River and Lake Ecological Restoring and Environmental Dredging

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With deep research on water environmental harness, people have recognized that it is a basic way of headstream cutting, inner headstream cleaning, ecological restoring in order to achieve river and lake water restoring. When the contaminated headstream is cut, the environmental dredging that combined with ecological restoring will play an important role in harnessing the water environment. The ecological restoring and dredging effects are discussed in the paper, and the structural characteristic of the environmental cutter is also analyzed.

1. Abstract

Sediment is an important depository of river and lake nutrients，the discharge of domestic sewage and industrial wastewater. The injection of surface runoff and the dead remains of aquatic life in the lake, together with plant dead leaves, dust and garbage, often lead to the gradual accumulation of nutrients in sediments in the river system, especially the internal load of nutrients in urban rivers and lakes.

When the pollution source of the river and lake is reduced or completely cut off, the pollutants deposited at the bottom of the river and lake continuously release nutrients and pollution components to the water body can still make the water body eutrophication, even the death of fish, water odor and other phenomena. Under certain conditions, nutrient salt of sediments may be the leading factor of eutrophication of rivers and lakes. Due to the invasion of external pollution, the water body has formed an internal source of pollution, that is, a large number of sediments — polluted sediment. Just like a healthy person suffering from acute gastroenteritis due to not paying attention to hygiene and diet at ordinary times, the emphasis on diet and hygiene has no effect. The key is to treat gastroenteritis and make it recover, and then pay attention to diet and hygiene so that it will not relapse. Therefore, while controlling the external source, the improvement of water environment cannot ignore the role of the internal source of the pollution of river and lake bottom sediment. As a measure of river and lake environment improvement, the sediment that has no self-purification ability can be removed which plays a great role in reducing the nutrient load of the water area.

In the treatment of rivers and lakes, measures of “internal and external control” must be taken, that is, to control the input of exogenous nutrients by changing the production and consumption model and constructing sewage treatment facilities. Dredging of sediment can eliminate endogenous pollution and create conditions for ecological restoration. To achieve the goal of water environment restoration through ecological restoration, the long-term improvement of river and lake water environment is realized through management. Environmental dredging of sediment is the fastest and most practical method to eliminate endogenous pollution, especially for shallow rivers and lakes.

2. Environmental dredging creates conditions for the restoration of aquatic plants

Aimed at weeding out environmental dredging rivers water body pollution in sediment, remove the endogenous pollution, reduce sediment release of pollutants into any water body, and the combination of ecological restoration, environmental dredging has both features of environmental dredging, clear lakes pollution, at the same time create conditions for the recovery of aquatic ecosystems and realize the improvement of the shallow water environment of lakes. Plants need suitable soil, water, light and other conditions for growth, while different plants need different living conditions. The main environmental factors that restrict the growth of aquatic plants are water temperature, light condition, PH value, nutrient salt content in water, geological condition of sediment, etc.

 2.1 Suitable water depth conditions

A healthy, stable ecosystem is made up of many organisms. Different aquatic plants need different water depth conditions. On the one hand, since certain plants have specific tolerance to water, too deep or too shallow water depth conditions may affect their normal and healthy growth. On the other hand, the water depth also has an impact on the illumination conditions at the bottom. The illumination gradually declines after entering the water body, and too much or too strong illumination is not suitable for the growth of aquatic plants.

In the arrangement of dredging, it can be selective dredging according to the living conditions of aquatic plants. If the water depth is small, the contaminated sediment may not be removed completely, or even be backfilled by dredging means to meet the water depth conditions of plant growth. As long as the water is not polluted, the thin layer of the eutrophic layer may be better able to provide adequate nutrients for plant growth, which is conducive to the vigorous demand for nutrients at the early stage of plant growth. If a large depth of water is needed, it can be dug moderately deep, provided that the soil should be suitable for the growth of aquatic plants.

 2.2 Suitable soil conditions

Polluted sediment often has high water content and fine particle content, soft soil, rich in nitrogen, phosphorus and organic matter. Such sediment has the following disadvantages:

(1) The soil is soft, aquatic plants are not easy to take firm roots, and the ability to resist wind and waves during the growth of aquatic plants is poor.

(2) Due to the high content of fine particles, small pores between the soil, poor permeability, water permeability, aquatic plants difficult to breathe, not conducive to the growth of plants.

(3) Since the sediment is rich in nitrogen and phosphorus, the PH value is unbalanced, which is not conducive to the survival of fish.

Therefore, soil conditions can be improved by environmental dredging. According to the requirements of different plants on the soil, specific dredging can be carried out to obtain appropriate soil conditions.

 2.3 Suitable lighting conditions

The water body is turpid, the transparency is low, and the bottom light intensity is insufficient. This cannot maintain the plant the survival. This is the heavy pollution water body common character. The causes of water turbidity are the release of nutrient salts from polluted sediment and the dissolution and re-suspension of semi-suspended colloid-like substances at the soil and water interface. Removing the floc near the bottom of the river and lake together with the sediment can cut off the main source of suspended pollutants in the water, which is conducive to improving the water quality and light conditions, and creating conditions for the growth of submerged plants.

To sum up, the implementation of environmental dredging projects can reduce the toxic and harmful pollutants in rivers and lakes. Transform topography, geomorphology and water depth to improve water quality and lighting conditions. Ecological restoration should be carried out in time after dredging to restore stable and healthy aquatic plant communities, which is the basis for the balance of the river and lake ecosystem.

