A method for promoting ecological restoration of subtropical freshwater lakes by dredging

Abstract

The application provides a method for promoting the ecological restoration of subtropical freshwater lakes, and the key innovative technology is: according to the dynamic division of the narrowest length of the water area, the segmented rule is used to determine the operation width; the dredging depth is determined by the depth of the contaminated bottom mud layer; the uncontaminated bottom mud is collected, the background values of total nitrogen, total phosphorus, organic carbon, sulfide and heavy metal indicators are determined, the chemical content and dry bulk density of the dredging layer and the normal layer are analyzed, and the exceeding standard coefficient and the compensation coefficient are calculated; the dredging width is derived by formula, the dredging belt and the reserved belt width are determined, and then the dredging is implemented; the bottom mud of the reserved belt is reclamation to the dredging belt; finally, by planting submerged plants and putting microbial agents, the bottom mud is effectively fixed and the ecological system is reconstructed. The method scientifically divides the operation unit, quantifies the pollution evaluation index and accurately controls the dredging range, and takes into account the effective removal of contaminated bottom mud and the rapid recovery of the ecological system, and is especially suitable for the restoration of subtropical freshwater lakes.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of water ecological management, and particularly relates to a dredging method for promoting ecological restoration of subtropical freshwater lakes. BACKGROUND

[0002] Dredging is a complex systematic project, and the geographical location, climate, water body characteristics, types and contents of pollutants and the like of the project area should be comprehensively considered. Different dredging technologies have great differences in the effects of dredging and the recovery of the water ecological environment after dredging. At present, there are mainly two methods for dredging of lake sediment, one is dry water dredging after the lake water is pumped out, and the other is water dredging by directly dredging mud underwater. The former method has strong disturbance to the habitat of benthic organisms, and the ecological restoration effect is poor; the latter method is water dredging, and has less negative impact on the ecological function of the lake after dredging, and is the development direction of current ecological dredging of sediment. For the latter dredging method, the current method mainly adopts complete removal of sediment, which has complete dredging effect, but has serious damage to benthic aquatic organisms and their habitats, and is not conducive to maintaining the stability of the lake ecological system. After complete deep dredging, the original lake ecological system may collapse, and it is difficult to rely on natural forces for restoration succession after dredging. Therefore, it is necessary to seek a dredging method which can achieve the purpose of dredging, maintain the stability of the original lake ecological system, and promote natural recovery after dredging.

[0003] The subtropical region is controlled by monsoon climate, and the seasonal distribution of precipitation is uneven. There are more rainstorms in summer and less rain in winter, resulting in large interannual and intra-annual changes in lake water level. This hydrological fluctuation requires that the restoration measures must consider the adaptability to water level changes, such as the construction of elastic wetland buffer zone or the restoration of drought-tolerant-flood-tolerant alternating vegetation community. The high temperature in summer (such as extreme weather under the control of subtropical high pressure) in subtropical region can accelerate water evaporation, exacerbate lake water reduction and salinity change, and high temperature may promote the outbreak of blue-green algae. The introduction of aquatic vegetation or artificial wetland cooling measures should be given priority to in restoration. Due to large diurnal temperature difference, lake-land wind is significant in subtropical freshwater lakes. The wind blows from the lake to the land during the day, promoting water vertical mixing and increasing dissolved oxygen; at night, the wind direction is opposite, which may lead to local hypoxia. Restoration should strengthen the design of water circulation system to avoid bottom layer hypoxia caused by stratification. Therefore, the restoration of subtropical freshwater lakes has unique requirements, and needs to comprehensively respond to the influence of hydrological fluctuation, high temperature eutrophication and intensive human activities. The core is to build a climate-adaptive ecological system, and to realize dynamic balance through multi-level biological regulation and engineering measures. Its particularity requires that the restoration scheme should have flexibility and long-term nature, which is in sharp contrast with the "static restoration" mode in other climate zones. SUMMARY

[0004] The present application aims to overcome the deficiencies in the prior art, and provides a dredging method for promoting ecological restoration of subtropical freshwater lakes, so as to promote the recovery of the lake ecosystem through scientific technical means.

