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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 4: 727 - 732

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.04.005

Theory and Technology of Mine Terrain Reshaping

Restoration of Water Systems in Typical Open-pit Coal Mines in the Arid Desert Area of Northwest China

  • WANG Mingxin , 1, 2, 3, * ,
  • ZHAO Yiping 1, 2, 3 ,
  • LIU Yanping 1, 2 ,
  • ZHAO Shuyin 1, 2
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  • 1. Yinshanbeilu Grassland Eco-hydrology National Observation and Research Station, China Institute of Water Resources and Hydropower Research, Beijing 100038, China
  • 2. Institute of Water Resources for Pastoral Area, Ministry of Water Resources, Hohhot 010020, China
  • 3. Collaborative Innovation Center for Grassland Ecological Security (Jointly Supported by the Ministry of Education of China and Inner Mongolia Autonomous Region), Hohhot 010021, China
*WANG Mingxin, E-mail:

Received date: 2022-09-16

  Accepted date: 2022-12-30

  Online published: 2023-07-14

Supported by

Key Research and Development Program of China(2017YFC0504405)

Collaborative Innovation Center for Grassland Ecological Security(MK0143A032021)

China Institute of Water Resources and Hydropower Research Basic Scientific Research Projects(MK2021J08)

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Abstract

The huge pit formed after open-pit mining can partially change the local water system. Taking a typical open-pit coal mine in Wuhai City as an example, this study used survey data, hydrological analysis technology and a Rainfall-Run model to analyze the changes in the water system and runoff. The results indicate that the water system in the coal mining site has changed in the confluence path because of the mining pits and dumps formed by coal mining operations. Taking the local conditions into account, a water system restoration scheme using the pit for floodwater storage is proposed, that is, using the pit to retain upstream flood on the basis of an unobstructed downstream river flow. This scheme has several benefits. First, it can reduce the pressure of downstream flood control. Second, the sediment brought by the flood will be stored in the pit, which is conducive to reducing soil erosion. Third, it is conducive to the conservation of regional groundwater. Fourth, the retained water can be used for irrigation, which is conducive to the improvement of the surrounding ecology. The results of this study can provide references for the restoration and management of mining areas and ecological restoration in the arid desert area of Northwest China.

Key words： northwestern arid desert; open-pit coal mine

Cite this article

WANG Mingxin , ZHAO Yiping , LIU Yanping , ZHAO Shuyin . Restoration of Water Systems in Typical Open-pit Coal Mines in the Arid Desert Area of Northwest China[J]. Journal of Resources and Ecology, 2023 , 14(4) : 727 -732 . DOI: 10.5814/j.issn.1674-764x.2023.04.005

1 Introduction

The arid desert area in Northwest China belongs to the “Northern sand-prevention belt”, which is one of the main ecological security strategic patterns in China, and it is located in the key construction area of the national “Belt and Road” initiative. However, this area has very little precipitation, strong wind, degraded vegetation, serious land desertification and an extremely fragile ecosystem (Rong et al., 2018). At the same time, the regional coal reserves are rich, and many large-scale open-pit coal bases are located there. With the continuous increase in the scale and intensity of open-pit coal mining in the area, its ecological security problems are becoming increasingly serious (Zhao et al., 2018).
Because open-pit mining has the advantages of a high resource recovery rate, safe production conditions and high production efficiency, it is the main coal mining method used today. However, the characteristics of open-pit mining dictate that a series of environmental and ecological problems will inevitably occur. With the continuous improvement of the ecological environment and an increasing requirement for development quality, people are now paying more attention to the comprehensive ecological improvement of abandoned open-pit mining areas. Tian and Wang expounded the concept of environmental disturbance caused by open-pit mining, put forward a classification method of the disturbance behavior based on the disturbance object and the developmental stage of the ore field, and proposed that the disturbance behavior has the characteristics of self-recovery, process and controllability (Tian ang Wang, 2018). Through the implementation of comprehensive measures such as backfilling and slope cutting, the geological environmental problems related to the pits and slag piles left by open-pit mining were restored in Gongyi City (Zhang and Han, 2020). Wang and Huang (2003) proposed the use of coal mining subsidence areas to build fish ponds, cultivate as square farmland, and construct irrigation and drainage systems. Kong and Li (2006) conducted research on the two aspects of engineering reclamation technology and biological reclamation technology in land reclamation technology in coal mining subsidence areas.
Coal mine open pits are generally large enough in scale that they may affect the connectivity of water systems, especially in arid desert areas where rivers are seasonal and inadequate attention is paid to water system restoration. Therefore, this study took the watershed in which the coal mine is located as the research object, analyzed the impact of coal mining on the drainage system of the watershed by using hydrological tools through the terrain data before and after coal mining, and estimated the flood volume by using the rain flood model. On this basis, a comprehensive utilization of water system restoration scheme for flood storage in the mine pit is proposed, which provides a reference for the rehabilitation and ecological restoration of the mining area in the arid desert area of Northwest China.