3. Features of environmental dredging

Environmental dredging is an important measure to solve the internal pollution of rivers and lakes. The main purpose of dredging is to remove the pollutants contained in the sediment, remove the internal sources of polluted water bodies, reduce the release of sediment pollutants to water bodies, improve water quality, and promote the recovery of aquatic ecological environment. Environmental dredging is different from ordinary dredging construction. The latter is mainly for dredging purposes such as dredging channels and increasing storage capacity. For their different purposes, environmental dredging has many unique features.

 3.1 High construction accuracy

Generally, the bottom of the river and lake is irregular, the distribution of sediments is uneven, and the thickness of silt varies from place to place. The environmental dredging should not only remove the polluted sediment on the bottom of the river and lake but also avoid over digging and leakage digging and reduce the transportation and treatment cost of the sediment as far as possible. Therefore, the construction precision of environmental dredging is much higher than that of ordinary dredging project. Normally, the construction accuracy of environmental dredging is 5 ~ 10cm, while the construction accuracy of dredging is 50cm.

 3.2 Reasonable control of dredging depth

In environmental dredging, shallow dredging may not have the desired effect, while too deep dredging may damage the existing ecosystem, making ecological restoration more difficult. Therefore, scientific experiments should be carried out before dredging, and the dredging depth should be reasonably selected after considering not only the vertical distribution characteristics of sediment but also the living conditions of plant restoration at the bottom of the river and lake.

 3.3 Avoid secondary pollution

Secondary pollution in the dredging process is mainly reflected in two aspects:

(1) During dredging, the stirring of reaming knife leads to the release and diffusion of pollutants in the sediment in local areas, and the release rate of nitrogen and phosphorus pollutants in the sediment is several times higher than that in the static state, resulting in secondary pollution of water body.

(2) Heavy metals, nitrogen, phosphorus and other nutrient-rich elements are contained in the mud of the bottom sludge storage site. Under the circumstances of natural rainfall and surface erosion, runoff may infiltrate down or sideways, which may cause secondary pollution to groundwater, surface water environment and surrounding environment.

In order to prevent secondary pollution of water body, dredging equipment with an anti-proliferation device should be used to treat the surplus water in the storage yard, and some anti-seepage measures should be taken in the storage yard.

4. Conclusion

In the process of river and lake eutrophication treatment, it is not possible to solve the problem of shallow river and lake eutrophication simply by taking measures to control exogenous nutrients, and must pay attention to the control of internal nutrients. As one of measures to control lake internal pollution, environmental dredging should be taken seriously. The restoration of aquatic vegetation can not only control the suspension of sediment due to the dynamic action, so as to release the endogenous nutrients, but also absorb the nutrients in the water and reduce the endogenous load. Only the comprehensive application of measures such as cutting off external sources, clearing internal sources, ecological restoration and long-term management is the fundamental way to realize the restoration of river and lake water environment.

 

Discussion on construction technology of channel dredging project

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In the process of social and economic development, the transportation industry occupies a crucial position, and waterway is one of the important transportation channels. In order to improve the construction quality of waterway dredging project, this paper discusses the construction technology of waterway dredging project based on the characteristics of waterway operation.

Based on the characteristics of channel dredging projects with intensive capital and technology, it is very important to make a reasonable allocation of relevant construction technology in the course of channel dredging project implementation. The construction personnel of channel dredging should make a comprehensive analysis according to the construction standards of channel dredging engineering and the planning of channel construction area, and formulate a perfect construction design scheme based on the specific construction environment, so as to minimize the risk of channel dredging construction and improve the environmental benefits of the project.

1. Preparation for waterway dredging

 1.1 Setting-out survey

In the course of setting-out survey, the channel depth is evaluated in detail by using relevant measuring equipment, such as SDH-13d sounder, global positioning system, etc. Before the setting-out survey is carried out, the hydrological weather should be chosen as smooth as possible to ensure that the measuring 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.

 1.2 Site layout

Site layout shall be conducted in accordance with the relevant specifications of the bidding materials for waterway dredging project, and in combination with the guiding principles of safety, reliability, ease of management, reduction of interference and smooth transportation. 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 is very professional. In the actual construction layout process need to choose reasonable dredging equipment. Choosing the appropriate wringing suction ship and it needs to be reasonable configuration grab ship, mud pump, blowing mud ship, etc., to ensure the smooth development of the overall construction at the same time according to the blowing distance and contract requirements.

2. Construction technology of channel dredging project

 2.1 Construction process management

First of all, in the process of starting the trial excavation, it is necessary to organize personnel with strong practical operation skills. Comprehensively consider the dredging impacts and quality of the channel soil, and reasonably regulate the parameters of the soil excavation equipment such as the main engine speed, lateral movement speed, gear shifting, mud concentration, cutting thickness, etc.

Secondly, the lowering depth of grab, reamer, and other components of the dredging equipment should be reasonably controlled according to the relevant data in the dredging process. Meanwhile, measures such as dredging leakage and back silt cleaning can be used to increase the excavation depth. The super-depth groove has a great influence on the actual construction. Therefore, according to the characteristics of the scale of dredging project, the upper layer can be used to dig the first and later dredging process. In the process of the trenching process, the lateral moving speed of trenching equipment and the ultra-deep groove should be gradually slowed down to maintain the stable operation of the whole system. In order to ensure the accuracy of groove excavation, after the relevant groove excavation is completed, it shall be overlapped with other grooves, and then layered construction shall be carried out successively. In order to ensure the trench excavation depth to meet the requirements, different back silting depths can be reserved according to the time limit of trench excavation to the completion of construction and combined with the timely quality inspection to ensure the quality of trench excavation in line with the construction standards.