[0005] The present application is achieved by the following technical solutions, which include the following steps:

[0006] S1. Determining the working unit: determining the dredging working unit according to the target water area, wherein the narrowest length of the water surface of the target water area is not less than 33 m (the minimum power dredging and filling equipment requires a working width of 23 m, and the working safety distance should be greater than or equal to 5 m from the shore);

[0007] S2. Determining the dredging depth: measuring the bottom mud depth of the dredging working unit and determining the dredging depth;

[0008] S3. Measuring the background value: taking the unpolluted bottom mud of the target water area to measure the background values of total nitrogen, total phosphorus, organic carbon, sulfide and heavy metals;

[0009] S4. Determining the dredging zone and the width of the reserved zone: according to the contents of total nitrogen, total phosphorus, organic carbon, sulfide and heavy metals in the dredging layer (polluted bottom mud layer (A layer) and polluted transition layer (B layer)) and the normal layer of the bottom mud of the dredging working unit, and the dry bulk density of the soil, and in combination with the background values of each chemical substance of the unpolluted bottom mud of the target water area measured in step S3, the width of the dredging zone and the reserved zone is determined, and the dredging working unit is composed of the dredging zone and the reserved zone;

[0010] S5. Dredging: dredging according to the dredging depth determined in step S2 and the width of the dredging zone determined in step S4;

[0011] S6. Blowing and filling: determining the dredging depth of the reserved zone, dredging in the reserved zone, and blowing and filling the dredged soil to the dredging zone;

[0012] S7. Post-processing: planting submerged plants according to the water habitat, and then putting microbial agents for degrading pollutants to promote the fixation of the bottom mud and ecological restoration.

[0013] Further, the determination method of the dredging working unit in step S1 is: assuming that the narrowest length of the water surface of the target water area is W 目标 , and the width of the working unit is M,

[0014] (1) If W 目标 ≥ 110 m, M is 50 m, and if it is less than 50 m, it is processed according to (3);

[0015] (2) If W 目标 < 110 m and greater than or equal to 60 m, M = (W 目标 - 10) / 2;

[0016] (3) If W 目标<60m, M = W 目标 -10.

[0017] Further, if the difference between the widest and narrowest parts of the water surface of the target water area is greater than 10 m, the water surface of the target water area is divided into multiple regular areas to determine the work unit again, ensuring that the difference between the widest and narrowest parts of the water surface of each area is not more than 10 m.

[0018] It should be noted that the work unit should be at least 5 m away from buildings such as shore slopes, bridge piers, and lock stations to ensure the stability of the existing shore slopes, bridge piers, and lock stations.

[0019] Further, the dredging depth in step S2 is the sum of the depths of the contaminated bottom sediment layer and the contaminated transition layer, and the minimum dredging depth is ≥10 cm. The bottom sediment depth is based on the actual depth of the bottom sediment survey, and the bottom sediment levels include the contaminated bottom sediment layer (A layer), the contaminated transition layer (B layer), and the normal layer (C layer). Sample multiple bottom sediment depths within each work unit, and take the average as the final bottom sediment depth value. Take the average of the total depth of the contaminated bottom sediment layer (A layer) and the contaminated transition layer (B layer) as the dredging depth value.

[0020] In the step of determining the background value, the sediment coring method is used to collect sediment samples at different depths; the chemical content in fine-grained sediment that is not contaminated or disturbed by biological interference is selected as the background value. If there is an uncontaminated area in the lake area, the sediment in the uncontaminated area of the lake area is used as the background value reference sample.

[0021] Further, the heavy metals include copper, zinc, lead, cadmium, nickel, chromium, arsenic, and mercury.

