【Objective】Gravity is an important factor affecting development of runoff sediment and rills on slopes. Large piles of soil, rock and slag generated as waste and formed during the processes of coal mine construction are liable to have inductive geological disasters such as collapse, landslides and debris flow, occur under the action of water and gravity, causing serious impact or even damage to surrounding water resources, land resources, vegetation resources, hydrological cycle and ecological environment quality.【Method】 A few of such piles in a coal mining site of a karst zone were selected as object of the study. Field scouring experiments were conducted on slopes different in condition with varying flow to explore how erosion occurs and proceeds, and how gravity affects development of runoff and sediment and morphology of the rills formed. Field investigations found that the piles in the coal-mining areas of Chongqing varied in slope in the range between 25.5°and 38°. To objectively reflect characteristics of erosions on such piles different in stacking condition, the experiments were carried out on slopes different in gradient, i.e. 25°、30°、 35° and 40°, and had five flow rates, i.e. 10, 15, 20, 25 and 30 L?min^-1 designed according to the characteristics of the per-unit width discharge caused by local rainstorms relative to duration and frequency and each round of the scouring experiment lasted 60 min. 【Result】Results show:(1) The erosion on the slopes of the piles could be divided into two phases, sheet erosion (within the initial 3 min after runoff started) and rill erosion which could be further divided into three stages, i.e. rill expansion (3～24 min after runoff started), rill transition (24～30 min after runoff started) and rill stabilization (30 min after runoff started); the time of rill appearance on the slope was negatively related to the discharge flow rate and the slope; rills appeared the fastest in 4s and the slowest in 97 s, and the latter appeared about 24 times later than the former. (2) Gravity was an important factor affecting runoff-sediment yield and its fluctuations on the slopes of the piles. During slope erosion, runoff rate varied in the range between 7.77% and 374.25% in variation coefficient, while sediment yield did in the range of 1.75% ~ 1021%. Gravity could explain more than 17.41% or even up to 99.60% of the total sediment yield during slope erosion. It was tentatively concluded that gravity started to act on development of rills, when runoff and slope reached their respective critical value of 20 L?min-1 and 35°. And (3) Runoff mainly caused rills to develop in depth, while gravity mainly did in width. The two leading functions were quite similar in degree of their effects on development of rills. Gravity turned short, shallow and narrow rills on slopes of loose deposit into long, deep and wide ones, which ranged between 7.89 and 19.73 cm in width between 2.17 and 7.73cm in depth, between 2.12 and 4.36, in width/depth ratio and between 1.35～3.00 m?m^-2 in density’ . Mean depth and width of the rills increased with increasing flow rate and with increasing slope gradient, too. Under the same runoff conditions, the sediment yield increased with increasing density of the rills. However, density of the rills first increased and then decreased with increasing flow and slope gradient.【Conclusion】All the findings in this experiment are of important scientific significance to understanding correctly how runoff and sediment occurs and their influencing factors, to establishing models to predict soil erosion on slopes of the mine-engineering waste piles, and to elucidating . scientifically the roles of gravity in inducing collapse and landslide and maintaining slope stability of mine-engineering waste piles.