Thirdly, in the mud transportation process, the operation of channel dredging vessels should be controlled, and the underground water pipeline should be laid to facilitate the orderly transportation of soil. Generally, the water diving pipe adopts the flexible connection mode, which is not only easy to sink and heave, but also can make the water diving pipe and water terrain cooperate well. In the laying process, the underwater submarine pipeline usually needs to be arranged above the water body for stage sinking. Before the actual laying, relevant equipment can be used to measure the underwater topography of the channel, so as to maintain the depth of the underwater submarine pipe above 2.6m. If the water depth is insufficient, the groove can be treated or re-dug. In order to ensure the continuous mud transportation, mud pump can be connected with mud conveying pipe to ensure the effective mud storage and transportation.

Finally, according to the characteristics of dredged soil, it can be treated by blow fill measure, mud disposal measure, overflow measure and side disposal measure. The side throw measure mainly uses the auxiliary dredging measure. In the process of the friction reaction between mud and water, the energy loss is gradually reduced, and the mud and soil particles continue to sink from coarse to fine. After the overall energy consumption, the mud is deposited and becomes an integral part of the riverbed. However, some mud particles with small mass expand into the water with the increase of flow velocity, and the mass of mud particles is inversely proportional to the bypass effect of mud. Therefore, in order to improve the mud deposition rate, on the basis of increasing the flow rate, a certain inclination Angle can be set between the channel and the flow direction to improve the mud edge casting effect. In the underwater mud throwing measure, attention should be paid to the selection of water areas. Generally, the water areas with large volume and small velocity should be selected, and the surrounding building facilities should be considered to be in good operation and free from congestion, such as hydraulic structures, grooves and waterways. Short distance excavation sites should be selected as far as possible to provide efficiency of soil delivery. In order to facilitate the effective operation of mud throwing equipment, attention should be paid to the mud throwing area of water depth and water area. The overflow measure mainly uses the mud pump to suck out the mud and make it enter the mud tank, and then the extra mud can enter the underwater from the overflow port on both sides of the mud tank. This measure can effectively prevent small and medium particles of mud from entering the mud tank. When the mud tank reaches the rated limit, it can be treated by mud throwing measure, so as to avoid the excavation of groove back to silt. Due to the change of function and property in the process of discharging mud, it can be diffused in a certain water basin, providing a guarantee for the improvement of the quality of side throwing construction. The dredging measure is mainly used to transport the excavated soil to the filling area. The dredging measure can not only effectively treat the dredged soil, but also improve the comprehensive utilization efficiency of soil and reduce the risk of dredged soil returning to the silt channel. The excavation direction should be reasonably selected before the implementation of the blowdown measure.

 2.2 Quality acceptance

After the completion of the channel dredging project, it is necessary to carry out such operations as the drawing of the punching surface of the completed and accepted section of the channel, internal data processing and external measurement. The section drawing for completion and acceptance of waterway mainly includes section drawing and section plan. The CAD network platform is mainly used to construct the spatial data of the overall construction topography, engineering application, geological culture, earth and stone works and other data. In this way, the data mapping and database establishment can be effectively integrated to ensure the smooth drawing of channel section after the completion of channel dredging project. In the field survey, the distance information of channel dredging section is compiled by collecting the topographic data of water shorelines and underwater topographic data, which should be combined with the effective application of channel section plans and sections such as triangle network and contour line.

3. Conclusion

In conclusion, the smooth implementation of channel dredging project is of great significance to the effective improvement of channel operating environment. Based on the characteristics of waterway operation, in the actual construction process, construction management personnel should carry out reasonable application of various construction technologies in combination with the planning situation of waterway water, so as to guarantee the construction quality in an all-round way.

Environmental Protection Concept under the Port Channel Dredging Project Supervision

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Dredging of harbor channel is a huge project, which is time-consuming and has a certain impact on the environment. Ordinary dredging measures cause widespread and violent agitation of the bottom sludge, leading to the spread of pollutants and deterioration of water quality. The waterway has the function of transportation and flood control. Dredging of the channel will lead to the pollution of the surrounding ecological environment and adversely affect its function. The environmental protection concept of port dredging projects requires the environmental protection of dredging equipment, the environmental protection of the dredging process, and the environmental protection of polluted sediment treatment. This paper analyzes the importance and impact of the port-channel dredging project on the environment and discusses the key points of port-channel dredging project supervision under the concept of environmental protection.

Considering the importance of environmental protection in today’s society, it is of great significance to analyze and discuss the dredging project of port-channel under the concept of environmental protection. In port, waterway dredging construction application of green concept not only can satisfy the demand of the protection of the ecological environment and realize the harmonious development of ecological environment and further but also can let the dredging construction companies keep up with the needs of the development of the era and increase their efficiency and core competitiveness.

1. The importance of port-channel dredging project supervision

The influence of harbor channel dredging projects on environment cannot be ignored, so the harbor channel dredging project under the environmental protection concept is analyzed carefully. The primary task of waterway dredging engineers is to strengthen the application of environmental protection concepts in port and waterway dredging projects and further strengthen the protection of the ecological environment. The concept of environmental protection refers to keeping the intact ecological environment as much as possible and taking effective measures to restore the damaged ecological environment to the natural state before the destruction. The concept of environmental protection has certain physical and chemical characteristics. The specific content of the environmental protection concept includes ecological protection within the concept of relative scope, there will be no absolute protection, only relative protection, ecological protection has dynamic characteristics. Ecological protection cannot be done once and for all. It is constantly changing with the change of the natural environment. In addition, ecological protection is closely related to the natural environment and social and economic development. And the two have certain coordination. The natural environment will not be destroyed because of social and economic development which will not be restricted because of the natural environment.