[0022] Further, the method for determining the dredging width in step S4 is:

[0023] S4.1. Determine the content of each type of chemical in the dredged bottom sediment layer, the soil dry bulk density, and the content of each chemical in the normal bottom sediment layer, the soil dry bulk density of the work unit; the chemicals are total nitrogen, total phosphorus, organic carbon, sulfide, and heavy metals; S4.2. Calculate the exceeding standard coefficient of each type of chemical in the dredged bottom sediment layer according to formula (1), which is the exceeding standard degree of a certain chemical in the bottom sediment:

[0024]

[0025] Where, α i is the exceeding standard coefficient of the current chemical; γ1 is the soil dry bulk density of the dredged bottom sediment layer; H1 is the dredging depth; is the measured content of the chemical in the dredged bottom sediment layer; is the background value of the chemical in the uncontaminated bottom sediment; i is the sequential number of the chemical;

[0026] S4.3. Calculate the compensation coefficient of the bottom mud according to formula (2), i.e. the content of a certain chemical substance in the normal layer of the bottom mud:

[0027]

[0028] wherein β i is the compensation coefficient of the current chemical substance; γ2 is the dry bulk density of the soil in the normal layer of the bottom mud; H0 is the thickness of the bottom mud; is the measured content of the chemical substance in the normal layer of the bottom mud; H1 is the dredging depth; is the background value of the chemical substance in the unpolluted bottom mud;

[0029] S4.4. If when the bottom mud layer should be completely removed;

[0030] If when calculate the dredging width according to formula (3) and formula (4):

[0031] X = max (X 1 , X 2 ,..., X i ) formula (3)

[0032]

[0033] wherein X i is the dredging width determined according to the current chemical substance; M is the width of one working unit; X is the dredging width; α i is the exceeding standard coefficient of the current chemical substance; β i is the compensation coefficient of the current chemical substance; i is the serial number of the chemical substance.

[0034] Further, the dredging depth of the reserved zone in step S6 is calculated according to formula (5):

[0035]

[0036] wherein H2 is the dredging depth of the dredging ship in the reserved zone; H1 is the dredging depth; M is the width of one working unit; X is the dredging width.

[0037] Further, the submerged plants in step S7 are Myriophyllum verticillatum L., Ceratophyllum demersum L. and Hydrilla verticillata.

[0038] (1) Myriophyllum verticillatum L.: Take 5-10 cm healthy stem segments and insert them into the water bottom soil, bury 3-5 cm, keep the stem top exposed to the mud surface, and the plant spacing is 15-20 cm.

[0039] (2) Ceratophyllum demersum L. : No need to bury, directly place the stem section in the water body, or use fine stones to fix the base of the stem section to grow stably, with a plant distance of 20-30 cm.

[0040] (3) Hydrilla verticillata (L.f.) Royle: Insert healthy stem sections or top buds into the soil at the bottom of the water, bury to a depth of about 3-5 cm, with a plant distance of 20-30 cm.

[0041] Further, the microorganism agent for degrading pollutants in step S7 is Rhodospirillum, Nitrosomonas, and Streptomyces.

[0042] (1) Rhodospirillum: Rhodospirillum mainly degrades organic matter through anaerobic photosynthesis in ecological restoration, improves the oxidation-reduction conditions of the sediment, and its metabolic activity can consume harmful substances (such as sulfides and ammonia nitrogen), and release oxygen to promote the development of plant roots, while also promoting nitrogen cycling and reducing the risk of sediment eutrophication. The amount of Rhodospirillum to be added is 0.5-1 kg of bacterial liquid per mu, and the concentration is diluted 500-1000 times (according to the volume ratio of the original microorganism solution to water), and the concentration of the bacterial liquid usually contains 10 7 -10 8 CFU / ml; frequency: once every 10 days, continuously for 2-4 times, and the specific amount is adjusted according to the water body conditions.

[0043] (2) Nitrosomonas: Nitrosomonas can oxidize ammonia nitrogen in water and sediment to nitrite through the process of ammonia oxidation, reducing the toxicity of ammonium nitrogen to water and sediment organisms, and providing a nitrogen source for plants and other microorganisms, especially suitable for water or sediment remediation with severe nitrogen pollution. The amount of Nitrosomonas to be added is 1-1.5 kg of bacterial liquid per mu, and the concentration is diluted 300-500 times, and the concentration of the bacterial liquid usually contains 10 8 -10 9 CFU / ml; frequency: once every two weeks, continuously for 3 times, and the amount and frequency are adjusted according to the ammonia nitrogen concentration in the water body.

