Three gorges reservoir area sediment dredging opportunity determining method based on asynchronous water and sediment transportation

Abstract

The invention discloses a three gorges reservoir area sediment dredging opportunity determining method based on asynchronous water and sediment transportation. The method comprises the following steps: firstly, determining initial working time meeting construction conditions based on a water level process and ship operation parameters; furthermore, the core of the method is to quantitatively calculate the time lag of a sand peak reaching a target dredging river reach relative to a water peak in combination with a water-sand asynchronous transportation rule. And the initial work starting time and the lag amount are superposed and corrected to obtain the optimized dredging starting opportunity capable of avoiding the sediment deposition peak, and the required total construction period is calculated according to factors such as the work amount and the equipment efficiency. According to the method, the back-silting peak period is scientifically avoided, the dredging efficiency can be remarkably improved, the effective period of the dredging effect is prolonged, and the engineering cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of waterway dredging engineering, in particular to a construction timing optimization method suitable for large reservoir area dredging engineering, and especially suitable for solving the reservoir sedimentation problem under the "water-sediment asynchronous transport" phenomenon caused by the combined action of reservoir regulation and basin water and sediment. BACKGROUND

[0003] In natural river channels, the transport processes of flood and sediment are usually coupled, that is, high flow corresponds to high sediment concentration, and the time of flood peak and sediment peak is basically synchronous. However, after the operation of the Three Gorges Reservoir, due to the regulation and storage effect of the reservoir, the water and sediment processes in the upstream are separated in the reservoir area, forming a unique "water-sediment asynchronous transport" phenomenon: the time of flood peak reaching a certain section of the waterway is no longer consistent with the time of sediment peak reaching. This phenomenon makes the erosion and deposition law of the reservoir waterway, especially the tail and the variable backwater area, become extremely complex, and in a certain period and area, the sedimentation problem is increasingly prominent, becoming a major factor affecting the water depth and navigation safety of the waterway.

[0004] In order to ensure that the reservoir waterway meets the design water depth requirements, regular maintenance dredging must be carried out. At present, the selection of dredging time largely depends on engineering experience, or the traditional method suitable for natural river channels or other types of water areas is used. For example, in the middle and lower reaches of the Yangtze River or estuary areas, the determination of dredging time mainly considers the convenience of construction in dry season or tidal, wave and other marine hydrodynamic conditions. Some existing researches, such as the dredging discussion of sand and gravel channels such as Taipingkou and Lujiahe in the Yangtze River, or the optimization of sediment throwing time based on certain criteria (such as minimum sediment loss rate), although they have certain reference value in certain areas and certain aspects, but their core mechanism is essentially different from the main control factor of "water-sediment asynchronous transport" in the Three Gorges reservoir area.

[0005] These existing technologies and methods do not fully consider the non-synchronous characteristics of water flow and sediment transport under the operation mode of the Three Gorges Reservoir. If the dredging time is not properly selected, for example, dredging is carried out when the sediment peak is about to arrive or is passing through, the dredged channel will be quickly filled with subsequent sediment, resulting in a significant reduction in dredging effect, causing huge economic waste, and may have adverse effects on navigation due to frequent operations. On the contrary, if the water and sediment transport processes can be accurately predicted, dredging can be carried out in the favorable time window of "clear water flushing", which can achieve the best maintenance effect with the least engineering quantity, and significantly improve the dredging effect.

[0006] Therefore, the prior art has obvious deficiencies in dealing with the problem of siltation in the Three Gorges Reservoir area, and lacks a systematic method that can scientifically and accurately determine the dredging opportunity. In order to solve this technical problem, it is urgent to develop a method for determining the dredging opportunity of the Three Gorges Reservoir area, which is specially designed for the characteristics of water and sediment in the Three Gorges Reservoir area, can comprehensively consider the reservoir regulation, the process of interval inflow and sediment, and take the "water-sediment asynchronous" as the core mechanism, so as to improve the scientificity, economy and effectiveness of channel maintenance. SUMMARY

[0007] The purpose of the present application is to overcome the deficiencies of the prior art and provide a method for determining the dredging opportunity of the Three Gorges Reservoir area based on water-sediment asynchronous transport.

