Influence of Estuarine Dredging on Environment

Dredging is conducive to flood discharge and navigation and can eliminate pollution hazards after removing silt containing pollutants. At the same time, it will have many adverse effects, including local turbidity, saltwater wedge retrogression, ecological environment changes, etc.

1. Abstract

The estuary is the confluence area of the sea and river. The dynamic conditions and the evolution process of the riverbed are complicated due to the dual action of tidal wave and river. When the estuary is dredged and the soil is disposed of, it will have more complicated impacts on the surrounding environment. With the development of dredging technology, the concept of dredging not only refers to the navigation dredging at the shoal reach but also is an important measure for flood control, disaster reduction and elimination of environmental pollution. However, during the dredging process of estuaries, it is inevitable to have negative impacts on the environment. For example, mechanical agitation or excavation may cause the bottom sand to rise, resulting in local turbidity and so on. In addition, the dredging depth of the channel in the estuary will affect the upstream distance of the salt wedge, thus changing the distribution of salt in the upstream, and having some adverse effects on the surrounding ecological environment.

2. Environmental impact of dredging

Dredging is the use of dredging equipment to move sediment underwater to other waters or land. Since dredging changes the local environment, flood control dredging in water conservancy, navigation dredging in shipping, and environmental dredging in environmental protection will have a positive or negative impact on the environment.

 2.1 The positive dredging effects

2.1.1 Dredging for flood control

Estuarine shrinkage is a serious sediment problem in recent years. All estuaries have tidal gates to hold back the tide, to control salinization, and to store freshwater. The establishment of tidal sluice changed the dynamic condition of the estuary area and brought serious siltation to the downstream of the sluice. In order to be able to open the sluice normally to drain flood, the silt under the floodgate must be removed every year. After dredging the river mouth, it can stabilize the river mouth, and ensure the smooth discharge of the flood of these rivers, protect the safety of people’s lives and property, and avoid the water logging disaster.

2.1.2 Navigation dredging

With the development of economy and trade, the demand for water transportation is constantly expanding, especially with the increasingly busy coastal transport and Marine transport and the transport ships are gradually developing to large scale, while the lack of water depth in the channel at the mouth of the river often becomes the bottleneck of regional social and economic development.

P.Brunn (1976) pointed out that it is impossible to maintain navigable water depth in estuaries without necessary dredging. It can be seen that dredging is a major measure to improve the navigable water depth of estuaries. The deepening of navigable water depth will certainly produce better social benefits and, from a certain point of view, will also have a beneficial impact on the environment. For example, the natural depth of the Yangtze estuary channel is only 6m, 1000 tons of ships have to enter the port by the tide or load reduction. After the first phase of the Yangtze estuary deepwater channel regulation project, the water depth has been increased to 8.5m, and the fourth-generation container ship carrying 4,000 teu only needs to unload 1,000 teu before entering the port. Before regulation, 2000 teus need to be removed. Meanwhile, the channel after regulation can be two-way navigation, and the speed is also increased from 8kn to 12kn. It is expected that after the second and third phases of the dredging project, the water depth can continuously reach 10.0m and 12.5m, and the fifth and sixth generation containers and 10,000-ton cargo ships can enter the port by the tide.

2.1.3 Environmental dredging

With the enhancement of environmental protection awareness, the harm of underwater pollutants to the environment has gradually attracted people’s attention. Environmental dredging is an important measure in water environment comprehensive remediation, and its status and function have been revealed. Environmental dredging plays an important role in ecosystem restoration. Dredging also removes heavy metal pollution and reduces the release of nutrient solutions over time. Estuarine is the intersection of river and sea. Pollutants from river runoff and ocean tide often accumulate and deposit in the estuarine area, causing environmental pollution. However, estuarine areas， where the economy is prosperous and the population is dense will cause greater harm and people should pay attention.

 2.2 Negative dredging effects

2.2.1 Suspension of bottom sediment

Mechanical agitation during dredging will cause the suspension of bottom sediment, and during the transfer of dredged material, the sand will also cause local water turbidity. There are main measures to control such pollution: (1) the dredging with the rake suction dredger (the rake head is installed with movable closing plate) can not only avoid the sediment from falling, but also prevent the water from flowing inwards. (2) Using the often-used lake dredging methods, a closed net curtain is used to enclose several layers in the dredging area to prevent muddy water from flowing out. However, since the high flow velocity in the estuarine area, the practicability of this method needs to be discussed.

2.2.2 Retransport of polluting substances

During dredging and dredging operations, bottom sediments are disturbed and re-suspended. Suspended particles may release toxic substances and remain in the water, posing a potential hazard to local aquatic life. Moreover, the dual effects of the tidal waves and rivers in estuaries are more likely to cause the re-transport of toxic substances. At the same time, excavated silt containing toxic substances will also cause secondary pollution if not properly handled.

