Method for improving water quality of large and medium shallow lakes based on hydrodynamic optimization and vegetation community recovery

Abstract

The invention discloses a large and medium shallow lake water quality improving method based on hydrodynamic optimization and vegetation community restoration, and relates to the technical field of lake treatment and ecological restoration. Comprising the following steps: step 1, reducing lake-entering pollution load; arranging an in-and-out river mouth sediment settling reservoir to increase a river mouth water-passing section; 2, high-concentration water entering the lake and low-concentration water in the lake body are isolated; remodeling a near-shore area terrain of a shore zone where the lake-entering river channel is located, and arranging an ecological submerged beach, a drainage groove and a mud flow capturing groove; 3, improving the habitat of the aquatic plants on the large water surface; designing matrix purse seine units for flow choking and wave dissipation; a plurality of matrix purse seine units are designed to spatially separate a large water surface, and floating plants are fixed in a local area by using the matrix purse seine units; 4, preparing an aquatic plant community; and recovering pioneer floating-leaf plants and submerged plants. According to the method, a hydrodynamic optimization project and a vegetation community reconstruction project are combined, the water quality is improved, and the ecological system structures of large and medium-sized shallow lakes are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lake management and ecological restoration, and in particular to a method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration. Background Art

[0002] Lakes in economically developed areas are easily affected by human activities and have high external pollution loads. At the same time, shallow lakes are significantly affected by wind and waves and have active endogenous releases. Therefore, eutrophication of water bodies, degradation of aquatic vegetation, and imbalance in ecosystem structure have become widespread problems facing shallow lakes.

[0003] The results of relevant databases and online searches at home and abroad show that: Bian Bo et al., with the water quality safety project of Tangma Reservoir Water Source in Liyang City, Jiangsu Province as the background, aimed at the main problems of surface source pollution in reservoir-type water sources, constructed a composite pre-reservoir partition purification system with sedimentation area, shallow water area, deep water area, subsurface artificial wetland and biological filler to strengthen the purification area, and studied the removal rules of major non-point source pollutants. Song Jie of Changsha University of Science and Technology, based on the basic characteristics of rivers in urban and rural areas of Hunan Province, restored the vegetation of riverbank buffer zone, built the original fragile exposed slope into an ecological rough revetment with certain anti-scouring ability, set up a large-scale riprap phase combination in the straight river section, built a hydrophilic landscape area and an ecological cultural park, built a submerged dike in the river channel, submerged spur dike and other remediation measures. Based on the dilution and self-purification mechanism of river water quality improvement by live water diversion, Kong Xiaolu from Yangzhou University explored the dilution, diffusion and purification laws of pollutants in river networks, established a calculation model for the amount of water required for dilution in the urban river network of Yangzhou, and calibrated the relevant model parameters according to the planned water diversion and water quality protection goals and water diversion constraints in the study area. Li Junfei and others from Guangdong Provincial Architectural Design Institute set up large sewage interception channels at the bottom of the river to completely intercept dry season sewage and initial rainwater, reduce water pollution, and improve river water quality. The Chinese patent application number 201520442432.8 discloses a fence partition device used for lake and reservoir zone ecosystem restoration, which includes a fence and a nylon soft net. The fence is set at the boundary between the ecosystem restoration area and the unrestored area, a gabion is set at the lower end of the fence, and a first float is set at the upper end of the fence. The nylon soft net is laid at the bottom of the lake and reservoir in the direction extending from the boundary between the ecosystem restoration area and the unrestored area to the unrestored area. The Chinese patent application number 200810019140.8 discloses a method for ecological restoration of the lakeside zone of a shallow lake, which includes the following steps: a. Selecting an area: Selecting an area with eutrophication, rich sediment nutrients, and serious water pollution as the restoration area; b. Setting a wave-breaking layer; c. Setting a wave-breaking layer; d. Seasonal ecological restoration. E. Jeppesen et al. studied a method for restoring shallow lakes by reducing nutrient loads. The US patent application number US07550079B2 discloses a method for water body restoration, including a positive water flow device that works in conjunction with a negative water flow device, wherein the positive water flow device and the negative water flow device are separated from each other in a pool or water area. The Japanese patent application number JP2007154647A discloses a method for building a rigid underwater breakwater with high wave stability at low cost. The international patent with publication number WO2021098731A1 discloses a wastewater interception centralized treatment system and treatment method for river regulation, including a wastewater interception pipeline network, a comprehensive enhanced treatment component and an ecological restoration component.The international patent with publication number WO2020051198A1 discloses a system and method for restoring lakes, including dredging, island creation, water treatment, real estate development, computer modeling of environmental conditions, wave height reduction, sediment removal and packaging, bathymetric contour measurement, coastal zone restoration, plant restoration and / or fish restoration. The US patent with publication number US20220073369A1 discloses a multi-interface governance and restoration method for urban river and lake water environment, which is based on the multi-interface coordinated governance and restoration technology of "controlling the bottom layer, regulating the middle, and governing the top layer", including: "controlling the bottom layer" - controlling the discharge of sediment nutrients and the dormancy and recovery of algae; "middle regulation" - regulating the primary productivity of the water body and inhibiting the recovery of algae; and "top governance" - reducing nitrogen and phosphorus nutrients at the air-water interface to control the reproduction and growth of algae.