2. Environmental impact analysis of dredging project of the harbor channel

 2.1 The diffusion of pollution during the dredging project

Sediments are important depositing carriers of nutrients and pollutants. Sewage discharge, dust fall, the inflow of surface runoff and dead debris of aquatic organisms often lead to the gradual accumulation of nutrient salts in sediments in the channel, forming an internal load of nutrient salts. If there is a high temperature or flood, the pollutants in the sediment of the port-channel will continue to surface. Once the pollutants are too much to be independently evolved by the water body, eutrophication will occur and the organisms in the water will be polluted and die. The water quality will also change and smell bad. Once the channel dredging is carried out without effective treatment of the polluted bottom sediment in the waterway, the phenomenon of the polluted bottom sediment spreading in the water is easy to occur, which will worsen the water quality. Dredging abandoned soil should also be reasonably disposed of, and should not be directly filled, or it may cause adverse effects on groundwater.

2.2 Inorganic suspensions can damage aquatic ecosystems

In the dredging process of the harbor channel, the inorganic suspended matter is common pollution in the water area. Inorganic suspended matter mainly increases turbidity in water, and influences the growth and reproduction of a large number of aquatic organisms by its burial after settlement, leading to the destruction of the aquatic ecosystem. In the dredging process of the harbor channel, the concentration of suspended matter in the water is related to the position of the dredger.

 2.3 Noise pollution and ship pollution during dredging

During dredging and filling, the noise emitted by dredger will further affect the surrounding ecological environment. And the ships involved in the dredging process of the harbor channel will also have an adverse impact on the ecological environment. When a ship is sailing or stopping, it is easy to find sewage or oil at the bottom of the cabin. During the leakage process, such sewage and oil may be carried out in the surrounding area of the channel, which will affect the surrounding water quality. The sewage generated in the process of ship construction will also adversely affect the aquatic ecological environment.

3. Key points of port-channel dredging project supervision under the concept of environmental protection

 3.1 Time selection of ort channel dredging

It is necessary to select the dredging time of harbor and channel scientifically and reasonably. It is not suitable to dredge the harbor and channel in the season with the frequent biological activities in the nearby benthic habitat to prevent the adverse effects on the normal activities and survival of benthic habitat. From this point of view, the dredging project of port-channel should be carried out in the period with little risk and environmental impact as far as possible, and the best dredging period is winter. In addition, to minimize the impact on benthic organisms, the dredging of ports and channels should be carried out during the dry season to avoid the growth period of benthic larvae. At the same time, the flow rate at high tide and low tide should also be taken into account.  At this time, the precipitation of suspended and overflow is more difficult.

 3.2 The port-channel dredging plan should be scientific and reasonable

The environmental pollution near the port-channel should be closely examined to observe the causes and characteristics of the polluted sediment. Based on the analysis of the results, a specific dredging scheme for the port-channel should be worked out. The bottom sediment should be calculated according to the survey data, and then every link of the port and channel dredging project should be studied and formulated, and the scale, scope, and method of the port and channel dredging project should be formulated and divided respectively. In the dredging process of the harbor channel, the appropriate dredging equipment and tools should be selected. In view of the work management in the dredging process of port-channel, it is necessary to formulate certain management methods and regulations to ensure the normal operation of the dredging work of port-channel.

 3.3 Supervision of dredging construction of ports and waterways

The following three problems should be paid attention to the dredging process of port-channel to ensure effective control of the channel dredging construction.

(1) Further, improve the positioning of the dredging industry of port-channel and enhance the precision of excavation. The depth of pollutants deposited in the bottom layer of the water is usually 10 to 15 cm thick, ensuring a range of 1 m. In the process of harbor channel dredging, the concrete distribution of sand and stone polluted by the bottom should be analyzed and studied, so as to define the scope of dredging. In the course of dredging, the contaminated sediment should be completely removed, and the dredging quantity of the non-contaminated sediment should be controlled within a certain standard range. Under the concept of environmental protection, all aspects of the dredging work of the port-channel are different from the usual dredging requirements and significantly higher than usual.

(2) In the dredging process of the port channel, the possibility of secondary pollution must be reduced. A large part of the channel dredging does not pay attention to the re-emergence of suspended or deposited material after dredging, and on the premise of ensuring the amount of dredging work, often habitually ignore some of the leaked sediment. A small amount of fine sediment is rapidly added during the dredging of the channel, and after the dredging is completed, some of the suspended material will fall to the bottom or be taken out of the dredging area. In the dredging operation of ports and waterways under the environmental protection concept, in order to avoid the bottom sediment pollution or mud pollution, some relevant operation equipment in the dredging of ports and waterways should be installed and the use of certain methods should be used to completely eliminate most of the pollutants. At the same time, prevent the water ecological environment from suffering from secondary pollution.

(3) In order to deal with the pollution deposition materials removed from the dredging project of the port and waterway scientifically and reasonably, scientific and reasonable advanced technologies should be adopted to properly deal with the pollution deposition materials after the pollution deposition materials are removed, so as to prevent them from bringing different degrees of pollution to the groundwater around the storage yard and the new environment. In the sludge disposal site, the discharge of sewage concentration should also be effectively controlled.

4. Conclusion

The proposal of the environmental protection concept has brought a great impact on the construction of the dredging project of port-channel. Meanwhile, due to the characteristics of a long construction period and a large amount of engineering, the construction process often has different degrees of impact on the ecological environment. In order to integrate the environmental dredging project of port-channel, it is necessary to further strengthen the research on construction technology, combine with the characteristics and requirements of project construction, clarify its impact on the environment, take effective measures to reduce the impact and meet the requirements of environmental protection on the basis of guaranteeing the construction quality.

