River ecological restoration and habitat function restoration method based on cascade reservoir dismantling

Abstract

The invention discloses a riverway ecological restoration and habitat function restoration method based on cascade reservoir dismantling, and belongs to the technical field of water conservancy projects. The method comprises the following steps: step 1, silt conveying; step 2, restoring the river channel; thirdly, the sand flushing gate is rebuilt or complementarily built; and 4, adjusting the discharged flow of the cascade power station. The method comprises the following steps: starting from the whole watershed step, after silt conveying and to-be-dismantled dam dismantling are completed, restoring and restoring the ecological and habitat functions of river channels at the upstream and downstream of the to-be-dismantled dam, providing a suitable habitat for aquatic animals and plants, and reconstructing or supplementing a sand flushing gate B of a next-stage dam at the downstream of the to-be-dismantled dam so as to improve the sand adjusting and flushing capacity of the next-stage dam; and the risk of dam break of the next-stage dam is reduced, and the discharge flow of the upper-stage dam and the next-stage dam of the to-be-disassembled dam is adjusted so that the discharge flow can meet the ecological flow requirement.

Description

Technical Field

[0001] The invention relates to a method for river ecological restoration and habitat function recovery based on the removal of cascade reservoirs, and belongs to the technical field of water conservancy engineering. Background Art

[0002] With the advancement of ecological civilization construction, environmental issues have received widespread attention from all walks of life, among which the fish habitat problem of reservoir dams has always been the focus of attention. Reservoir dams block river connectivity, fragment natural river channels and fish habitats, and interfere with fish foraging and migration; the storage and operation of reservoir dams affect the natural rhythm of rivers, change the water flow signals that stimulate fish spawning and hatching, interfere with fish reproduction, and thus threaten the continuation of fish populations.

[0003] In order to reduce the impact of fish passages blocked by cascade reservoirs or dams, the current measures to restore and connect fish passages focus on the construction of fish passage facilities or the addition of fish breeding stations. However, according to incomplete statistics, there are 6,539 reservoir dams that have been downgraded and scrapped in my country, a large number of which 99% are medium and small reservoir dam projects, most of which were built in the 1970s to 1990s. As of the end of 2020, 3,528 small hydropower reservoir dams in the Yangtze River Economic Belt have been withdrawn. Due to the blocking effect of reservoir dams on fish migration channels, some reservoirs or dams are about to be demolished. The removal of cascade reservoir dams faces a series of problems: first, it affects the transport of river sediment. The soil erosion below the removed reservoir dam is serious, the river dies, and then gullies are formed, the upstream flow velocity increases, the sediment carrying capacity increases, and the riverbed and embankments on both sides of the downstream are eroded; second, it affects the river landform. After the dam is removed, the upstream river channel will undergo upstream erosion, causing the upstream river channel to become narrower and deeper, the slope to increase, the riverbed to decrease, and the bank collapse to cause the river channel to widen, which in turn causes the sediment washed downstream by the upstream to be deposited downstream of the reservoir area, making the riverbed and embankments on both sides of the downstream to be eroded. The lateral and vertical directions of the river channel are adjusted, and the river channel becomes shallower and wider; thirdly, it affects the water quality of the river. After the dam is removed, pollutants in the reservoir water will no longer accumulate, and the water quality of the reservoir will be affected downstream as the dam is removed. The polluted sediment and suspended matter in the reservoir area have the risk of causing water pollution downstream, and too much silt will consume a large amount of oxygen in the river channel, causing an oxygen-deficient environment, which is not conducive to the growth of fish. Excessive decomposition of organic matter will also produce toxic gases, which will directly cause fish poisoning, lower the pH value of the water, and increase the acidity, causing harm to fish.

[0004] At present, the research on cascade reservoirs or dams focuses on the construction method of demolition, such as the method of demolishing old dams provided in the patent document with the announcement number CN111765824B, which determines the demolition method of old dams from the aspects of determining the demolition elevation of the water retaining sill, block blasting, blasting height and demolition height, and realizes the control of the gap size to prevent large water flows when the dam is demolished on a large scale. However, it does not focus on the research and discussion of ecological restoration methods after dam demolition.