[0004] A comprehensive analysis of domestic and foreign literature has been conducted, and the existing technologies of using estuarine sedimentation reservoirs, drainage channels, ecological submerged dikes or flow-blocking nets to improve water quality and restore vegetation have been explored. However, for large and medium-sized shallow lakes, large and medium-sized shallow lakes are dominated by the monsoon climate, wind-driven hydrodynamic disturbances are frequent, the water-soil interface is often in an unstable state, the lake hydrological, chemical, and biological processes have strong interactions and are highly complex, and it is difficult to restore aquatic plants, especially submerged plants. There have been no reports on water quality improvement technical methods based on hydrodynamic optimization and vegetation community restoration. Summary of the invention

[0005] The main purpose of the present invention is to provide a method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration, so as to solve the three major technical problems faced by the improvement of the water quality of large and medium-sized shallow lakes: 1. How to reduce the direct impact of high nitrogen and phosphorus nutrients and other pollutants carried by rivers entering the lake on the large lake, thereby achieving the separation of high-pollutant concentration entering the lake and low-concentration lake water, thereby improving the targeted water quality management; 2. How to use a near-natural method to reduce the disturbance of wind and waves to the bottom mud, reduce endogenous release and promote the sedimentation of suspended matter, and create a habitat for the recovery of aquatic plants; 3. How to achieve artificial and natural recovery of aquatic plants over a large area without affecting water exchange, thereby improving the self-purification capacity of lakes.

[0006] To achieve the above-mentioned purpose, the present invention provides a method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration, comprising the following steps:

[0007] Step 1: Reduce the pollution load entering the lake; set up sediment sedimentation reservoirs at the river mouth and increase the water flow section of the river mouth;

[0008] Step 2: Isolate the high-concentration water entering the lake from the low-concentration water in the lake body; reshape the nearshore terrain of the riverbed where the river entering the lake is located, set up ecological submerged beaches, diversion channels, and mud capture channels to guide the high-concentration water entering the lake to flow southward;

[0009] Step 3: Improve the habitat of aquatic plants on large water surfaces; design matrix seine units to block currents and eliminate waves, and improve the local hydrodynamic environment; multiple matrix seine units are designed to spatially separate large water surfaces, and use matrix seine units to fix floating plants in local areas;

[0010] Step 4: Configure aquatic plant communities; restore pioneer floating-leaf plants and submerged plants. Both floating-leaf plant restoration areas and submerged plant restoration areas are located within the matrix enclosure units, forming floating-leaf plant restoration units and submerged plant restoration units.

[0011] Furthermore, in step three, the matrix fence unit is made of bamboo and nylon net as main materials.

[0012] Furthermore, in step four, European chestnut is used as the pioneer species.

[0013] Furthermore, in step four, floating leaf plants are planted artificially in units of a single matrix enclosure.

[0014] Furthermore, in step four, the plant species and community construction planted in the submerged plant configuration area are based on the submerged vegetation of the lake in the historical period. Species with high pollution tolerance are planted in the initial stage, and species with strong water purification capabilities are gradually introduced in the later stage.

[0015] Furthermore, in step 4, the submerged plant restoration unit and the floating leaf plant restoration unit are arranged at intervals.

[0016] The present invention has been implemented and verified in Changdang Lake, a medium-sized shallow lake, and can achieve the following effects:

[0017] 1. Hydrodynamic optimization improves water quality:

