The biggest difference between polluted sediment dredging and ordinary engineering dredging is that the former is integrated with the environmental protection concept and requirements, aiming at the removal of pollutants from the sediment and the change and improvement of water environment indicators. But the majority of the planning and implementation of the environmental protection dredging process, the starting point for the sediment pollutant removal and control of pollutants in the process of dredging relief, rarely considers the ecological problems caused by dredging and ecological protection. Study on ecological environmental protection dredging optimal control factor, water ecological restoration and protection of aquatic is conducive to the follow-up research, it is extremely necessary and urgent. Based on the focus on environmental protection dredging sediment pollutant removal currently, water ecological restoration after the dredging reconstruction are concerned about the problem of not enough analysis filter control factor affecting aquatic ecosystem survival, growth. And combined with the new ecological dredging method of terrain model is put forward for the subsequent ecological restoration build appropriate depth and basal conditions are needed to provide the solution.

1. Environmental dredging and ecological conservation paradox

In dredging construction practice, some ecological and environmental problems are increasingly prominent. Rational analysis, deep reflection, and comprehensive understanding of the impact of dredging are of great significance to the revision or promulgation of dredging regulations for sediment removal.

 1.1 Full section removal, surface removal and seed bank retention of sediment

The spatial difference in sediment pollution determines the layout of dredging. Under natural conditions, the pollutants in the bottom sediment are mainly concentrated in the sediment layer with a thickness of 20~50cm on the surface layer. However, due to the influence of external pollution and hydrodynamic environmental conditions, the distribution characteristics of pollutants in different regions are different, and the vertical horizon of the pollution layer is different. The thin layer of surface dredging not only removes the pollution but also completely removes the aquatic plants and large benthic seed banks that live in this layer, affecting the germplasm resources for subsequent ecological restoration. The larger the dredging section and scale, the more serious the influence is.

 1.2 Dredging depth and ecological restoration difficulty

There are no mature methods and accepted standards for the determination of ecological dredging depth of rivers and lakes for the purpose of reducing endogenous pollution. The sediment deposition rate and stratification characteristics can be used as reference factors to determine dredging depth. At present, the reference methods mainly include empirical value method, concentration control value method, background value comparison method, inflection point method and stratified release rate method. Too shallow dredging cannot effectively remove pollutants, and too deep dredging will change the shape of the bottom and adding the difficulty of repairing aquatic plants after dredging. The deeper the dredging, the more difficult the ecological restoration is under the same conditions.

 1.3 Minimize dredging size with maximum removal benefit

The ideal dredging scale of sediment is to remove the heavily polluted part of sediment with a small amount of dredging on the basis of full protection of the existing ecological good section, and to seek the best dredging effect, so as to realize the unification of environmental benefits, economic benefits and ecological benefits. The scale is a function of the dredging area and dredging depth. The more sample points and sections are monitored, the higher the upfront cost will be and the higher the construction accuracy will be required. The precision of dredging is closely related to the advancement of dredging equipment and the level of construction technology. According to the investigation and analysis of dredging equipment of ecological dredging projects at home and abroad, the control precision of vertical depth is generally 5~10 cm in practical construction.

 1.4 In situ cleaning and remote pollution

The pollutants in the bottom and overlying water in the ecological dredging area of the river and lake have been effectively controlled. However, some surface pollutants in the dredging process will have adverse effects on the surrounding water during the agitation and diffusion of construction. The dredged sludge and tail water of the sludge disposal site are external pollution to the storage site and the receiving water body, and there is a risk of leakage in long-distance transportation. The larger the scale of sediment removal, the higher the degree of pollution to other places, and the greater the risk of pollution to the water around the pipeline.

1.5 Dredging projects are lagging behind the scientific research support

After partial and small scale demonstration or experimental dredging, there is a trend of large-scale dredging in some areas. However, the research on key and mechanical problems, such as pollution division of supporting sediment, dredging scale and layout indicators and methods, is still in progress, especially on the evaluation of the dredging effect and monitoring of ecological restoration, which is relatively backward. Of course, this is related to the fact that river and lake dredging is an organic part of comprehensive water treatment, and it is difficult to quantify the comprehensive effect of dredging on water quality improvement and its contribution rate. However, it is the best way to make up for the lack of support and guide ecological dredging.

2. Research on key issues of dredging measures based on ecological protection and subsequent ecological restoration

 2.1 Ecological assessment objectives of environmental dredging

The effect of dredging should be evaluated comprehensively by the degree of completion of dredging target and the effect of dredging. While affirming its positive effects on the effective removal of pollutants and the increase of flood control and storage capacity, the adverse impact on ecology should not be ignored or avoided. At present, environmental protection dredging can only be used as an effective means to remove pollutants in an emergency. The long-term maintenance and improvement of water quality mainly depend on the purification of water quality of the water ecosystem and the play of ecological carrying function. Specifically, the proportion of aquatic vegetation must be certain after dredging. Therefore, the planning and design should focus on the dredging disturbance area and the area suitable for habitat construction after dredging, compare the changes of the habitat suitable for ecological restoration before and after dredging, increase the assessment of the suitable habitat construction after restoration and make up for the ecological damage caused by dredging. In principle, the area suitable for the growth of aquatic organisms should be maintained unchanged or improved to some extent.