[0044] (3) Streptomyces: Streptomyces is a kind of actinomycete with strong organic matter degradation ability, especially in decomposing complex organic matter such as cellulose and lignin, and inhibiting pathogenic microorganisms in the sediment, which can reduce the threat of harmful bacteria to the rhizosphere environment of submerged plants. In addition, Streptomyces can also fix heavy metals in the sediment through chelation, reducing their bioavailability and reducing the impact of heavy metal pollution on the water ecosystem. The amount of Streptomyces to be added is 0.5-1 kg of bacterial liquid per mu, and the concentration is diluted 400-800 times, and the concentration of the bacterial liquid usually contains 10 7-10 8 CFU / ml; frequency: once every two weeks, continuous 4-5 times, according to the sediment pollutants (such as organic matter and heavy metal content) to determine whether to increase the frequency of release.

[0045] In order to realize the multiple effect synergy, the microbial agent for degrading pollutants is preferably mixed and released in a mass ratio of 1:1:1, is diluted 500 times and is sprayed, so that the total release amount is not more than 2 kg / mu. The release is started on the third day after the dredging and filling is completed, and is continuously released for 3 times with an interval of 10 days. Submerged plants are planted, and the microbial agent for degrading pollutants is released, so as to quickly cover the bare lake bed, and the sediment is fixed through the plant root system, and the secondary pollution is reduced.

[0046] The present application has the following advantages: the present application provides a strip dredging method for promoting subtropical freshwater lake ecological restoration, the working unit is dynamically divided according to the narrowest length of the water area, and the working width is determined by using a segmented rule; the dredging depth is determined by the depth of the contaminated sediment layer; the uncontaminated sediment is collected, the background values of total nitrogen, total phosphorus, organic carbon, sulfide and heavy metal indicators are determined, the chemical content and dry bulk density of the dredging layer and the normal layer are analyzed, the exceeding standard coefficient and the compensation coefficient are calculated; the dredging width is derived by formula, the dredging strip and the reserved strip width are determined, and then the dredging is implemented; the sediment in the reserved strip is filled to the dredging strip; finally, the submerged plants are planted and the microbial agent is released, so as to effectively fix the sediment and promote the reconstruction of the ecological system. Compared with the prior art, the present application takes the uncontaminated sediment as the background sample, comprehensively evaluates the sediment pollution characteristics of the target water area, determines the sediment dredging strip and the reserved strip with appropriate width on this basis, and promotes the ecological restoration of the dredging strip by using the filling technology. The present application combines the dredging measures for reducing the internal source load and the ecological measures for protection and restoration, can reduce the content of the surface newly generated pollutants at the sediment-water interface, retains the original sediment biological community of the aquatic ecological system, and realizes the dredging of the freshwater lake. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 is a schematic diagram of the present application, wherein a is a horizontal schematic diagram of water surface dredging working unit division, b is a cross-sectional schematic diagram of water surface dredging working unit division, and c is a cross-sectional schematic diagram of the dredging working unit; in the figure, M is the width of a dredging working unit; H0 is the sediment depth; H1 is the dredging strip depth, and X is the dredging strip width. DETAILED DESCRIPTION

[0048] The present application is further described below in combination with the drawings and examples, and the protection scope of the present application is not limited to the following: example 1: a certain lake area needs to be dredged. The water area of the lake is large, and the sediment pollution problem is serious. According to the previous ecological investigation and evaluation, it is determined that the lake sediment is mainly polluted by heavy metals (copper, chromium) and nutrients.

[0049] 1. Basic information of the lake:

[0050] (1) Lake width: 80m;

[0051] (2) Water area: 2500m 2 ;

[0052] (3) Lake functions: mainly used for urban landscape, rainwater storage, ecological protection and wildlife habitat.

[0053] 2. A dredging method comprising the following steps:

[0054] S1. Determine the operation unit: Determine the dredging operation unit based on the target water area, such as Figure 1 As shown in Figures a and b, the method for determining the dredging operation unit is as follows: let the narrowest length of the water surface of the target water area be W 目标 , the width of the operating unit is M,

[0055] (1) If W 目标 ≥110m, M is 50m, if less than 50m, proceed as (3);

[0056] (2) If W 目标 <110m and greater than or equal to 60m, M=(W 目标 -10) / 2;

[0057] (3) If W 目标 <60m, M=W 目标 -10;

[0058] Lake to be desilted: water surface width W 目标 : 80m; operating unit width M: according to (2), M is 35m; number of operating units: water surface width 80 / 35≈2 operating units.

