Resistance to sheet flow induced by raindrop impact on rough surfaces

Raindrops impact can affect the flow resistance to sheet flow by interfering with flow velocity profile. However, quantitative information regarding the flow resistance induced by raindrops impact to sheet flow (Δf) under the influence of different surface roughnesses is limited. In this study, multiple experimental simulations of rainfall and inflow were conducted on flumes with five unit width inflow discharges ranging from 0.02-0.1 m2 min−1, five rainfall intensities ranging from 40 to 120 mm h−1, and three levels of surface roughnesses ranging from 0.009 to 0.25 mm to identify flow resistance due to raindrop impact under different surfaces. Results showed that surface roughness was able to increase the total flow resistance without raindrop impact (fo) and with raindrop impact (fr), respectively. Under rainfall conditions, raindrop impact was able to increase fr on different surfaces. The surface roughness played a stronger role on contributing the fr than the raindrop impact. The major control of fr was the Reynolds number (Re), and fr decreased with the increasing Re. Δf were positively affected by surface roughness and rainfall intensity. Rough surface can increase the effects of raindrop impact on water flow by reducing the flow velocity of base flow or inflow, which leads to the increase in Δf. The major control of Δf was the Re, and Δf decreased with Re. Flow resistance induced by raindrop impact was larger for shallower and slower moving sheet flow with the low Re. In addition, a new equation has been developed to predict Δf under the conditions of Fr < 2.16 by using rainfall intensity, surface roughness, velocity, and water depth. This study helps to estimate the flow resistance induced by raindrops impact to sheet flow under surface roughness and provide the scientific insights for improving hydrologic and soil erosion models.