Rills have been recognized as main channels for both sediment and solute transport on slopes. Studies on rill morphology and hydrodynamic characteristics on red soil slopes can reveal the erosion mechanisms, and also have great significance on soil and water conservation in red soil hilly region. In order to investigate the rill cross-sectional morphology and the hydrodynamics of rill flow along the slope, an experiment considering the interaction of simulated rainfall(0, 60, 90, 120 mm/h) and scouring flow(5, 7.5 L/min) was carried out in a quaternary-derived red soil flume(10°, 3-m length and 0.5-m wide and 0.4-m deep). The whole slope was divided into three parts, which represented the up-slope(1 m from the top slope), middle-slope(1-2 m from the top slope) and down-slope(2-3 m from the top slope). Rill cross-sectional width-depth ratios(C) and cross-sectional morphology coefficient(η) were employed to quantify the rill morphology at three different slope positions. The results show that most of the rill cross-section presented a “V” or “U” shape, while a few of them presented a bowing bank shape. C varied from 0.93 to 3.52, and η varied from 0.30 to 0.60. The C values of up-slope were larger than those of down-slope for the same rill, while the η showed no definite trend with the rill length change. The mean rill flow velocity(V) along the slope was: down-slope>middle-slope>up-slope, Froude number(Fr) and Reynolds number(Re) were basically followed the same trend as velocity. Finally, V and Re were positively related(p<0.01) to the unit discharge, while the increased value of V and Re from the up-slope to down-slope were positively related(p<0.01) to rainfall intensity and mean C, respectively. The results can provide a reference for further studies on rill erosion in red soil hilly region.

Key words：red soil; rill; cross-sectional width-depth ratios; cross-sectional morphology coefficient; hydrodynamics