(A) Ecological slope protection and its advantages

1. Significance and mechanism of ecological slope protection

Ecological slope protection is an important measure of river ecological management, and it is a new slope protection technology combining water conservancy projects with ecological environment protection. It combines modern water conservancy engineering, biological science, environmental science, ecology, landscape science, aesthetics and other disciplines, breaking the traditional pattern of three-sided lining and uniformity of revetment cement, and at the same time, it no longer only emphasizes the anti-scouring ability of revetment, but requires the perfect combination of stability, landscape and ecology. Therefore, ecological slope protection technology can be described as "water conservancy engineering technology to reduce the erosion degree of water flow, wind and rain, improve the stability of slope and toe, and realize the evolution of riparian ecological community to top community".

Ecological slope protection technology is a new technology which uses the principle of vegetation culvert to stabilize geotechnical slope and beautify the environment at the same time. It is a comprehensive engineering technology involving geotechnical engineering, soil and water conservation, environmental protection, eco-botany, horticulture, soil fertilizer and other disciplines.

According to the above ideas, the ecological slope protection mode should have the following characteristics:

1) emphasizes the coordination of the two standards in terms of concept and method, which requires not only putting forward the standard of river protection system according to the local natural and socio-economic development, but also understanding the natural ecosystem of the river itself, so as to restore the ecological balance of the river system as soon as possible after the implementation of the project. It involves more fields, more technical means and more complicated methods.

2) It enriches the original slope protection mode, including not only the traditional protection and greening mode, but also the development mode of artificial wetland system, golf course and beach planting and breeding.

3) The selection of materials is diversified, including soil, stone, concrete, ecological concrete products, geosynthetics and suitable plants. As long as it meets the requirements of ecological protection, concrete is not excluded for protection.

4) Different from the traditional greening and beautification, ecological slope protection emphasizes the use of natural forces to realize the ecological balance and natural restoration of vegetation system and restore the natural four-season landscape. Experience shows that the system that can't balance the natural ecology needs manual maintenance, so the management and maintenance cost is high.

The function of ecological slope protection is mainly through the interweaving of underground roots and aboveground stems and leaves of slope plants, which greatly increases the stability and cohesion of soil, and the stems and leaves of aboveground parts can reduce or prevent the direct impact and splash erosion of rainfall on the ground. From the above point of view, the slope protection mechanism should be considered from the following two aspects: ① the mechanical mechanism (mechanical effect) of vegetation system to control erosion; ② Hydrological mechanism (hydrological effect) of vegetation system controlling erosion. Zhou Xijiu, Zhang Junyun, Wang Wensheng and others have done a lot of research work in this field, and summarized the mechanism of ecological slope protection as shown in figure 10- 1.

Figure 10- 1 Ecological Slope Protection Mechanism

In figure 10- 1, the main functions of deep root anchorage and transpiration drainage are to reinforce the slope, while the main functions of shallow root reinforcement and hydrological action of stems and leaves are to protect the slope.

Mechanical effects of vegetation. Plants are divided into two categories: herbaceous plants and woody plants. Because the biological characteristics of these two kinds of plants are different, their root morphology is also different, so their action mechanisms are also different. According to the morphological characteristics of roots, plant roots can be divided into three types: herb roots, woody roots, horizontal roots and vertical roots. From the mechanical point of view, there are two types of action: shallow root reinforcement and deep root anchorage. Its mechanism is as follows: ① shallow root strengthening. The roots of the planted grass are intertwined in the soil, so that the slope soil becomes a composite material of soil and grass roots within its extension range. Grass roots are regarded as three-dimensional reinforced materials with prestress. ② Deep root anchorage. The main root of the vertical root system of woody plants is thick and deep into the soil layer. Vertical deep roots pass through the soft layer or sliding surface of the slope, and the slope is reinforced by anchorage and pile side resistance, similar to anchor rod or anti-slide pile.

