Wind effects on sediment transport by raindrop-impacted shallow flow: a wind-tunnel study

摘要

In wind-driven rains, wind velocity and direction are expected to affect not only energy input of rains but also shallow flow hydraulics by changing roughness induced by raindrop impacts with an angle on flow and the unidirectional splashes in the wind direction. A wind-tunnel study under wind-driven rains was conducted to determine the effects of horizontal wind velocity and direction on sediment transport by the raindrop-impacted shallow flow. Windless rains and the rains driven by horizontal wind velocities of 6 m s~(-1), 10 m s~(-1), and 14 m s~(-1) were applied to three agricultural soils packed into a 20 by 55 cm soil pan placed on both windward and leeward slopes of 7 per cent, 15 per cent, and 20 per cent. During each rainfall application, sediment and runoff samples were collected at 5-min intervals at the bottom edge of the soil pan with wide-mouth bottles and were determined gravimetrically. Based on the interrill erosion mechanics, kinetic energy flux (E_(rn)) as a rainfall parameter and product of unit discharge and slope in the form of q~bS_o~c as a flow parameter were used to explain the interactions between impact and flow parameters and sediment transport (q_s). The differential sediment transport rates occurred depending on the variation in raindrop trajectory and rain intensity with the wind velocity and direction. Flux of rain energy computed by combining the effects of wind on the velocity, frequency, and angle of raindrop impact reasonably explained the characteristics of wind-driven rains and acceptably accounted for the differences in sediment delivery rates to the shallow flow transport (R_2 ≥ 0.78). Further analysis of the Pearson correlation coefficients between E_(rn) and qS_o and q_s also showed that wind velocity and direction significantly affected the hydraulics of the shallow flow. E_(rn) had a smaller correlation coefficient with the q_s in windward slopes where not only reverse splashes but also reverse lateral raindrop stress with respect to the shallow flow direction occurred. However, E_(rn) was as much effective as qS_o in the sediment transport in the leeward slopes where advance splashes and advance lateral raindrop stress on the flow occurred.