Abstract
Rainfall and sheet flow (SF) on gentle slopes always appear concurrently. However, the combined effects of rainfall and flow depth on SF resistance are still unclear for gentle slopes. In this study, two sets of experiments, namely, upstream inflow with rainfall and upstream inflow only, were conducted to investigate the resistance characteristics of SF on a flat and smooth flume with three gentle slopes (3°, 5° and 7°). The experimental results indicated that in the tests with upstream inflow only, the Darcy–Weisbach resistance (f) and Manning coefficient (n) decreased following a power function with increasing flow depth. When the Reynolds number (Re) was less than 1070, the n and f of the SF in the upstream inflow alone decreased with increasing Re following the power function. Both the n and f of the SF increased with increasing rainfall intensity for the upstream inflow with the rainfall tests. The rainfall significantly affected the f and n of the SF in the low Re zone (Re < 1070), inflow discharge (<0.06 m2 min−1) and flow energy for all inflow with rainfall cases. Under the conditions of upstream inflow with rainfall, both the f and n of the SF decreased with the increase in the ratio between the flow depth and median raindrop diameter (H/D50) in the power function. This result indicated that a deeper flow could dissipate more raindrop energy. In addition, empirical equations among Re, hydraulic slope and H/D50 were established to predict the n and f of the SF for upstream inflow with rainfall tests. This study will be helpful in understanding the combined effects of rainfall and flow depth on SF and could provide a scientific basis for improving hydrological and soil erosion models.
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