片沙覆盖黄土区沙土二元结构坡面土壤侵蚀机理研究

The sand-covered loess region is the heart area of the water-wind erosion crisscross region on the Loess Plateau and also is the main source of the coarser sediment in the downstream bed of Yellow River.The dual structure slope in the sand-covered loess region is different from the unitary structure slope whose soil eroison have been studied abundantly in the past. The special eroison type which is named the underflow ersion in sand-covered loess slope exist. There are few studies for this special eroison type on the dual structure slope of sand-covered loess.These have as a result to limit the intensive study for soil erosion in this region and further threat the ecological safety in the downstream bed of Yellow River.In this project, based on the sand-covered loess slope and loess slope, using the artificial rainfall and the three-dimensional laser scanner, the underflow erosion process will be systematically studied; the underflow erosion dynamic mechanism will be found; the evolution process of slope erosion micro-topography and its dynamic mechanism will be discovered; and then the corrected coefficient of predicted soil eosion on the sand-covered loess will be proposed. This project is very important to study intensively the characteristics of soil erosion in sand-covered loess region and is the basis to deeply analyze the characteristics of soil erosion and the forming mechanism of hyperconcentrated flow in water-wind erosion crisscross region. The studying results will be used to serve the soil erosion prediction and prevention in this region. In the other hand,the soil erosion on the dual structure slope will enrich the soil eroison theory on the slope.

片沙覆盖黄土区是黄土高原水蚀风蚀交错带水蚀风蚀交互作用的核心区域和黄河下游河床粗泥沙的主要源区。区域内存在明显不同于一元结构坡面的片沙在上，黄土在下的沙土二元结构坡面，可发生特殊的沙土界面潜流侵蚀，目前对这种二元结构坡面土壤侵蚀的了解甚少，这严重制约了对该区土壤侵蚀的深入研究和科学防治，进而威胁到黄河下游生态安全。本项目拟以概化的片沙覆盖黄土坡面和黄土坡面为研究对象，基于野外调查与模拟降雨试验，结合三维激光扫描技术，系统研究片沙覆盖黄土坡面沙土界面潜流侵蚀过程；揭示片沙覆盖黄土坡面沙土界面潜流侵蚀动力学机理；探寻片沙覆盖黄土坡面侵蚀形态演化过程及动力机制；确定片沙覆盖黄土坡面土壤侵蚀预报修正系数。本项目的研究将有助于对片沙覆盖黄土坡面土壤侵蚀特点的深入了解，深化对水蚀风蚀交错带土壤侵蚀特征和该区内高含沙水流形成机理的认识，为区域土壤侵蚀预报和防治提供支撑。同时也推动坡面土壤侵蚀理论的发展。

黄土高于水蚀风蚀交错带片沙覆盖黄土地貌土壤侵蚀研究比较薄弱。本研究基于野外调研，进行了大量室内模拟降雨试验，以揭示片沙覆盖黄土坡面侵蚀机理。研究发现：（1）片沙覆盖黄土坡面与黄土坡面产流产沙差异明显，黄土坡面属超渗产流，侵蚀过程为溅蚀―片蚀―细沟侵蚀，而典型片沙覆盖黄土坡面则为垂直渗流-坡面潜流-崩塌-径流冲刷，且降雨过程中产流产沙速率变异性大，呈明显单峰分布。（2）片沙覆盖改变坡面产流方式，较薄覆沙坡面同时存在超渗产流和蓄满产流，覆沙较厚坡面则在沙土界面产生渗流。片沙覆盖能够延长初始产流时间，减少产流总量，增大降雨过程中产流速率峰值。随覆沙厚度、坡度、雨强增加，片沙覆盖黄土坡面产流速率明显增加，峰值变大，变异性增强，径流系数出现大于1的现象，甚至大于2。覆沙厚度、坡度、雨强对产流速率的影响存在交互作用和临界值。相同降雨历时下，片沙覆盖黄土坡面产流量与覆沙厚度、雨强和坡度的偏相关系数分别 -0.516、0.831和0.43，双侧显著性检验呈显著。（3）片沙覆盖能够增加产沙量，增大产沙速率和产沙峰值。片沙覆盖黄土坡面产沙速率随覆沙厚度、雨强、坡度的增加整体表现出递增趋势，但存在临界坡度。相同降雨历时下，覆沙坡面产沙量与覆沙厚度、雨强和坡度的偏相关系数分别0.532、0.599和0.342，双侧显著性检验呈显著。（4）随覆沙厚度、坡度和雨强增加，降雨过程中产流速率和产沙速率变化的一致性加强，产沙和产流速率都出现先增大再减少单峰分布，产沙峰值前后产流速率和产沙速率及最大产沙速率和产流速率都具有显著的正相关关系；分析发现径流速率、坡度和覆沙厚度能够表征坡面产沙速率的变化，回归关系式的决定系数达0.926。（5）5-10 cm内可能存在一个临界覆沙厚度，大于临界覆沙厚度后坡面产流产沙方式、水沙关系及坡面形态演化过程都发生质变，临界覆沙厚度随坡度和雨强的变化而变化。该项目的研究为揭示水蚀风蚀交错带土壤侵蚀机理及黄河下游粗泥沙来源提供了数据支撑。