[0005] Another example is the patent document with the announcement number CN116145610B, which provides a method for ecological demolition and ecological restoration of silt-accumulated reservoirs. Through pre-dam demolition observation and evaluation, sand discharge and flow gathering, mechanical sand removal, dam section demolition, exposed section survey, dam section repair and other steps, the safe implementation of small hydropower dam demolition on small rivers is achieved in a safe and orderly manner, and the habitat at the dam site is restored by using the original terrain of the river channel. Although the invention considers ecological restoration of the dam site and the exposed sections upstream and downstream of the adjacent dam site, it only restores the ecology from the perspective of a single demolished dam site, and does not consider the ecological restoration methods and theories after the demolition of cascade dams from the perspective of the overall cascade of the basin. Therefore, it has the disadvantage of not being applicable to the restoration of the ecology after the demolition of cascade dams. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a method for river ecological restoration and habitat function recovery based on the demolition of cascade reservoirs.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for river ecological restoration and habitat function recovery based on the removal of cascade reservoirs, comprising the following steps:

[0009] Step 1: Sediment transportation: In the non-flood season and before the demolition of the dam to be demolished, the sand flushing gates of the reservoir dams at all levels are opened to discharge sediment to fill the impact pit backfill area at the tail of the reservoir dams at all levels, and gradually reduce the amount of sediment in front of the dam before the demolition of the dam to be demolished;

[0010] Step 2: River restoration: remove the dam to be demolished, and then repair and restore the river ecology and habitat functions upstream and downstream of the dam to be demolished;

[0011] Step 3: Reconstruction or supplementation of the sand flushing gate: Reconstruction or supplementation of the sand flushing gate B of the next dam downstream of the dam to be demolished;

[0012] Step 4: Adjust the downstream flow of cascade power stations: According to the ecological flow demand, adjust the downstream flow of the upper and lower dams of the dam to be demolished.

[0013] In the step 1, the sand flushing gate C of the dam to be demolished is first opened, and the gate opening of the sand flushing gate C is controlled, so that the amount of sediment in front of the dam before the demolition of the dam to be demolished is gradually reduced, and in this process, the impact deep pit backfill area at the tail of the next dam reservoir downstream of the dam to be demolished is backfilled with the discharged sediment;

[0014] Next, the sand flushing gate A of the upper dam upstream of the dam to be demolished is opened, and the gate opening of the sand flushing gate A is controlled so that the sediment discharged from the upper dam is just flushed to the impact pit backfill area at the tail of the dam to be demolished;

[0015] Finally, the sand flushing gate B of the next-level dam is opened, and the gate opening of the sand flushing gate B is controlled so that the sediment discharge from the next-level dam can be flushed to the impact pit backfill area behind the dam.

[0016] The gate opening of the sand flushing gate A is smaller than the gate opening of the sand flushing gate B.

[0017] The original riverbed line upstream and downstream of the dam to be demolished will be lowered after the sediment transportation is completed. After the dam to be demolished is demolished and the cleaning is completed, the impact deep pit backfill area at the tail of the dam to be demolished will be further backfilled, and the exposed convex reduction area will be rectified to form a riverbed line with a stable slope after demolition, and then the river ecology and habitat functions upstream and downstream of the dam to be demolished will be repaired and restored.

[0018] In the second step, the river ecology and habitat functions upstream and downstream of the dam to be demolished are repaired and restored, including bank protection repair, riverbed repair and fish habitat habitat function repair.

[0019] The bank protection and repair method is: using gabion stone cage structures to form retaining walls to enhance the natural properties of bank slope protection. The gabion stone cage structure is formed by filling blocks of stone, pebbles, wood piles or living tree stumps in the gabion net.

[0020] The riverbed restoration method is: on the basis of the natural morphology of the river channel upstream and downstream of the dam to be demolished, and after completing the cleaning of the dam to be demolished and its reservoir area, the river channel morphology is prevented from natural reshaping, specifically, the river channel is set into an asymmetric section by backfilling and leveling, deep pools and shallow shoals are constructed, and sand, pebbles and gravel of various specifications and particle sizes are added to the deep pools and shallow shoals to form a sand, pebble and gravel bottom material, so as to change and adjust the local erosion and siltation of the river, so that silt is deposited to form a natural sandbar, shape a variety of cross-sectional flow velocities and water depths, and provide a variety of habitats for aquatic animals and plants.