[0059] S2 determines the dredging depth: Determine the depth of the dredging operation unit determined in step S1, and determine the dredging depth;

[0060] The sediment depth is based on the actual depth of the sediment survey. The sediment layers include the contaminated sediment layer (Layer A), the contaminated transition layer (Layer B), and the normal layer (Layer C). Within each operating unit, multiple sediment depths are sampled and surveyed, and the average value is taken as the final sediment depth. The average of the total depth of the contaminated sediment layer (Layer A) and the contaminated transition layer (Layer B) is taken as the dredging depth. Sediment coring is used to sample sediment depths at each operating unit in the target area to determine the dredging depth for each operating unit.

[0061] Test results: sampling survey in a certain work unit obtained five places of bottom mud depth of 1.2 m, 1.1 m, 0.9 m, 0.8 m and 1 m respectively; the total depth of the contaminated bottom mud layer (A layer) and the contaminated transition layer (B layer) is 55 cm, 52 cm, 60 cm, 65 cm and 68 cm respectively;

[0062] Bottom mud depth H0: H0 = (1.2 + 1.1 + 0.9 + 0.8 + 1) / 5 = 1 m;

[0063] Dredging depth H1: H1 = (55 + 52 + 60 + 65 + 68) / 5 = 60 cm = 0.6 m.

[0064] S3. Measure the background value: take the uncontaminated bottom mud of the target water area, measure the background value of total nitrogen, total phosphorus, organic carbon, sulfide and heavy metals (copper, chromium); according to the sediment sampling, analyze the chemical background value of the uncontaminated area Total nitrogen 300 mg / kg, total phosphorus 100 mg / kg, organic carbon (TOC) 40000 mg / kg (4%), copper (Cu) 20 mg / kg, chromium (Cr) 50 mg / kg.

[0065] S4. Determine the width of the dredging zone and the reserved zone: according to the content of each chemical substance in the bottom mud dredging layer and the normal layer of the bottom mud of the dredging work unit and the dry bulk density (dry bulk density = dry weight / total volume, kg / m 3 ), combined with the background value of each chemical substance of the uncontaminated bottom mud of the target water area measured in step S3, the width of the dredging zone is determined; the dredging work unit is composed of the dredging zone and the reserved zone; as Figure 1 shown in -c, a dredging work unit, the bottom mud (depth H0, m) is a silt layer, which contains a pollution layer, a transition layer and a normal layer, the dredging zone (depth H1, m) contains a pollution layer and a transition layer, and the reserved zone (width m) is located on both sides of the dredging zone.

[0066] S4.1. Test the content of each chemical substance in the bottom mud dredging layer and the dry bulk density γ1, the content of each chemical substance in the normal layer and the dry bulk density γ2:

[0067] Test results of bottom mud dredging layer (A layer + B layer): and γ1:

[0068] Total nitrogen 600 mg / kg, total phosphorus 200 mg / kg, organic carbon 55000 mg / kg (5.5%), copper 40 mg / kg, chromium 80 mg / kg, γ1: 1.2 kg / m 3 ;

[0069] Normal layer (C layer) test results and γ2:

[0070] Total nitrogen 280 mg / kg, total phosphorus 90 mg / kg, organic carbon 480 mg / kg, copper 12 mg / kg,

[0071] Chromium 40 mg / kg, γ2: 1.5 kg / m 3 ;

[0072] S4.2. Calculate the over-standard coefficient (α i ) of each type of chemical substance in the dredged sediment layer according to formula (1), i.e. the over-standard degree of a certain chemical substance in the sediment:

[0073]

[0074] Dry bulk density of the dredged sediment layer soil: γ1=1.2 kg / m 3 , sediment thickness: H0=1 m (including the total thickness of the pollution layer, transition layer and normal layer), according to formula (1):