[0008] As pointed out in the background art, after the operation of the Three Gorges Reservoir, the law of water and sediment movement in the reservoir area has changed fundamentally, forming a unique phenomenon of "water-sediment asynchronous transport", that is, the transmission of flood peak and sediment peak in time and space is no longer synchronized. However, the existing methods for determining the dredging opportunity are mainly for natural rivers, estuaries or port areas, and do not fully consider this core water-sediment characteristic of the Three Gorges Reservoir area.

[0009] Therefore, the current dredging operation in the Three Gorges Reservoir area mainly relies on engineering experience for the selection of opportunity, and lacks scientific and quantitative decision-making basis. This leads to the possibility that the dredging operation is carried out just before or during the arrival of the sediment peak, so that the dredged channel is quickly filled up by the subsequent sediment, not only causing the dredging effect to be greatly reduced and the channel maintenance period to be shortened, but also leading to a great waste of engineering investment and unnecessary interference to normal navigation.

[0010] Therefore, the core technical problem to be solved by the present application is: how to accurately predict and avoid the peak of sediment transport, scientifically determine the "time window" of dredging operation, and thus maximize the dredging efficiency, prolong the effective period of dredging effect and reduce the engineering cost.

[0011] To achieve the above-mentioned purpose, the present application provides a method for determining the dredging opportunity of the Three Gorges Reservoir area based on water-sediment asynchronous transport, which combines the traditional construction period selection based on water level process with the unique water-sediment asynchronous transport law of the reservoir area, and systematically optimizes the starting time and duration of dredging operation through quantitative calculation. The technical scheme of the present application specifically includes the following steps: Step one: based on the water level process, calculate the initial dredging opportunity T L ) This step first determines a preliminary construction window from the perspective of meeting the basic hydrological conditions of dredging operation. The core that affects this opportunity is the water level process, because the water level determines whether the channel is navigable and whether the dredging ship can operate smoothly.

[0012] Specifically, based on the navigation grade requirements and hydrological characteristics of the target dredging area, a suitable water level range for dredging is set. a , b The lower limit of this interval ( ). a (Minimum water level) and upper limit ( b The highest water level is determined by the following factors: the dredging design water level ( H s), dredging to maintain water depth ( h w ), dredging and maintenance with reserved water depth ( h y) and the dredging vessel's own operational parameters, mainly the maximum underwater vertical depth of the vessel's boom ( L ).

[0013] Meanwhile, considering the need for construction coordination and operational continuity, the duration for which this water level condition must be met is required ( T A It must be greater than a preset minimum construction period threshold. N By analyzing the historical water level processes of the target waterway in recent years, the earliest time point that satisfies both the aforementioned water level range and the minimum construction period threshold is identified, and this point is determined as the initial dredging opportunity. T L ).

[0014] Step 2: Based on the asynchronous transport pattern of water and sediment, calculate the time difference ( ΔT y) This step is the core of the invention, aiming to quantify the impact of "asynchronous water and sediment transport" on dredging timing. Water peaks (flood peaks) propagate in the river channel as dynamic waves at a relatively high speed; while the movement of sediment peaks (peak sediment transport) is closely related to the actual flow velocity, and its propagation process is influenced by the characteristics of the incoming water and sediment, as well as the river channel boundary conditions. In the Three Gorges Reservoir area, due to the significant increase in water depth and the reduction in water surface gradient, the flow velocity decreases substantially, and the transport velocity of sediment particles is much slower than the propagation velocity of the water peaks.

[0015] This step involves collecting and analyzing historical hydrological and sediment data from the target dredging area and its upstream major control hydrological stations (such as Cuntan Station) over many years. It calculates the time required for both water and sediment peaks to propagate from the upstream control stations to the target dredging area under different flow rates. By calculating the time difference between the two, a representative value is obtained representing the time difference between the sediment peak and the water peak in reaching the target area. ΔT (y). This difference reflects the degree of delay in the main sediment deposition period relative to the flood period.

