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。

Effects of surface roughness and vegetation coverage on Manning's resistance coefficient to overland flow

Resistance to flow of surface water determinesthe study hydrodynamics in the slope. The common resistance types in the wild are grain resistance that mainly exerted by soil particles,and thevegetation resistance that exerted by the vegetation belonging to form resistance. However, there are no consensus conclusions about the laws of overland flow that influenced by the grain and vegetation resistance, especially for the resistance to overland flow that calculated by manning’s resistance coefficient. Simultaneously, the superposition principle that used to calculate composite resistance needs to be verified in these situations for the overland flow. Therefore, the artificial scouring experiments in the fixed bed have beenconducted at the slope gradient of 5° in Jinyun Forest Ecosystem Research station, Chongqing Province, China. The waterproofs with different roughness were selectedto simulate the surface roughness, whilethe circular cylinders with different diameterswere used to simulate the vegetation coverage. In this study, 9 different unit discharges varyingfrom 0.2′10^-3 to 0.5′10^-3 m^3/(m · s) were set as the water inflow;3 different grain sizes ksof surface roughness of 0.12, 0.18, 0.38 mm were selected to simulate the grain resistance, while the ksof smooth flume bed equaled to 0.009;and 4 different vegetation coverage Cr of 0, 4.0%, 6.6%, 12.2%were selected to simulate the vegetation resistance. To calculate more accurately, the resistance was calculated by the equivalent manning’s resistance coefficient newhich usedthe equivalent hydraulic radius, instead of hydraulic radius. The equivalent hydraulic radius were considered the contact area of vegetation and flow, while the hydraulic radius did not. The velocity of overland flow were measured using a trace method, and repeated for 10 times. Afterwards, the flow depth h and newere calculated. Results showed that 1) the nenegatively relates with discharge for non-vegetated slope, while positively relates with discharge for vegetated slope. The neincreases as the increasing ksand Cr. 2) Assuming the composite resistance equals to the sum of grain resistance and vegetation resistance, the equivalent manning’s resistance caused by grain resistance neband caused by vegetation resistance nevcan be deduced. The nebnegatively relates with h, while positively relates with ks.The nevlinearly positive relates with h for larger h, while the nevis larger for the lower h. This phenomenon indicates that the linear superposition principle wasnot suitable for calculating the overland flow resistance, because the vegetation resistance should be linearly positively related with h for the fully flow depth if the linear superposition principle was suitable based on the results of previous works. The larger nevfor the lower hcan beattribute to the effect of additional resistance. Because of the shallow flow depth of the overland flow, the region impact of surface roughness was overlapped with the region impact of vegetation. Therefore, twotypes of the resistance interfered each other, resulting the additional resistance. Afterwards, the equivalent manning’s resistance that caused by additional resistance nawas used to verify nev, resulting in nevlinearly increased as the increasing h. The nanegatively related with h, while positively related with ksand Cr. 3) The multiple regression analysis was used to simulate ne, and the results was well accordance with the observed ne(correlation coefficient 0.98).Finally, by comparing correlation coefficient R after rejecting corresponding resistance components, the vegetation resistance is the major factor of neand the grain resistance is the second major factor, while additional resistance has the smaller impact on ne. This finding provides sound supports for building the model of soil erosion, and for the conservation of soil and water on the slope.