 

Characteristics and Development of Environmental Dredging

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With the continuous improvement of people’s awareness of environmental protection, great progress has been made in the control of exogenous pollution in rivers, lakes, oceans and other waters. After the external pollution is effectively controlled, it is one of the effective methods to remove the internal pollution by environmental dredging. This paper discusses the basic concept and technical requirements of environmental dredging and introduces the engineering characteristics of environmental dredging.

1. Introduction

Environmental protection is one of the hottest topics in this century. All countries and regions in the world have invested a lot of manpower, material resources and financial resources in resource protection and environmental protection. China is a developing country with a large population, with many lakes and rivers, among which there are serious water pollution problems. As people pay more and more attention to environmental protection, the construction concept of environmental dredging comes into being. Environmental dredging is a newly developed industry, which belongs to the cross-discipline and construction technology of ocean engineering, water transportation engineering, environmental engineering and dredging engineering. It is one of the most effective ways to improve the water environment by environmental dredging after the pollution from external sources is effectively controlled in rivers, lakes and oceans. Environmental dredging refers to the overall planning and comprehensive consideration of general dredging activities and environmental protection contents such as soil care, ecological reconstruction, environmental improvement and resource utilization. By means of treatment, the pollution of internal sources of water can be removed and the ecological restoration condition of water environment can be created.

2. Technical requirements for environmental dredging

The purpose of environmental dredging is to remove the polluted sediment in oceans, lakes, rivers and other water bodies, strictly control the precision of dredging, and focus on the prevention and control of secondary pollution such as the rediffusion of polluted sediment during dredging and the leakage of polluted sediment during transportation.

Key technologies in the process of environmental dredging include dredging scale design, dredging measure of polluted sediment, accurate positioning of dredging space, residual water treatment after dredging, and storage and disposal of dredged sediment, etc.

 2.1 Choice of environmental dredging equipment

The choice of dredging equipment is directly related to the type and technology used: (1) Different dredging equipment has different disturbance degree to sediment, and the mechanism and characteristics of preventing diffusion and leakage are also different. Therefore, the pollution of dredging materials caused by different dredging equipment is different in the dredging process. (2) Generally, the control technology of environmental protection dredging equipment should be considered, because environmental dredging has high precision requirements for spatial positioning and depth control, the dredging section should be close to the bottom sediment pollution surface, otherwise, it will cause secondary pollution, and the over-excavation and leakage should be avoided and reduced as much as possible.

 2.2 Dredging technology

In the process of environmental dredging, a suitable storage site for contaminated sediment should be selected according to the requirements, and then the cleaned contaminated sediment should be transported to the selected location. Generally speaking, there is a certain distance between the pollutant storage yard and the construction area, which may cause secondary pollution along the process of transportation, which should be avoided as far as possible. Therefore, the key technology for the transportation of polluted sediment in the environmental protection dredging process is how to solve the secondary pollution problem in the dredging process.

 2.3 Treatment technology of polluted sediment

In the course of dredging construction, the polluted sediment removed from the coastal areas of rivers, lakes and oceans contains a large number of pollutants that may cause harm to the environment, which cannot be directly discharged into the water body of the outland reclamation or stored in the space of the land storage yard. Ecological risk assessment should be carried out for polluted sediment in strict accordance with relevant regulations. According to the requirements, appropriate treatment plans should be made for the pollution hazard and degree of sediment to avoid pollution and harm to the surrounding water and land environment.

3. Environmental characteristics of dredging projects

Due to different dredging purposes, environmental protection dredging has different engineering characteristics from conventional dredging, which is mainly used to dredge channels and increase storage capacity. Environmental dredging has many technical characteristics different from ordinary dredging because of the different purposes of the two dredging measures. The environmental protection dredging construction has the following characteristics: the dredging quantity is relatively small. The pollutant content in dredging sediment is high. The dredging boundary, dredging depth and elevation requirements are special and should be determined according to the distribution of pollutants and other conditions. Avoid diffusion during dredging. Dredged soil is relatively soft. Environmental dredging is generally non-repetitive.

These characteristics of environmental dredging determine that environmental protection dredging requires high precision, which needs to be carried out carefully, secondary pollution should be avoided as far as possible, and its impact on the environment should be positive. These technical characteristics of environmental dredging also pose challenges to dredging equipment and dredging technology.

4. The development of environmental dredging equipment

Up to now, the development of environmental dredging in China has only a decade of history and engineering experience. The research and application technology of the environmental underwater dredging equipment are still in the preliminary stage, and many technical means and methods have some defects and are not mature yet. At present, there is a big gap between the research and development of environmental protection dredging equipment and technology in China and the developed countries. At present, the research and development of environmental dredging equipment in the world can be summarized into two categories: First, the existing dredging equipment can meet the requirements of environmental dredging. Second, based on the concept of environmental protection, the special dredging equipment developed for environmental dredging. Relatively speaking, in the above two research and development methods of environmental dredging equipment, the transformation and upgrading of existing dredging vessels and dredging equipment with environmental protection functions are one of the most common and commonly used methods at present, and also the most economical and practical methods. At present, the transformation and upgrading of dredger mainly focus on dredging equipment and positioning control, that is to replace the original dredging equipment with a new environment-friendly reamer, and equipped with advanced high-precision positioning and monitoring system to reduce secondary pollution in the dredging process and meet the requirements of environmental dredging.

