A Brief Analysis of the Effect of Dredging Project on the Environment

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At present, people pay more and more attention to environmental problems. The impact of dredging projects on the environment has aroused the concern of many organizations and countries. Since environmental dredging can completely remove the polluted sediment and eliminate the potential factors affecting the water quality, it has become an important measure to treat the water. However, disturbance and improper disposal of polluted sediment in environmental dredging projects will cause secondary environmental pollution. Various technologies to reduce sediment disturbance and resource utilization of sediment have become research hotspots. Therefore, it is of great significance to study and analyze the factors and extent of dredging projects affecting the environment. This paper explores the dredging effect on the environment.

Dredging is divided into environmental dredging and engineering dredging. Engineering dredging is mainly for the purpose of capacity enhancement, navigation, and fishery application. Environmental dredging aims to improve water quality. At present, environmental pollution is becoming more and more serious, and people’s environmental awareness is becoming stronger, so the prospect of environmental dredging is broad.

1. Effect on dredging construction

For different dredging projects, different construction techniques and equipment are often adapted according to the dredging purpose, sediment characteristics, and hydrological environment, and construction conditions. The process of dredging is also varied and generally consists of major steps or stages such as underwater sediment excavation, vertical lifting of dredged sediment, horizontal transport, and disposal of sediment. Different stages of construction will have different effects on the environment.

 1.1 Subsea sediment excavation

The main purpose of this step is to remove the sediment to be dredged from its original position and to excavate it with diggers, buckets, reamers, and harrows. Under the extrusion and cutting action of the above device, the cohesive force of sediment on the bed surface is destroyed and displaced, and some sediment is transported to other places by the dredging equipment for further treatment. Some of the sediment is scattered near the dredging site and some of it is suspended in the water.

Environmental impacts during the excavation of sediment include: increasing the concentration of suspended sediment. Under the action of rotary cutting of dredging equipment, some sediment is suspended. The suspended sediment concentration depends on the basic characteristics of the sediment, the type of dredging equipment, excavation energy, vertical lifting method, and other construction parameters. Pollutants suspended in the water will be released into the water by desorption. The release intensity and rate are influenced by suspended sediment concentration, sediment background adsorption value, water physicochemical environment, and adsorption kinetics. Dredging sediment remains a new source of pollution if it contains pollutants.

 1.2 Dredging up the sediment vertically

The second key step in the dredging process is to lift the dredged sediment to the surface, either mechanically or hydraulically, depending on the dredging vessel used. The second key step in the dredging process is to lift the dredged sediment to the surface, either mechanically or hydraulically, depending on the dredging vessel used. When a mechanical dredger is used, dredged sediment is lifted to the surface in a bucket, while when a hydraulic dredger is used, dredged sediment is usually drawn through a suction pipe. Under the action of a centrifugal pump, the mud and water mixture is sucked into the suction pipe and transported to the destination through a mud discharge pipe.

Sediment impact on the environment includes: in the process of ascension if the scoop for mechanical transmission of type opening open that is, the process of ascension will be inevitable in contact with water, under the effect of water dilution and scour, the suspended sediment concentration in water will increase, suspended sediment concentration by dredging bucket type and capacity, speed and other factors. When hydraulic transport is adopted, the main influencing factor is the suction capacity of the dredging pipe. If the suction capacity is not enough to transport all the excavated sediment, a large amount of dredging residue will be generated to pollute the environment. When a harrow suction dredger or barge is used to load the mud-water mixture, the overflow will increase the concentration of suspended sediment and pollutants in the water.

 1.3 Dredging sediment disposal

After dredging sediment is transported to the destination, further disposal is carried out according to dredging purpose, sediment characteristics, and availability. Including land reclamation, beach maintenance, sea disposal, land disposal, isolation disposal, etc. Dredging sediment disposal is the final stage of a dredging project. The environmental impact varies depending on the location. Under the hydrodynamic action, if the use of underwater disposal, fine particles of sediment will be different degrees of diffusion and movement. If land disposal is adopted, such as land reclamation and beach maintenance, the main pollution mainly comes from residual water discharge and overflow in the process of sludge and water separation. In addition, the onshore treatment of dredged sediment may have a negative impact on the quality of groundwater near the disposal site if no protective measures are taken.

2. Optimize utilization technology and related processing characteristics

 2.1 Optimal utilization technology

In order to minimize the negative impact of dredging on the environment, the following points should be controlled during dredging of polluted sediment:

(1) Reduce the agitation of polluted sediment, and take anti-proliferation and leakage measures to ensure a high concentration of inhalation, so as to avoid the pollution of suspended pollutants to the surrounding water.

(2) Monitoring work should be done well to improve dredging accuracy, so as to completely remove the contaminated sediment, ensure the environmental protection dredging effect, and reduce the over dredging quantity as far as possible, so as to reduce the project cost.

(3) Reduce leakage during transportation and avoid secondary pollution to the water body and another environment.

(4) Dredging sludge should be treated safely to avoid secondary pollution near the storage yard. And it can be properly processed into available resources, with a view to turning waste into treasure.

(5) The dredging period should avoid the larval regeneration and breeding period of benthic organisms (usually the warmer months), and winter is the season with the least environmental risk for dredging of sediment, so the dredging project should be carried out in winter.

(6) Before the construction, the dredging vessel shall be comprehensively inspected to prevent oil leakage, and the household garbage generated by the dredger shall not be directly discharged into the water body to prevent the pollution of the water body.

(7) Do a good job in anti-noise work, avoid night construction, appropriate installation of sound absorption, and sound insulation materials.

