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.