[0016] Step 3: Integrate the above factors to determine the final timing for starting dredging. T f ) Initial dredging opportunities will only consider water level factors.T L ) and the time difference considering the sediment transport law ( ΔT y ) are superimposed to obtain the final optimized dredging start time ( T f ). The calculation method is: T f = T L + ΔT y .

[0017] By increasing a delay of ΔT y , it is ensured that the start time of dredging operation has missed the peak period of sediment transport following the flood, and the construction is carried out in the favorable period of "clear water flushing".

[0018] Step four: based on the engineering parameters, the duration of dredging is determined ( T A ) After determining the optimal start time, the total time required to complete the entire dredging project needs to be calculated. The duration of dredging is mainly determined by three factors: total dredging amount ( V ): it is composed of two parts, one part is the planned excavation amount calculated according to the topographic survey before construction ( V d ), and the other part is the increment estimated considering the inevitable back silting during construction, which is usually corrected by the dredging back silting rate ( ɑ ). Therefore, the actual total dredging amount V = (1 + ɑ ) * V d .

[0019] daily average dredging efficiency ( V h ): the average daily effective dredging volume determined according to the type and performance of the dredging ship and the geological conditions of the construction area (such as gravel layer, bedrock, etc.).

[0020] construction extension time ( ΔT T ): the additional time delay caused by unforeseen factors such as the contradiction between navigation and construction operation, bad weather, equipment failure, complex geological conditions (such as hard bedrock difficult to dig), etc.

[0021] Based on the above factors, the calculation method of the duration of dredging ( T A ) is: T A= V / V h + ΔT T 。

[0022] Through the above four steps, the application provides a complete and quantifiable opportunity decision scheme for the channel dredging project in the Three Gorges Reservoir area, including scientific answers to two key problems of "when to start" and "how long it needs".

[0023] The application has the following significant beneficial effects compared with the prior art: scientificity and accuracy are significantly improved: the application first takes the unique "water-sediment asynchronous transport" law of the Three Gorges Reservoir area as the core decision basis and quantifies it, and builds a binary coupling decision model including the water level process and the sediment transport process. This makes the determination of the dredging opportunity change from the traditional experience judgment to the scientific calculation based on the water-sediment movement law, and the decision is more accurate and reliable. The dredging effect and the channel maintenance period are significantly prolonged: by determining the dredging opportunity through the method, the sediment transport peak can be effectively avoided, and the operation is carried out in the relatively stable "clear water scouring" period. This fundamentally avoids the passive situation of "dredging before siltation and siltation while dredging", so that the channel formed by dredging can be maintained for a longer time, effectively guaranteeing the channel navigation conditions and prolonging the channel maintenance period. The economic and social benefits are huge: due to the consolidation of the dredging effect, the engineering quantity of invalid dredging and repeated dredging is reduced, which directly saves huge engineering cost and energy consumption. At the same time, the longer channel maintenance period also means less construction operation time, reduces the interference to the normal navigation of the Yangtze River golden waterway due to construction, and brings significant economic and social benefits. It has good popularization and application value: the theoretical framework and calculation method established by the application are not only suitable for the channel dredging project of different river sections in the Three Gorges Reservoir area, but also provides an important technical reference for the dredging and silt reduction and channel maintenance work in other large reservoirs, downstream of cascade hubs and other water areas with similar water-sediment asynchronous transport characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical scheme of the disclosed embodiment, the drawings of the embodiment will be briefly introduced below, which are only used for the purpose of illustration, and are not intended to limit the protection scope of the application.

[0025] Figure 1 is a runoff channel dredging water level relationship diagram of the application.

[0026] Figure 2 is a method flowchart of the application. DETAILED DESCRIPTION

[0027] The technical solutions of the present application (including the preferred technical solution) will be described in further detail below by means of the accompanying drawings and by listing some optional embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of the present application.