 2.2 Characteristics of environmental dredging

The choice of dredging equipment directly determines the form of dredging: different dredging equipment has different disturbance degree to sediment, different diffusion and leakage performance, and different absorption concentration. Therefore, the pollution caused by the construction of the surrounding water is different. Currently, the research and development of international environmental dredging equipment fall into two categories:

(1) Make use of the original dredger to carry on the transformation to meet the requirements of environmental construction.

(2) Special environmental protection dredging equipment.

Using the original dredger is a widely used method at present, which is economical and practical The reform of dredging vessel focuses on dredging equipment and positioning control, that is, replacing the original dredging equipment with an environment-friendly reamer, and equipping with advanced high-precision positioning and monitoring system, so as to reduce the secondary pollution in the dredging process and meet the requirements of environmental dredging. From the technical point of view, the design and construction of special environmental protection equipment are not difficult, but from the economic point of view, the design and construction of special equipment are much higher than the transformation of traditional dredging equipment, and different projects need to build different equipment, for smaller projects, the economy is worse. Environmental dredging equipment, the more advanced is the use of suction dredger with an environmental reamer. In this way, the surface sediment is sucked up by the pipeline under the action of a dredging pump and transported to the land for a long distance. All these dredging vessels are equipped with an automatic control system and monitoring system, and DGPS is adopted for precise three-dimensional positioning, which greatly improves the dredging accuracy. This method can reduce sediment re-suspension and release of pollutants.

3. Conclusion

In a dredging project, detailed investigation and environmental impact assessment are carried out to further improve the design, select appropriate dredging equipment and take certain control measures to optimize the construction scheme, so as to minimize the adverse impact of the dredging project on the environment.

Discussion on Dredging Construction under the Concept of Environmental Protection

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The application of environmental protection concept in port and waterway dredging project can effectively reduce the impact and damage of port and waterway dredging construction on natural ecological environment, so as to realize the harmonious and sustainable development of ecological environment and social economy. In the actual dredging process of the waterway, construction management personnel should combine with the planning situation of waterway water and carry out the reasonable application of various construction technologies, so as to guarantee the construction quality in an all-round way. Based on this, this paper analyzes the dredging construction under the concept of environmental protection.

1. The role of environmental dredging construction

 1.1 Develop effective and suitable dredging schemes for ports and waterways

In the project, the possible pollution situation of a river channel can be investigated carefully, and the construction organization and design scheme can be arranged more reasonably according to the analysis and investigation results. Based on the analysis and investigation results, the pollutant is calculated. The dredging project construction plan is reasonably planned. The construction area of dredging work is scientifically and reasonably divided. The appropriate dredging technique is selected. The corresponding working system is formulated according to the plan, and the appropriate professional mechanical facilities are selected to ensure the normal dredging project of port and channel.

 1.2 Choose the right dredging time

In order to avoid the impact on the living environment of benthic organisms in the harbor, the construction should be carried out in a reasonable season and time. Another factor to consider is that the speed of seawater in coastal areas varies between high and low tides, making it difficult for suspended solids to sink.

 1.3 Choose the scientific method to treat sludge

The residual water and accumulated sludge must be filled with a scientific method. In the process of dredging and blowing-filling, the area of the mud discharge field should be increased to extend the natural precipitation time, and the vertical and horizontal ridges should be added in the mud discharge field to effectively control the construction surplus water. Sludge treatment methods mainly include biological dressing, centralized treatment, recycling, and closed stacking.

2. Environmental dredging construction

 2.1 Design scientific and reasonable dredging scheme of the harbor channel

First, the environment around the port-channel should be carefully investigated, and the factors and characteristics of the polluted sediment should be analyzed, and the preliminary paragraphs should be implemented according to the corresponding data. After that, a comprehensive evaluation is carried out according to the final result, and the corresponding construction scheme of port and channel dredging project is optimized according to the final result. The design work of each link should be carried out strictly in accordance with the corresponding order, scientific planning of the construction scope of the dredging project of port-channel, optimal selection of the most suitable construction method, and operation of dredging vessels and dredging equipment.

 2.2 Preparation for dredging construction

2.2.1 Setting-out survey

In the course of the lofting survey, the channel depth is evaluated in detail by using relevant measuring equipment, such as SDH-13d sounder, global positioning system, etc. Before the lofting and measuring work is carried out, the hydrological weather should be chosen as smooth as possible to ensure that the accuracy of relevant measuring instruments is not affected. In the actual measurement process, the precast slab or concrete can be used to make buoys, which can not only ensure the measurement accuracy but also reduce the cost of the channel dredging project.

2.2.2 Site layout

The site layout shall be conducted according to the relevant specifications of the bidding materials for the waterway dredging project. Considering regional water conservancy culture, urban overall planning, lake protection planning, and other factors comprehensively, the construction site layout is carried out on the premise of maintaining the previous water system ecosystem. Channel dredging project has a strong professional and layout process that needs to choose reasonable dredging equipment in the actual construction. According to the blowing distance and the requirements of the contract, select the appropriate wringing suction boat. At the same time, reasonable configuration of grab boat, mud pump, blowing mud boat, etc. are to ensure the smooth implementation of the overall construction.