[0021] The method for restoring the habitat function of fish habitats is to set up a variety of ecological habitat reinforcement structures inside the river channel to increase the diversity of water flow, so that the water flow will produce different degrees of scouring on the riverbed, so as to improve the micro-topography inside the river channel, strengthen the habitat function of the river channel, and at the same time improve the habitat diversity level and biodiversity level in the river channel.

[0022] The ecological habitat enhancement structure is a small groyne, weir, tree stump, gravel pile or shelter.

[0023] The method for reconstructing or supplementing the sand flushing gate B of the next dam downstream of the dam to be demolished in step three is: enlarging the size of the sand flushing gate B of the next dam downstream of the dam to be demolished, or increasing the number of sand flushing gates B of the next dam downstream of the dam to be demolished.

[0024] The beneficial effects of the present invention are:

[0025] 1. From the perspective of the overall cascade of the river basin, after the completion of sediment transportation and the demolition of the dam to be demolished, the river ecology and habitat functions upstream and downstream of the dam to be demolished shall be repaired and restored to provide a suitable habitat for aquatic plants and animals. The sand flushing gate B of the next dam downstream of the dam to be demolished shall be rebuilt or supplemented to improve the sand regulating and flushing capacity of the next dam and reduce the risk of dam breach of the next dam. The discharge flow of the upper dam and the next dam of the dam to be demolished shall be adjusted to adapt to the ecological flow demand.

[0026] 2. Through sediment transportation, the amount of sediment accumulation in front of the dam to be demolished is decomposed or reduced, which helps to improve the demolition efficiency of the dam to be demolished and reduces the construction difficulty of demolishing the dam to be demolished. In addition, during the demolition of the dam to be demolished, the sediment in front of the upper and lower dams is cleared.

[0027] 3. Use natural materials such as blocks of stone, pebbles, wooden piles, and living tree stumps to construct porous "flexible ecological revetments" instead of non-porous steel bars and concrete hard revetments. Flexible ecological revetments have good stability and durability, increase the porosity of the river bank, provide a porous habitat for organisms, and integrate with the surrounding natural environment to reflect the natural landscape of the river.

[0028] 4. On the basis of the natural morphology of the river channel upstream and downstream of the dam to be demolished, and after the cleaning of the dam to be demolished and its reservoir area, the river channel morphology will be prevented from natural reshaping. Specifically, the river channel will be set into an asymmetric section by backfilling and leveling, and deep pools and shallows will be constructed. Sand, pebbles and gravel of various specifications and particle sizes will be added to the deep pools and shallows to form a sand, pebble and gravel bottom, so as to change and adjust the local erosion and siltation of the river, form natural sandbars by siltation, and shape diverse cross-sectional flow velocities and water depths, provide a variety of habitats for aquatic plants and animals, and facilitate a variety of fish to rest and hide and complete their life history.

[0029] 5. Set up a variety of ecological habitat reinforcement structures inside the river channel to increase the diversity of water flow, so that the water flow will have different degrees of scouring on the riverbed, so as to improve the micro-topography inside the river channel, strengthen the habitat function of the river channel, and at the same time improve the level of habitat diversity and biodiversity in the river channel, and realize the restoration of the habitat function of fish habitats.

[0030] 6. On the basis of normal construction of cofferdam, a sand flushing gate for the next level dam is constructed on the next level dam to improve and meet the sand regulating and flushing capacity between the previous level dam and the next level dam, and to prevent the next level dam from bursting due to excessive siltation.

[0031] 7. After the demolition of the dam to be demolished, the water area at the dam site will expand, resulting in an increase in the number of water reduction sections, which will further lead to the water area being unable to continue to be coordinated with the downstream flow of the upper-level dam. Therefore, the downstream flow of the upper-level dam and the lower-level dam has been adjusted to meet the ecological flow needs upstream and downstream of the dam to be demolished. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the longitudinal section structure of the cascade reservoir of the present invention;

[0033] Figure 2 It is a schematic diagram of the top view structure of the cascade reservoir of the present invention;

[0034] Figure 3 It is a schematic diagram of the longitudinal section structure when the riverbed is repaired during the process of dismantling the dam to be demolished according to the present invention;

[0035] Figure 4 for Figure 3 Section view along AA;