[0018] Through the construction of estuary sediment sedimentation reservoirs, ecological submerged beaches, diversion channels, and mud capture channels, the underwater topography of the estuary and nearshore areas has been reshaped, the wind and wave conditions of the lake have been changed, and the sedimentation of suspended particles in the nearshore area has been promoted. The wave-breaking effect of the ecological submerged beach has greatly reduced the disturbance of the bottom sediment inside the submerged beach, and the total suspended matter concentration in the water body has been reduced by 28.69% compared with the outside of the submerged beach, and the transparency has been improved by 11.67%, which has improved the water quality and underwater lighting conditions in the nearshore area. The diversion effect of the underwater topography guides the high-turbidity water flow of the rivers entering the lake to flow southward, avoiding the water flow entering the lake directly into the lake body. The TN on the east side of the ecological submerged beach is reduced by 28.36% compared with the west side, and the DTN is reduced by 54.74%. The nitrogen in the river entering the lake mainly exists in the form of nitrate nitrogen (NO3-N). The NO3-N on the east side of the ecological submerged beach is reduced by 68.34% compared with the west side, and the total nitrogen in the lake area has been effectively reduced. The TP on the east side of the ecological submerged beach is 13.07% higher than that on the west side. The reason is that the phosphorus in the Changdang Lake water body mainly exists in particulate form. The wave-breaking effect of the ecological submerged beach has greatly reduced the sediment disturbance on the west side, while the wind and wave disturbance on the east side is strong and the TP is high. The soluble total phosphorus (DTP) reflects this phenomenon to a certain extent. The DTP on the east side of the ecological submerged beach is 6.0% lower than that on the west side of the submerged beach, indicating that the DTP ratio on the east side of the submerged beach is greater than that on the west side of the submerged beach, and the particulate phosphorus on the west side of the submerged beach has been effectively reduced. In general, the concentration of water pollutants in the space shows a decreasing trend from the river channel entering the lake-the estuary entering the lake-the west side of the diversion submerged beach-the east side of the diversion submerged beach-the center of the lake-the east area of ​​the lake, which has initially achieved the engineering goal of absorbing the pollutants entering the lake in the nearshore area and reducing the pollution load of the large lake.

[0019] 2. Hydrodynamic optimization improves the effects of wind, waves and lake currents:

[0020] Before hydrodynamic optimization, the wind and waves in the nearshore area were still of high intensity under different prevailing winds, and the area of ​​wind and wave suitable for the growth of aquatic vegetation was small, distributed only in shallow waters near the coast; after the topography was reshaped, the intensity of wind and wave in the western nearshore zone, especially in the ecological submerged beach waters, was significantly weakened, which was conducive to the improvement of physical habitat conditions for the growth and survival of aquatic vegetation. Under the high probability wind speed of the prevailing wind, the effective wave height in the ecological submerged beach area was reduced by a minimum of 10.23%, a maximum of 85.18%, and an average of 43.21%; while the ecological submerged beach protection area was reduced by a maximum of 47.63% and an average of 17.55%.

[0021] Before hydrodynamic optimization, the lake current was mainly wind-driven current and compensation current. After entering the lake, the water flowing into the lake quickly merged with the wind-driven current and compensation current and flowed toward the interior of the lake, which would cause the highly polluted water entering the river to quickly transfer toward the interior of the lake; after the topography was reshaped, the water flowing into the lake was blocked by the ecological submerged beach and guided by the diversion channel, flowing from north to south along the west side of the submerged dike, and then flowing northward to the large lake after bypassing the southern part of the submerged dike. As a result, the highly polluted water entering the river would quickly transfer to the interior of the lake after passing through the longer flow path on the west side of the submerged dike.

[0022] 3. Improve the habitat and restore aquatic plants:

[0023] The matrix seine unit has a 7.0% reduction rate for effective wave height and a 10.6% reduction rate for bottom shear stress. By laying out multiple matrix seine units, the rooting of aquatic plants is promoted and mechanical damage to aquatic plants is reduced. The area of ​​aquatic plants successfully restored by using matrix seine units in the center of the lake is 8.6 km 2 The total area of ​​aquatic plants in the central part of the lake before restoration is less than 100m 2 . 12 species of aquatic plants were restored: European water chestnut, Nymphoides, Ceratophyllum, Vallisneria, Myriophyllum paniculate, Potamogeton, Elodea, Hydrilla verticillata, Trionyx, Alternanthera philoxeroides, Eichhornia crassipes, with European water chestnut as the dominant species. The coverage of aquatic vegetation in the restoration area is >90%, with a total biomass of 36,788t.

[0024] 4. Restoration of aquatic plants on large water surfaces to improve water quality

