Impact analysis of dredging stages on environment

For different dredging project, often according to the purpose of dredging, sediment characteristics, hydrologic environment and construction conditions, and so on various aspects of factors and adopt different construction technology and equipment, the process of the dredging operations are different, generally speaking, all include underwater silt sediment vertical ascending mining, dredging and level of sediment transport and disposal of such main links or phase, different stage of the construction work will have a different impact on the environment.

1. Underwater sediment mining

The main purpose of this step is to remove to be 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 shift, are part of the sediment by dredging equipment transfer to other places for further processing, some scattered near the point of dredging sediment, sediment suspension parts into the water.

In the process of dredging sediment, the environmental impacts include :

(1) increasing the concentration of suspended sediment. Under the rotary cutting action of dredging equipment, part of the sediment enters a suspended state. The concentration of suspended sediment depends on the basic characteristics of bottom sediment, type of dredging equipment, excavation energy, vertical lifting method, and other construction parameters;

(2) Release pollutants. The polluted sediment suspended in the water will release pollutants into the water through desorption. The release intensity and rate are affected by suspended sediment concentration, sediment background adsorption value, water physical and chemical environment, adsorption kinetics characteristics;

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

2. Dredged sediment lifts vertically

The second key part of the dredging process is to raise the dredged sediment to the surface, either mechanically or hydraulically, depending on the vessel being dredged. When a mechanical dredger is used, the dredged sediment is placed in a bucket and lifted to the surface. When a hydraulic dredger is used, the dredged sediment is usually absorbed by 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 sludge discharge line.

Harvest of sediment in the process of ascension's impact on the environment includes:

(1) 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;

(2) In the case of hydraulic transportation, the main influencing factor is the suction capacity of the suction pipe. If the suction capacity is not enough to transport all the excavated sediment, a large number of dredging residues will be generated to pollute the environment.

(3) if the suction dredger or mud barge is used to load the mud-water mixture, the overflow will increase the concentration of suspended sediment and pollutants in the water.

3. Dredged sediment horizontal transportation

From the bed, the surface to dig and rose to the surface of the sediment transport to destination for further disposal or treatment, the level of transportation way including the trailing suction hopper dredger, barge transportation, and hydraulic pipeline, in general, adopt mechanical dredging barge transportation, hydraulic dredge pipeline, trailing suction hopper dredger can be direct with the delivery of the ship. Compared with other stages, the horizontal transport of dredged sediment has less impact on the environment, and its main risk is from the leakage of dredged sediment.

4. Dredged sediment disposal

After dredged sediment is transported to the destination, it is further disposed of according to the purpose of dredging, sediment characteristics, and availability, including land reclamation, shoreline maintenance, offshore disposal, land disposal, isolation disposal, etc. Dredged sediment disposal is the last stage of the dredging project.

Different disposal locations have different impacts on the environment :

(1) Under the hydrodynamic action, fine sediment will spread and move around to different degrees;

(2) If land-based treatment is adopted, such as land reclamation and shoreline maintenance, the main pollution is mainly from residual water discharge and overflow in the process of mud-water separation;

(3) In addition, if no protective measures are taken, dredged sediment from onshore treatment may also have a negative impact on groundwater quality near the disposal site.

Impact analysis of different types of dredging vessels on the environment

With the booming development of the dredging market and the rapid progress of science and technology, in order to meet the construction requirements of different projects, various shipping companies have developed different types of dredging vessels and put them into use. The dredging vessels which are common in the project can be divided into mechanical and hydraulic types. The mechanical dredger mainly uses the excavator to carry on the underwater excavation, uses the machine itself to carry on the vertical lifting to achieve the dredging purpose, mainly includes the grab dredger, the bucket dredger, and the chain bucket dredger, etc. Hydraulic dredger is the use of mechanical rotation to cut the underwater soil layer, the sand, and water mixed to form a certain concentration of mud, through the mud pipeline delivery ashore, including suction dredger, cutter suction dredger, sand suction dredger.

Ocelot Technology Dept

       2020.02.18