However, the region is dominated by a subtropical monsoon climate, with an annual average rainfall higher than 2000 mm. In recent years, with the development of the Guangdong-Hong Kong-Macao Greater Bay Area, infrastructure construction in Guangzhou and surrounding areas has ushered in a new round of development opportunities. Under the effect of long-term geological and climatic conditions, the granite surface is generally covered with a thick layer of weathered eluvial soil, namely, granite residual soil, which is a characteristic regional soil and one of the main soils encountered in engineering construction in the coastal areas of the Yuemin region. Granite is widely distributed in southeastern China, especially in Guangdong and Fujian provinces, where the exposed area accounts for 30–40% of the total area. Moreover, the occurrence of road surfaces and channels also accelerates the water collection process and runoff velocity 12, which generates adverse impacts on the ecological environment and even threatens the stability of road slopes 13, 14, 15, 16, 17. In addition, road subgrades with a high density and low infiltration are an important source of excess infiltration and surface runoff in mountainous areas 11. When gully erosion develops on a slope surface, the number of erosion gullies sharply increases 10. Due to the unconsolidated soil quality and poor erosion resistance of filled slopes, rill or gully erosion easily occurs under the influence of rainfall and road catchment water runoff 9. Specifically, high and steep cutting slopes are intensely scoured by the incoming water originating from upward slope areas, and the areas along roads exhibit a high incidence of geological disasters such as collapses and landslides. Under the effect of heavy rainfall, serious erosion phenomena often occurs in the slope areas on both sides of roads 6, 7, 8. Moreover, vegetation is readily damaged, and a large number of exposed slopes are typically generated in road construction. Their effects include landscape fragmentation, alteration of the topography and watershed hydrological processes, and water quality change 3, 4, 5. Existing studies have indicated that roads, especially highways, inevitably impact the surrounding ecological environment. However, the development of human economy and society is difficult to avoid the destruction of ecological environment, especially in the field of traffic engineering construction 1, 2. Protecting, restoring and promoting the sustainable use of terrestrial ecosystems, combating desertification, and halting and reversing soil degradation are among the 17 global development goals set by the United Nations by 2030. Only on slopes with no stability issues should Nephrolepis cordifolia be considered to preserve soil and water. Nephrolepis cordifolia should be cautiously planted as slope protection vegetation. (4) Zoysia is a suitable vegetation species in terms of slope protection because of its comprehensive slope protection effect. The root system significantly enhances the water storage capacity of the slope, and greatly increases the permeability of the slope soil in the second rainfall, which is totally different from that of the exposed and Zoysia japonica slopes. The stem leaf can stabilize the infiltration of rainwater, and successfully inhibit the surface runoff under continuous secondary heavy rainfall. (3) Both the stem leaf and root system of Nephrolepis cordifolia have important roles on the hydrological effect. (2) The multiple hindrances of the stem leaf of Zoysia japonica plays a leading role in regulating the hydrological effect of slope, the root system has little effect on the permeability and water storage capacity of slope soil, but improves the erosion resistance of it. (1) The exposed slope has weak resistance to two consecutive heavy rains, the degree of slope scouring and soil erosion damage will increase greatly during the second rainfall. The results indicate that surface vegetation first effectively reduces the splash erosion impact of rainwater on slopes and then influences the slope hydrological effect through rainwater forms adjustment. The variation in slope surface runoff, soil moisture content and rain seepage over time was recorded during the tests. Against the background of two heavy rainfall events occurring during a short period due to a typhoon making landfall twice or even two typhoons consecutively making landfall, laboratory model tests were carried out on the hydrological effects of the granite residual soil slope considering three vegetation types under artificial rainfall.
The problems caused by the interaction between slopes and hydrologic environment in traffic civil engineering are very serious in the granite residual soil area of China, especially in Guangdong Province.
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