpose of the treatment is to prevent the boundary area of ​​the anti-seepage layer 3 from being directly washed away by the river flow and damaged, thereby destroying the entire anti-seepage layer 3, thereby affecting the service life of the project.

[0019] (6) Lay an underwater concrete or fluidized solidified soil protective layer 4 on the surface of the anti-seepage layer 3 to increase the weight of the anti-seepage layer 3, reduce the scouring effect of the river flow, and increase its service life. The coverage area of ​​the protective layer 4 should be slightly larger than the coverage area of ​​the anti-seepage layer 3. The edges of the protective layer 4 are in the shape of a large gentle slope with a slope ratio of 1:10 to 1:15. The front corners of the gentle slope overlap the surface of the base bed 1 at a small angle, and there is no boundary scouring point to reduce the generation of turbulence.

[0020] like Figure 3 As shown, the implementation case of the two-dimensional numerical model is as follows: According to the hydrogeological data of the coastal estuary area of ​​Jiangsu Province, a method provided by the present invention for preventing and controlling saltwater intrusion into the aquifer at the bottom of the estuary by constructing an impermeable layer was adopted, and a two-dimensional numerical model was established using COMSOL Multiphysics software to test the effect of the method of the present invention.

[0021] The two-dimensional numerical model in this embodiment is established by using the "Free and Porous Media Flow, Brinkman" interface and the "Transport of Diluted Species in Porous Media" interface in COMSOL Multiphysics software.

[0022] The control area of ​​the simulated area includes the river channel, the impermeable layer and the shallow aquifer adjacent to the riverbed, which is simplified into three rectangular areas with a model length of 200 m and a total height of 20 m. The upper layer is the river water body with a thickness of 5 m; the middle layer is the impermeable layer with a thickness of 1 m; and the lower layer is the bed (i.e. the shallow aquifer at the bottom of the river) with a thickness of 14 m.

[0023] The upstream boundary is a freshwater boundary with a fixed water level and concentration. The boundary water level and concentration are 20 m and 0 mol / m 3The downstream is the tidal level constant concentration seawater boundary, and its boundary concentration is 600 mol / m 3 , the average sea level is 18.5 m, the tidal range is 2 m, and the tidal period is 1 h; the bottom boundary of the aquifer is set as a no-flow boundary.

[0024] The impermeable layer and the aquifer are assumed to be homogeneous and isotropic, and the permeability of the impermeable layer is 1×10 -19 m 2 , the porosity is set to 0.01; the permeability of the aquifer is 1×10 -10 m 2 , porosity is 0.4; fresh water density is 1000 kg / m 3 , the density of seawater is 1025 kg / m 3 ; The longitudinal diffusion coefficient and lateral diffusion coefficient of the impermeable layer and the aquifer are 0.1 and 0.01 respectively.

[0025] The model without an impermeable layer was set as the control condition for simulation, and the simulation time was 400 h.

[0026] The simulation results are shown in Figure 3. Compared with the case results without an impermeable layer, in the case of the model with an impermeable layer, the total length of the salt water wedge in the aquifer was reduced by 56.8%, the degree of salt water intrusion in the adjacent aquifer at the bottom of the river channel where the salt tide goes up was reduced, and the salt water intrusion area with a salinity of more than 0.45‰ in the shallow aquifer was reduced by 31.6%. The comparison results show that laying an impermeable layer at the bottom of the river channel according to the method provided by the present invention can significantly reduce the degree of salt water intrusion in the shallow aquifer and force the salt water wedge at the bottom of the shallow aquifer to retreat to the sea side. Although the numerical model is highly simplified, the above simulation results show that the method provided by the present invention for preventing and controlling salt water intrusion in the aquifer at the bottom of the estuary river channel by constructing an impermeable layer is effective.

Claims

1. A method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary by constructing an impermeable layer, characterized in that: The following steps are involved: S1. Obtain the hydrogeological data of the control area and determine the size of the area to be controlled based on the expected effect of the project; S2. Use curing agent materials to cure the base bed in the prevention and control area; S3, leveling the solidification area; S4. Laying an anti-seepage layer on the surface of the solidified base bed; S5. Smoothing the connection area between the anti-seepage layer and the surrounding base beds; S6. Lay a protective layer on the surface of the anti-seepage layer.

2. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer according to claim 1, characterized in that: The hydrogeological data in S1 include the permeability and porosity of the aquifer, the average water level of the river channel, the upstream distance of the salt tide, the toe position of the salt wedge of seawater intrusion, the geological and geomorphological characteristics of the riverbed, and the type of sediments at the bottom of the riverbed.

3. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer according to claim 1, characterized in that: In S2, the curing agent material is selected according to the type of sediment at the bottom of the riverbed in the control area.

4. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer according to claim 1, characterized in that: In S2, a suitable solidification treatment construction process is selected based on the geological and geomorphological characteristics of the riverbed and the type of sediment at the bottom of the riverbed.

5. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer according to claim 1, characterized in that: The material of the anti-seepage layer in S4 is selected from geomembrane, bentonite waterproof blanket or underwater concrete.

6. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer according to claim 1, characterized in that: Specifically, in S5, the anti-seepage layer is extended obliquely to the inside of the surrounding base bed, and horizontally extended for a certain distance inside the base bed.

7. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary river channel by constructing an impermeable layer according to claim 1, characterized in that: The protective layer in S6 is an underwater concrete or fluidized solidified soil protective layer.

8. The method for preventing and controlling saltwater intrusion into the bottom aquifer of an estuary by constructing an impermeable layer according to claim 1, characterized in that: In S6, the coverage area of ​​the protective layer is slightly larger than that of the anti-seepage layer. The edges of the protective layer are in the shape of a gentle slope with a slope ratio of 1:10-15. The front corners of the gentle slope overlap the base bed surface at a small angle.

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