[0025] Two large water surface aquatic plant restoration projects were implemented in the lake area. The TP range of the aquatic plant restoration area on the east side of the ecological beach was 0.05-0.09 mg / L, with an average of 0.07 mg / L, which generally met the Class IV water standard and the Class III water standard in some areas; the TN range was 0.43-1.03 mg / L, with an average of 0.64 mg / L, which generally met the Class III water standard; the NH3-N range was 0.01-0.09 mg / L, with an average of 0.03 mg / L, which met the Class I water standard; the SS range was 1.50-9.0 mg / L, with an average of 6.58 mg / L; the Chla range was 4.35-17.10 μg / L, with an average of 10.85 μg / L. The TP range of the lake center aquatic plant restoration area was 0.04-0.08 mg / L, with an average of 0.06 mg / L, which generally reached the Class IV water standard and the Class III water standard in some areas; the TN range was 0.53-0.93 mg / L, with an average of 0.70 mg / L, which generally reached the Class III water standard; the NH3-N range was 0.01-0.04 mg / L, with an average of 0.02 mg / L, which reached the Class I water standard; the SS range was 0.50-26.50 mg / L, with an average of 6.78 mg / L; the Chla range was 1.76-38.56 μg / L, with an average of 13.31 μg / L. In general, the concentrations of TN, TP, NH3-N, SS, and Chla in the large water surface aquatic plant restoration area decreased by 28.7%, 21.6%, 11.2%, 42.5%, and 10.9% compared with the area without vegetation restoration, and the local water bodies reached the Class III water standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a technical roadmap for the method of improving water quality in large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration.

[0027] Figure 2 This is a schematic diagram of an ecological submerged beach of the method for improving water quality in large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration of the present invention.

[0028] Figure 3 It is a schematic diagram of the visual matrix enclosure unit of the method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration of the present invention.

[0029] Figure 4 It is a schematic cross-sectional diagram of a matrix enclosure unit of the method for improving water quality in large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration of the present invention.

[0030] Figure 5 This is a schematic diagram of Example 1 of the method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration of the present invention.

[0031] Figure 6 This is a schematic diagram of Example 2 of the method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration of the present invention. DETAILED DESCRIPTION

[0032] In order to achieve the above-mentioned purpose and effect, the technical means and structures adopted by the present invention are described in detail with reference to the accompanying drawings for the features and functions of the preferred embodiments of the present invention.

[0033] like Figure 1-4 As shown, the present invention provides a method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration, including the following aspects:

[0034] (1) Reducing the pollution load entering the lake

[0035] In order to address the problem that the backflow of water from rivers entering and leaving the lake carries high levels of sediment and its adsorbed pollutants into the lake, leading to increased concentrations of nutrients and suspended matter in the open lake area and deterioration of underwater light conditions, sediment sedimentation reservoirs are set up at the river estuaries to increase the water-passing section of the estuary, thereby slowing down the flow rate, reducing the sediment carrying capacity of the water flow, promoting the rapid sedimentation of coarse-grained sediment that adsorbs nutrients, reducing the content of suspended particles and their nitrogen and phosphorus nutrients in the water of rivers entering and leaving the lake, and reducing the pollution load in the nearshore zone and lake area, providing the first line of defense for improving the transparency of the lake and enhancing the self-purification capacity of the lake ecosystem.

[0036] (2) Isolating high-concentration water entering the lake from low-concentration water in the lake

[0037] In response to the problems of large and medium-sized shallow lakes carrying high concentrations of nitrogen, phosphorus and other pollutants flowing directly to the center of the lake, as well as shallow water, large waves and easy suspension of bottom mud in open waters, the nearshore terrain of the shore zone where the river entering the lake is located is reshaped, and ecological submerged beaches, diversion channels and mud capture channels are set up to guide the high-concentration water entering the lake to flow southward. The retention time of high-concentration water in the coastal area after entering the lake is greatly increased, the path to the center of the lake is extended, and the purification capacity of nearshore water bodies is improved. The water quality of large and medium-sized lakes is generally better than that of rivers entering the lake. Therefore, the reshaping of the nearshore terrain has achieved the isolation of high-concentration water from low-concentration water, significantly reducing the adverse effects of high-concentration water entering the lake on the water quality of large lakes. On the other hand, ecological submerged beaches weaken the intensity of wind and waves in open waters, can inhibit the resuspension of bottom mud, improve water transparency, and create good physical habitat conditions for the restoration of aquatic vegetation. The soil used for the ecological submerged beach is the soil produced by excavating drainage channels and estuary sedimentation reservoirs. No foreign soil is introduced or other wave-breaking equipment is used. It is a near-natural lake hydrodynamic optimization method with obvious long-term effectiveness and economy.

[0038] (3) Improving the habitat of aquatic plants on large water surfaces

[0039] In view of the low survival rate of floating leaf plants and submerged plants planted directly in the center of large and medium-sized shallow lakes due to turbid water and large waves, a matrix enclosure unit is designed to block flow and eliminate waves, improve the local hydrodynamic environment, and further carry out vegetation restoration on this basis. The matrix enclosure unit is made of bamboo and nylon nets as the main materials to spatially separate large water surfaces. The floating plants are fixed in local areas by the matrix enclosure unit to reduce friction damage caused by wind and wave disturbances, prevent aquatic plant propagules from floating to waters that are not suitable for growth, and maintain the stability of germplasm resources in the enclosure. The blocking and wave-breaking effect of the matrix enclosure unit can promote the rooting of aquatic plants, reduce mechanical damage to aquatic plants, prevent fish from gathering in aquatic plant areas, and reduce fish grazing pressure. The biological membrane formed by the enclosure is used to absorb suspended matter, improve the water quality of the vegetation planting area, and provide a suitable habitat for the restoration of vegetation on large water surfaces.