Compared with the traditional dredging equipment, the economic efficiency of the special environmental dredging equipment is poor due to the high production cost and the uniqueness of the environmental dredging project. In addition, in general, most of the special equipment for environmental dredging has low dredging capacity, which cannot meet the requirements of large-scale environmental dredging projects. Therefore, in the practical dredging engineering application, the special environmental protection dredging equipment is still rare. Since the beginning of the 21st century, China’s research and development institutions and other dredging groups began to actively develop environmental protection dredging equipment and has also achieved some remarkable results through research, and successfully applied in practical projects.

5. Conclusion

Although due to our increasing awareness of environmental protection, rivers, lakes, oceans and other waters have made great progress in the control of exogenous contamination and have been well controlled. However, after the external pollution is controlled, how to adopt effective environmental protection dredging engineering measures is the main problem that should be considered to solve the problem of internal pollution.

 

Environmental Dredging impact analysis and countermeasures

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Environmental dredging is an important measure to treat water because it can completely remove the polluted bottom sediment and eliminate the potential factors affecting water quality. However, disturbance and improper disposal of contaminated sediment in environmental protection dredging projects will lead to secondary environmental pollution. Various technologies to reduce sediment disturbance and resource utilization of sediment have become research hotspots. Reducing pollution sediment disturbance is mainly achieved by improving dredging equipment. At present, the main environmental protection dredging equipment used in China is the cutter suction dredge and environmental protection reamer. The resource utilization of sediment can not only improve the secondary pollution to the environment but also turn waste into treasure and increase resources. At present, the main ways of resource utilization include land utilization, building materials, filling materials, sewage treatment materials and hydrogen production technology.

Most of the rivers flowing through cities in China are seriously polluted, especially the bottom sediment. When the external pollution sources are controlled, once the water environment of rivers and lakes is changed, the nitrogen and phosphorus nutrient elements, heavy metals and refractory organic compounds deposited in the sediment will be released into the water again, affecting the water quality of the overlying water, and forming secondary pollution. In addition, sediment is the main living place and food source of benthic organisms and the pollutants can directly or indirectly produce toxic and harmful effects on benthic organisms or overlying aquatic organisms. Moreover, it further influences terrestrial organisms and human health through biological enrichment, food chain amplification and other processes. Sediment dredging can permanently remove contaminated sediment from the water body and prevent the secondary pollution of contaminated sediment to the water body, so it is commonly used to treat the water body.

Dredging is divided into engineering dredging and environmental dredging. The main purpose of engineering dredging is navigation, capacity enhancement 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. Environmental dredging impacts

Dredging can effectively reduce the concentration of pollutants such as nutrients, heavy metals and persistent organic compounds in sediments. However, during the dredging process, pollutants will be released into the water, and the interface process after dredging may have a great impact on the dredging effect. Whether dredging techniques can fundamentally improve the water environment remains a matter of great debate. The environmental impacts of dredging are mainly as follows:

(1) Sediment is the main storage site of hydrophobic pollutants, a large part of which adsorbs on suspended particles and settles into the sediment. Dredging projects cause sediments to be re-suspended so that pollutants are released into the water as suspended particles, increasing the level of pollution in the water.

(2) The sediment can absorb pollutants in the water and release pollutants into the water. When the environment changes little, the adsorption and release of pollutants in the water reach a dynamic balance. If the dredging depth is lower than the polluted sediment depth and the polluted sediment is not completely removed, the new sludge layer will be exposed to the sediment surface, breaking the dynamic balance between sludge and pollutants and aggravating water pollution. Excessive digging will damage the original soil and increase the project cost.

(3) In the process of sediment transport, leakage will cause secondary pollution to the water body and may pollute other environments.

(4) Contaminated sediment usually contains a lot of pollutants. If the dredged sediment is simply stacked without any measures, on the one hand, it will occupy a large amount of land, and on the other hand, it will produce secondary pollution due to the erosion of rain.

(5) Sediment dredging leads to changes in the physical and chemical properties of sediments (such as grain size composition and organic matter content), which can lead to the replacement of benthic populations and thus change the composition of benthic communities. In the aquatic ecosystem, the microorganism is not only an important part of the benthic community but also plays a key control role in material degradation, transformation and energy flow in the aquatic ecosystem. The sediment dredging changes the physical and chemical properties of the surface sediments, destroys the stable structure of the surface microbial community before dredging, and changes the community composition or community structure of the microbial community in the sediment.

(6) Oil spills from dredgers during dredging and the discharge of household waste can also pollute water bodies.

(7) The construction process of dredging will produce some noise pollution to the surrounding environment.

2. Correlation processing characteristics and optimization utilization techniques

In order to minimize the negative impact of dredging on the environment, the following points should be controlled during contaminated sediment dredging: (1) Reduce the agitation of contaminated 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 water body and another environment. (4) Dredging sludge should be treated safely to avoid secondary pollution near the storage yard. And it can be appropriately processed into available resources. (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 dredger 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-absorbing, and soundproof material. The characteristics of environmental protection dredging are introduced in detail from the aspects of resource utilization of dredging equipment and sediment.

 2.1 Dredging equipment

The choice of dredging equipment directly determines the form of dredging: different dredging equipment has different disturbance degrees, different diffusion and leakage performance and different suction 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: The first is to use the original dredger for transformation to meet the requirements of environmental protection construction. The second is a special environmental dredging equipment. Using the original dredger to carry on the transformation is the common method at present, which is economical and practical. The reform of dredger 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 protection dredging. From the technical point of view, the design and construction of special environmental protection equipment is not difficult. But from the economic point of view, the design and construction of special equipment is much higher than the transformation of traditional dredging equipment, and different projects need to build different equipment. The economy is worse for smaller projects.