 2.3 Choose dredging time and season scientifically

Choose the appropriate dredging period and season without affecting the survival of the habitat. Therefore, all kinds of work should take into full consideration the living creatures, and give priority to the winter with low-risk coefficient and less impact on the benthos. Choose a dry season to implement necessary dredging to avoid benthic growth time and reduce adverse factors to benthic. It is worth noting that it is also necessary to avoid as far as possible in the flow of relatively large spring tide or ebb time to be implemented. The dredging time of the port channel is determined mainly on the basis of the corresponding area, but it is generally chosen to be implemented in winter. The main reason is that winter is the lower stage of biological activities in the benthic habitat in port and waterway waters. In this period, the ecological and environmental protection of the whole water area was analyzed, and it was also the safest and most reliable construction stage. According to the requirements of environmental protection, dredging in winter should try to land the bottom mud, so that the diffusion degree of particles in the water area is relatively small.

 2.4 Strengthen water quality monitoring in dredging projects

During the dredging process, it is strictly forbidden for non-staff to carry out the construction area to minimize the impact of the outside on the surrounding water quality. At the same time, we should increase the detection of water quality in the construction area to master the water quality. The monitoring scope includes the concentration of water and mud, effective measures must be taken to strengthen the treatment of contaminated sediment, comprehensively improve the purification efficiency of water quality, and avoid secondary impact on the water quality around the dredging construction site of port and waterway.

 2.5 Strictly control the precision of dredging construction

In the dredging process of the harbor channel, dredging should be carried out reasonably according to the distribution of sediment. To avoid over-excavation, polluted sediment can be filled at the bottom of the filling area, followed by uncontaminated dredged soil covering the contaminated soil layer. The protection of the surrounding ecological environment can be strengthened from the source by reducing the quantity of sludge treatment. Therefore, we must strictly control the precision of construction, so as to ensure the scientific dredging technology of the port channel. For example, in the dredging process of the harbor channel, effective measures should be taken to avoid the spread of polluted sediment. In view of the leakage of polluted bottom sediment, the times of stirring polluted bottom sediment at the bottom of ports and rivers can be reduced to strengthen control and minimize the pollution caused by the leakage of bottom sediment. At the same time, the construction quality control should be strengthened in strict accordance with relevant laws and regulations to comprehensively improve the precision of the dredging technique of port and waterway. Due to the sediment thickness of polluted sediment in port and river waters ranging from 10cm to 15cm, the sediment thickness in some areas may exceed 1m. Therefore, the construction schedule is highly required.

conclusion

In short, with the rapid development of China’s economy, the importance of ports and waterways is increasing day by day, and many economic activities are highly dependent on ports. To improve the construction technology of port and waterway, the most direct effect is to improve the construction quality of port and waterway engineering, so as to provide the safest guarantee for the port’s shipping capacity. To a large extent, the port’s shipping capacity is directly reflected in the channel’s shipping capacity. The stronger the channel’s shipping capacity is, the stronger the port’s shipping capacity will be. Especially in the ecological environment, it is necessary to focus on strengthening control, so as to better meet the development requirements.

Research on Environmental Dredging and Technical Key Points

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Ecological environment protection is the theme of social development and construction in recent years. The concept of environmental protection is reasonably applied in dredging projects of ports and waterways. It can not only meet people’s requirements for the protection of the ecological environment but also enable dredging engineering enterprises to keep up with the needs of the times, so as to realize the sustainable development of ecological environment and social economy in modern life in China. Based on this, according to the characteristics of environmental dredging construction, this paper analyzes several factors of environmental impact caused by dredging construction of port-channel, so as to realize the purpose of environmental protection construction by strengthening the effective control of dredging construction technology.

1. Environmental dredging effect in ports and waterways

 1.1 Diffusion of pollutants in sludge substratum

Industrial wastewater, domestic sewage and dead biological debris discharged from the area near the port are discharged into the port with the help of surface runoff, and gradually accumulate sediments in the port channel. These sediments often contain nutritive salts, which tend to produce eutrophication when their content exceeds a certain level. This is not only easy to cause the body of water odor, aquatic life death, and damage to the water environment, but also easy to affect the normal operation near the port. Under normal conditions, the water body itself has a purification effect, as long as the content of nutrients in the water within a certain range, there will be no eutrophication phenomenon. However, if the sludge layer in the channel is not well treated before the port dredging project, the sludge will be stirred during the dredging process, which will aggravate the diffusion of pollutants in the sediment, release the nutrient salts in the pollutants, and easily produce eutrophication.

 1.2 Damage of inorganic suspended matter to the aquatic ecosystem

In the dredging project of the harbor channel, the inorganic suspended matter is the most common pollutant and the main pollution source in the water area. The dredging project will stir up the inorganic suspended matter in the water, and the closer it is to the dredging vessel, the higher the concentration of inorganic suspended matter will be, which will reduce the light transmittance, increase the turbidity of the water, and have an adverse impact on the aquatic ecosystem.

 1.3 Noise pollution and pollution of dredging vessels and dredging equipment

Dredging vessels will produce noise pollution during dredging and dredging construction, which will affect the surrounding ecological environment. And the dredging vessels and dredging equipment used in the port dredging project will also have an adverse effect on the ecological environment. For example, vessels and equipment may leak oil during construction, causing pollution to water bodies. At the same time, construction vessels will also discharge a certain amount of domestic sewage to the water body, which will adversely affect the water body.

2. The construction characteristics of the environmental dredging project

 2.1 Improve the precision of engineering construction

According to previous experience, the pollution blockage layer in waterway waters is generally between 10cm and 15cm. The positioning accuracy and excavation accuracy of engineering construction should be constantly improved during actual construction. Before excavation, a detailed field survey should be carried out to draw the corresponding schematic diagram of river excavation, and the size, depth, and width of the trench should be designed, so as to ensure that no leakage excavation, no over-excavation, and at the same time to prevent the production of greater pollution and damage to the balance of the surrounding ecological environment.

