黄土坡面细沟形态变化及其与流速之间的关系

Morphological changes of rill on loess slope and its relationship with flow velocity

研究细沟的形态变化特征是认识细沟侵蚀的重要基础,细沟发育过程中细沟形态变化与水流动力学特性之间存在相互影响和相互作用的关系,研究细沟发育过程中细沟形态与水动力学之间的关系,有利于更好地了解细沟侵蚀过程和侵蚀机理。该研究通过室内人工模拟降雨试验,对黄土坡面细沟发育过程中的细沟形态变化及其与流速的关系进行了研究。结果表明：坡面侵蚀过程呈明显的阶段性,坡面细沟形态变化过程与坡面径流含沙量的变化情况基本一致;坡面跌坎发生的临界流速为0.19-0.21 m/s,当坡面径流流速大于这个临界值的时候,坡面会出现跌坎;细沟发育初期,细沟间的距离一定程度上影响细沟的分布,最早出现的细沟之间不会再出现新的跌坎,这一间距范围在12.5-17.5 cm之间;细沟侵蚀过程主要以下切侵蚀和溯源侵蚀为主,沟壁坍塌的侵蚀作用相对较小;细沟流速随时间的变化大致呈先增后减的趋势,细沟流速随细沟宽度的增加而显著减小,这一趋势在4 m坡段尤为明显,二者之间存在显著负相关关系（r=-0.348,P=0.04）。受试验条件所限没有研究细沟深度和流速等其他水动力学参数,以后需要不断改进试验方法来准确测量流速、水深等指标,进一步研究细沟发育过程。

Rill erosion is usually identified as a series of little rills up to 30 cm deep that can be obliterated by cultivation. Rills are small channels which function as both sediment sources and sediment transport vehicles on hill slopes. It is a quantum leap during the process of soil erosion on slopes, and is also the beginning of qualitative changes in the process of soil erosion. As an intermediate stage between overland and gully erosion, theories for soil detachments by rill erosion are different with that by inter-rill erosion. The loss of the topsoil and nutrients caused by rill erosion can reduce soil productivity, and the deposition of off-site sediments can bring sedimentation and water-quality deterioration in streams and reservoirs. Rill erosion is the main way of slope farmland erosion and plays an important role in soil erosion process on the Loess Plateau. On the Loess Plateau of China, the contribution of rill erosion can be up to more than 70% of slope erosion and about 50% of the total erosion. Studies on rill erosion can help control soil erosion on sloping land, and facilitate the development of agricultural production, and are a main content of studies on the development process of soil erosion. Rill morphology over the soil surface plays a significant role in determining the runoff and soil loss from sloping farmland. However, few attempts have been done on accurate measurement of the rill throughout the study area for that eroding rill evolves morphologically in time and space. The objectives of this study were to quantitatively measure the detailed rill morphology under controlled laboratory conditions and to provide more insight and detail for later experiments. The soil used in this study was the loess soil from Yangling, Shaanxi province in China. In this paper, in conducting indoor simulation rainfall experiments using deionized water at different slopes（10°, 15°, 20°, and 25°） and different rainfall intensities（90 mm/h and 120 mm/h）, morphological changes of rill development on loess slope and its relationship with flow velocity were discussed. The results showed that： 1） Slope erosion had periodic development, and the changes of morphology of rill erosion process and changes in runoff sediment concentration was consistent; 2） the critical velocity when backward-step occurred under the experiment conditions was 0.19-0.21 m/s, and the backward-step could occur only when the flow velocity was greater than the critical values; 3） In the early stage of rill erosion development, the distance among rills affected its distribution, and backward-step did not occur among the earliest developed rills. The distance here was 12.5-17.5 cm. The rill erosion process was mainly dominated by cutting and headward erosion, and little by bank landslip; and 4） the changes of rill flow velocity with time substantially increased and then decreased, the flow velocity increased with decreased rill width, and this trend was particularly obvious on the 4 m slope where the flow velocity and width of rill were significantly（P＆lt;0.05） negatively correlated.