Environmental dredging equipment is more advanced by using suction dredger with an environmental reamer. In this way, the surface sediments are sucked up by pipes under the action of mud pumps and transported to the land for a long distance. All these dredging vessels are equipped with an automatic control system and monitoring system and adopt DGPS for precise 3d positioning, which greatly improves the dredging accuracy. This method can reduce the re-suspension of sediment and release of pollutants.

 2.2 Sediment dredging

The conventional disposal methods of dredging sediment mainly include stacking, blowing and ocean dumping. These disposal methods not only solve the problem of dredging outlet but also generate various problems. Stacking treatment is mainly the problem of stacking. Stacking will take up a lot of lands, and may produce secondary pollution. Blowout treatment is a method of comprehensive utilization of soil, which transports the excavated soil to the filling point. After water loss, the silt is left to increase the elevation of the filled land. However, the backfill foundation is usually very weak, which is not conducive to the later development and use of the backfill land. Meanwhile, backfill construction is prone to cause secondary pollution of the surrounding environment. Marine slinging, though cheap, can pollute the marine environment and damage fishing grounds. Many countries have banned it. Dredging mud contains rich secondary minerals and is a good renewable resource. As the pollution source, dredging mud can be comprehensively developed and reused through a series of harmless and stable treatment methods, which can not only avoid the harm to the environment but also produce newly available resources.

3. Conclusion

Sediment deposition affects the normal operation of the waterway, and the contaminated sediment will release pollutants into the water, making the water quality worse. Environmental dredging has become an important measure to control the polluted sediment in the waterway because it can remove the polluted sediment directly and thoroughly from the water. However, the sediment disturbance, sludge leakage and unreasonable sludge treatment caused by environmental protection dredging will also cause certain pollution to the environment. With the enhancement of people’s awareness of environmental protection, the prevention and control of water pollution caused by the dredging process have been gradually developed.

 

Discussion on Influence Factors of Environmental Dredging

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The lake is the main reservoir of sediment pollutants. Water pollution is divided into endogenous pollution and exogenous pollution. Studies show that even if the polluted water is effectively controlled from exogenous sources, the nutrient salts in the sediment will be released gradually, which will still make the rivers and lakes eutrophication. Therefore, when the exogenous pollution of rivers and lakes is basically controlled, it is necessary to take measures of environmental protection dredging to remove the serious pollution and release heavy sediment. Environmental dredging is one of the most effective ways to remove endogenous pollution quickly. However, due to the influence of dredging precision, dredging diffusion, dredging sludge drying, the resources of sediment dehydration utilization, resource utilization of sediment dehydration, residual water treatment, biological activation of the new interface layer after dredging, back silting and other problems. It is still difficult to achieve the expected effect in practical engineering.

With the development of human society, the speed of urbanization is getting faster and faster, the industrialization degree is higher and the increasing influence of human activities on the natural environment. As a result of a weak human environmental protection meaning before, a lot of industrial wastewater and living sewage discharged into rivers and lakes, many rivers and lakes water bodies of soil pollution, cause the eutrophication of water bodies or adverse effects such as water quality deterioration and has the serious influence human survival environment.

1. Environmental impact factors of dredging

Environmental dredging aims to remove pollutants from the water. It is a science which combines the technology of removing the internal pollutant from polluted water with the technology of ecological restoration. Environmental dredging technology is a comprehensive science of environmental engineering, port and navigation engineering and water conservancy engineering. In the process of environmental protection dredging, there’re some technical defects of dredging accuracy and the diffusion in the dredging process, drying of dredged sediment, sediment over the dehydration of resource utilization, water treatment.

 1.1 Dredging accuracy

The aim of environmental dredging is to remove contaminated sediment from the water, and the depth of excavation depends on the distribution of contaminated sediment. On the other hand, it is necessary to avoid the over digging of non-polluted sediment as far as possible and control the treatment quantity and cost of polluted sediment within a certain range. Therefore, the dredging precision of environmental dredging equipment is much higher than that of ordinary waterway or water conservancy dredging equipment. The bottom mud is composed of floating mud layer, silt layer and old soil layer from top to bottom. Environmental dredging requires dredging the upper layer of mud which is prone to contaminate the overlying water. The intermediate silt layer is also removed during dredging. The old soil layer at the bottom is a natural structural layer, which should be retained during dredging. It is not difficult to see that environmental dredging has a high technical standard for dredging accuracy, and the dredging depth error is controlled within 5cm~10cm. Normally, the accuracy that the dredging control system can demonstrate and achieve refers to the positioning accuracy of the dredging head, that is, the locating point of the dredging head. It cannot be understood that the depth reached by dredging is the corresponding depth of the dredging head. According to the basic information such as the angle of the dredging head support, the vertical position of the ship (determined by RTK-GPS slipway) and the structural parameters of the ship, the depth of the dredging head was calculated by comprehensive processing.

It is proved by practice that the error of the ship’s vertical position and the accuracy of RTK-GPS itself are the important factors affecting the positioning accuracy of digging head. Therefore, to improve the positioning accuracy of the digging head and optimize the accuracy of RTK-GPS are system technologies, the key lies in avoiding the vertical error of ships and the accuracy of RTK-GPS itself.

At present, the most commonly used methods for vertical positioning of dredging vessels are as follows: (1) The conversion and positioning of the water surface have certain limitations. Generally, it can meet the requirements of environmental protection dredging under suitable working conditions, but its accuracy will be affected to some extent when the water environment is narrow, closed, or stormy. (2) RTK-GPS positioning, using RTK-GPS positioning can reach the accuracy of water surface conversion positioning. Its advantages can overcome the limitations of water surface conversion positioning.