[0036] Figure 5 for Figure 4 A partial enlarged schematic diagram of

[0037] Figure 6 It is a schematic diagram of the top view structure when the riverbed is repaired during the process of dismantling the dam to be demolished according to the present invention;

[0038] Figure 7 for Figure 6 Sectional view along BB;

[0039] Figure 8 It is a schematic diagram of the plan layout of the sand flushing gate of the next-level dam of the present invention;

[0040] Fig. 9 This is a schematic diagram of the longitudinal section structure after the dam to be demolished is removed according to the present invention;

[0041] Fig.10 This is a schematic diagram of the top view of the structure after the dam to be demolished is removed according to the present invention.

[0042] In the figure: 1-dam to be demolished, 2-water surface line before demolition, 3-original riverbed line, 4-silt in front of the dam before demolition, 5-impact deep pit backfill area, 6-deep pool and shallow area, 7-sand flushing gate A, 11-previous dam, 12-next dam, 13-dam bedrock, 14-mountains on both sides, 21-water surface line after demolition, 31-riverbed line after demolition, 42-silt in front of the dam after demolition, 51-naked convex reduction area, 61-bank protection repair area, 62-sand and gravel bottom, 63-gabion stone cage structure, 71-sand flushing gate B, 72-previous dam modified / supplementary sand flushing gate, 73-next dam modified / supplementary sand flushing gate, 731-gate pier, 74-overflow dam section. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further described below, but the scope of protection claimed is not limited to the description.

[0044] like Figures 1 to 10 As shown, the method for river ecological restoration and habitat function recovery based on cascade reservoir demolition of the present invention comprises the following steps:

[0045] Step 1, sediment transport: In the non-flood period, and before the demolition of the dam to be demolished 1, the sand flushing gates of the reservoir dams at all levels are opened to fill the impact pit backfill area 5 at the tail of the reservoir dams at all levels by discharging sediment, and gradually reduce the amount of sediment 4 in front of the dam before the demolition of the dam to be demolished 1. Under the premise of river hydropower development, multiple cascade reservoir dams are arranged, including the dam to be demolished 1, the upper dam 11, the lower dam 12, the dam bedrock 13, the mountains on both sides 14 and the original riverbed line 3.

[0046] Step 2: River channel restoration: demolish the dam 1 to be demolished, and then repair and restore the river channel ecology and habitat functions upstream and downstream of the dam 1 to be demolished.

[0047] Step 3: Reconstruction or supplementation of the sand flushing gate: Reconstruction or supplementation of the sand flushing gate B71 of the next-level dam 12 downstream of the dam 1 to be demolished.

[0048] Step 4: Adjust the discharge flow of cascade power stations: According to the ecological flow demand, adjust the discharge flow of the upper dam 11 and the lower dam 12 of the dam to be demolished 1. After the demolition of the dam 1 to be demolished, the water area at its dam site will expand, resulting in an increase in the water reduction section, which will further cause the water area to be unable to continue to coordinate with the discharge flow of the upper dam 11. Therefore, it is necessary to adjust the discharge flow of the upper dam 11 and the lower dam 12 to meet the ecological flow demand of the upstream and downstream of the dam 1 to be demolished.

[0049] In the step 1, the sand flushing gate C of the dam 1 to be demolished is first opened, and the gate opening of the sand flushing gate C is controlled, so that the amount of sediment 4 in front of the dam 1 to be demolished is gradually reduced before the demolition of the dam 1 to be demolished, and in this process, the impact deep pit backfill area 5 at the tail of the next level dam 12 downstream of the dam 1 to be demolished is backfilled with the discharged sediment;

[0050] Next, the sand flushing gate A7 of the upper dam 11 upstream of the dam to be demolished 1 is opened, and the gate opening of the sand flushing gate A7 is controlled so that the sediment discharged from the upper dam 11 is just flushed to the impact pit backfill area 5 at the tail of the dam to be demolished 1;

