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.