Hydrological effects of vegetation. The stems, leaves and litters of the aboveground parts of vegetation can reduce or prevent the direct impact and splash erosion of rainfall on the ground. Plants, especially herbs, have the ability to control soil erosion. Through the functions of rainfall interception, runoff delay, soil infiltration and soil consolidation, the microclimate on slope surface can be improved and soil surface erosion can be slowed down. ① Reduce the pore water pressure of slope and improve the shear strength of soil. Rainfall is one of the important factors inducing landslide, and slope instability is closely related to slope water pressure. By absorbing and evaporating the water in the slope, plants can reduce the pore water pressure of the soil, increase the soil suction and improve the shear strength of the soil, which is beneficial to the stability of the slope. (2) rainfall interception, weaken the splash erosion. Part of the rainfall was intercepted by vegetation before reaching the slope, re-evaporated into the atmosphere or fell on the slope. When the falling raindrops hit the slope, they transfer momentum to the soil, and the resulting splitting force separates and splashes the soil particles. In the process of dripping, the greater the momentum of raindrops, the greater the impact splitting force and the more soil particles splashed. Vegetation can intercept falling raindrops at high speed, reducing the number of raindrops, splashing energy and splashing soil particles. ③ Restrain runoff erosion. Surface runoff takes away soil particles separated by dripping and splashing, which further leads to sheet erosion and gully erosion. Vegetation can inhibit surface runoff, reduce drip and splash erosion, and thus control soil particle loss. Usually, the amount of soil loss decreases exponentially with the increase of vegetation coverage.

2. Advantages of ecological slope protection compared with traditional slope protection

In the traditional urban river regulation, simple engineering measures are generally adopted for slope protection, mainly including mortar or dry block stone slope protection, cast-in-place concrete slope protection, precast concrete block slope protection and so on. Most of the protection forms are trapezoidal or rectangular sections, and some even are full-face lining. These traditional slope protection structures have played a certain role in maintaining the stability of slope bank structure, preventing soil erosion and flood control and drainage, but they have had negative effects on landscape, environment and ecology to varying degrees. At present, ecological slope protection generally adopts plant slope protection or composite slope protection combining plant and engineering measures. Because of the existence of vegetation, the ecological function of slope protection has been restored, which is greatly improved compared with traditional slope protection.

(1) Improve water quality and water self-purification ability.

Smooth and hard traditional slope protection, without plants as a barrier, will make the garbage on the shore and pollutants brought by surface runoff easily enter the water body, accelerate the deterioration of water quality and cause pollution. Ecological slope protection plays a role in protecting river water, because vegetation can retain garbage and particulate matter, and filter pollutants in highland and surface runoff. Cai Jing's research found that the interception rate of suspended solids and nutrients by soil bioengineering firewood cages, shrub mats and traditional grass planting slope protection reached more than 80%. In addition, the traditional hard revetment without grass destroys the material and energy circulation system between water, soil and organisms, which is not conducive to the growth of aquatic organisms with water purification function and weakens the self-purification ability of water bodies. Vegetation and new materials used in ecological slope protection enhance the water permeability and air permeability of slope protection, maintain the material and energy circulation among water, soil and organisms to a certain extent, and protect the self-purification ability of water.

(2) Adjust the change of water temperature to maintain the balance of water ecosystem.

Traditional slope protection enhances the absorption of light, which leads to excessive temperature change of water body, which is not conducive to maintaining the ecological balance of water body. The water temperature changes too much, which is not conducive to the survival of temperature-sensitive species; It has a great negative impact on fish spawning, because the normal reproduction of fish is very sensitive to temperature; In the hot season, infectious bacteria will also multiply. Ecological slope protection by introducing vegetation, shrubs or forests has played a certain role in regulating light absorption and preventing excessive changes in water temperature. Therefore, from this perspective, ecological slope protection has improved the balance of aquatic ecosystem to a certain extent.

③ Protect the ecological diversity of river system.

The traditional slope protection repaired with concrete and gravel accelerates the flow velocity of water body and increases the scouring intensity of water flow to a certain extent. Some creatures in the water will be washed away by the current, which will reduce the number of creatures in the water, and the lack of natural plants on the shore will directly affect the survival of wild species along the river, such as waterfowl. According to statistics, at present, after the river is crossed, the species along the river have decreased by more than 70%, and the aquatic life is only 50% of the original. In addition, the heterogeneity of riverbed in natural river system no longer exists, and many aquatic organisms cannot settle down and form a variety of habitat combinations. The water permeability of ecological slope protection is conducive to maintaining the original velocity of river water, providing a favorable waterside environment for the growth of various organisms, the habitat of insects and birds, and the reproduction of fish, and protecting the biodiversity of river water system.

(4) Restoration of slope protection and greening landscape.

There was no green traditional slope protection in the past, which made the river lose its original vitality. The green ecological slope protection is added, the green landscape of slope protection is restored, and the pursuit of modern people returning to nature is satisfied. Ecological slope protection is an improvement on the basis of traditional slope protection, which is not only limited to the application of new environmental protection materials or technologies, but also attaches importance to exploring the ecological functions of traditional artificial materials and technologies and improving traditional slope protection methods, and pays more attention to the needs of environment and biology in design and construction. Therefore, ecological slope protection is not only the derivative of traditional slope protection, but also different from traditional slope protection, and it is a brand-new stage of slope protection engineering development.