 2.2 Effectively reduce re-pollution during dredging

It is easy to cause sediment suspension during dredging and sediment leakage may occur during sediment transport. After the sediment deposition, attached to the above pollutants will be suspended and water, leading to insufficient sunlight, biological growth, and reproduction hindered. Therefore, special facilities are needed to treat the bottom sludge to prevent the pollutants from contaminating the water again.

 2.3 Professional monitoring of pollution sources

The pollution elements in the construction project are closely monitored to ensure that the water quality changes or pollution problems can be solved in time. In the channel dredging project, the common pollution factors that need to be monitored and analyzed usually include: mud diffusion degree, mud concentration, backwater concentration, suspended pollutant particle quantity, etc. While monitoring, the tailwater concentration in the discharge field should be strictly controlled.

 2.4 Cleaning of polluted sediments

Dredged sediment can also have a serious impact on the surrounding ecological environment, so appropriate measures should be taken to deal with the sediment. In order to prevent these sediments from polluting the accumulation site or nearby waters, it is necessary to adopt effective purification measures such as chemical decomposition or physical decomposition to avoid secondary pollution. At the same time, measures should be taken to control the concentration of polluted sediments to prevent polluting the ambient air.

3. Analysis of technical control points of dredging construction of harbor channel

 3.1 Port channel dredging construction control

Technical control of dredging construction is the most important prerequisite for the development of technology. In terms of construction control, the following points should be done:

First, technical agitation should be carried out on the bottom contaminated sediment of the channel, but the number of agitation should be controlled within a certain range (generally the agitation range is 4~8m), which is also to prevent further diffusion and aggravation of pollutants, affecting the ecological environment in other areas of the water. At the same time, certain non-proliferation construction technology measures should be taken to ensure that a high concentration of pollutants can be removed quickly, so as to avoid the transformation of pollutants into a suspended state and minimize the pollution level of surrounding water bodies.

Second, accurate positioning should be carried out for the dredging construction of port-channel, which should not only include the enhancement of the excavation accuracy (control the bottom layer of pollutants in the depth of 15cm~1m) but also should clarify the scope of dredging and excavation. If it is a waste polluted sediment, its dredging control should also be planned within the standard range. The general idea is to raise the technical standards of dredging significantly above those of ordinary dredging. Thirdly, the noise of large equipment such as diesel engines should be reduced as far as possible. During the dredging operation, the engine room door of the ship should be closed to ensure that the dredging operation will not disturb the construction and affect the construction quality due to too much noise. Due to the large pollution caused by the dredging of the harbor channel, the direct discharge of sewage and garbage from dredgers should be strictly controlled during the dredging operation, especially for some sensitive waters, the real-time monitoring of the pollution level should be strengthened to ensure that the water quality is always within the qualified standard range.

 3.2 Environmental technology analysis of dredging construction

As described in the paper, the dredging construction of port-channel must focus on avoiding sensitive areas of water, and prevent the diffusion of suspended substances, so as to do good real-time monitoring. Of course, the construction technology and progress should also be adjusted according to these monitoring conditions, so as to give a reasonable play to the technical content. Specifically, the following two technical points are given.

(1) Reasonable arrangement of mud discharge pipeline

In the construction area of the port channel, the layout of the mud discharge pipeline on the water should take the lead to investigate the flow and wind direction, and according to them, the mud discharge pipeline should be set into an arc shape and anchored at the same time. At the same time, connect the waterway pipeline with the underwater pipeline, set a two-way pipe anchor for the channel, and fix it with a three-way pipe anchor. In the construction process, it is generally selected to install the night light display for the water pipeline, and at the same time set the pipe anchor drift. Finally, when considering the floor-suction dredging vessel, consideration should be given to whether the dredging pipeline is too long. If it is too long, it may lead to the fracture of the dredging pipeline when the wind waves or the flow velocity is too high. Generally speaking, the length design of the dredging pipeline should be controlled within the range of 300~500m. In addition, during the construction process, the tightness of the mud conveying pipe should be ensured to avoid any leakage during mud conveying. The mud is mainly sprayed from the interface, so it should be well checked and maintained on a daily basis to avoid large-scale environmental pollution caused by the break and leakage of the mud pipeline.

(2) Environmental protection construction treatment of blowing-fill overflow in the blowing-fill area

Environmental protection countermeasures for the blowing-filling overflow in the blowing-filling area of the port-channel should be carried out based on three points. Firstly, the flow direction of the spilled mud should be strictly controlled during the blowing-filling process to prevent a large amount of mud from flowing into the water and causing serious water pollution and even affecting the water quality outside the blowing-filling area. Therefore, we should try our best to control the discharge sludge concentration of the overflow port, so we can consider setting separate cofferdam, increase the distance of the overflow port of the blow filling point, increase the flow rate of mud in the blow filling area, but reduce the flow rate at the same time, so as to improve the settlement effect of suspended solids and effectively control the actual concentration of suspended solids. Second, technological transformation should be carried out on the basis of the traditional dredger, and environmental protection measures should be set for dredging construction. For example, the anti-mud curtain can be set in the construction area for the grab dredger, with the purpose of preventing a large number of sediment pollutants from spreading. At the same time, it can set the antifouling cover on the reamer head of the cutter suction dredger, and realize effective control of suspended matter pollution in waterway waters with reamer head. Third, in the process of land reclamation work, should be regularly to the exhaust pipe and dredge the maintenance inspections on the connection between the two points, the first time found between pipeline there may be a bad connection or damage problem, actively adopt corresponding remedial measures, and in order to effectively avoid the happening of mudflow pollution accident.