[0075] Total nitrogen α 1 =1.2×0.6×(600-300)=216 mg / kg;

[0076] Total phosphorus α 2 =1.2×0.6×(200-100)=72 mg / kg;

[0077] Organic carbon α 3 =1.2×0.6×(55000-40000)=10800 mg / kg;

[0078] Copper α 4 =1.2×0.6×(40-20)=14.4 mg / kg;

[0079] Chromium α 5 =1.2×0.6×(80-50)=31.6 mg / kg;

[0080] S4.3. Calculate the compensation coefficient (β i ) of the sediment according to formula (2), i.e. the content of a certain chemical substance in the normal layer of the sediment:

[0081]

[0082] Normal layer soil dry bulk density: γ2= 1.5 kg / m 3 According to formula (2), we have

[0083] Total nitrogen β 1 = 1.5 x (1.4 - 0.6) x (300 - 280) = 24 mg / kg;

[0084] Total phosphorus β 2 = 1.5 x (1.4 - 0.6) x (100 - 90) = 12 mg / kg;

[0085] Organic carbon β 3 = 1.5 x (1.4 - 0.6) x (40000 - 480) = 47424 mg / kg;

[0086] Copper β 4 = 1.5 x (1.4 - 0.6) x (20 - 12) = 9.6 mg / kg;

[0087] Chromium β 5 = 1.5 x (1.4 - 0.6) x (50 - 40) = 12 mg / kg;

[0088] S4.4. Due to The dredging width is calculated according to formula (3) and formula (4):

[0089] X = max(X 1 , X 2 ,..., X i ) formula (3)

[0090]

[0091] The settlement result is:

[0092] Total nitrogen X 1 = (1 - 24 / 216) x 35 = 31.11 mg / kg;

[0093] Total phosphorus X 2 = (1 - 12 / 72) x 35 = 29.17 mg / kg;

[0094] Organic carbon X 3 = (1 - 47424 / 10800) x 35 = -118.69 < 0 (indicating that the organic carbon content is lower than the background value);

[0095] Copper X 4 = (1 - 9.6 / 18) x 35 = 16.33 mg / kg;

[0096] Chromium X 5 =(1-12 / 31.6) x 35 = 21.7 mg / kg

[0097] The dredging width: X = max(X 1 , X 2 , …, X i ) = 31.11 m.

[0098] S5. Dredging: dredging according to the dredging depth determined in step S2 and the dredging width determined in step S4.

[0099] S6. Blowing filling: determining the reserved zone dredging depth, using a cutter suction dredger to dredge the reserved zone and blowing fill the mud to the dredging zone to accelerate the ecological restoration of the dredging zone. The dredger is 23 m long, 6 m wide, 0.9 m deep in heavy load, and the maximum dredging depth is 6 m. The required water area conditions for construction are water depth ≥1.1 m and width ≥23 m. The lake meets the conditions, and the dredged mud is blown to the dredging zone to accelerate the ecological restoration of the dredging zone.

[0100] The reserved zone dredging depth is calculated according to formula (5):

[0101]

[0102] Reserved zone dredging depth:

[0103] S7. Post-processing: after the blowing filling work is completed, Myriophyllum spicatum is planted, and 5-10 cm healthy stem segments are inserted into the soil under the water, buried 3-5 cm, keeping the stem top exposed to the mud surface, with a plant spacing of 15-20 cm. And the diluted Streptomyces (containing 10 7 -10 8 CFU / ml of effective bacteria in the bacterial solution) is put on the surface of the lake bed after dredging, 1 kg of bacterial solution per mu, and is put every two weeks for 4 times, in order to fix the bottom mud and the pollutants therein and promote ecological restoration.

[0104] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can make equivalent replacement or change according to the technical solution and the inventive concept of the present application within the technical range disclosed by the present application, which is covered by the protection scope of the present application.