[0051] Finally, open the sand flushing gate B71 of the next-level dam 12, and control the gate opening of the sand flushing gate B71 so that the sediment discharged from the next-level dam 12 is just flushed to the impact pit backfill area 5 behind the dam. Before demolishing the dam 1 to be demolished, it is necessary to decompose or reduce the amount of sediment 4 deposited in front of the dam before demolition. Therefore, first control the gate opening of the sand flushing gate C to gradually reduce the amount of sediment 4 in front of the dam 1 to be demolished, which will help improve the demolition efficiency of the dam 1 to be demolished and reduce the construction difficulty of demolishing the dam 1 to be demolished. When the water level is low and suitable for large-scale mechanical construction operations, mechanical sand clearing can also be combined. It is necessary to reasonably adjust the gate openings of the sand flushing gates A7 and B71 to prevent too much sediment from entering the impact pit backfill area 5. In addition, compared Figure 1 and Fig. 9 It can be found that the amount of sediment 42 in front of the dam after the demolition of the upper dam 11 and the lower dam 12 is significantly less than the amount of sediment 4 in front of the dam before demolition, that is, the present invention completes the sediment transportation during the process of demolishing the dam 1 to be demolished.

[0052] The gate opening of the sand flushing gate A7 is smaller than the gate opening of the sand flushing gate B71.

[0053] The original riverbed line 3 upstream and downstream of the dam 1 to be demolished will be lowered after the sediment transportation is completed. After the dam 1 to be demolished is demolished and cleaned up, the impact pit backfill area 5 at the tail of the dam 1 to be demolished will be further backfilled, and the bare convex reduction area 51 will be rectified to form a riverbed line 31 with a stable slope after demolition, and then the river ecology and habitat functions upstream and downstream of the dam 1 to be demolished will be repaired and restored. Figure 4 As shown, the riverbed line 31 after demolition is much lower than the original riverbed line 3. With the clearing of sediment, the water depth l is further reduced, and the water surface line 21 after demolition is also lower than the water surface line 2 before demolition, which increases the water volume ratio of the basin where the dam 1 to be demolished is located.

[0054] In step 2, the river ecology and habitat functions upstream and downstream of the dam 1 to be demolished are repaired and restored, including bank repair, riverbed repair and fish habitat habitat function repair. The upstream and downstream river of the dam 1 to be demolished refers to the river section between the upper dam 11 and the lower dam 12.

[0055] The bank protection repair method is: using gabion stone cage structure 63 to pile up to form a retaining wall (i.e. Figure 4The gabion gabion structure 63 is formed by filling blocks of stone, pebbles, wood piles or living tree stumps in the gabion net. When in use, other structures with the same function can also be used to replace the gabion gabion structure 63. The blocks of stone are purchased, or the available materials produced by excavation are transported to the construction area by dump trucks for storage. During the construction process, the blocks of stone, pebbles, wood piles or living tree stumps are transported to the working surface by manual rubber-wheeled carts. After the gabion gabion structure 63 is manually manufactured, the gabion gabion structure 63 is hoisted into place by machinery.

[0056] The silt deposited in the reservoir area of ​​the dam to be demolished, especially in the water depth l section, has been immersed in water all year round and the soil texture is loose. After the water surface line is lowered, it is easy to form river bank collapse, and at the same time it will cause the river to be isolated from the original ground and riverbank of the riverbed, destroying the basis for the survival of riverbank vegetation and aquatic plants and animals. The solidified revetment blocks the water and air circulation of the river channel and riverside vegetation, which not only deprives terrestrial plants of their living space, but also makes some aquatic animals lose their living and refuge places, making them easily washed away by floods, making it difficult for aquatic plants and animals to survive, resulting in the loss of the natural purification capacity of the river basin and the destruction of the river ecology. Therefore, the revetment needs to be repaired. The reservoir area of ​​the dam 1 to be demolished should embody the concept of "near-natural ecological restoration" during the ecological restoration process. Natural materials such as blocks of stone, pebbles, wooden piles, and living tree stumps should be used to construct porous "flexible ecological revetments" to replace non-porous steel bars and concrete hard revetments. Flexible ecological revetments have good stability and durability, increase the porosity of the river bank, provide a porous habitat for organisms, and integrate with the surrounding natural environment to reflect the natural landscape of the river.