2.2.3 The saltwater wedge goes up

After the estuary is dredged, the siltation position will move up with the saltwater wedge upstream. Usually, the upstream of the saltwater wedge is controlled by many factors, but water depth is the most important. The lower reaches of tidal estuaries are usually dredged to improve navigation conditions. But any deepening and small changes in the channel affect the distribution of salt in the upper reaches and may lead to greater changes in the length of the saltwater wedge upstream. For example, the Mississippi River in the United States is a high-stratified river estuary. Before treatment, the gate sand ratio is relatively stable, the water depth can be maintained at 2.74m, and the saltwater wedge is not far upstream. After the estuary was dredged to a depth of 12.2m (southwest passage), the saltwater wedge went up to 160~200km, and the silt position also moved up and down with the saltwater wedge. However, the change of the distribution of the upstream salt content will inevitably affect the freshwater supply and drinking water quality of towns on both sides of the river, and cause the corresponding land salinization and ecological environment changes. To control or improve these problems, some guidance projects must be taken. It means that the combination of regulation and dredging is an effective way to control tidal estuary, but the negative impact of dredging on the environment and the countermeasures taken still need to be further studied.

2.2.4 Enormous pressure on the environment generated by dredged material

Dredging is accompanied by a large amount of dredging spoil. For example, the whole project in the Yangtze estuary deepwater channel regulation will produce 243.86 million cubic meters of dredging spoil. If placed on land, it will require a large area of compressed wasteland, and the fine particles in the dredging soil after drying will move with the wind, causing air pollution; If cast into the water, it will lead to the increase of sediment concentration in local waters of the sludging area, and may also cause sediment deposition in other estuaries under the action of coastal sediment transport.

2.2.5 Fertilization of water quality

When dredging, mechanical agitation causes fine particles to be suspended, this may release ammonia and phosphates and “fertilize” the water, resulting in environmental dredging not achieving the desired results. For example, during environmental dredging in the Taihu lake basin, although certain measures have been taken to prevent the spread of suspended sediment, the “fertilization” of water quality has not been effectively prevented, causing the proliferation of plankton and algae, which consume a large amount of dissolved oxygen in the water, resulting in hypoxia of water quality and aggravating other types of pollution. Therefore, dredging sediment is not a necessary and sufficient condition for controlling lake eutrophication. Moreover, in estuaries, where the flow rate is relatively high, dissolved contaminants are more likely to be carried away. It’s equivalent to diluting the concentration of dredged areas which results in more dissolved contaminants into the water and causes more widespread pollution. Therefore, attention should be paid to the cost-benefit ratio of dredging as an environmental engineering and its possible negative impact on ecological restoration.

2.2.6 Other effects

The environmental effects of dredging estuaries are complex and have many aspects. The change of runoff into the sea with the nutrient salt and pollutant flux it carries, and the change of bed bottom will also lead to the change of the ecological environment will affect the reproduction and growth of aquatic animals and plants or the migration conditions of birds. There are also environmental problems caused by dredging: (1) Oxygen consumption. (2) Reduce sea life. (3) Changing the nutrient levels of seawater, etc.

3. Environmental impact of dredging material

There is a lot of dredging waste with the dredging process. The environmental impact of dredging results is obvious: proper disposal can turn waste into treasure and obtain huge economic and social benefits; Bad or improper treatment can become a burden, causing greater damage to the environment. Therefore, the disposal and utilization of dredged soil should be emphasized when discussing the impact of dredged soil on the environment. This research emphasizes on how to minimize the adverse impact of dredged soil on the environment and maximize its utilization value.

 3.1 Make the most of dredging soil

Dredging has many uses and many benefits. In determining the dredging scheme, the properties of dredging soil should be carefully analyzed and the utilization value of dredged soil should be explored as much as possible. Generally speaking, dredging uses can be divided into three categories: engineering uses, agricultural and product uses and landscaping uses. Its uses can be divided into the following categories: (1) Construction and other engineering uses, including port, airport construction, urban and residential construction, etc. Such as Bordeaux, Le Havre, Rouen and other ports in France, dredging soil will be used to fill the land for port construction and development of industrial and agricultural land. (2) Building materials, such as cement added to solidify them, are made into building blocks. (3) Replacement backfill. (4) Coastal protection and erosion control. (5) Artificial fill beach. (6) Covering, i.e. using clean dredged material to cover the contaminated dredging material in the offshore backfill. (7) Wildlife habitat expansion. (8) For fishing. (9) Building national parks and recreation areas. (10) Used in agriculture, forestry and horticulture. (11) Backfill of open-pit mine and management of solid waste.

With the deepening of the work, the utilization scope of dredging soil will be expanded, and the economic and social benefits will be improved.

 3.2 Problems to be paid attention to when dredging soil utilization

In some estuarine areas, pollutants are found in underwater sediments due to the influence of industrial wastewater and pesticides. In the process of dredging soil utilization, we should not ignore the possible pollution and simply pursue dredging soil utilization. Direct application of contaminated dredged soil, resulting in the diffusion of pollution; the negative effects caused by environmental changes should not be emphasized unilaterally. Dredging soil should be treated as secondary pollution. This requires a correct assessment of the degree of contamination of dredged materials, and appropriate treatment of dredged soil that is indeed contaminated, such as the application of restrictive dredging technology, so that it can be applied in the construction of harbors and cities.