坡面流阻力研究进展

主要从坡面流流态对坡面流阻力的影响、坡面流阻力与雷诺数的关系、降雨对坡面流阻力的影响、坡面流阻力公式4个方面对前人关于坡面流阻力的研究成果进行了总结。结果表明:坡面流流态的不确定性导致了坡面流阻力变化更为复杂;坡面流阻力与雷诺数之间并无良好单一的相关关系;降雨对坡面流阻力有很大的影响作用但具体影响机理并未有明确的结论;坡面流阻力公式表征仍用明渠水流概念及表达方式,并且3个阻力系数n、f、C哪个更适合表征坡面流阻力尚无统一的结论。进而对目前存在的问题进行了分析,对今后的发展进行了展望,认为需在研究方法和实验设备及量测技术方面有所突破,才可以更好的对坡面流阻力进行全面系统的研究。

降雨强度和单宽流量与地表粗糙度交互作用下坡面流阻力特征

为探明降雨和地表颗粒共同影响下的坡面流阻力变化特征及形成机理,在15?定床条件下,分别研究了4个粗糙度(0.009,0.18,0.25,0.425 mm)和4个降雨强度(0,60,90,120 mm/h)在9个单宽流量(0.397~2.049L/(m·s))冲刷下的坡面流流速、流态指数、阻力系数、雷诺数等水动力学参数间的关系及变化特征。结果表明:1)坡面流流速与粗糙度呈反比,与降雨强度呈正比。流态指数m的计算结果显示(m<0.5),坡面水流能量主要转化为动能形式;2)试验条件下,雷诺数变化范围为300~2 300,达西阻力系数变化范围为0~3.0,颗粒阻力和降雨阻力皆随着雷诺数的增加而减少,随着水深的增加,降雨对坡面流施加的影响逐渐减小。坡面总阻力系数与粗糙度成正比,与雷诺数和雨强成反比,降雨具有"减阻"效果;3)计算降雨阻力与颗粒阻力线性叠加值与坡面流总阻力的差异,结果表明坡面流总阻力大于线性叠加的结果,t检验结果显示差异显著(sig.0.003<0.05),表明将坡面流阻力分量线性叠加计算坡面流总阻力的方法具有一定局限性;进一步构建降雨及地表颗粒影响下的坡面流阻力通式,具有良好的模拟效果。研究成果为深入探明坡面流阻力形成机理和构建坡面侵蚀模型提供科学依据。

地表粗糙度及植被盖度对坡面流曼宁阻力系数的影响

为探明地表颗粒和植被共同影响下的坡面流曼宁阻力系数变化规律,在5°缓坡定床条件下,以曼宁阻力系数表征坡面流阻力,利用等效水力半径计算等效曼宁阻力系数,等效水力半径考虑了水流与植被的接触面,分别研究了4个地表粗糙度(0.009,0.12,0.18,0.38 mm)和4个植被盖度(0,4.0%,6.6%,12.2%)在9个单宽流量(0.2×10^-3~0.5×10^-3m^3/(m·s))冲刷下的坡面综合等效曼宁阻力系数、颗粒等效曼宁阻力系数及植被等效曼宁阻力系数的变化特征及相互之间的关系。结果表明:1)在坡面没有植被时,坡面综合等效曼宁阻力系数与流量呈负相关,当坡面有模拟植被时,两者间呈正相关。坡面综合等效曼宁阻力系数随着地表粗糙度和植被盖度的增加而增加;2)当地表粗糙度和模拟植被存在的坡面,线性叠加原理不适用于坡面综合阻力的计算,在水深较小时,会出现附加阻力。附加等效曼宁阻力系数与粗糙度和盖度呈正相关,而与水深负相关;3)通过多元回归分析得到坡面综合等效曼宁阻力系数的计算式,模拟效果较好(相关系数R=0.98),并分析得到各阻力分项等效曼宁阻力系数的计算式。分别剔除各阻力分项后将相关系数比较,植被阻力对坡面流综合阻力的影响最大,颗粒阻力次之,而附加阻力最小。研究成果将为构建坡面侵蚀模型和防治坡面侵蚀提供科学依据。

不同覆盖条件下坡面流水力学特征试验研究

【目的】坡面流水力学特性对阐明土壤侵蚀和坡面产沙机理具有重要意义,本试验就不同覆盖条件下坡面的坡面流阻力及相关水力学特征进行了研究,探究不同覆盖条件下坡面流水力学特征,以期为指导水土保持坡面措施合理配置提供理论支持。【方法】试验使用自制定床冲刷水槽,选用不同覆盖度(0%、10%、15%、20%)砾石、草被床面,在不同流量(15~120 L/(min·m))、不同坡度(3°~15°)条件下进行室内定床冲刷试验。【结果】结果表明:(1)不同坡面上流速随着流量、坡度的增大而增大,且相同覆盖度下草被床面流速大于砾石床面流速,最高可达其2.2倍。(2)不同覆盖条件下弗劳德数(Fr)随着坡度的增大而增大,Darcy-weisbach阻力系数(f)随着覆盖度的增加而增加,相同覆盖度下砾石床面f大于草被床面,最高可分别为草被床面的10.9倍。(3)不同覆盖条件下坡面流阻力与流量呈线性正相关关系,相同覆盖度下砾石床面坡面流阻力大于草被床面,最高可达草被床面的2.2倍。(4)坡度与覆盖度对坡面流阻力均有显著影响,坡面流阻力随坡度的增大而增大,随着覆盖度的增大而增大,且呈较好的线性相关关系。【结论】本试验通过对不同覆盖条件下坡面流阻力的研究,可为建立基于径流动力学过程的山坡水文模型和侵蚀物理模型提供一定基础,为指导水土保持坡面措施合理配置提供理论支持。

Effects of simulated submerged and rigid vegetation and grain roughness on hydraulic resistance to simulated overland flow

Better understanding of the role of vegetation and soil on hydraulic resistance of overland flow requires quantitative partition of their interaction. In this paper, a total of 144 hydraulic flume experiments were carried out to investigate the hydraulic characteristics of overland flow. Results show that hydraulic resistance is negatively correlated with Reynolds number on non-simulated vegetated slopes, while positively on vegetated slopes. The law of composite resistance agrees with the dominant resistance, depending on simulated vegetation stem,surface roughness, and discharge. Surface roughness has greater influence on overland flow resistance than vegetation stem when unit discharge is lower than the low-limited critical discharge, while vegetation has a more obvious influence when unit discharge is higher than the upper-limited critical discharge. Combined effects of simulated vegetation and surface roughness are unequal to the sum of the individual effects through t-test, implying the limitation of using linear superposition principle in calculating overland flow resistances under combined effect of roughness elements.

Effects of Roughness Elements Distribution on Overland Flow Resistance

Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.