[0028] The present application takes a typical navigation-obstructing shoal in the Fuling-Fengdu reach of the Three Gorges reservoir area as an example to elaborate the application process of the method for determining the dredging opportunity provided by the present application. After the impoundment of the Three Gorges reservoir, the sedimentation problem is typical due to the effects of perennial backwater and asynchronous sediment transport.

[0029] Step A, calculation of water level process influence condition The water level process is an important factor affecting the navigation maintenance dredging opportunity. Through the aforementioned analysis of the dredging opportunity influence factors, the dredging water level of the inland waterway should meet the conditions that the mechanical arm of the ship can be extended into the river bottom and the water depth is sufficient for the ship to normally enter, which is related to the parameters such as the dredging design water level, the dredging maintenance water depth, the maintenance reserved water depth, the full load draft of the dredging ship, and the maximum underwater vertical depth of the ship with the arm extended. Through the analysis of the dredging water level relationship (see Figure 1 ), it is assumed that A The event is the basic requirement of the water level suitable for dredging, which can be calculated according to formula (5-1): (5-1) In the formula: H ʹ The water level suitable for dredging; a The minimum water level value suitable for dredging; b is the maximum water level value suitable for dredging; H S The dredging design water level, which can be determined by the guarantee rate method, the minimum water level method, the minimum water level plus the reserved water depth, etc. in the inland waterway maintenance dredging; h w The dredging maintenance water depth, which is generally published by the waterway operation and maintenance unit; h y The dredging maintenance reserved water depth, which can be selected according to the characteristics of the river channel, and is generally 0.3-0.5m; L The maximum underwater vertical depth of the dredging ship with the arm extended.

[0030] Considering the time required for construction to develop, after the water level reaches the requirement, it should be sustained for a certain period of time to meet the basic requirement of construction time, therefore, the sustained time required is set as T (A) is the sustained time required to meet the requirement, and the condition should meet formula (5-2): A T (A) ≥ N (5-2) In formula (2): N To consider the factors such as ship deployment, the minimum sustained time of construction water level is met.

[0031] Step B, calculation of riverbed erosion and deposition conditions: riverbed erosion and deposition is an important factor that determines the deposition of shallow areas and is also an important factor that determines the starting time of dredging. It is mainly caused by water and sediment transport. In this method, the asynchronization of water and sediment caused by water and sediment transport is mainly considered. The difference between the time of water and sediment reaching the shallow area is denoted as .

[0032] Step C, calculation of dredging start time: the dredging start time should be calculated in combination with the two factors: = (5-3) In the formula: is the dredging start time; is the start time considering only the water level process. In combination with the water level process of the waterway in recent years, the earliest start time of dredging is determined under the conditions of meeting formula (1) and formula (2); is the difference between the time of water and sediment reaching the shallow area.

[0033] Step D, determination of dredging duration: the dredging amount is generally composed of two parts, the surveying dredging amount and the back deposition amount. The formula for calculating the actual total dredging amount is as follows: (5-4) In the formula: V is the actual total dredging amount, V d is the surveying dredging amount, ɑ is the dredging back deposition rate.

[0034] The dredging duration calculation mainly considers three factors, the total dredging amount, the actual dredging efficiency per day, and the dredging construction extension time due to navigation conditions, geology, etc. It is calculated by the following formula: +Δ (5-5) In the formula: ​The actual dredging duration; V The total amount of dredging; The daily dredging amount; Delta The dredging construction extension time due to navigation conditions, geological conditions and the like, and the dredging is recommended to be determined according to the actual dredging situation in the past.

[0035] The dredging opportunity determination method of the application is executed in the following steps: Step 1: Calculate the initial startable dredging time based on the water level process T L This step aims to determine a preliminary construction window that meets the hydrological conditions of dredging operation.

[0036] Determine the dredging operation parameters: Target river section: a shoal in the Fuling-Fengdu river section.

[0037] Channel maintenance requirements: It is assumed that the dredging maintenance water depth required to be met in the river section h w is 5.0 meters. According to the “Technical Specifications for Inland Waterway Maintenance”, and combined with the erosion and deposition characteristics of the river section, the dredging maintenance reserved water depth h y is set to 0.5 meters.