2 Materials and methods

2.1 Study area

The selected study area is a complete small watershed unit located in a typical open-pit coal mine in the eastern part of Wuhai, which is located in the arid desert area of Northwest China, and its geographic location is shown in Fig. 1. The gray area in the figure is the coal mining area. The topography of the study area is high in the east and low in the west. The precipitation is limited, with an average annual precipitation less than 160 mm, and the evaporation is strong, averaging more than 3000 mm. The study area has a typical continental climate, with a short spring and autumn, long winter and summer, large temperature differences between day and night, and long sunshine hours. The area between the upper reaches of the river basin and the mining area consists of rocky mountains, and the main lithology is limestone. The downstream area is the Quaternary loose layer, which is mainly composed of gravel, sand, clay and aeolian sand. According to an investigation of the study area, these rivers are mountain torrent ditches, also known as Kabuqi Valleys. The characteristics of the mountain torrent gully floods are abrupt suddenness, short duration, fast flow velocity and strong destructiveness.
The light blue dot-dash line in Fig. 1 is the boundary of the small watershed, and the black solid line is the current channel. The typical open-pit mining area is located in the middle and lower reaches of the watershed, as shown in Fig. 1. Typical coal mine pits are located on the confluence path, blocking the natural channel confluence process.
Fig. 1 Map of study area location and features

Note: Regarding the issue of monitoring wells, considering the significant impact of coal mines and other factors within the study area, the water level is not representative within its scope of influence. So the monitoring well is located outside the small watershed, as it belongs to the same hydrogeological unit as the study area, and its water level is more representative in the region.

2.2 Data sources

Crucial data collected from various sources, including digital elevation model (DEM), topographic map (1978), rainfall frequency atlas, precipitation data and geological map, were developed to be used for the runoff assessment in this study. The digital elevation model (DEM) was obtained from 91wemap (12.5 m×12.5 m resolution, acquired by Advanced Land Observing Satellite) to conduct the watershed analysis. The rainstorm was designed using the results of Atlas of Statistical Parameters of Rainstorm in China (2006). The geological map was acquired from the National Geological Archives data center (http://dc.ngac.org.cn). The topographic map of the typical coal mine pit was mapped by an unmanned aerial vehicle (UAV) in 2019. The groundwater level data were obtained from monitoring wells in 2019.

2.3 Research methods

2.3.1 Model introduction

As a runoff calculation model widely used throughout the world, the SCS model was proposed by researchers from the Soil Conservation Service of the United States Department of Agriculture. The physical parameters of the model are clear, and its dimensionless runoff curve number CN is its only parameter, so it is very suitable for areas with incomplete hydrological data. The SCS-CN equation is usually a relationship between runoff volume and rainfall volume. The SCS-CN technique for explaining the water balance equation (Kumar et al., 2021) can be articulated as:
$Q=\frac{{{(P-{{I}_{a}})}^{2}}}{P-{{I}_{a}}+S}$
where Q is the runoff depth (to obtain volume, multiply by the basin area); P is the rainfall depth; Ia is the initial abstraction; and S is the basin storage. All units of depth are mm.
The amount of rainfall that falls before runoff is initiated can be conceptualized as the initial abstraction (Ia) and this is generally assumed to be 0.2S, therefore:
$Q=\frac{{{(P-0.2S)}^{2}}}{P+0.8S}$
with the condition that the CN is related to S as:
$S=\frac{25400-254}{CN}$
The CN, which is a dimensionless number ranging from 0 to 100, is determined from a table based on the data for land-use/land cover (LU/LC), hydrological soil group (HSG), and Antecedent moisture conditions (AMC). The hydrologic Soil Group (HSG) is generally divided into four groups (A, B, C, and D) based on the rate of the soil’s infiltration in the watershed area. Because of the sparse precipitation in the study area, the Antecedent Moisture Condition (AMC) is not considered. The CN value was adopted from the USDA Technical Release (USDA, TR-55, 1986). The SCS technique was initially designed for use in watersheds of 15 km2, but it has been modified for application to larger watersheds/basins by weighing curve numbers related to the watershed/basin area.
$CNw=\frac{\mathop{\sum }^{}\left( C{{n}_{i}}\times {{A}_{i}} \right)}{A}$
where CNw is weighted curve number (dimensionless); Cni is curve number (dimensionless); Ai is area with curve number Cni (km2); A is area of all watersheds (km2).

