间歇降雨对铁尾砂重构坡面侵蚀水动力学特性的影响

黄土高原夏季降雨集中且多暴雨,铁尾矿砂自身结构松散,堆存坡度大,雨季通常遭受多次暴雨侵蚀,严重限制植被恢复。为有效减少铁尾矿的水土流失,促进生态恢复,在干排铁尾矿砂中添加不同比例土壤和菌糠,形成6种重构坡面:松散铁尾矿砂(LT),结壳铁尾矿砂(CT),铁尾矿砂与菌糠混合物(TM),铁尾矿砂、菌糠和土壤混合物(TSM),铁尾矿砂和土壤混合物(TS),纯土(S)。采用室内人工模拟降雨技术,研究重构坡面在间歇性降雨下的侵蚀特征和水动力因素。结果表明:1)随降雨场次增加,6种坡面的产流率均持续增大,而产沙率表现出明显的分异:LT、CT和TS产沙率持续增大,表现为高产沙型,TS、TSM和S产沙率整体上是先增加后减少,表现为低产沙型;2)随降雨场次增加,6种坡面侵蚀形态也出现分异,可分为2类:LT和CT和TS坡面侵蚀形态严重,细沟发展快,演化为宽深比较小的细沟;TM和TSM和S坡面侵蚀形态轻微,细沟发展缓慢,S坡面演化为宽深比较大的细沟,TM和TSM坡面细沟很少,多为均匀分布的菌糠小坑穴。据此表征坡面侵蚀微地形变化的沟壑面积和地形起伏度,随降雨场次增加持续增大;沟壑密度则随降雨场次先急剧增加,随后由于沟壑形态差异,部分坡面减少;3)6种坡面的坡面流流型均为层流,流态大多为急流;随降雨场次增加,流速、水流功率整体表现为增大的趋势,径流剪切力无明显趋势,阻力系数、曼宁粗糙系数表现出减小的趋势;除径流剪切力,不同降雨场次间水动力学特征差异显著(P0.05)。6种坡面侵蚀产流产沙可用对数、线性或幂函数较好地模拟。纯尾矿砂坡面侵蚀最为严重,坡面覆土具有明显的减流效果,但添加菌糠的减沙效果最好,因此对于土源缺乏的铁尾矿砂复垦区,同时添加菌糠和土壤可以有效减少侵蚀,易于植被修复。

Effects of intermittent rainfall on hydrodynamics characteristics of erosion on reconstructed slopes of iron tailings

Abstract: Rainfall is concentrated in summer and rainstorm often occurs in the Loess Plateau of China. Iron tailings dump with loose structure and steep slope is repeatedly eroded by rainstorms in rainy seasons, which severely affects the vegetation restoration. This study explored the influence of intermittent rainfall on hydrodynamics characteristics of erosion on reconstructed slopes of iron tailings in order to decrease soil and water loss of iron tailings. Soil or mushroom residue was added to tailings in order to improve the properties of iron tailings. A total of six reconstructed slopes were constructed and they included loose tailings (LT), crusty tailings (CT), tailings incorporating mushroom residues (TM), tailings incorporating soil (TS), tailings incorporating soil and mushroom residues (TSM) and soil (S). Indoor artificial simulated rainfall was used to study the erosion characteristics and hydrodynamic factors of the six reconstructed slopes at 25o, 30 o and 35 o under intermittent rainfall. Three rainfall events of 60, 90 and 120 mm/h were carried out on the same slope. The results showed that 1) the runoff rate of six substrates increased continuously with the increase of rainfall events but the sediment yield rate was different: sediment yield rate of LT, CT and TS increased continuously with high-sediment yield and that of TS, TSM and S increased first and then decreased as a whole with low sediment yield. The difference in sediment yield among six slopes was significant, and the difference of runoff was not significant. 2) Under intermittent rainfall condition, the differentiation in slope micro-topographic morphology was much distinct. LT, CT and TS easily produced rills and the rills were developed rapidly, evolved into the rills with smaller width-depth ratio; the rills on TM, TSM and S evolved slowly, and finally the width-depth ratio of rills on S was larger; TM and TSM produced many small pits of mushroom residues evenly, and the rills were rare and shallow. The gully area and relief amplitude continued to increase with the increase of rainfall events while the gully density sharply increased then some of the substrates decreased and the others still increased. 3) The slope of the six slopes produced laminar flow, and most of substrates were supercritical flow. With the increase of rainfall events, the flow velocity and runoff power showed increasing trend while the Darcy-Weisbach resistance coefficient and Manning roughness coefficient showed decreasing trend. The runoff shear stress didn''t show obvious trend. The hydrodynamic driving factors of erosion were different among the six slopes and the runoff and sediment yield were well simulated by logarithmic, linear or power functions. LT and CT had the most serious erosion morphology on the slope and S had the obvious effect on reducing runoff. TM, S and TSM could reduce sediment loss. Among TM, S and TSM, TM was the best for reduction of sediment loss. Therefore, in the iron tailings reclamation area lack of soil resources, the addition of mushroom residues and small amount of soils is an effective way to decrease erosion and facilitate vegetation restoration.