黄土坡面侵蚀沟沟岸扩张过程

Gully Sidewall Expansion Process on Loess Hill Slope Erosion

坡面细沟、浅沟及切沟侵蚀是黄土高原土壤侵蚀的常见类型.为了研究坡度和流量对侵蚀沟沟岸扩张的影响,获得侵蚀沟道断面宽度随时间和空间的动态变化过程,从而预测侵蚀沟道最大宽度,设计了3个试验坡度:12°、20°、30°,3个试验流量:12、20、30L/min,进行了9个坡度流量组合的径流冲刷试验,结合高清摄像监测的方式,记录冲刷过程中沟道侵蚀量、沟道扩张过程.在Matlab中,利用图像自动边缘检测算法提取沟边沿并转换为矢量文件,得到从沟头到沟尾,间隔50mm、每min的沟道断面宽度数据,通过Matlab编程计算,得到了在断面最大沟宽位置处,沟宽随时间变化曲线,从而建立了沟宽随时间变化方程,得到最大沟道宽度及不同时刻的沟道宽度,该方程形式简单,容易求解;沟宽随时间变化曲线符合S型曲线的发展规律,对应着沟岸扩张发生-快速发育-稳定-崩塌-再稳定的周期性过程,沟岸扩张过程中,随着跌坎发育为侵蚀沟,断面侵蚀沟宽度随冲刷时间的变化存在激增点,当沟宽随时间推移趋于无穷时,宽度趋于定值;侵蚀沟沟道发展的最大宽度也可用流量Q和坡度S表示,建立了沟宽最大宽度预测方程(R^2=0.91,RMSE=0.0338);最后,结合断面侵蚀沟宽度的变化曲线,分析了侵蚀过程中坡面沟道侵蚀量与断面沟宽变化的关系,结果表明:流量和坡度是侵蚀过程的动力因素,沟道断面最大宽度与坡度及流量有关.

Gullies are large,deep depressions or channels formed by erosion,and they are a common type of soil erosion on the Loess Plateau.In this study,the experiments were conducted at the Laboratory of Hydrology and Water Resources in Xi’an University of Technology,China.We designed a non-recirculating,3-meter-long,1-meter-wide,0.6-meter-deep,tilting hydraulic flume,For each experimental treatment,9 continuous experimental runs were performed and featured three different flow rates( 12 L/min,20 L/min,30 L/min),three different slopes(36.39%,46.63%,57.73%).The gully width changed with time and cross section for different slopes and flow rates,and the changes were recorded with a high-resolution camera.The image edge detection algorithm was utilized to extract gully edge and converted to vector file from gully head to gully outlet in 50 mm,per min intervals of gully cross-section data.We calculated the maximum gully width in all cross sections with Matlab code based on the recorded gully width. The results showed that the flow discharge and gully slope gradient are driving factors facilitating the process of gully widening,and the evolution of the maximum cross-sectional gully width exhibited an S-shaped curve. The main process in overall gully widening process evolves via a cycle of initiation( headcut emergence),high activity( sidewall collapse) and stability. In the early stage of gully development,gully headcuts were very active and played a crucial role in sediment detachment. In the active stage,sidewall extension became predominant in the gully development cycle in the presence of a ploughed layer. The gully width and sediment yield rapidly increased after a collapse,followed by a gradual decrease in sediment yield to a relatively stable state. As the gully erosion transitions into the stable stage,the rates of increasing width of the erosional gully decrease,gully sidewalls collapse episodically,causing fluctuations in the sediment yields. Based on the S-shaped curve fit,a predictive equation for gully width change over time at the maximum cross-sectional was established and the form of the equation is simple and easy to apply. The maximum gully width can also be expressed by flow rate Q and slopes S,and a power function dependent upon the flow rate and slope were established and fits the data well( R^2= 0. 91,RMSE = 0. 0338). The results showed that the flow discharge and gully slope are driving factors that facilitate the water erosion process and that the maximum width of the cross section is related to the slope and flow rate.