| Road system erosion is a very important component of the regional scale soil erosion, its formation and development are the combination consequences of the natural and human influence. Road building in mountain areas has a notable geomorphological impact. Several factors, such as breaking of the hillslope profile, lack of vegetation protection on road sides and alteration of hillslope hydrogeomorphological functioning, suggest an increase of soil erosion processes, both variety and intensity, and in sediment yield, particularly in those hill roads that were built with low budgets, carry heavy traffic and do not include even minimal conservation techniques. Road sides slope are the most important sources of the watershed soil erosion. Thence, the research of road side slope of soil erosion processes and vegetation protection give a further approach to recognize the mechanism of road side slope protection and pattern of sediment transport, and have its significant practical value on road building, side slope protection patterns designing and maintaining, road web reasonable programming.This paper focused on a fresh built rural road side slopes, combination with the methods of rainfall simulation and soil shear strength test to study the erosive and stability characteristics of the road slope after ecological protection. The main results as followed:(1)The test results of the physical and chemical properties showed that:as one of the particular land utilization systems of the road system, road slope soil has its special soil structure and difference compared with others, especially on the bulk density, saturated hydraulic conductivity and total porosity. The cut slope for the bigger bulk density, more difficult revegetation and higher slope degree, its sediment and runoff yield values are superior to the fill slope, its frequency and intensity are also higher than the fill slope. During the soil erosion impacted factors of road side slope, the vegetation coverage, soil bulk density, saturated hydraulic conductivity and total porosity are the most important.(2)The rainfall simulation results suggested that:on the cut slope of 45 degree, the erosion track efficiency(TE) of stone dike terrace with vegetation are higher than the grass and bush combination, through the runoff volume increase in the later, but the sediment concentration is still very low. On the cut slope of 30 degree, the TE is grass and bush combination> herbaceous>sodded strip>control. On the fill slope of 30 degree, the herbaceous treatment for a good vegetation coverage, its erosion track efficiency is close to the grass and bush combination, and both significant higher than sodded strip and control treatment. During all the vegetation patterns, the pattern of combination of grass and bush has the best protecting effect for dense vegetation coverage and high total porosity and saturated hydraulic conductivity. This phenomenon reflected especially on the plot of fill slope of 30 degree, it got a start in runoff yield after 10 min of rainfall simulation, and had a maximum accumulated rainfall of 19.13 mm before runoff generation. The herbaceous pattern also has a good effect on reducing runoff and sediment. On a moderate degree of fill slope, sloping farmland combinated with a appropriate width herbaceous buffer strip can significant reduce the soil erosion.(3) The scouring test of simulated the runoff of the road surface impacted on the fill slope suggested:when the rainfall intensity is 90 mm/h, all of the fill slope plots have a very low sediment and runoff yield for the dense vegetation coverage, except the plots of control and sloping farmland. While the road surface runoff's interfering enhance the slope runoff generation capability, at the poor vegetation recover plot, the peak runoff coefficient is 1.9 to 7.1 times higher when there is no road runoff exist, the sediment generation capability significant increased accompany with the increased runoff. When there is no road runoff, the herbaceous buffer strip of the verges has a remarkable effect on soil erosion delay, however, when the road surface runoff entered into the fill slope increased, the buffer effect of runoff vanished away, but for sediment, there is still a preferable value for the peak intercept efficiency of 62.6 percent, and stationary phase value of 45.9 percent.(4)There is a following observation on the road side slope when the road construction finished, and the observation results showed that:the slope soil erosion concentrated on the first half year after the construction. Extreme rainfall has a very significant impact on the road side slope erosion, for the severest plot of 45 degree control plot, individual storm event erosion modulus come to 12.22 g.m-2.min-1, and individual storm event total sediment is 51937.2 gram, occupied 46.73 percent of the yearly total sediment yield. During the extreme rainfall observated, the combination of grass and bush plot of 30 degree fill slope, individual storm event sediment yield is 12606.2 gram, takes a proportion of 78.90 percent of yearly. Time factor is a very important influence factor of road side slope soil erosion, in process of time, the rainfall contribution to side slope runoff and sediment generation sharply decreased, and the rainfall erosivity showed a exponential decline with the observated days.(5)When it comes to the comparison of natural rainfall and simulated rainfall, the results showed that:there is no significant difference of the runoff, but the sediment yield of the observated is higher than the simulated, and the vegetation factors does not show a remarkable effect on the erosion interception. During the natural rainfall, sediment generation stem from the sediment transported by the road surface runoff, when it deposited on the slope, with the dual function of natural rainfall and road surface runoff, it becomes the rejuvenated erosion again.(6)The vegetation can protect the slope against erosion, the canopy, leaves and stems above the ground intercept the rainfall and dissipat the energy, and the roots below the ground is also a very important part to reduce the soil erosion, except for the improvement of the soil structure, more for the inhance of mechanics intensity and stability of the surface soil.This paper tested the undrained shear strength before and after the simulated rainfall by a portable vane tester. The results show that:the soil water content is a very important factor to influence the soil shear strength, but the existence of vegetation roots can significant reduce the impact of soil water caontent on shear strength. The treatments of grass and bush combination and herbage for a dense root density, its impact of water on shear strength are lower than the sodded strip and control treatment. The dense root density not only improve the strength, infiltration and rougthness of the surface soil, so the runoff and sediment of grass and bush combination and herbage treatments are lower than sodded strip, slope farmland and control. The runoff coefficient and soil detachment rate show exponential decline with the shear strength. Integrated the results above, when we come to the protection of the side slope of mountain rural road. Plant measurements of the native species are still the priority choises. When the degree is moderate, it can consider as slopeing land combination with vegetation buffer strip. The intervence of human activity not only can cut down the period time of vegetation rehabilitation, but also can rational exploit and protect the road system zone as a ecotone. That are very important to reduce the impact of road construction on ecological environment. |
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