Claims

1. A dredging method for promoting ecological restoration of subtropical freshwater lakes, characterized in that: It includes the following steps: S1. Determine the operation unit: Determine the dredging operation unit based on the target water area, where the narrowest length of the target water area is ≥33m; S2. Determine the dredging depth: Measure the bottom mud depth of the dredging operation unit and determine the dredging depth; S3. Determine background values: Take uncontaminated sediment from the target waters and determine the background values ​​of total nitrogen, total phosphorus, organic carbon, sulfide, and heavy metals; S4. Determine the widths of the desilting and retention zones: Determine the widths of the desilting and retention zones based on the total nitrogen, total phosphorus, organic carbon, sulfide, and heavy metal contents and soil dry bulk density in the desilting and normal sediment layers of the desilting unit, combined with the background values ​​of various chemical substances in the uncontaminated sediments of the target waters as determined in step S3. The desilting unit consists of the desilting and retention zones. S5. Desilting: Desilting is performed according to the desilting depth determined in step S2 and the desilting width determined in step S4; S6. Fill by blowing: Determine the dredging depth of the retention zone, dredge the dredged soil in the retention zone, and blow the excavated soil into the desilting zone; S7. Post-treatment: Plant submerged plants according to the water habitat, and then release microbial agents that degrade pollutants to promote sediment fixation and ecological restoration.

2. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The method for determining the dredging operation unit in step S1 is as follows: assuming that the narrowest length of the target water surface is W 目标 , the width of the operating unit is M, (1) If W 目标 ≥110m, M is 50m, if less than 50m, proceed as (3); (2) If W 目标 <110m and greater than or equal to 60m, M=(W 目标 -10) / 2; (3) If W 目标 <60m, M=W 目标 -10.

3. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: If the difference between the widest and narrowest parts of the water surface in the target water area is greater than 10m, the water surface of the target water area shall be divided into multiple areas and then the operation units shall be determined to ensure that the difference between the widest and narrowest parts of the water surface in each area does not exceed 10m.

4. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The dredging depth in step S2 is the total depth of the contaminated sediment layer and the contaminated transition layer, and the minimum dredging depth is ≥10 cm.

5. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The heavy metals include copper, zinc, lead, cadmium, nickel, chromium, arsenic and mercury.

6. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The method for determining the width of the desilting belt in step S4 is: S4.

1. Determine the content of each chemical substance and the soil dry bulk density in the desilting layer of the silt and the content of each chemical substance and the soil dry bulk density in the normal silt layer of the silt desilting unit; the chemical substances are total nitrogen, total phosphorus, organic carbon, sulfide, and heavy metals; S4.

2. Calculate the excess coefficient of each chemical substance in the desilting layer according to formula (1), that is, the degree of excess of a certain chemical substance in the silt: Among them, α i is the excess coefficient of the current chemical substance; γ1 is the dry bulk density of the soil in the dredging layer; H1 is the dredging depth; is the measured content of the chemical substance in the dredged sediment layer; is the background value of the chemical substance in uncontaminated sediment; i is the sequential number of the chemical substance; S4.

3. Calculate the sediment compensation coefficient according to formula (2), that is, the content of a certain chemical substance in the normal layer of the sediment: Among them, β i is the compensation coefficient of the current chemical substance; γ2 is the dry bulk density of the normal layer of sediment; H0 is the sediment thickness; is the measured content of the chemical substance in the normal layer of sediment; H1 is the dredging depth; is the background value of chemicals in uncontaminated sediment; S4.

4. If The bottom mud layer should be completely removed; Ruodang Calculate the desilting width according to formula (3) and formula (4): X = max(X 1 , X 2 ,..., X i ) Equation (3) Among them, X i is the dredging width determined according to the current chemical substances; M is the width of one operation unit; X is the dredging width; α i is the excess coefficient of the current chemical substance; β i is the compensation coefficient of the current chemical substance; i is the sequence number of the chemical substance.

7. The dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The dredging depth of the reserved zone in step S6 is calculated according to formula (5): Among them, H2 is the dredging depth of the dredger in the retention zone; H1 is the dredging depth; M is the width of an operating unit; and X is the dredging width.

8. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The submerged plants in step S7 are foxtail algae, hornwort and black algae.

9. A dredging method for promoting ecological restoration of subtropical freshwater lakes according to claim 1, characterized in that: The microbial agents for degrading pollutants in step S7 are Rhodospirillum, Nitrosomonas and Streptomyces.

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