Brief Discussion on the Construction Method of River Dredging Project

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The main purpose of the dredging project is to dig a shallow section of the river or bay, which can greatly improve the flood discharge capacity of the river, improve the irrigation guarantee rate of the river network, improve the water supply conditions and water ecological environment, and improve the navigation capacity. This paper mainly discusses how to carry out reasonable river dredging construction.

With the progress of science and technique, the dredging technique changes with each passing day. Various new and advanced special dredging equipment for different soil quality and working conditions have appeared one after another, greatly improving the construction efficiency and the dredging effect. In short, according to different operating conditions and construction requirements, the construction method with advanced technique, convenient construction, and low cost should be flexibly selected in practice to achieve the expected dredging effect.

1. Selection of dredging measures

 1.1 Selection of construction method for cutter suction dredger

(1) Main pay pile transverse excavation

The key points of construction are: take one steel pile as the main positioning pile, and the other steel pile as the supporting pile. When the main pile moves forward, it always stays on the centerline of the excavation channel. This construction method is different from projects of different soil quality.

(2) Double main pile excavation

This construction method takes two steel piles in turn as the swing center, which is suitable for digging loose soil. It is not suitable for those projects with higher digging quality. The advantage is that the operation is relatively simple, but due to the inconsistency of the swing center, it is easy to cause two disadvantages, such as digging and leakage.

(3) Anchor cable construction technique

Concrete can be divided into four anchors horizontal excavation methods, five anchors horizontal excavation methods, and six anchors horizontal excavation methods. Concrete construction technique for dredger cast the main bow anchor. Side anchors are set at the front and back respectively, with the bow anchor as the swing center. This construction technique is suitable for the dredger with main and side cable winch, and the flow rate is large, the direction of flow is basically consistent with the trenching direction. The advantages are wind-wave resistance, water flow adaptability, and the width of excavation is larger than the steel pile positioning method.

(4) Shallow area piling method

First, the steel pile positioning method is adopted to excavate the production forward. When half a ship’s position is put in, half a ship’s position is put back, and then the excavation is carried forward. In such a reciprocating cycle, the rotation pendulum method can be used to reverse the position. This construction method is suitable for the projects that need to catch the tide, as well as the projects where the excavation area is weak and the designed excavation depth cannot be reached due to the leakage of piles. And the designed excavation depth exceeds the maximum excavation depth of the dredging vessel and the water depth varies to a certain extent.

 1.2 Grab dredger

This construction method can include row bucket dredging measure, quince-shaped dredging measure, angle cutting dredging measure, and stem retaining dredging measure. The scope of application of bucket dredging measure is the project of soil compactness, the soil layer is thicker, and the quality requirement is higher. The plum-shaped dredging is suitable for soft soil and small thickness of the mud layer. The method of cutting angle and retaining stem dredging is suitable for hard soil.

 1.3 Bucket dredger

The commonly used construction methods are inclined transverse excavation method, more special parallel transverse excavation method, sector transverse excavation method, and cross-shaped transverse excavation method. The oblique cross-section method is suitable for water area and hydrologic conditions. And the advantages of higher quality excavation projects are less excavation resistance, less mud flushing, and easy to meet the quality requirements when digging the edge.

The parallel cross-cutting method is suitable for water areas with large flow velocity and has the advantage of good adaptability to flow. The scalloped method is suitable for the situation where the water depth at the boundary is less than the draft of the dredger. The crosswalk method is suitable for the situation where the depth at the edge of the trench is smaller than the draft of the dredger and the width of the trench is smaller than the length of the dredger.

 1.4 Dipper dredger

In this case, the back dredging method and the horizontal excavation method can be adopted. The main construction points of the back dredging measure are: After lowering the bucket, use the back rope to pull the bucket as far as possible to the hull, push the bucket with the weight of the hull, cut into the river bottom for digging. This is suitable for a thicker soil layer with a thickness of 3~4m, which has the advantage of higher efficiency. The main point of the horizontal excavation method is that the rotary bench dredger pushes and presses the bucket at any time during the cutting process to keep the bucket track level.

 1.5 Pneumatic pump

It includes three construction methods: hole excavation, cross excavation, and drag excavation. Among them, the hole excavation method adopts the plum-shaped arrangement of the hole side, and the air pump is lowered at the same time to move after reaching a certain depth. It is suitable for loose flow plastic silt, low compacting soil, and obstacles in the soil layer, and has the advantages of convenient operation and high production efficiency. Cross excavation method means that digging another hole after digging a hole and dredge until the cave been soft and collapse. This method is suitable for sandy clay. The main technical points of the drag method are to install a shovel knife and traction rope at the pump mouth. The shovel knife excavates the soil layer by traction, which is suitable for soil, sand, and clay with a high compacting degree and good excavation quality.

2. Selection of construction machines and tools

Different types of dredging machine have different soil and have their respective adaptability and limitation, so in the process of construction should be based on dredging soil can be dug and transportability chooses different dredging machines. Thus achieve the purpose of improving the dredger construction progress. Certain principles should be followed in the selection of dredger tools for cutter suction dredger and grab dredger.

In some dredging and hydraulic reclamation projects often encounter excavation above the water, such as excavation canal, dock, and channel. There are anchoring, transverse movement of difficult operations, and high slope collapse safety problems.  Too much collapsed soil will not only bury the machine and tools but also produce adverse shock waves and produce destructive power. The main technical measures adopted in the construction are as follows:

(1) When the water above exceeds 4m, measures shall be taken to lower its height before excavation to ensure safety. Common methods are: Reduce the height by land mechanical excavation and loose blasting reduces the height of the landslide in advance.