 2.2 Selection of suitable ecological factors and construction of suitable habitat for aquatic organisms

2.2.1 Selection of ecological optimal control factors

The function of water ecological purification is closely related to the biological area after dredging. The ecological limiting factors affecting the water ecological restoration in dredging areas are the external habitats closely related to their living and growing, including water quality, water depth, bottom quality, illumination, wind waves, etc. For the aquatic organisms in the dredging area, the main difference is the change of water depth and the physical and chemical properties of the substrate before and after dredging. Under the same conditions, if the water depth is increased than before dredging, the underwater light field will be insufficient, making the underwater vegetation restoration more difficult. However, for shallow water bodies, the increase of water depth means that the disturbance effect of wind waves on sediment can be reduced to some extent, and the transparency will be improved accordingly. Underwater light intensity is closely related to water transparency, water depth and physicochemical properties of water quality, which can be reflected by water quality and water depth. The nutrient concentration in the water will become the threshold value for the growth and survival of aquatic organisms. Dredging can alleviate this situation to a certain extent, so it is not an important limiting factor after dredging. Under the influence of wind and waves, the bottom material will be suspended or disturbed and released to the overlying water. Its physical and chemical properties will affect the growth and distribution of immortal organisms. To sum up, the water depth and bottom quality can be used as the optimal control factors for ecological restoration after environmental dredging.

2.2.2 Aquatic organisms are suitable for deepwater habitat.

Biology and environment are coevolutionary, and aquatic plants and benthic animals in particular water bodies have their suitable living conditions. In rivers dredging depth, all kinds of aquatic plants arguably suitable growth conditions of water depth data are its pre-conditions. Rich and varied underwater terrains should be determined and shaped based on the current topography, bottom elevation, current water depth and the changes before and after dredging, so as to provide a good external habitat for the water ecosystem to play its ecological purification function.

2.2.3 Basal-textured habitat

Physical and chemical properties of substrates, such as hardness, the thickness of loam, and sediment characteristics, all affect the biological distribution of growth and habitat on them. At low nutrient concentration, organic substances, mineral elements and nutrients in the substrate are necessary for the normal growth of submerged plants. But at high nutrient concentration, the substrate may exert stress and inhibition on the growth of submerged plants. According to research, too hard or too soft sediments are not conducive to the growth of aquatic plants, high nutrient concentration will cause damage to aquatic plants, and eutrophication water is not conducive to the growth of aquatic plants to a large extent. When dredging depth, suitable depth and basement properties cannot be taken into account, the following treatment methods and sequence are recommended: It is better to meet the maximum pollution removal requirements first and then backfill loam according to the appropriate water depth. Next, according to the physical and chemical properties of the soil in the dredging area, the dredging depth should be reduced appropriately, and the suitable soil layer inherent in the bottom mud should be used as the new growing substrate after dredging. Finally, it is necessary to remove pollutants to the maximum extent, so that the water ecosystem can recover slowly after the water quality improves and transparency increases in the future. However, phased dredging should be adopted to create conditions and buffer time for water ecological restoration.

 2.3 Relationship between dredging pattern, layout and scale, and ecological protection and restoration

2.3.1 Relationship between dredging and other control measures

Sediment dredging is an organic unit in the comprehensive measures of river and lake pollution. On the basis of economic benefit analysis, the sequence and combination of various control projects in the comprehensive treatment of river and lake water environment are reasonably arranged, and the external source control and management are studied to ensure the dredging effect.

2.3.2 Influence of dredging layout on water ecosystem construction

The function of water ecological purification after dredging is closely related to the total area and layout of the water ecosystem. Under the premise that guarantees effectively remove endogenous pollution of lakes, combined with good ecological protection, is advantageous to the aspects such as ecological restoration after dredging, choose chip dredging, spacing strip dredging, local dot pit way dredging in rivers upstream and downstream, the full reasonable layout, forming different shades, habitat for a variety of underwater terrain to lay the foundation for the follow-up of ecological restoration.

2.3.3 The establishment of a water ecosystem in water level fluctuation zone combined with bank line regulation and bank revetment construction.

Under the influence of water diversion in the plain area, river basin flood control and season, the water level of rivers and lakes fluctuates within and between years, forming the water-level fluctuation zone. In combination with the regulation of river and lake shorelines, the form of bank protection is adjusted. The dredged silt is used in the underwater shoreline shorelines to build the habitat of different shorelines, so as to form the natural wetlands of river and lake shorelines and make up for the impact of bank protection construction on the encroachment of natural shoal wetlands.

3. Conclusion

The underwater topography is an important habitat for aquatic life. River and lake dredging will change the underwater topography. The reconstruction is an important means to adjust the elements of lakes and rivers. On the basis of analyzing the purpose, effect and influence of dredging, and from the perspective of environmental protection and subsequent ecological restoration, the optimal control factors of environmental dredging of sediment are selected as water depth and substrate. A new type of dredging measure, which can be combined with the dredging of sediment, is put forward to provide a new idea for the transition from environmental protection dredging to ecological dredging.