Study On Runoff Generation And Confluence Mechanism Of Special Underlying Surface In Goaf And Establishment Of Hydrological Model

Coal mining causes the subsidence and fractures of rock strata,surface above goaf,with the expansion of goaf water-conducting fissure zone and ground subsidence area,the formation of river runoff and floods is closely related to the hydraulic connection between surface water and groundwater in goaf in many regions.Therefore,it is necessary to deeply understand the generation and confluence mechanism of goaf to accurately predict the flood process of special undersurface beneath goaf.This paper,the control basin of Fenhe reservoir,which is badly damaged by coal mining in Shanxi province,is taken as the research area.Firstly,a field survey was conducted to find out the soil texture types in the study area and the development characteristics of fissures on the special underlying surface of goaf.Based on the research results,the experiment of the convergence production of special undersurface in goaf was designed to analyze the influence mechanism of the special undersurface in goaf during the convergence production process,and the mechanism was parameterized.Fenhe reservoir control basin is divided into the control basin of Jingle Hydrology Station,the control basin of Shangjing You Hydrology Station and Loufan Hydrology Station according to the damage degree of goaf,and hydrological simulation of each basin was carried out.Loufan hydrological station's control basin is less affected by coal mining,so it is applicable to evaluate the hydrologic model in the control basin of Fenhe reservoir.The following important conclusions are drawn:(1)In the horizontal direction,the value range of surface crack rate of goaf is 0? 2%.In the vertical direction,fractures develop in different depths,which can be generally divided into two categories: Fractures does not cut-through waterresisting layer and develop on the surface;The other type of fracture cut-through water-resisting layer and connects directly to the goaf.When the crack does not cut-through to underground goaf: In the process of slope runoff production and confluence,the appearance of fissures in the early stage of rainfall reduces the surface runoff and lengthens the outflow time of the interflow.In the course of river confluence,the water flow into the fissure so that leads to the decrease of river confluence flow in the early stage,but the flow rate of channel sink will eventually be maintained at the level without fissure development.When the crack cut-through to underground goaf: During the flow generation process,fractures become channels for surface runoff,interflow and river flow to recharge underground water flow in goaf,make the surface runoff,interflow and river flow reduce and the groundwater flow increases.(2)Linear fitting was performed for the correlation between surface runoff and runoff reduction,as well as the decrease rate of surface runoff and fracture rate in the influence period of fracture,shows that surface runoff and fracture rate show strong negative correlation relationship,surface runoff reduction and the decrease rate of surface runoff assumes the strong positive correlation with fracture rate,in addition,fractures have different influence degrees on surface runoff and runoff reduction at different times,which is a quantity that changes with time and has stable influence degrees on surface runoff reduction rate at different times.Linear fitting was performed for the correlation between interflow and interflow reduction,as well as the decrease rate of interflow and fracture rate in the influence period of fracture,shows that interflow and fracture rate show strong negative correlation relationship,interflow reduction and the decrease rate of interflow assumes the strong positive correlation with fracture rate,in addition,fractures have different influence degrees on interflow and interflow reduction at different times,which is a quantity that changes with time and has stable influence degrees on interflow reduction rate at different times.Linear fitting was performed for the correlation between river flow and river flow reduction,as well as the decrease rate of river flow and fracture rate in the influence period of fracture,shows that river flow and fracture rate show strong negative correlation relationship,river flow reduction and the decrease rate of interflow assumes the strong positive correlation with fracture rate,in addition,fractures have different influence degrees on river flow,river flow reduction and interflow reduction rate and at different times,the influence of fissure leading to underground goaf on river flow reduction rate decreases and tends to be stable with the passage of time.(3)For the the hydrological simulation of special underlay surface in goaf,it can be considered as the early rainfall period before the maximum surface runoff reaching.The reduction rate of surface runoff before the maximum surface runoff reaching represents the reduction effect of fractures that do not cut-through to underground goaf in the early stage of rainfall.The surface runoff reduction rate represents the reduction effect of the fractures leading to the underground goaf on surface runoff during the whole rainfall production process;The retention time can be used to indicate the time that the formation of fissures in the early stage of rainfall increases the outflow time.The usable interflow reduction rate represents the reduction of the interflow in the whole rainfall process by the fissures leading to the underground goaf;The available pre-peak flow reduction rate indicates that the fissure that does not lead to the underground goaf reduces the river flow at the early stage of the confluence.The available river flow reduction rate represents the reduction effect of the fissure leading to the underground goaf on the river flow during the whole course of river confluence.(4)In the basin controlled by Jingle hydrological station,taking the period from 1951 to 1990 as the research period,rainfall and runoff showed a high cycle synchronization,and the cycle stability and differentiation were small.However,taking the period from 1991 to 2017 as the research period,that is,after largescale coal mining in the basin,the cycle of rainfall and runoff is out of sync,the periodical change of rainfall is stable and shows little difference,but runoff no longer obey the periodic change rule.The correlation coefficient of rainfall and runoff in the basin controlled by Jingle hydrological station decreased by 0.0052 annually from 1951 to 2017,by 0.0002 annually from 1951 to 1990,and by0.0082 annually from 1991 to 2017.In the basin controlled by Shangjing You hydrological station,the correlation coefficient decreased by 0.0034 on average from 1959 to 2017,by 0.0016 from 1959 to 1990,and by 0.0057 from 1991 to2017.After the large-scale coal mining in 1990,the correlation of rainfall and runoff in the control basin between Jingle and Shangjing You hydrological station decreased significantly.(5)Taking Loufan hydrological station control basin as an example,25 floods in the basin were simulated.It is obtained that the qualified rate of HECHMS model simulated floods is 80.00%,the qualified rate of double-excess distributed model simulated floods is 84.00%;The qualified rate of two model simulation of peak flow are all 96.00%;The qualified rate of two model simulation of peak show time are all 92.00%;The qualified rate of two model simulation of runoff depth are all 100%;The floods with the DC value of the HEC-HMS model's certainty coefficient reaching 0.5 accounts for 80.00% of the total simulated floods;The floods with the DC value of the double-excess distributed model's certainty coefficient reaching 0.5 accounts for 84.00% of the total simulated floods.The double-excess distributed constructed in this paper is superior to the HEC-HMS model in simulating floods in Loufan hydrological station control basin.(6)The 25 floods in Jingle hydrological station control basin were selected to simulate,and the simulation results of 25 floods in Jingle hydrological station basin show that: For the 13 floods before 1990,the qualified rate of simulated floods is 76.92%,the results of 12 floods simulation after 1990 were poor and could not meet the requirements of the specifications of water situation prediction.Considering the influence of special undersurface in goaf in the confluence process,and introducing the generalized parameters in this paper to recalculate the floods process after 1990,the qualified rate of the simulation results of 12 floods were simulated again by introducing goaf parameters in Jingle Hydrology Station was increased from 0 to 66.67%.The 25 floods in Shangjing You hydrological station control basin were selected to simulate and the simulation results of 25 floods in Shangjing You hydrological station basin show that: For the 14 floods before 1990,the qualified rate of simulated floods is 85.71%,the results of 11 floods simulation after 1990 were poor and could not meet the requirements of the specifications of water situation prediction.Considering the influence of special undersurface in goaf in the confluence process,and introducing the generalized parameters in this paper to recalculate the floods process after 1990,the qualified rate of the simulation results of 11 floods were simulated again by introducing goaf parameters in Shangjing You Hydrology Station was increased from 0 to 81.82%.The introduction of goaf parameters after large-scale coal mining after 1990 effectively improved the floods simulation accuracy of Jingle and Shangjing You hydrology station.