[0040] (4) Configure aquatic plant communities

[0041] Planting ecological submerged beach emergent plants - Planting lake native emergent plants on ecological submerged beaches, using the bottom mud fixation of the emergent plant roots to enhance the submerged beach's ability to resist water flow erosion, improve the stability of the submerged beach, and reduce soil loss and collapse caused by long-term erosion by wind and waves. The stems and leaves of emergent plants form resistance to the wind speed on the lake surface, reduce the wind shear stress on the water surface in the downwind direction, and reduce water disturbance.

[0042] Restoration of pioneer floating leaf plants - The restoration area of ​​floating leaf plants is located in the matrix enclosure unit, and artificial planting is carried out in a single matrix enclosure unit. The species is the European water chestnut as the pioneer species. Its adventitious roots are produced at the base of the degenerated radicle, hypocotyl and lower stem nodes, and can grow downward into the bottom mud. It has the function of fixing vegetation and absorbing nutrients, and has a high light tolerance range. The friction of the stems and leaves of floating leaf plants on the water flow can create an environment with weak water flow disturbance. At the same time, its stems and leaves have a good adsorption effect on suspended matter. High-density floating leaf plants can significantly improve the transparency and water quality of local water bodies, creating a favorable habitat for the restoration of submerged plants.

[0043] Restoration of submerged plants - Submerged plant restoration units and floating leaf plant restoration units are arranged alternately. After the floating leaf plants eliminate waves and filter and purify the peripheral water body, the habitat of the internal matrix enclosure unit is effectively improved, which is conducive to the restoration of submerged plants and further improves the diversity of aquatic vegetation and water purification capacity. The plant species and community construction planted in the submerged plant configuration area are based on the submerged vegetation of the lake in the historical period. Species with high pollution tolerance are planted in the initial stage, and species with strong water purification ability are gradually introduced in the later stage. Through the combination of vegetation with different life histories and canopy types, the water body is stabilized and plant diversity is increased.

[0044] Example 1

[0045] like Figure 5 As shown, taking Changdang Lake as the implementation object, the following water quality improvement methods are adopted:

[0046] (1) Optimization of Changdang Lake hydrodynamics

[0047] In response to environmental problems such as shallow water, large waves, easy suspension of bottom mud in Changdang Lake, and high turbidity water flow from Dapu Port and Baishi Port directly rushing to the center of the lake, the terrain of the western and southwestern parts of Changdang Lake was reshaped. The high turbidity water flow from rivers entering the lake such as Dapu Port and Baishi Port was guided to flow southward, the path of the high turbidity and low transparency Danjin Licao River entering the lake to the center of the lake was extended, the amount of water directly flowing to the aquatic germplasm resources protection area was reduced, the transparency of the large water body of Changdang Lake was improved, and good physical habitat conditions were created for the restoration of aquatic vegetation. The following hydrodynamic optimization projects were implemented in Changdang Lake: 1. Underwater ecological submerged beach project in the western nearshore area; 2. Underwater ecological submerged beach project in the southwestern nearshore area; 3. Mud capture trough project; 4. Southwestern nearshore diversion trough project.

[0048] ①Construct underwater ecological beach in the western coastal area

[0049] The base of the underwater ecological submerged beach is located on the south side of the Shuibagua wetland and the east side of Baishi Port, and the estuary diversion submerged beach is located on the east side of the Dapu Port estuary. The submerged beach will have an effective blocking effect on Dapu Port and Baishi Port, and the water flow will no longer flow directly eastward into the aquatic germplasm resource protection area and the open lake. Waves will deform and break when they reach the submerged beach, so the slope of the base toward the shore will form a good habitat condition for vegetation restoration. The profile of the submerged beach base is trapezoidal, with an elevation of 3.20m, an upper top width of 40 to 60m (average width of 50m), and a lower bottom width of 80 to 120m (average width of 100m). The length of the underwater ecological submerged beach is about 5700m (2307m+3393), and the earthwork volume is 30.8×104m 3 The river mouth diversion beach is circular, with an elevation of 3.4m, a diameter of 400m to 500m, and an earthwork volume of about 15.0×104m 3 .

