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The spoil dumps are the main sediment sources in the open-pit mining area. The erosion control is of great significance to the high-quality development of the energy zone. This article aims to analyze the gully development and sediment process of spoil dumps. The plot construction of platform-steep slope system and field scouring experiments were also carried out, where the flow rate was ranged from 60 to 80 L/min, and the duration was 45 mins each run. The results showed that: 1) The gully was developed by the combination of different erosion progresses, including the headcut migration, bed incision, and lateral erosion on the platform-steep slope system. Notably, there were the stage differences in the topographic characteristics of gully, due to the variation of dominant process. A three-stage development was experienced for the gully on the platform, including the headcut formation, migration-expansion, and stable stage. In the gully on the steep slope, four developmental processes were divided into the incision of sandy loam layer, expansion of sandy loam layer, incision of clay loam layer, and deceleration of erosion. No significant differences were found between the various flow rates of gully development speed on the platform. Whereas, the transformed time increased with the flow rate between adjacent gully development stages on the steep slope. 2) In terms of sediment process, the migration - expansion stage was the main erosion period on the platform. The incision of sandy and clay loam layer were the main erosion stages on the steep slope, particularly with the accumulated sediment yields accounting for 29.72%~53.36% and 19.06%~48.88%, respectively. Spatially, the runoff shear force and stream power increased by 7.11-120.86 times, and 7.59-239.59 times after the runoff flowed from the platform into the steep slope, and the erosion rate increased by -0.84-66.20 times. Correspondingly, the steep slope was the main sediment source of the platform and steep slope system. The cumulative sediment yield was accounted for 88.15 %-90.16 % of the total amount of platform and steep slope system. Hence, the separation and control of runoff on the platform was a vital way to control the gully erosion in the platform-steep slope system of spoil dumps. 3) In terms of the flow hydraulics, the platform velocity decreased first and then increased with the gully development. Conversely, the slope velocity decreased gradually. Meanwhile, the runoff shear stress and the runoff stream power increased gradually with the gully development on the platform, but increased first, finally decreased on the steep slope. 4) Comparatively, the erosion rate presented a more sensitive response to the stream power than the runoff shear force, where the response regularity was different among the gully development stages. The erosion rate was linearly responded to the stream power in the migration-expansion stage and stable stage of platform. On the steep slope, the responses were still linear in the expansion of sandy loam layer and deceleration of erosion, but the exponential in the incision of sandy and clay loam layer. Therefore, it is necessary to consider the difference of responses in the various gully development stages, in order to improve the applicability of sediment transport model for the gully erosion. As such, the results revealed the staged development of gully and sediment process on the platform-steep slope system of spoil dumps. The finding can also offer the strong reference for the gully process, further to implement the soil and water conservation measures in the spoil dump. [ABSTRACT FROM AUTHOR]