Research On The Distributed Water Cycle Model Of Coal Mining Subsidence Area

It is a new attempt to utilize the coal mining subsidence area as a plain reservoir in the comprehensive treatment of the coal mining subsidence area, which can achieve a win-win situation in both the development of mining and the water resources exploration. However the current study in this area is in the exploratory stage, the related basic research is weak and fragmented, and there are few referable results to date due to the strong comprehensiveness and difficulties of the subject. Especially the understanding of coal mining subsidence area ponding mechanism and the overall process of water circulation is not very clear, and there’s no reliable fundamental theorems and quantitative technology system to guide the practice of subsidence area management and the comprehensive utilization of water resources. In this work, Huainan coal mining subsidence area is taken as an example due to the high groundwater level, abundant coal reserves and frequent flood disasters in Huaihe river basin. Starting from the subsidence area water cycle simulation, a distributed"river-subsidence area-groundwater"coupling simulation model has been established, and the water cycle mechanism and the effect of flood storage and waterlogged control have been investigated in order to clarify the water transformation process and regularity and water-ponding mechanism of subsidence area. The impact of flood storage and waterlogged control based on the development of mining subsidence area has also been evaluated, providing theoretical support and quantitative fundamentals for comprehensive treatment of coal mining subsidence area and water resource utilization.The main achievements are as follows:(1) The distributed "river-subsidence area-groundwater" coupling simulation model of Huainan coal mining subsidence area has been established.Based on the principles of water balance, the distributed "river-subsidence area-groundwater" coupling simulation module with a strong physical mechanism has been developed considering the water cycle processes of ponding zons and non-ponding zone. The module couples hydrologic processes of river, ponding, ground water and soil water. Huainan coal mining subsidence area as the research object, the spatial data and water cycle driving factors were dealt with to develop the distributed simulate of Huainan coal mining subsidence area. Based on the works to date, the proposed model has been evaluated through four comparisons with experimental results and published data, including the comparison between simulated values and the ones measured of the surface and the underground water level in the study area, the balance test of water cycle, the comparison of macroscopic hydrology characteristic parameters between the predicted results and the results from some published literatures. The comparisons indicates that the simulation results are in good agreement with that of actual situation, therefore the model is reasonable to reflect the ponding mechanism and strongly applicable in Huainan coal mining subsidence area.(2) The water sources have been analyzed and the amount of water resources has been evaluated in Huainan coal mining subsidence areaAccording to the water supply ratio of Huainan coal mining subsidence area, the river confluence accounts for 84.6% as the main source of water in subsidence area, followed by the surface precipitation which accounts for about 8.8%, the ratio of groundwater recharge including water area and non-ponding area is smaller as only 3.6% while the total water resources is only about 32 million m~3/yr, and the runoff of non-ponding zone accounts for about 3%. It could be conclude that the proportion of the subsidence area water supply amount from groundwater in its own scope is small, while the river confluence associated with subsidence area provides more resources for the present water system situation in the study area. In this situation, total amount of the water resources in the subsidence area is about 905 million m~3/yr, while the available part is 831 million m~3/yr after deducting the evaporation from water surface.(3) The water-ponding mechanism of coal mining subsidence area has been discriminated and analyzed.Based on the analysis of model simulation and theoretical derivation, the water supply and drainage characteristics of isolate subsidence area have been described and the relationship between the ponding water and groundwater has been dissected. The quantitative analysis has shown that the recharge of groundwater accounted for only 4.0%, and the fraction of the surface precipitation is 87%, and that of the non-ponding area runoff is about 9%. The evaporation from water surface accounts for 92.6%, while the seepage from subsidence area to the groundwater accounts for 7.4% of total drainage. Studies have shown that the large amount of accumulation of water in the studied subsidence area could be primarily attributed to the meteorological conditions that the precipitation closed to the evaporation capacity, and the water supply source for the most part is precipitation rather than the groundwater recharge in isolated subsidence area. The high groundwater level in the study area only can provide an environment of conserving water and prevent the subsidence area water from leakage.(4) The flood storage and waterlogged control utility of Huainan coal mining subsidence area in subsidence situation of 2010 and 2030 has been evaluated.Located in the middle reaches of the Huaihe River, "closed flood" (occurrs when the flood water level of Huaihe River is higher than the ones of branches for a long time until the waterlogging hard to discharge to the river through the branches) is a major problem of Huainan in flood season. It could cause more flood damages by the accumulation of waterlogging for a longer time "closed flood". The use value of mining subsidence is important in "closed flood" areas as the disaster mitigation mainly depends on the local storage capacity. Based on the meteorological and hydrological data in the studied area, the combination frequency of flood and waterlogging in studied area has been analyzed. According to the disaster level of the "closed flood", combinations of unfavorable years are sorted, and three typical flood years (1991,2003 and 2007) have been chosen as the assessment years to analyze the effect of coal mining subsidence area on flood storage and waterlogged control in subsidence situations of 2010 and 2030. Conclusions could be obtained that:in subsidence situation of 2010, the waterlogged control ability of subsidence area and lakes in study area was inadequate to deal with the typical floods in 1991 and 2003, and the lack of storage space was remarkable in downstream of Xifei River. On the other hand, it’s able to meet the demand of 2007 typical flood and waterlogging, therefore the study subsidence area and lakes had a waterlogged control status about 10 to 15 years. Besides, in the year 2010, only Ni River confluence area had available flood storage because of the small capacity of the subsidence and the negligible flood storage potential, and the capacity was between 18 and 49 million m~3 in three typical years. In subsidence situation in 2030, it could be expected that three typical floods can be successfully dealt with due to the increased subsidence capacity, the simultaneous construction and reconstruction of the valve and pump station engineering. It’s foreseeable that the waterlogged control status of the study subsidence area and lakes would be increased to about 25 to 30 years. At the same time, the flood storage potential of subsidence area and lakes would be increased to 276 million m~3,270 million m~3 and 407 million m~3 while the potential of subsidence area alone would be 222 million m~3,217 million m~3 and 315 million m~3 respectively in 1991,2003 and 2007. Finally, by considering the terrain and distribution of flood storage space in the study area, the project vision for the object of flood storage and waterlogged control has been proposed, making full use of the flood and waterlogged defense ability of the coal mining subsidence area.