2.3.2 Parameter calibration

Through the flash flood simulations of small watersheds distributed in different regions, the results of previous studies have shown that the SCS model has good applicability for flash flood simulation in various typical watersheds (Wang et al., 2017; Zhang et al., 2021). As the only comprehensive parameter of the model, the value of CN is related to LU/LC, HSG and AMC. The coefficient can be calibrated by looking it up in the comparison table of LU/LC, HSG and SCS-CN values for an un-gauged watershed.
According to the field survey and remote sensing analysis of this small watershed, the results are shown in Figs. 2-4.
The images in Figs. 2-4 show that the upstream and midstream of the watershed are covered by Ordovician and Cambrian limestone, and the downstream area is covered by Carboniferous and Permian sandstone. The surface is scattered with aeolian soil, and the soil layer is very thin. The vegetation coverage is low, with an average coverage of only 0.11.
Fig. 2 Photo of the watershed surface
Fig. 3 Geological map of the watershed

Note: The figure is a geological map. The specific strata (eg. P1, P2, etc.) will not be explained here as legend. Please refer to the standard geological map for relevant strata information.

Fig. 4 Results of vegetation coverage in the watershed
The soil type classification of SCS takes into account the soil texture classification of the United States Department of Agriculture, but the soil classification standard adopted by SCS is not consistent with the system used in China. If the SCS is used directly, there will be some systematic deviation. However, in the study area, the surface is all rock, so there is no involvement of the classification of soil types. From the perspective of safety, the impacts of fissure leakage and AMC are not considered. According to the data in Table 1, the CN value of the watershed is determined to be 93.
Table 1 Runoff curve numbers for arid and semiarid rangelands①
Cover description Curve numbers for hydrologic soil group
Cover type Hydrologic condition② A③ B C D
Herbaceous (mixture of grass, weeds, and low-growing brush, with brush, maple, and other bush plants) Poor 80 87 93
Fair 71 81 89
Good 62 74 85

Note: ① Average runoff condition, and Ia=0.2S. ② Poor: <30% ground cover (litter, grass, and brush overstory). Fair: 30% to 70% ground cover. Good: >70% ground cover. ③ The curve numbers for group A have been developed only for desert shrub.

3 Results

3.1 Analysis of the influence of open-pit coal mining on the water system

By dividing and extracting the water system in the small watershed before and after mining, the superimposed comparison of the results of these two phases is shown in Fig. 5. The map in Fig. 5 shows that the water system in the coal mining site has changed in the confluence path and is now blocked. The main reason is that the mining pits and dumps formed by coal mining operations have encroached on the river course, causing the river course to become blocked or silted up and atrophied, resulting in the change in the confluence path.
Fig. 5 Changes of the catchment path before and after open-pit coal mining

3.2 Runoff forecast

Rainstorm precipitation values for different durations (1 hour, 6 hours and 24 hours) and different frequencies (1%, 2%, 5% and 10%) were obtained from the rainfall frequency atlas. The SCS model was used to calculate flood peak and flood volume, and the analysis results of typical river basins are shown in Table 2.
Table 2 Runoff calculation results
Duration Category Calculated values
1% 2% 5% 10%
1 h Flood peak (m3 s-1) 97 74 46 27
Flood volume (104 m3) 64 52 36 24
6 h Flood peak (m3 s-1) 174 140 95 65
Flood volume (104 m3) 159 131 92 67
24 h Flood peak (m3 s-1) 125 101 72 50
Flood volume (104 m3) 212 174 127 92

3.3 Coal mine pit volume analysis

According to the results of the survey, the cumulative volume was calculated from the bottom of the mining pit, and the results are shown in Fig. 6.
Fig. 6 Relationship between depth and typical coal mine pit volume
The curve in Fig. 6 shows that the maximum volume of the current typical coal mine mining pit is 3.2 million m3, which is greater than the flood volume generated by P=1% rainfall in the small watershed, so its capacity is large enough for the regulation and storage of the floodwater.