[0057] The riverbed restoration method is: on the basis of the natural morphology of the upstream and downstream river channels of the dam 1 to be demolished, and after the cleaning of the dam 1 to be demolished and its reservoir area is completed, the river channel morphology is prevented from natural reshaping, specifically, the river channel is set into an asymmetric section by backfilling and flattening, a deep pool and shallow beach area 6 is constructed, and sand, pebbles and gravels of various specifications and particle sizes are added to the deep pool and shallow beach area 6 to form a sand, pebble and gravel bottom material 62, so as to change and adjust the local erosion and siltation of the river, so that the silt is deposited to form a natural sandbar, shape a variety of cross-sectional flow rates and water depths, provide a variety of types of habitats for aquatic plants and animals, and facilitate a variety of fish to rest and hide, and complete their life history. When in use, the upstream and downstream riverbeds of the dam site of the dam 1 to be demolished will present complex river channel geomorphological conditions, which need to be cleaned and revetted to restore the natural river channel. During the river channel regulation process, construction should be avoided as much as possible during the flood season and the fish breeding season, and earthwork is excavated by excavators, supplemented by manual cleaning. In addition, slag that can be used for backfilling should be temporarily stored at a location selected within the construction site.

[0058] The method for restoring the habitat function of fish habitats is to set up a variety of ecological habitat reinforcement structures inside the river channel to increase the diversity of water flow, so that the water flow will produce different degrees of scouring on the riverbed, so as to improve the micro-topography inside the river channel, strengthen the habitat function of the river channel, and at the same time improve the habitat diversity level and biodiversity level in the river channel.

[0059] The ecological habitat enhancement structure is a small groyne, weir, tree stump, gravel pile or shelter.

[0060] The method for reconstructing or re-building the sand flushing gate B71 of the next dam 12 downstream of the dam 1 to be demolished in step 3 is: enlarging the size of the sand flushing gate B71 of the next dam 12 of the dam 1 to be demolished, or increasing the number of sand flushing gates B71 of the next dam 12 of the dam 1 to be demolished. With the increase of the water storage time of the reservoir dam and the influence of the reservoir dam barrier, the sediment will accumulate more and more in front of the dam. Although the construction of the reservoir dam will consider the construction of sand flushing gates during the construction process, so as to discharge the sediment through the sand flushing gates to reduce its impact on the reservoir dam, the reduction or removal of the cascade reservoir dams in the mainstream of the river will lead to the weakening of the step-by-step barrier effect on the sediment, which will indirectly lead to the increase of the sediment deposition pressure of the downstream reservoir dam, and the increase of sediment pressure will bring the risk of dam failure. It can be seen that when the dam 1 to be demolished is demolished, the sand storage pressure of the next-level dam 12 will increase, resulting in problems such as low sand flushing efficiency or insufficient space of the original sand flushing gate B71 of the next-level dam 12; therefore, on the basis of the normal construction cofferdam, it is necessary to build a next-level dam modification / supplementary sand flushing gate 73 on the next-level dam 12 to improve and meet the sand regulation and flushing capacity between the previous dam 11 and the next-level dam 12, and prevent the next-level dam 12 from bursting due to excessive siltation; in the process of building the next-level dam modification / supplementary sand flushing gate 73, the overflow dam section 74 can be modified and gate piers 731 can be added to meet the construction conditions of the next-level dam modification / supplementary sand flushing gate 73, and its gate operation can use the gate opening and closing equipment of the overflow dam section 74 to reduce operating costs.

[0061] Similarly, when the upper-level reservoir dam of the upper-level dam 11 is demolished, the same problem will be faced, and it is necessary to build the upper-level dam modification / supplementary sand flushing gate 72 on the upper-level dam 11.

[0062] The invention is mainly applicable to the ecological optimization of reservoir dams that have fallen into disrepair and have no hydropower and water conservancy functions, and the demolition of tributary cascade power stations for mainstream hydropower development and construction.

Claims

1. A method for river ecological restoration and habitat function recovery based on the removal of cascade reservoirs, characterized by: The following steps are involved: Step 1: Sediment transportation: During the non-flood period and before the demolition of the dam to be demolished (1), the sand flushing gates of the reservoir dams at all levels are opened to discharge sediment to fill the impact pit backfill area (5) at the tail of the reservoir dams at all levels, and gradually reduce the amount of sediment (4) in front of the dam before the demolition of the dam to be demolished (1); Step 2: River channel restoration: dismantle the dam to be demolished (1), and then repair and restore the river channel ecology and habitat functions upstream and downstream of the dam to be demolished (1); Step 3: Reconstruction or supplementation of the sand flushing gate: Reconstruction or supplementation of the sand flushing gate B (71) of the next dam (12) downstream of the dam to be demolished (1); Step 4: Adjust the outflow of the cascade power stations: According to the ecological flow demand, adjust the outflow of the upper dam (11) and the lower dam (12) of the dam to be demolished (1).

2. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 1, characterized in that: In the step 1, the sand flushing gate C of the dam to be demolished (1) is first opened, and the gate opening of the sand flushing gate C is controlled, so that the amount of sediment (4) in front of the dam to be demolished (1) is gradually reduced before demolition, and in this process, the impact deep pit backfill area (5) at the tail of the reservoir of the next level dam (12) downstream of the dam to be demolished (1) is backfilled with the discharged sediment; Next, the sand flushing gate A (7) of the upper dam (11) upstream of the dam to be demolished (1) is opened, and the gate opening of the sand flushing gate A (7) is controlled so that the sediment discharged from the upper dam (11) is just flushed to the impact pit backfill area (5) at the tail of the dam to be demolished (1); Finally, the sand flushing gate B (71) of the next-level dam (12) is opened, and the gate opening of the sand flushing gate B (71) is controlled so that the sediment discharged from the next-level dam (12) is just flushed to the impact pit backfill area (5) behind the dam.

3. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal as claimed in claim 2, characterized in that: The gate opening of the sand flushing gate A (7) is smaller than the gate opening of the sand flushing gate B (71).

4. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 1, characterized in that: The original riverbed line (3) upstream and downstream of the dam (1) to be demolished will be lowered after the sediment transportation is completed. After the dam (1) to be demolished is demolished and cleaned up, the impact deep pit backfill area (5) at the tail of the dam (1) to be demolished is further backfilled, and the exposed convex reduction area (51) is rectified to form a riverbed line (31) after demolition with a stable slope, and then the river ecology and habitat functions upstream and downstream of the dam (1) to be demolished are repaired and restored.

5. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 1, characterized in that: In the second step, the river ecology and habitat functions upstream and downstream of the dam (1) to be demolished are repaired and restored, including bank protection repair, riverbed repair and fish habitat function repair.

6. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 5, characterized in that: The bank protection and repair method comprises: using a gabion stone cage structure (63) to form a retaining wall to enhance the natural property of bank slope protection; the gabion stone cage structure (63) is formed by filling a gabion net with blocks of stone, pebbles, wood piles or living tree stumps.

7. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 5, characterized in that: The riverbed restoration method is as follows: based on the natural morphology of the river channel upstream and downstream of the dam to be demolished (1), and after the cleaning of the dam to be demolished (1) and its reservoir area is completed, the river channel morphology is prevented from being reshaped naturally, specifically, the river channel is set into an asymmetric section by backfilling and leveling, a deep pool and shallow beach area (6) is constructed, and sand, pebbles and gravel of various specifications and particle sizes are added to the deep pool and shallow beach area (6) to form a sand, pebble and gravel bottom material (62), so as to change and adjust the local erosion and siltation of the river, so that the silt is deposited to form a natural sandbar, and a variety of cross-sectional flow velocities and water depths are formed, providing a variety of types of habitats for aquatic animals and plants.

8. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 5, characterized in that: The method for restoring the habitat function of fish habitats is to set up a variety of ecological habitat reinforcement structures inside the river channel to increase the diversity of water flow, so that the water flow will produce different degrees of scouring on the riverbed, so as to improve the micro-topography inside the river channel, strengthen the habitat function of the river channel, and at the same time improve the habitat diversity level and biodiversity level in the river channel.

9. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 8, characterized in that: The ecological habitat enhancement structure is a small groyne, weir, tree stump, gravel pile or shelter.

10. The method for river ecological restoration and habitat function recovery based on cascade reservoir removal according to claim 1, characterized in that: The method for reconstructing or supplementing the sand flushing gate B (71) of the next dam (12) downstream of the dam (1) to be demolished in step 3 is: enlarging the size of the sand flushing gate B (71) of the next dam (12) of the dam (1) to be demolished, or increasing the number of the sand flushing gates B (71) of the next dam (12) of the dam (1) to be demolished.

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