Influence of longwall mining on surface runoffs in gully terrain area

Based on the exploitation background of gully terrain area in western China,the relationship between strata movement and groundwater level decline was established by adopting theoretical analysis and simulation methods,and then the influence of longwall mining on surface runoffs in gully terrain area was discussed. The results show that if the aquifer is not damaged by mining-induced fractures,the basin morphology and catchment area will vary due to mining subsidence,which would subsequently affect the distribution of surface runoffs and water environment. With the in- creasing intensity of underground mining,four types of surface runoffs distribution caused by mining in gully terrain ar- ea will appear including “ stable” type,“ derivative” type,“ transfer” type and “ deterioration” type. When mining height and mining area are relatively small,groundwater and surface water level decline is not obvious and regional water environment has minor changes. When mining height is large but mining area is small comparing to the basin area, groundwater level decline is not obvious but surface subsidence is considerable. Hence,the buried depth of groundwa- ter level is shallower than before in mining area. When there are gullies overlaying on the surface of the mining area, groundwater may outcrop in the gully as its low elevation,from which new surface runoff would derive. As the mining area enlarges comparing to the basin area,groundwater level in the whole area obviously declines;thus the original sur- face runoffs outside the mining area disappear and are transferred to the mining area because of its large surface sub- sidence. Finally,groundwater level declines rapidly due to larger mining height and mining area. Eventually,all the surface runoffs disappear and are converted into groundwater,causing regional ecological environment deterioration. This research provides a theoretical basis for water-preserved mining practice in arid and semi-arid areas of western China.