(2) The delamination thickness of excavation should be reasonable, and the excavation thickness of the first layer should be reduced as far as possible under the condition of ensuring the dredger’s draft and minimum excavation depth.

(3) The distance and thickness of the dredger should be less than the normal value. Soil collapse can be reduced by reducing the distance forward and the thickness of excavation.

(4) Flexible strip excavation is a short strip excavation to reduce the impact of soil collapse on both sides of the dredger and to reduce the lateral tension.

(5) In the area affected by damp, the upper layer should be excavated by high tide, and the lower layer should be excavated by low tide. Excavation should be arranged as far as possible when the conditions are better.

(6) It is necessary to strengthen the observation of prow and bank slope, master the law of soil collapse, find out the problem, and take timely measures to avoid it.

3. Environmental dredging

Compared with ordinary dredging techniques, the environmental protection dredging technique has the following features: It can effectively remove polluted sediment from rivers and lakes and create conditions for the recovery of aquatic life. In the process of environmental protection dredging construction, the appropriate environmental protection vessel should be used to automate the equipment configuration and achieve a high degree of accuracy. And the dredging process has higher requirements. The corresponding sludge will be different degrees of special treatment. At the same time, there are also special analysis and strict control. Environmental dredging projects usually use the cutter suction dredger construction.

4. Conclusion

This paper discusses the construction technique of different river dredging. According to different soil conditions, river conditions, and other environmental conditions, it is necessary to select the river construction technique scientifically and reasonably. Reasonable dredging methods should also be used to maximize the effectiveness of the corresponding mechanical equipment. In this way, good social and economic benefits can be achieved and corresponding project quality requirements can be met.

 

Environmental impact analysis of dredging projects

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As people growing emphasis on environmental problems, the environmental impacts of dredging project cause the attention of many countries and organizations. The paper from dredging engineering-type construction features and main phase, dredging vessels dredging project major impact on the environment are analyzed, in order to design and develop a dredging construction plan, a decrease of the adverse impact of dredging on the environment to provide the reference.

1. Abstract

Since the main object of the dredging project is sediment at the bottom of the water, dredging projects and dredging activities will have a certain impact on the environment. The vast majority of dredging projects will increase the depth of water at the construction site, change the river or coastal topography and biological habitat, and have a certain degree of impact on the local hydrodynamic conditions, thereby changing the rules of pollutant migration movement. The impact on the environment is often long-term. And dredging the activity itself will bring certain influence to the environment. Relatively speaking, the short-term impact is often more obvious. Such as dredging water suspended sediment concentration increased dramatically and release all kinds of pollution elements, different degrees of the environmental impact of the different dredging projects, affect the way in the different stages of dredging construction form is different. This article from the main stage of ship type and dredging project aspects analyzes the dredging project’s major impact on the environment. It provides a reference for designing and formulating a dredging construction scheme and reducing the adverse impact of dredging on the environment.

2. The impact of different dredging stages on the environment

For different dredging projects, according to the purpose of dredging, sediment characteristics, hydrologic environment and construction conditions, and many other various aspects of factors and adopt different construction technology and dredging equipment. The processes of the dredging operations are different, generally speaking, all include underwater silt sediment vertical ascending mining, dredging, sediment transport, and disposal or not.

 2.1 Undersea sediment excavation

The main purpose of this step is to remove dredged sediment from the original position, the digging bucket, bucket, reamer, rake, top tools for mining under the effect of extrusion, cutting off the device. The bed surface sediment cohesion is destroyed and the shift is part of the sediment by dredging equipment transfer to other places for further processing, part of the scattered points in dredging silt floating to the water.

During the excavation of sediment, the environmental dredging effects include:

(1) increase the suspended sediment concentration. Under the effect of rotary cutting of dredging equipment, some sediment enters the suspended state. The suspended sediment concentration depends on the basic characteristics of sediment, type of dredging equipment, excavation energy, vertical lifting method, and other construction parameters.

(2) Release pollutants. Polluted sediment suspended in the water will release pollutants to the water through desorption. The release intensity and rate are determined by suspended sediment concentration, sediment background adsorption value, the physical and chemical environment of water, and adsorption kinetic characteristics.

(3) Dredging sediment residue will become a new source of pollution if it contains pollutants.

 2.2 Dredging up the sediment vertically

The second key step in the dredging process is to lift the dredged sediment to the surface, either mechanically or hydraulically, depending on the dredging vessel used. When a mechanical dredging vessel is used, dredged sediment is lifted to the surface in a bucket. While when a hydraulic dredging vessel is used, dredged sediment is usually drawn through a suction pipe. Under the action of a centrifugal pump, the mud and water mixture is sucked into the suction pipe and transported to the destination through a mud discharge pipe.

Dredging sediment effect on the environment in the process of ascension includes:

(1) I use open dredging bucket for mechanical transport, the process of ascension will be inevitable in contact with water. Under the effect of water dilution and scour, the suspended sediment concentration in water will increase, suspended sediment concentration by dredging bucket type and capacity, speed, and other factors.

(2) When hydraulic transport is adopted, the main influencing factor is the suction capacity of the mud suction pipe. If the suction capacity is not enough to transport all the excavated sediment, a large amount of dredging residue will be generated to pollute the environment. (3) When a rake suction dredging vessel or barge is used to load the mud-water mixture, the overflow will increase the concentration of suspended sediment and pollutants in the water.

