Characteristics of overland flow resistance under interaction of rainfall intensity and unit discharge and surface roughness

Abstract: Overland flow resistance is one of key parameters to study the feature of eroding process on the slope. Grain resistance mainly exerted by soil particle and rainfall resistance exerted by precipitation are the common resistance types in the wild. However, few attentions were focused on the characteristics and mechanism of overland flow resistance under interaction of grain resistance and rainfall resistance. Therefore, artificial precipitation was conducted to simulate rainfall resistance in Jinyun Forest Ecosystem Research Station, Chongqing. This research conducted fixed bed scouring experiments at slope gradient of 15°. In this study, 9 different unit discharges varying from 0.397 to 2.049 L/(m·s) are set as water inflow, 4 different grain sizes of waterproof of 0.009, 0.18, 0.25 and 0.425 mm are selected to simulated surface grain roughness, and 4 different rainfall intensities of 0, 60, 90 and 120 mm/h are selected, based on the conditions of the Loess Plateau where water erosion dramatically occurs on slope. The flow depths of 3 cross-sections along flume were measured, and afterwards, velocity and Darcy-Weisbach resistance would be calculated. Results show that the velocity of overland flow is positively correlated with rainfall intensity while negatively with surface roughness. The major form of energy consumption is transforming to kinetic energy based on flow-state indicator (it is smaller than 0.5 in all cases). Darcy-Weisbach resistance varies from 0 to 3.0 while the Reynolds number varies from 300 to 2300. Overland flow resistance is positively correlated with surface roughness while is negatively with rainfall intensity as well as Re. Effect of rainfall and surface roughness on overland flow resistance becomes slight as the increasing of flow depth. There are many doubts about the role of rainfall resistance on overland flow resistance, such as rainfall resistance reducing or enhancing overland flow resistance, even having no effect on overland flow resistance. In this research, rainfall resistance would reduce the total resistance, which may be attributed to component of rain with positive gradient slope more than with reverse gradient slope. Superposition principle that sum of resistance elements is usually applied to estimate the composite resistance of overland flow. However, this method is still doubted for overland flow due to that the flow depth of overland flow is significantly less than open channel flow; herein, limitations may exist for overland flow. Here, the discrepancy between sum of rainfall resistance and grain resistance and composite resistance is verified through t-test (P=0.003, P<0.05), and the results indicate that the discrepancy is significant and composite resistance is larger than sum of rainfall resistance and grain resistance. Therefore, there is the limitation for using superposition principle in calculation of overland flow resistances under the combined effect of rainfall and surface roughness. Because of low flow depth of the overland flow, the region impact of surface roughness is overlapped with region impact of rainfall. Hence, these 2 types of resistances interfere with each other and enhance the total resistance. In addition, according to non-superposition principle, an equation is summarized to calculate the composite resistance (R2=0.79). This research is helpful to understand the characteristics of overland flow resistance and provide support for building soil erosion model.