西南山区土质边坡防护措施的减流减沙效益评估

Benefit evaluation of runoff and sediment reduction measures on the earth slopes in mountainous areas,Southwest China

山区强降雨频率高,工程创面边坡坡度高陡,因此采用水土保持措施以防治和减少土质边坡土壤侵蚀尤为重要。目前裸露或具防护措施的土质边坡土壤侵蚀特征与过程、影响因素尚缺乏更多研究。本文通过设置微型径流小区结合模拟降雨,评估西南山区道路边坡裸坡、阶梯细沟、砾石、三维网、植被+三维网、不同植被覆盖度(25%、50%、75%和90%)等措施对减流、减沙效果以及边坡表土紧实度与产沙量的关系。研究表明:除表土砾石覆盖措施外,实施防护措施后的边坡产流量和产沙量显著低于裸坡。其中,以植被加三维网和植被覆盖度75%以上的减流、减沙效果最显著。随着植被覆盖度由0增加至90%,边坡产流及产沙量均有显著下降甚至无产出的趋势。道路工程裸坡0~10 cm深度土壤平均紧实度值差异极为显著,在400~2000 kPa之间波动。经历降雨后,0~10 cm深度土壤平均紧实度下降4~373 kPa。土壤紧实度变化会引起土壤孔隙度以及土粒黏结力等力学性质相应变化,当紧实度超过200 kPa时,裸坡产沙量随表层(<2.5 cm)土壤紧实度增加而出现降低的趋势。研究表明土质边坡植被覆盖度75%以上的措施减流、减沙效果最显著;另外,土壤紧实度与裸坡产沙量具有非线性关系。

[Background]In view of high and steep slope with frequent heavy rainfall,the measures against extreme soil erosion have been taken commonly on the cut-slope in mountainous areas.However,the characteristics,process and influencing factors of soil erosion on the earth cut-slope with or without soil and water conservation measures are rarely reported.The earth cut-slopes in four mountainous regions of Sichuan and Yunnan provinces,Southwest China were selected to evaluate the benefit of runoff and sediment reduction resulted from soil and water conservation measures under multiple rainfall intensity,and examine the effect of soil compactness on sediment yield.[Methods]Here,the effects of nine measures,i.e.,bare slopes,step rills,gravel,three-dimensional(3D)network,vegetation plus 3D network,different vegetation coverage(25%,50%,75%and 90%)against soil erosion on runoff and sediment reduction as well as the relationship between surface soil compactness and sediment yield have been executed on forty-five road cut-slopes at 12 sites in Panlong and Qiling counties of Yunnan province and Wenchuan and Songpan counties of Sichuan province,southwestern China,by setting up micro-runoff plots with simulated rainfall.The surface runoff and sediment yields produced in 45 earth slopes under various measures were determined by runoff plots with an area of 2 m^(2).The soil pH,organic matter content,particle and aggregate size distribution on each earth slope also were analyzed.[Results]The tested slope soils generally have coarser texture relative to undisturbed slope soils,and are dominated with the particle size fraction of sand and silt,occupying an averaged range of 36.9%to 54.1%,and 18.3%to 50.4%,respectively.Structural failure rate of the soil aggregates varies greatly from 20.8%to 63.3%,indicating the erosion durability of soils with the highest rate in Songpan is relatively low.Except the treatment that the surface soil covered with gravels,the runoff and sediment yields on slopes with protection measures are significantly lower than those on bare slope,with the runoff and sediment yields of higher than 1700 mL and 100 g,respectively.Among them,the measures,i.e.,vegetation plus 3D network and vegetation with the coverage more than 75%,are the most effective in mitigating runoff and sediment yields.With the increase of vegetation coverage from 0 to 90%,in general,runoff and sediment yields on cut-slopes significantly declined.The compacted degree values of the bare soil within a depth from the surface to 10 cm on the earth cut-slope varied greatly,fluctuating between 400 and 2000 kPa.After rainfall,the average compactness of the soil within a depth from the surface to 10 cm dropped by 4 to 373 kPa.The variation of soil compactness would cause the changes in porosity,grain cohesion,and other mechanical properties of soils.In case the soil compactness exceeds 200 kPa,sediment yields on the bare slope would reduce as the soil compactness increases.[Conclusions]The measure with the vegetation coverage more than 75%is the most effective in mitigating runoff and sediment yields,while the nonlinear relationship between soil compaction and sediment yield on bare slope is found.