 2.3 Dredge sediment for horizontal transport

Sediment dredged from the riverbed and rose to the surface needs to be transported to its destination for further disposal or treatment. The horizontal transport modes include rake suction dredging vessel transport, mud barge transport, and hydraulic pipeline transport. Generally speaking, mechanical dredging is carried by barge, hydraulic dredging vessel by pipeline, and rake suction dredging vessel by ship. Compared with other stages, the horizontal transport of dredged sediment has little impact on the environment, and the main risk comes from dredged sediment leakage.

 2.4 Dredging sediment disposal

After dredging sediment is transported to the destination, further disposal is carried out according to dredging purpose, sediment characteristics, and availability, including land reclamation, beach maintenance, sea disposal, land disposal, isolation disposal, etc.

Dredging sediment disposal is the final stage of dredging engineering. Different disposal locations have different impacts on the environment: (1) if underwater disposal is adopted, under the action of hydrodynamics, fine particles of sediment will spread and move around to different degrees. (2) If land disposal is adopted, such as land reclamation and beach maintenance, the main pollution mainly comes from residual water discharge and overflow in the process of separation of sludge and water. (3) In addition, the onshore disposal of dredged sediment may have a negative impact on the quality of groundwater near the disposal site without appropriate protection measures.

3. Environmental impact analysis of different types of dredging vessels

With the rapid development of the dredging market and the rapid progress of science and technology, in order to meet the construction requirements of different projects, different types of dredging vessels have been developed and put into use by various shipping companies. Dredging vessels commonly used in engineering can be divided into mechanical and hydraulic types. The mechanical dredging vessel mainly uses the excavator and tools to carry out underwater excavation and uses the machine and tools to lift vertically to achieve the purpose of dredging, including grab dredging vessel, bucket dredging vessel and bucket dredging vessel. Hydraulic dredging vessel uses mechanical rotation to cut the underwater soil layer and mixes the salt and water to form a certain concentration of mud, which is transported ashore through the mud pipeline, including suction dredging vessel, cutter suction dredging vessel and rake suction dredging vessel.

4. Conclusion

4.1 No matter what kind of dredging project is, no matter what kind of dredging equipment and preparation is adopted, each dredging link will inevitably have a certain negative impact on the environment during the dredging process. The main impact is the increase of suspended sediment concentration in the water body and the diffusion and transportation of pollutants.

4.2 Besides the influence of the above, the excavation effect will undermine the survival of the benthic environment, the implementation of the dredging projects often produce noise, dredging sediment in transportation, and disposal process will be unpleasant smell or release of volatile pollutants, in addition, the dredging engineering for underwater topography influence brought about by the hydrodynamic conditions change on pollutant transport also has a certain influence.

4.3 According to the different types of dredging projects and the dredging purpose, detailed investigation, and environmental impact assessment should be carried out in the design stage of the project, so as to further improve the design, select appropriate dredging equipment and take some control measures, optimize the construction scheme and minimize the adverse impact of the dredging project on the environment.

4.4 From the perspective of the impact of dredging on the environment, the main measures to reduce the impact of dredging on the environment are to improve dredging equipment, improve dredging precision, control sediment suspension, and dredging residue, and strengthen the management of dredging disposal.

 

Measures for Environmental Dredging of Harbor Channel

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In the process of implementing the dredging project of port and waterway, the technical operators and relevant departments need to set up the idea and concept of environmental protection strictly, and effectively promote the dredging construction of port and waterway under the guidance of reasonable cost and effective technical operation, so as to make a contribution to environmental protection. At the same time, it has been proved that only by fully following the principle of ecological maintenance can the dredging construction of ports and waterways be effectively promoted, so as to improve the navigation capacity of ports and waterways.

1. Overview of port-channel dredging project under the environmental concept

 1.1 Concept of the port-channel dredging project

The impact and development of harbor navigation dredging projects on the environment should not be underestimated. Therefore, before the implementation of the port-channel dredging project, it is necessary for relevant departments to analyze and predict the port situation in this region, so as to improve the environmental protection thoughts and concepts of all construction personnel, thus effectively strengthen the quality of ecological protection, which is also one of the important principles to promote the construction and development of ports and waterways. The concept of environmental protection is to protect the original state of the undamaged ecological environment and restore the damaged ecological environment as much as possible, so as to protect the local environment effectively. Therefore, environmental protection measures generally need to combine physical measures and chemical measures. But the port waterway dredging engineers should also be aware that in the construction of environmental protection is not once and for all, the environment will change over time, so you should try to promote the balance of the environmental construction and waterway dredging construction, thus effectively realize the unity of social benefit and economic benefit.

 1.2 Analysis of the characteristics of the environmental dredging project

The first is accurate positioning and excavation. Under normal circumstances, pollutants will be deposited in port waters, but their thickness will be thinner, generally between 10 and 50cm, but the maximum is no more than 1m. The scope of excavation depends on the distribution of sediment, and the excavation face should be consistent with the distribution of sediment. In the actual dredging process, we should not only remove the polluting substances in the sediment, but also minimize the development and destruction of non-polluted sediment, so as to effectively avoid the construction and treatment of channel sediment layer at the bottom of the channel, and thus control the cost of sediment mining within an effective range. It can be seen that the positioning accuracy and excavation accuracy of environmental dredging should be controlled within a reasonable range, and much larger than the dredging range of general waterway.