4 Discussion

The comprehensive management of mining pits formed after open-pit coal mining has always been the focus of ecological restoration in coal mines. In the mining areas of the arid desert areas in the northwest of China, the lack of water resources and suitable soil for crop growth are the main factors that limit the ecological restoration of the mining sites. These sites have no suitable place for large-scale soil sampling and long-term artificial irrigation is unsustainable, so the restoration of the mining area should be adapted to the local conditions. After years of open-pit mining, coal mine resources become depleted, and huge mining pits will be formed. According to local conditions, a water system restoration plan for the comprehensive utilization of the mine pit to store floodwater is proposed. In other words, the huge pit formed by coal mining can be used as a “reservoir”, with the downstream river channel acting as a “spillway”. On the premise of ensuring safety, the “spillway” of the mining pit, by measures of interception, drainage and guidance, intercepts the upstream floodwater and sediment and contributes to the comprehensive ecological restoration. The overall plan is illustrated in Fig. 7.
Fig. 7 Water system restoration plan for the comprehensive utilization of a mine pit to store floodwaters
For the safe operation of this scheme, it will be necessary to ensure that the downstream channel is unblocked. Then, the stability of the slope should be analyzed, and the terrain should be reshaped by adopting a near-natural terrain design. For the slope in the pit, the main consideration should be to reduce the slope, and refuse any further dumping into themine to maintain the stability of the slope. Finally, the ecological management should be mainly based on vegetation measures. These considerations are discussed in greater detail in the following list.
(1) Ensure that the downstream channel is unobstructed
Coal mine mining pits are dug down so they are lower than the surrounding area. Generally, there are no artificial dams and other water blocking facilities, and the safety is relatively high. However, in order to prevent floodwater from overflowing after the mining pits are full due to extreme weather, it will be necessary to first ensure that the downstream channel and the connection between the downstream channel and the mining pit are unobstructed. When the water level in the mining pit reaches the set height, it will be drained through the downstream channel.
(2) Safety assessment of coal mine pits
This consideration mainly involves evaluating the safety of water storage in the mining pit, including slope stability, etc., and it also delimits a reasonable buffer zone and sets up isolation measures.
(3) Investigation and clean-up of pollution sources in the mining pits
The focus of cleaning up pollution sources is on the materials used in coal mine production, in order to prevent them from infiltrating into the ground.
(4) Watershed ecological governance
The ecological governance of the watershed should be combined with the overall goal of ecological restoration in the mining area, and a governance plan should be scientifically formulated, with vegetation measures as the main focus and auxiliary engineering measures to dredge the river channel encroachment caused by coal mining activities.
Based on the specific conditions of the mining area, the proposed water system restoration plan for the comprehensive utilization of mines for floodwater storage is in line with local natural conditions. This plan has four main potential benefits: 1) Reduce the pressure of downstream flooding; 2) Keep the sediment brought by the flood out of the mine pit, which is conducive to reducing soil erosion; 3) Recharge of the groundwater will not cause secondary salinization, etc.; and 4) The amount of impounded water can be used for irrigation, which is conducive to the improvement of the surrounding ecology.
This scheme is applicable to the water system that can directly flow into the pit. In the study area, previous coal mining activity had caused significant changes in the confluence path, due to the prevention of flood flow into the pit during the mining period. After mining ceased, the mining pit has a large volume and small catchment area. To increase flood storage, recovering the previous catchment path is recommended so that the water entering the pit will be increased, thus reducing the downstream flood pressure. Due to the lack of data, this study cannot provide sufficient assessments of sediment production and pit operation lifespan.

5 Conclusions

The mining pits and dumps formed by coal mining have encroached on the river channel, causing the channel to shrink and become blocked, resulting in a change in the catchment path. Considering the lack of water and soil resources in the arid desert areas of Northwest China, combined with the calculations of the flood production in these small watersheds and the volume of the mining pits, a water system restoration plan for the comprehensive utilization of mine flood storage is proposed. That is, the huge mine pits formed during coal mining can be used as “reservoirs”, and the downstream river channels can be used as the “spillway”. On the premise of ensuring safety, the “spillway” of the mining pit, by measures of interception, drainage and guidance, intercepts the upstream floodwater and sediment and contributes to the comprehensive ecological restoration.

References

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