[0038] Dredging ship parameters: a 4m 3 hard arm type grab dredger is planned to be put into operation. According to its performance parameters, the maximum underwater operation depth L is 18.0 meters.

[0039] Minimum construction period requirement: considering the ship deployment, preparation for entry and construction efficiency, the duration of water level that meets the operation conditions should not be less than N = 5 days.

[0040] Calculate the suitable dredging water level interval a , b ]: The calculation of the interval is as follows: First, determine the dredging design water level H s . For the river section of the reservoir area, it is generally beneficial to dredge at a lower water level during the non-flood season, and it is assumed that the dredging design water level H s is selected as 160.0 meters (Wusong elevation).

[0041] Calculate the minimum dredging water level a = Hs - h w - h y = 160.0 - 5.0 - 0.5 = 154.5 meters. This water level is the critical water level to ensure the post-dredging channel depth meets the standard.

[0042] Calculate the highest dredging water level b = H s - h w - h y + L = 154.5 + 18.0 = 172.5 meters. This water level is the highest water level limit for the dredger's mechanical arm to effectively reach the designed excavation bottom boundary.

[0043] Therefore, the appropriate dredging water level interval H is [154.5 meters, 172.5 meters].

[0044] Determine the initial dredging opportunity T L : Retrieve the daily historical water level data of the target river section in the previous 3-5 years. Analyze the water level process curve and find the starting date that meets the condition of "water level located in the interval [154.5 meters, 172.5 meters], and the duration of this state is not less than 5 days". Through analysis, in typical hydrological years, this condition is usually met from the end of the flood season to the beginning of the pre-flood recession period. Assuming that after checking and analyzing, the earliest date that meets the condition is November 10 of the current year.

[0045] Therefore, the initial dredging opportunity T L is November 10.

[0046] Step 2: Based on the asynchronous transport rule of water and sediment, calculate the time difference ( ΔT y ) This step aims to quantify the delay of sediment transport peak relative to the flood peak.

[0047] Data sources and analysis: Through statistical analysis of the measured water and sediment process data from 2003 to 2018, the time of water peak and sand peak propagating to different sections under different flow levels is obtained.

[0048] Calculate the time difference ΔT y : For the target area of this example: The average time for the water peak to propagate from Cuntan to Fengdu is about 7.6 hours.

[0049] The average time for the sand peak to propagate from Cuntan to Fengdu is about 36.2 hours.

[0050] Therefore, the time difference between the arrival of the sand peak and the water peak at the Fengdu reach is ΔT y = 36.2 hours - 7.6 hours = 28.6 hours, approximately equal to 1.2 days.

[0051] This time difference ΔT y is the amount of delay in sediment transport that needs to be considered in this method.

[0052] Step 3: Integrate factors to determine the final dredging start time T f ) Integrate the results of the previous two steps.

[0053] Calculation formula: T f = T L + 1.2 days ΔT y Substitute the numerical values: T f = November 10 + 1.2 days Determine the timing: Considering the convenience of project arrangement, round off the calculation result and leave a small margin to determine the final optimized dredging start time T f as November 12.

[0054] Through this step, the starting time of dredging operations not only meets the water level operation requirements, but more importantly, effectively avoids the peak of sediment transport following the flood peak, creating a favorable environment of "clear water and less sand" for dredging operations.

[0055] Step 4: Determine the dredging duration based on project parameters T A ) Calculate the total duration required for the entire dredging project.

[0056] Determine the project parameters: Surveyed dredging volume ( V d ): Through pre-construction topographic survey and design calculation, it is determined that the planned excavation volume for this dredging project is 200,000 cubic meters.

[0057] Dredging siltation rate (ɑ ) According to the historical dredging experience and the characteristics of the sediment in the reach, the siltation rate during construction is expected to be 20%.

[0058] Daily dredging efficiency V h ) Considering that the riverbed of the reach is mainly composed of pebbles mixed with sand and occasionally exposed bedrock, the geological condition is medium-hard, and the selected 4m 3 The comprehensive daily dredging efficiency of the hard-arm grab dredger under the water depth and geological conditions is expected to be 2,500 cubic meters per day.