In addition, dredging accuracy is affected by other influencing factors, such as reamer lateral displacement, reamer speed, mud pump flow, dredging sediment physical properties, etc. There are many factors affecting the precision of environmental protection dredging. To enhance the application scope of the system, comprehensive research should be carried out from various factors.

 1.2 Sludge diffuses during dredging

Due to the disturbance of the excavation head and the flow of water, the particles of the dyed sediment will become smaller after being dredged. The sludge with smaller particles is easy to diffuse in the water, and the sludge particles are not inhaled by the mud suction pipe to diffuse around the reamer head, which is easy to cause secondary pollution and pollute the water quality of the surrounding waters, which is called diffusion.

1.2.1 The common problems in the dredging process of traditional dredging technology

First is the sediment disturbance during dredging. In the course of dredging, sediment will appear suspended diffusion phenomenon under the disturbance of external force. Many non-hydrophilic and persistent organic pollutants accumulated in the contaminated sediment. The suspension of sediments causes the release of ammonia and phosphates, resulting in eutrophication and secondary pollution of the water body, which cannot achieve the ideal environmental protection dredging effect. There are cases in the UK showing that the concentration of herbicide in the water after dredging is higher than that before dredging due to the disturbance of the sediment during dredging. The reason is that the sediment disturbance in the dredging process leads to the sludge suspension diffusion, and most of these suspended diffusion sludge contains some heavy metals and toxic and harmful substances with the sludge particles into the water, leaving pollution hazard to the local aquatic environment.

Second is the secondary contamination of dredged sediment. The dredged sludge is stored in the sludge storage yard before drying. In the rainy season, when the polluted sludge is washed by the rain, the toxic and harmful sludge in the sludge may be transferred and spread with the rain, releasing the toxic and harmful sludge into the surrounding environment, which is easy to cause secondary pollution. In particular, heavy metals contained in contaminated sediment have a lasting negative impact on the environment. Therefore, dredged sediment should be properly disposed of. If not handled properly, it will seriously affect the surrounding ecological environment. While Germany, Japan, the United States, and other developed countries continue to improve the technology of environmental protection dredging, they even require the implementation of ecological risk assessment before dredging.

The third is the physical structure at the bottom of the channel is destroyed during dredging. In general, the ancient river course accumulates for a long time and forms a fixed natural impervious layer. If the precision of dredging is not well controlled, the natural impervious layer may be destroyed by humans, thus damaging the original impervious structure at the bottom of the river course.

1.2.2 Key technologies to control the diffusion

(1) Research on the law of natural sedimentation of sludge particles during dredging.

(2) Research on the relationship between centrifugal force in the turning process of reamer cutting and the negative pressure in the suction process of the mud pump.

(3) Research on the law of sediment diffusion during environmental dredging.

(4) Research on environmental protection dredging construction technology: the capacity of mud pump, the capacity of the dredging head, reduce the residual amount of contaminated sediment dredging and limit the diffusion range of sediment particles.

 1.3 The dewatering and drying of dredged sediment

The storage yard treatment is one of the effective measures for the disposal of a large number of environmental dredging sludge. The storage yard sediment is characterized by small solid content, fine particles, negative electricity, and a large amount of nitrogen and phosphorus nutrients. The natural drying period of dredged sediment is long, which is because the dredged mud is mostly silty clay with poor permeability. There is no transpiration of water in the middle and bottom of the storage yard without external force. Because the infiltration rate of contaminated sediment is quite slow, the mud layer of the whole storage yard is in the state of saturation for a long time, the bearing capacity is always low, and its solid content cannot meet the transportation requirements, leading to the dredging sediment cannot be removed in time. In the event of precipitation, the dredging sludge drying rate is further reduced, and secondary pollution will be caused by the erosion and immersion of rainwater. A large amount of polluted sediment is stacked in the storage yard for a long time, which occupies land resources, and is not conducive to the subsequent treatment and resource utilization of dredging sediment, affecting or even restricting the extensive development of environmental protection dredging of lake reservoir sediment. Therefore, the improvement of the rapid dewatering and drying technology of dredging sediment is beneficial to the large-scale implementation of the environmental protection dredging project.

At present, the main methods of sediment dewatering in engineering practice include physical dewatering and chemical dewatering. The physical method is mainly used to remove capillary and binding water from the sludge. The chemical dewatering method changes the particle size of sediment by adding flocculants. Adding flocculants can increase the floc size, improve the sedimentation rate of sludge floc, and facilitate the rapid dewatering and drying of sludge. Increasing the effect of sediment dewatering and reducing the cost of sediment dewatering will promote the wide application of environmental protection dredging technology. It has been proved that chemical dehydration alone and physical dehydration alone cannot achieve the desired result. The chemical method of adding flocculants combined with the physical drainage method has a better treatment effect. The addition of flocculants increases the particle size of bottom sludge and improves the drainage channel of bottom sludge in the storage yard, which is conducive to physical dehydration. It increases the sludge drying rate and reduces the occupation time of the sludge storage yard, shortens the engineering cycle and indirectly reduces the treatment cost. The combination of chemical and physical dehydration gives full play to the advantages of the low cost of flocculants and the good effect of physical drying. In urban lake dredging projects, the limitations of sediment storage area, time limit, and economic index examination are all factors to be considered in environmental dredging projects. Chemical flocculation dehydration combined with physical dehydration is a very cost-effective dredging measure, which is also the development trend of large-scale environmental protection dredging technology applications in the future.