[0050] ②Construct underwater ecological beach in the southwestern coastal area

[0051] The underwater ecological beach is located in the open lake area east of Renhe Port and Zhuangyang Port. The base profile is trapezoidal, with an elevation of 3.20m, an upper width of 40 to 60m (average width 50m), and a lower width of 80 to 120m (average width 100m). The underwater ecological beach is 2038m long and has an earthwork volume of 8.4×104m 3 According to the results of the ecological model simulation, the waves will deform and break when they reach the submerged beach, so the slope of the base toward the shore will form a good habitat condition for vegetation restoration.

[0052] ③Construction of mud capture tank

[0053] In view of the increase in water turbidity and the release of nitrogen and phosphorus from the bottom mud into the water body during the ecological dredging project in the Changdang Lake area, the frequency of ecological dredging projects has been reduced. Based on the hydrodynamic distribution of the lake current and wind and waves in Changdang Lake, local troughs are pumped on the lake bottom to change the water flow dynamics, form a static water environment in the deep troughs on the lake bottom, increase the particle settling rate in the local water body, and transfer the low-density mud on the lake bottom around the deep trough to the water body in the deep trough through the repeated disturbance of wind and waves around the deep trough and the transfer of lake currents, thereby capturing endogenous pollutants. Due to the diversion effect of the underwater ecological submerged beach, the water passage between two adjacent underwater ecological submerged beaches will have a large flow rate, and the sediment is easy to float and be carried here by the water flow. Therefore, mud capture troughs are arranged in this area to improve the removal efficiency of polluted mud. The area of ​​the mud capture trough is about 120,000 m 2 , earthwork volume is about 180,000 m 3 .

[0054] ④Construction of a drainage channel near the southwest coast

[0055] A diversion channel for water entering the lake is arranged inside the underwater ecological beach in the southwest. The water entering the lake, especially the water entering the lake from Dapu Port and Baishi Port, flows southward along the nearshore area. Under the action of aquatic vegetation and pre-storage, the highly turbid and highly polluted water entering the lake is fully purified, reducing the impact of river pollution on the aquatic germplasm resource protection area and the large lake. The diversion channel for water entering the lake is 60 to 80 meters wide, 2.0 meters deep, and has an earthwork volume of 338,000 m 3 , arranged along the inner side of the entire southwestern underwater ecological beach.

[0056] (2) Improvement of physical habitat in plant restoration areas

[0057] The water in the center of the lake is turbid and the waves are large. The survival rate of directly planting floating leaf plants and submerged plants is low. Therefore, it is necessary to block the flow and eliminate the waves to improve the local hydrodynamic environment, and further carry out vegetation restoration on this basis. The physical habitat improvement facilities mainly include matrix enclosure units and floating plant belts. The floating plant belt is located outside the restoration area. It uses the floating plant root system to intercept and flocculate suspended particles and absorb and adsorb pollutants to intercept and purify the water outside the enclosure, thereby improving the water quality of the vegetation restoration area. On the other hand, the local hydrodynamic environment is improved by the blocking and wave elimination effect of the floating plant belt. The matrix enclosure unit spatially divides the large water surface, and uses the matrix enclosure unit to fix the floating plants in the local area, reducing the friction damage caused by wind and wave disturbance, preventing the propagules of aquatic plants from floating to waters that are not suitable for growth, and maintaining the stability of the germplasm resources in the enclosure. The blocking and wave elimination effect of the matrix enclosure unit can promote the rooting of aquatic plants, reduce mechanical damage to aquatic plants, prevent fish from gathering in the aquatic grass area, and reduce fish grazing pressure. The biological membrane formed by the fence is used to absorb suspended matter and improve the water quality in the vegetation planting area.

[0058] The floating plant belt of the Changdang Lake large water surface aquatic plant restoration area is located outside the restoration area, with a width of 70m to 100m, forming a siege around the restoration area. The total area of ​​the floating plant belt is 1.12 million m 2 The matrix seine units are made of bamboo and nylon nets as the main materials to separate the large water surface. There are 210 matrix seine units in total, and the area of ​​a single matrix seine unit is 20,000 m 2 .

[0059] (3) Restoration of aquatic plants on large water surfaces

[0060] ① Pioneer floating leaf plant recovery

[0061] The floating leaf plant restoration area is located inside the floating plant belt. Through the interception effect of floating leaf plants, the habitat conditions of submerged plants in the core restoration area are improved. The floating leaf plant is pioneered by the European water chestnut. Its adventitious roots are produced at the base of the degenerated radicle, hypocotyl and lower stem nodes. It can grow downward into the bottom mud and has the functions of fixing vegetation and absorbing nutrients. Its tolerance range for water transparency and water depth changes is greater than that of other native floating leaf plants in Changdang Lake, such as water chestnuts. The floating leaf plant restoration area covers an area of ​​2.12 million m 2 .