The second is to control secondary pollution. In the course of sediment dredging, the re-suspension of sediment during dredging is usually not considered. At the same time, a small amount of sediment is neglected in the course of sediment transportation without affecting the construction progress and quality. The main reason for this is that the sediment particles will increase greatly in a short time, but after the completion of the project, the suspended solids will be transported out of the construction area very quickly. Therefore, in order to comprehensively promote the quality of the port-channel dredging project, it is necessary to reduce the secondary pollution of sediments. In terms of construction equipment and technology, necessary measures and devices should be installed, so as to effectively reduce the secondary pollution of sediments to the port water body.

Thirdly, it is a safe disposal device for polluted sediments. The technical operators shall properly treat the removed sediments and carry out pollution prevention and recycling to prevent the re-destruction of groundwater and its environment. It should be noted that the discharge site should strictly control the concentration of drainage.

Finally, there should be professional monitoring and defensive measures. Technical operators need to strictly monitor and manage the diffusion range of mud, mud concentration, and suspended particle quantity and backwater concentration, so as to effectively promote the construction quality of the project.

2. Environmental dredging effects in ports and waterways

 2.1 It intensifies the stirring and diffusion of pollutants

One of the most important technical requirements for dredging ports is to clean up the sediment near the ports. The most common method is to dump the sediment into the ocean, greatly increasing the pollution in the ocean. Especially if the construction workers fail to deal with the pollutants in the waterway before the construction, the agitation and diffusion of pollutants in the water will be caused, thus greatly deepening the pollution level of the water. In addition, if there is no effective treatment and transportation of the abandoned soil in the channel dredging project, it will cause serious pollution to the groundwater of the abandoned soil disposal site, thus contaminating the nearby environment and affecting the local residents’ life and transportation development.

 2.2 Damage the aquatic ecological environment

In the dredging process of the harbor channel, the common pollution type is the pollution of water by suspended matter. The specific principle is that inorganic suspended substances mainly increase the turbidity and pollution degree in the water body, and pollute a large number of organisms in the water body by relying on the burial effect after settlement, thereby affecting the reproduction of organisms in the water body and seriously damaging the ecological balance system in the water body. Meanwhile, in the dredging project of the port channel, the concentration and pollution of suspended solids in the water are closely related to the location of the dredging vessel. The closer a boat gets, the higher the concentration of suspended matter in the water, and the lower the light transmittance, which poses a threat to the water ecosystem.

 2.3 Noise pollution and ship pollution

During the dredging project of the harbor channel, a large number of large dredging equipment will be used, which will cause serious noise. In the actual dredging and dredging project, once the dredging vessels make noise, they will also cause pollution to the local ecological environment. At the same time, the docking of dredging vessels will also pose a threat to the ecosystem in the water. In particular, once the leakage of oil and sewage occurs at the bottom of the cabin, the ecological environment around the waterway will be affected, leading to serious qualitative changes in the surrounding waters, thereby reducing the survival rate of organisms. In addition, in the construction process of ships, once sewage discharge occurs, it will also pose a threat to the biological and water quality in the water body, thus reducing the survival rate and reproduction rate of biological.

3. Environmental protection concept of the port-channel dredging measures

 3.1 Design reasonable dredging operation plan

Dredging engineers need to carry out strict analysis and investigation of the pollutants and deposition characteristics at the bottom of the sediment, especially the strict measurement of the total amount of sediment. Before carrying out actual port dredging, the construction personnel should make a full investigation of the local natural conditions and hydrogeology, so as to provide effective information guarantee for further dredging. To ensure the scale of the excavation work arranged, to divide the specific dredging work scope and to formulate reasonable dredging construction methods, so as to promote the maintenance of equipment and develop relevant construction regulations.

 3.2 Determine reasonable times and seasons

As for the creatures at the bottom of the harbor, they need to reduce their activities during the winter, i.e. hibernate, so dredging of the bottom of the harbor dredging should be done during the winter to minimize the risk. At the same time, to avoid the growth period of underwater organisms, the most important thing is to carry out practical construction in winter or in the dry season, without changing the growth cycle and survival rate of underwater organisms.

 3.3 Strengthen control during construction

First, in the dredging and excavation process, the amount of stirred contaminated sediment should be minimized, and the predicted leakage prevention and diffusion measures should be taken to ensure the concentration of water inhalation and prevent the suspension from causing serious pollution to the water. Second, secondary pollution should be prevented in the dredging process. On the premise of not affecting the quality and progress of the construction, the number of fine particles of sediment should be increased in a short time. After the completion of the project, suspended solids will be transported outside the construction area, so as to effectively reduce the pollution of sediments and suspended solids to water bodies and waterways. Meanwhile, the maintenance and supervision of construction equipment should be strengthened to prevent oil leakage from vessels and improve the safety of construction operations. Third, the contaminated sediments should be treated effectively. Reasonable operation techniques should be used to strictly control the concentration of discharged tailwater after they are removed.

 3.4 Scientific treatment of contaminated sediment

First, residual water should be comprehensively treated. Due to the large number of pollutant particles in the dredging process of the channel, suspended particles in the sediment can be effectively removed by intermittent operation and other forms, so as to improve the quality of the residual water. Secondly, promote the reasonable stacking of the construction site, and conduct the sealing and biological stacking and removal of the bottom sediment. According to the volume of the bottom sediment and the water content of suspended objects, the distribution of the bottom sediment is reasonably analyzed and predicted, so as to effectively improve the quality of channel dredging and promote the balanced development of local ecology.

4. Conclusion

To sum up, based on the concept of environmental protection, to improve port waterway dredging engineering quality, you need to improve the environmental awareness of all construction personnel, adopt a scientific and reasonable method to analyze and predict dredging, and formulate an effective method of dredging, sediment, and sediment, and suspended solids are removed, so as to promote the ecological balance of the near the harbor.