紫色土坡耕地土石复合坎结构与水土保持效应研究

Soil-capped stone ridge, combined stable performance of stone ridge with low cost characteristics of soil ridge, is a main engineering measure to control soil erosion on purple sloping farmlands in the Three Gorges Area, however, there reported a few studies on the ridges design and its soil conservation effects. It is significant to study soil conservation practices of ridges on the slope farmland and control the soil erosion and non-point pollution as to ensure regional ecosystem health and construct eco-civilization.Based on a large number of field investigations and in situ observation of sloping farmland soil stress, this proposed program uses artificial rainfall simulation and runoff plot observation combined with three-dimensional laser scanning technique, finite element numerical analysis and WEPP (Water Erosion Prediction Project) model simulation to study the soil stress distribution regularity after the purple-soil capped stone ridge. And then rational size of soil-capped stone ridge and intervals between ridges will be decided by the critical point of the soil stress-strain. Except for the ridge structures, its soil conservation effects in terms of soil particle redistribution, soil water content changes and sediment reduction, further the soil conservation mechanism will be studied, as to provide scientific basis for sound and efficient soil conservation design and plans on the purple-soil sloping farmlands.

土石复合坎是三峡库区坡耕地水土流失防治的重要措施，兼顾石坎的稳定性和土坎的低成本特点，但其结构、功能与效应研究较少。开展埂坎措施研究，控制坡耕地土壤侵蚀以减轻流域面源污染，对保障区域生态安全和构建生态文明具有重要意义。以紫色土坡耕地土石复合坎为研究对象，在野外埂坎现状调查和土体应力监测系统原位观测的基础上，开展人工模拟降雨实验和自然降雨径流小区观测，结合三维激光扫描、有限元数值分析等方法，进行紫色土坡耕地土石复合坎水土保持机理研究。揭示坎后应力分布规律，探讨石坎高度、土坎最优宽高比和埂坎合理间距(田面宽度)等结构问题，分析埂坎对坡耕地土壤颗粒再分布的影响、水分通道/阻隔效应及其减沙效益，以期深化坡耕地水土保持措施优化研究，为紫色土坡耕地水土流失防治措施的合理设计提供科学依据。

埂坎是紫色土坡耕地重要的水土保持措施，土石复合坎兼顾石坎的稳定性和土坎的低成本特点，其结构、功能与效应研究可为紫色土坡耕地土壤侵蚀控制提供依据。在野外调查、土体应力观测基础上，开展三轴试验、染色示踪、人工模拟降雨等手段，结合GeoStudio模拟等方法，研究紫色土坡耕地土石复合坎水土保持机理。主要结果：(1) 土石复合坎是三峡库区坡耕地4种基本埂坎类型之一，可分为上土下石、上石下土、土石混合等结构，其中，在上土下石复合坎种植植物篱常见于耕地资源紧张区域，下土上石复合坎主要用作田间便道。土石复合坎约占埂坎中7%。埂坎外边坡坡度56°~90°，高度0.55～2.2 m，顶宽0.3～0.8 m。(2) 含水率和干密度是影响埂坎稳定性的重要因子，含水率对土壤黏聚力、内摩擦角的影响程度远远高于干密度。杂草和黄豆等根系能明显提高0~10 cm和10~20 cm层紫色土抗剪强度，固结土壤效果好，有利于埂坎稳定。在埂坎土壤中加筋可以显著提高埂坎的土壤抗剪强度，加筋材料对加筋土壤黏聚力的影响大于对土壤内摩擦角的影响，常见加筋材料最优加筋含量分别为麦壳0.8%、竹丝0.5%、稻秆0.3%。(3) 高度和外边坡坡度对埂坎稳定性的影响极为显著(P<0.01)，坎后土体压力分布呈“中间大、两端小”，土壤含水率对浅层土体水平压力影响较显著。结合三峡库区坡耕地埂坎实际情况，0.6m以下土坎可以保留，土石复合坎的石坎高度宜为坎高的2/3，土坎部分高度宜为0.3~0.6m。(4) 埂坎入渗对稳定性影响较大，入渗速率与土壤理化性质具有较好相关性，黏粒和粉粒体积分数、初始含水率、容重与入渗速率呈负相关，砂粒体积分数、总孔隙度、有机质含量与入渗速率呈正相关。埂坎水分运移具有明显的各向异性，XZ、YZ方向的优先流呈明显的垂直带状形态，XY方向的水流向基质流转变。埂坎的水分阻隔效应明显，具有较好的水土保持作用， 20 cm左右形成阻隔层。土坎-基岩界面渗漏为第一水分运移通道，埂坎下渗和侧渗为第二水分运移通道。结合细沟侵蚀试验和雨季时作物覆盖情况，埂坎间距宜控制在7.3 m内。