Hydrodynamics of overland flow under combined effects of precipitation and slope discharge

To better understand the role of precipitation and slope discharge on hydrodynamic characteristics of overland flow, the simulated rainfall experiments were carried out at a steep slope (slope gradient was 20°) flume in Jinyun Forest Ecological Station, Chongqing of southwestern China. Velocity v and flow depth h were measured under different conditions of 5 unit discharges q (ranged from 0.486×10^-3 to 1.736×10^-3 m³/(s·m)), 4 typical rainfall intensities P (0, 30, 60, 100 mm/h) and 4 different surface roughness k_s(0.009, 0.180, 0.250, 0.425 mm). Furtherly, the hydraulic parameters, i.e., the mean velocity u, the velocity correction factor, flow-state indicator m, flow regime and overland flow resistance coefficient f were calculated. The results showed that: 1) mean velocity increased with increasing q in power function and v decreased with the increase of k_s. The velocity correction factor ranged from 0.04 to 0.37 in rainfall condition while ranged from 0.42 to 0.98 without rainfall. The overland flow was disturbed by rainfall. 2) The flow-state indicator ranged from 0.291 to 0.538. Without rainfall, flow-state indicator was mainly affected by roughness which was negatively correlated with m. The reduction rate was 21%, 28% and 39%, respectively. With medium-low rain intensity, no obvious variation pattern existed. When the rain intensity was 30 mm/h, the variation rate was 18%, -29% and 12%, respectively. When the rain intensity was 60 mm/h, the variation rate was 12%, -17% and -16%. With high rain intensity, the flow-state indicator significantly decreased with increasing roughness. The reduction rate was -6%, 1% and -11%, respectively. 3) The R_e ranged from 500 to 2 000, therefore the flow regime of overland flow was mainly transition flow. In most cases, flow pattern was supercritical flow. Without rainfall, the roughness was obviously related with the flow regime, that the overland flow tended to be supercritical with smaller roughness value, but this trend was not detecte with rainfall because of its disturbance effect. 4) Darcy resistance coefficient was positively related to k_s, while negatively correlated with unit discharge. Comparing the calculation formula of resistance coefficient with rainfall intensity to condition of without rainfall intensity, the correlation coefficient declined by 0.01%, showing that rainfall had no significant influence on resistance coefficient.