[0062] ②Configuration of submerged plant communities

[0063] Restoration area - submerged plant restoration units and floating leaf plant restoration units are arranged at intervals. After the floating leaf plants break waves and filter and purify the surrounding water bodies, the habitat of the internal matrix enclosure unit is effectively improved, which is conducive to the restoration of submerged plants and further improves the diversity of aquatic vegetation and water purification capabilities.

[0064] The plant species and community construction planted in the restored species-submerged plant configuration area are based on the submerged vegetation of Changdang Lake in the historical period, and refer to the submerged vegetation community structure of the healthy waters of the Taihu Lake Basin ecosystem. The main vegetation species restored are: Malayan Potamogeton, Vallisneria, Verticillata Hydrilla, Microdentata, Water Chestnut, Foxtail Algae and Ceratophyllum, and the main clusters restored are: Malayan Potamogeton + Verticillata Hydrilla + Vallisneria, Ceratophyllum + Verticillata Hydrilla + Vallisneria, Foxtail Algae + Verticillata Hydrilla + Vallisneria, and Microdentata will be gradually introduced. Through the combination of vegetation with different life histories and canopy types, the water body can be stabilized and plant diversity can be increased. The total area of ​​the submerged plant restoration area is 960,000 m 2 .

[0065] Example 2

[0066] like Figure 6 As shown, taking Gehu Lake as the implementation object, the following water quality improvement methods are adopted:

[0067] (1) Optimization of the hydrodynamics of Gehu Lake

[0068] In order to address the problems of large pollution flux entering Gehu Lake from the river, large wind and wave disturbance in the lake, large suspended matter content, low transparency and strong dynamic release of sediment, we use a combination of technical measures such as estuary sediment settling reservoirs, wave-breaking submerged shoals, diversion submerged dikes, matrix grid nets to reduce wind and waves, and mud capture troughs to reduce the intensity of wind and waves in the lake area, increase water transparency and improve the water quality of the lake area.

[0069] ①Estuary hydrodynamic optimization

[0070] A total of 584,000 m2 of water was constructed at the Huangli River, Tangmen Port, and Wunan River estuaries. 2 Pre-reservoir, sediment particles carried by the water flow into the lake are settled; Huangli River and Tangmen Port estuary construction 24.0m 2The diversion submerged dike can reduce the adverse effects of high-concentration water flowing directly into the lake area and increase the retention time of water in the estuary and coastal areas.

[0071] ② Optimization of hydrodynamics in the western coastal zone

[0072] The simulation results of wind and waves in Gehu Lake show that under the current terrain, the overall distribution of wind and waves in Gehu Lake is consistent with the terrain changes. The overall wind and wave intensity in the center of the lake is relatively large. The wind and waves in most areas of the center of the lake are in the large wind and wave area under this wind field, with large effective wave heights and wavelengths. The nearshore area is affected by the protection of the coastline and the shallowing of the water depth. In particular, although the wind and wave intensity in the waters near the leeward coastline is relatively small, the half-wavelength / water depth value of most waters in the lake is greater than 1.0, that is, the lake bottom is in a state that can be disturbed by wind and waves. The repeated suspension of sediments at the bottom of the lake leads to a decrease in water transparency, which is not conducive to the restoration of aquatic plants. At the same time, the lake flow situation of Gehu Lake will cause water bodies in different areas to exchange back and forth, which is not conducive to the rapid improvement of water bodies in key waters. Therefore, according to the key protection of water bodies in water sources, the restriction of the transfer of suspended particles in the lake, the reduction of wind and wave intensity in areas with thick sedimentation, and the wind and wave intensity requirements and the principle of minimum engineering volume in areas where aquatic vegetation restoration is planned, the calculation of wind and wave and lake flow distribution results under multiple schemes was carried out. Underwater submerged dikes were arranged at the mouth of Huangli River in Gehu Lake, the mouth of Zhonggan River, the north and south sides of the ancient river channel in the center of the lake, the west and south of the water source, and the northwest of Gehu Lake, with a top elevation of 3.17m (20cm below the average water level). Six underwater shoals were constructed in the whole lake, with a total length of 15.5km and a total earthwork volume of 3.425 million m 3 .

