山区流域坡面汇流时间参数优化试验研究

Experimental Study on Optimization of Flow Concentration Time Parameter in Mountainous Watershed

山区流域坡体陡峻、植被覆盖变化大,暴雨作用下的洪水响应过程复杂,坡面汇流时间是洪水过程模拟的关键参数。目前应用最为广泛的坡面汇流时间公式,以坡面长度L、糙率n、有效降雨强度i及坡度S的恒定指数形式表征其对汇流时间T的变化,公式形式为:T=L^(0.6)n^(0.6)i^(-0.4)S^(-0.3)。降雨强度对汇流时间的影响,以降雨强度的恒定指数-0.4来表征其对汇流时间变化,而较少反映坡度与植被覆盖度的影响。本研究基于不同坡度、不同植被覆盖度条件下坡面汇流的系列试验,结合专业的数据统计软件,对相关数据进行分类处理,深入分析了坡面汇流时间参数的变化规律。研究结果表明:无植被情况,不同坡度对雨强指数存在一定影响,在45°及30°坡面上,雨强对坡面汇流时间影响的雨强指数拟合值约为-0.40,这与常用的研究成果基本吻合,在15°及5°坡面上,雨强指数拟合值分别为-0.30及-0.25,比常用雨强指数值略微偏大;有植被覆盖情况,不同植被覆盖度对雨强指数的影响十分明显,45°、30°及15°坡面,植被覆盖度50%以上时其雨强指数拟合值平均为-0.63,明显小于常用雨强指数-0.4,20%覆盖度情况下,雨强指数均值约为-0.37,与常用雨强指数-0.4接近,5°坡面不同植被覆盖度对雨强指数影响不大,雨强指数基本在-0.25～-0.30之间。

The flood response process under heavy rains is extremely complicated due to steep slopes and variable vegetation coverage in mountainous area and the runoff convergence time in the slope has become a key parameter in flood process simulation.The most widely used flow concentration time formula at present is T=L0.6 n0.6 i–0.4 S–0.3,where T is the flow concentration time,L is the slope length,n is the roughness,i is the effective rainfall intensity and S is the slope.The current studies generallyly use a constant index of rainfall intensity to characterize runoff convergence time while few study considers the slope and vegetation abundance effects.In this work we conducted a series of indoor experiments to study overland flow concentration under different slopes and vegetation coverage conditions.In addition,the variation rules of runoff convergence time parameters are analyzed as well.The results show that the fitting value of rain intensity index represented for the impact of rain intensity on confluence time is about–0.40 on the 45°and 30°slope,which is consistent with frequently-used results of previous research.However,when it comes to the 15°and 5°slopes,the fitting values of rain intensity index turn into–0.30 and–0.25 respectively,which is slightly larger than that used in the common approaches.The further investigation under conditions with vegetation suggests that rain intensity index changes obviously with the existence of vegetation:while on slopes of 45°,30°and 15°,the average fitting value of rain intensity index falls to–0.63 when vegetation coverage is 50%or above,significantly less than the theoretical value of–0.4;the mean value of rain intensity index in 20%coverage is about–0.37,almost the same as that in vegetation-free situation.In addition,rain intensity index under 5°slope condition for different vegetation coverage is relatively stable at values between–0.30 and–0.25.