[0059] Construction extension time ΔT T ) Considering the busy navigation demand (avoidance navigation) of the Yangtze River trunk line, the foggy weather in winter, the regular maintenance of the equipment, and the special treatment of the local hard bedrock that may be encountered, the total construction extension time is expected to be 10 days.

[0060] Calculate the dredging duration T A : First, calculate the total dredging amount V = V d * (1 + 20%) = 240,000 cubic meters. ɑ Then calculate the dredging duration

[0061] A = T / V h + 10 = 96 + 10 = 106 days. V ΔT T

[0062] Based on the above, through the specific implementation of the present application, for the typical dredging project of the Fuling-Fengdu reach in the Three Gorges Reservoir area, the final scientific construction scheme is determined as follows: Optimal starting date: November 12 of the same year.

[0063] Expected total construction period: 106 days.

[0064] It has been proved that, compared with the experience-based judgment, the dredging opportunity determined by the method provided by the present application can arrange the dredging operation in the period with the lowest siltation intensity, extend the stable period of the dredging effect by about 20%-30%, and effectively save about 15% of the engineering rework cost, thereby achieving significant technical and economic benefits.

[0065] ​​The above merely illustrates the specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any changes or replacements within the technical scope disclosed by the present application can be easily conceived by those skilled in the art, and should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport, characterized in that, Includes the following steps: Step A: Based on the preset dredging design water level, dredging maintenance water depth, dredging maintenance reserved water depth, and dredging vessel operation parameters of the target dredging section, and combined with historical water level data of the river section for more than 5 years, determine the earliest possible start time that only considers water level factors and meets the water level window requirements for dredging operations; Step B: By analyzing the difference between the water peak propagation time and the sand peak transport time from the upstream control station of the target dredging section to the river section, determine the time lag of the sand peak relative to the water peak in reaching the target dredging section caused by the asynchronous transport law of water and sand; Step C: Superimpose the earliest possible start time with the time lag to correct the dredging start timing, thereby obtaining an optimized dredging start timing that avoids the passage of sand peaks; Step D: Based on the measured topography and dredging plan and parameters, calculate the dredging volume, dredging siltation rate, daily operating efficiency of dredging vessels, and construction extension time caused by navigation and geological conditions, and calculate and determine the total duration required for dredging operations.

2. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1, characterized in that, In step A, meeting the water level window requirements for dredging operations also includes ensuring that the duration of the water level window is not less than the minimum duration preset to ensure vessel deployment and construction continuity.

3. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1, characterized in that, The dredging vessel operation parameters in step A include at least the dredging vessel's full-load draft and the vessel's maximum underwater operating depth with its boom extended.

4. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1, characterized in that, In step B, the water peak propagation time and the sand peak transport time are obtained by organizing and analyzing historical hydrological and sediment monitoring data of the target dredged section and its upstream main reservoir control hydrological stations.

5. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1 or 4, characterized in that, The mechanism of asynchronous water and sediment transport is as follows: water peaks propagate in the form of waves at a relatively high speed, while the transport of sediment peaks is directly related to the flow velocity of water. Under the conditions of increased water depth and decreased flow velocity in the Three Gorges Reservoir area, the transport velocity of sediment peaks is significantly reduced compared to natural river channels, resulting in sediment peaks arriving at the same river section significantly later than water peaks.

6. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1, characterized in that, The time lag in step B is determined by segmented calculation or interpolation based on the distance between the target dredged section and the upstream control station.

7. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1, characterized in that, In step D, the dredging volume consists of the initial dredging volume calculated based on topographic mapping and the incremental siltation estimated based on the dredging siltation rate.

8. The method for determining the timing of dredging in the Three Gorges Reservoir area based on asynchronous water and sediment transport according to claim 1, characterized in that, The geological conditions in step D refer to the fact that the riverbed of the target dredging section is mainly composed of pebbles, and there are local areas where the bedrock is hard and difficult to excavate, which leads to changes in dredging efficiency and thus extends the construction time.

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