[0073] ③Construction of mud capture tank

[0074] Under the constraint of the underwater dike, the lake bottom flow velocity of Gehu Lake carrying high concentration of particulate matter will be transported along the reshaped path. For example, the junction of the submerged dike in the south of the water source is the water transfer channel for the exchange between the water source and the surrounding water body. The water body carrying particulate matter outside will flow into the water source through this channel; after the underwater dike is arranged in the northwest of Gehu Lake, it will block the water flow entering the lake from the northwest mouth, thereby causing the sedimentation of particulate matter. Based on the lake flow situation formed after the reshaping of the Gehu Lake terrain, according to the vertical flow velocity, sedimentation rate and horizontal flow velocity, and in accordance with the principle that particulate matter is not easy to re-suspend in the water body after settling in the capture tank, the depth, width and layout position of the mud capture tank are determined. According to the preliminary demonstration results, multiple mud capture tanks are arranged at the junction of the underwater dike in the water source, on the east side of the ancient river channel in the west of the water source, and on the inner side of the submerged dike in the northwest of Gehu Lake, with a depth of 2 to 3m, a width of 20 to 100m, and a length of 300 to 1000m. There are 10 mud capture troughs in the lake area, with a total area of ​​324,000 m 2 , total earthwork volume 9,700 m 3 .

[0075] (2) Improvement of physical habitat in plant restoration areas

[0076] Gehu Lake also faces the problem of turbid water and large waves in the center of the lake, and the low survival rate of directly planting floating leaf plants and submerged plants. Therefore, it is necessary to block the flow and eliminate the waves to improve the local hydrodynamic environment. The physical habitat improvement facilities mainly include matrix enclosure units and floating plant belts. The floating plant belt is located outside the aquatic plant restoration area, with a width of 70m to 100m, forming a siege on the restoration area. The total area of ​​the floating plant belt is 1.12 million m 2 The matrix seine units are made of bamboo and nylon nets as the main materials to separate the large water surface. There are 210 matrix seine units in total, and the area of ​​a single matrix seine unit is 20,000 m 2 .

[0077] (3) Restoration of aquatic plants on large water surfaces

[0078] ① Pioneer floating leaf plant recovery

[0079] The floating leaf plant restoration area is located inside the floating plant belt. Through the interception effect of floating leaf plants, the habitat conditions of submerged plants in the core restoration area are improved. The floating leaf plants are led by the European ling as the pioneer species. The restoration area covers an area of ​​2.12 million m 2 .

[0080] ②Configuration of submerged plant communities

[0081] The submerged plant restoration unit and the floating leaf plant restoration unit are arranged alternately. The main vegetation species restored by submerged plants are native species of Gehu Lake: Myriophyllum paniculate, Ceratophyllum, Vallisneria, and Potamogeton. The total area of ​​the submerged plant restoration area is 960,000 m 2 .

[0082] The above descriptions are only preferred embodiments of the present invention, not all embodiments. Anyone should be aware that any structural changes made under the inspiration of the present invention, and any technical solutions that are the same or similar to the present invention, belong to the protection scope of the present invention.

Claims

1. A method for improving the water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration, characterized in that: The following steps are involved: Step 1: Reduce the pollution load entering the lake; set up sediment sedimentation reservoirs at the river mouth and increase the water flow section of the river mouth; Step 2: Isolate the high-concentration water entering the lake from the low-concentration water in the lake body; reshape the nearshore terrain of the riverbed where the river entering the lake is located, set up ecological submerged beaches, diversion channels, and mud capture channels to guide the high-concentration water entering the lake to flow southward; Step 3: Improve the habitat of aquatic plants on large water surfaces; design matrix seine units to block currents and eliminate waves, and improve the local hydrodynamic environment; multiple matrix seine units are designed to spatially separate large water surfaces, and use matrix seine units to fix floating plants in local areas; Step 4: Configure aquatic plant communities; restore pioneer floating-leaf plants and submerged plants. Both floating-leaf plant restoration areas and submerged plant restoration areas are located within the matrix enclosure units, forming floating-leaf plant restoration units and submerged plant restoration units.

2. The method for improving water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration as claimed in claim 1, characterized in that: In step three, the matrix fence unit is made of bamboo and nylon net as main materials.

3. The method for improving water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration as claimed in claim 1 or 2, characterized in that: In step four, use European chestnut as the pioneer species.

4. The method for improving water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration as claimed in claim 3, characterized in that: In step four, floating leaf plants are planted artificially in single matrix enclosure units.

5. The method for improving water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration as claimed in claim 4, characterized in that: In step 4, the plant species and community construction planted in the submerged plant configuration area are based on the submerged vegetation of the lake in the historical period. Species with high pollution tolerance are planted in the initial stage, and species with strong water purification capabilities are gradually introduced in the later stage.

6. The method for improving water quality of large and medium-sized shallow lakes based on hydrodynamic optimization and vegetation community restoration as claimed in claim 4 or 5, characterized in that: In step 4, the submerged plant restoration unit and the floating leaf plant restoration unit are arranged at intervals.

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