摘要
路域系统侵蚀是区域土壤侵蚀的重要组成部分,其形成和发展是自然因素和人为因素共同作用的结果。山区道路建设对区域地形地貌有着非常显著的影响,从而导致道路系统土壤侵蚀类型发生转变、侵蚀强度和产流产沙增加,而道路边坡由于植被恢复困难、坡度陡,常常发生严重的水土流失现象,是流域中重要的侵蚀来源。因此,进行道路边坡的侵蚀过程特性和边坡防护试验研究,对进一步认识边坡防护机理、泥沙输移规律,及山区生态环境建设中的机耕道路及边坡防护设计与维护、道路网络合理规划有重要的实用价值。
本研究以新建土质道路边坡为研究对象,通过野外降雨试验和土壤力学试验等方法,研究进行边坡生态防护措施之后,道路边坡的侵蚀稳定特性,获得如下主要研究结果：
(1)通过对道路边坡土壤理化性质分析认为：道路边坡作为路域系统中一种特殊的土地利用方式,其土壤结构性质与其他土地利用相比,存在较大差异,土壤容重、饱和导水率和总孔隙度最为显著,自然情况下,路堑边坡由于土壤容重大、植被恢复比较困难、坡度陡,产流产沙效果要高于路堤边坡,侵蚀发生的频率和强度路堑都要比路堤高。在边坡小区的各项影响因子中,植被覆盖度、土壤容重、饱和导水率、总孔隙度等是影响边坡降雨侵蚀性的最主要因子。
(2)边坡进行防护之后,模拟降雨试验的结果显示：45°路堑边坡,梯坎+草灌措施在防止边坡侵蚀的效果上显著优于草灌结合,虽然降雨后期产流增加,但是径流中含沙量非常低；30°路堑边坡,防护效果依次为草灌结合>草本>植生带>对照；30°路堤边坡,草本措施长势非常好,其防护效果与草灌结合效果差异不大,但是都显著优于植生带和对照处理,在15°的缓坡路堤上,同是坡耕地处理,有路肩草本缓冲带可有效减少坡面径流75.93%,产沙量减少81.62%,在人地矛盾激烈的山区,缓坡路堤的顶部配合1-2m的植物缓冲篱带,可有效地解决路堤边坡农业耕作带来的水土流失问题。
(3)道路的形成一部分拦截道路上方坡面径流和壤中流,同时高度压实的路面本身也是流域重要的产流区,这些水流的去路,除沿路面和边沟渠排走之外,其余则是沿路肩溢流从路堤边坡排走,对路堤边坡形成严重的冲刷侵蚀。在对路堤边坡进行模拟路面来水冲刷试验,数据显示,在30°路堤边坡,路面来水的介入对植被覆盖良好的草本和草灌小区产流能力的增加高于产沙能力,但两者增幅都不大,而对于植生带和对照处理,有路面来水时径流系数峰值是无路面来水的1.9～7.1倍,产沙能力伴随着产流的增加显著增大。在15°路堤边坡,路肩草本缓冲带在无路面来水时表现出很强的滞流减沙效果,但是随着路面来水的加大,缓冲带对减少坡面产流的贡献基本上消失了,但是对于产沙的抑制作用还是比较理想,径流含沙量峰值为坡耕地的62.6%,平稳期为45.9%。
(4)对道路边坡自修建开始进行天然降雨侵蚀的跟踪观测,观测结果显示：边坡的水土流失主要集中在道路修建之后的0-6个月内,且极端降雨的影响非常显著,侵蚀最严重的45°路堑对照小区,次降雨侵蚀模数高达12.22g.m-2.min-1,次降雨总产沙量为51937.2g,占全年总产沙量的46.73%,30°路堤草灌结合小区该次次降雨总产沙量为12606.2g,占全年总产沙量的78.90%。时间因子是边坡侵蚀中一个重要的影响因子,随着时间的推移,降雨在坡面上的产流能力和产沙能力都显著下降,降雨侵蚀力与观测天数呈指数下降。
(5)自然降雨的观测结果与模拟降雨的结果还是存在差异,模拟试验的结果,路堤边坡几乎所有的处理,其产流产沙都要显著低于路堑边坡,但是自然观测的数据确与实验值有明显的差异,观测结果显示,路堤边坡各处理在产流产沙上与路堑边坡相差较小,且这种差异随降雨类型变化,在短历时、低雨强降雨条件下,路面的高含沙水流对下边坡的影响就越大,而对于降雨量大和降雨强度大的降雨类型,路面来水来沙对路堤的产流产沙贡献率就要小一些。
(6)生物措施对边坡的防护作用,除地上部分的冠层、茎、叶及枯落物对降雨的拦截消能之外,地下根系也是一个不能忽视的部分,地下根系除能显著改变边坡土壤的物理化学性质之外,更重要的是能增加土壤的力学强度,增加表层土壤的抗冲性及边坡的浅层稳定性。
本文采用野外十字板剪切仪,在雨前雨后分别测定边坡的土壤不排水剪切强度,数据显示,含水量的变化是影响抗剪强度的一个极为重要的因素,植被根系的存在能显著降低含水量的影响,对于根系含量比较高的草本和草灌结合处理,含水量的增加对抗剪强度的影响作用要显著低于植生带和对照处理。并且草本和草灌结合处理,由于表层土壤根系含量高,密度大,提高了表土抗冲性的同时还增加了降雨的入渗和地表粗糙度,导致这两个处理降雨产流产沙低于植生带、坡耕地和对照处理。径流系数与坡面土壤颗粒的剥蚀率都与抗剪强度呈负指数相关。
综合以上的研究结果,在对山区土质道路边坡进行防护时,植物防护是首选,措施选择上,以乡土草本和草灌结合为主,低缓边坡则可以考虑坡耕地和植物篱带相结合的方式以缓解山区激烈的人地矛盾,人为因素的介入不仅能显著缩短边坡植被的自然恢复周期,降低道路的水土流失,还能对路域这一环境脆弱区进行合理的开发利用和保护,这对于降低道路建设对山区生态环境的影响是极为重要的。
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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