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.