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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 4: 733 - 743

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.04.006

Theory and Technology of Mine Terrain Reshaping

Construction of a Parameter System for the Near-Natural Topographic Reconstruction of Abandoned Mines in the Arid Desert Region of Northwest China

  • YANG Gang , 1, 2, * ,
  • SONG Tongtong 1, 2 ,
  • ZHANG Chengliang 3
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  • 1. School of Information Science and Technology, Beijing Forestry University, Beijing 100083, China
  • 2. Engineering Research Center for Forestry-oriented Intelligent Information Processing of National Forestry and Grassland Administration, Beijing 100083, China
  • 3. Institute of Resources and Environment, Beijing Academy of Science and Technology, Beijing 100095, China
*YANG Gang, E-mail:

Received date: 2022-08-20

  Accepted date: 2022-12-30

  Online published: 2023-07-14

Supported by

Key Research and Development Program of China(2017YFC0504404)

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Abstract

Topographic reconstruction is the fundamental task of the ecological rehabilitation in abandoned mines, which affects the results of the whole ecological system rehabilitation. The technology of near-natural topographic reconstruction constructs the terrain by using the adjacent, undisturbed natural terrains as a reference, and it has become a hot research issue for land reclamation and ecological rehabilitation in recent years. But in reconstructing the near-natural terrain, one must first determine what kinds of characteristics of the natural terrain should be referred to, what necessary parameters should be used, and what indexes should be used to evaluate the results of the topographical design. All these issues still have not been investigated systematically. In this study, the arid desert region in Northwest China was taken as the target area and the theoretical analysis tool of the “Grounded Theory” was applied to discuss these issues systematically. Based on a large amount of literature research and the records obtained from the semi-structured interviews of 12 experts, a three-level parameter system for the near-natural topographic reconstruction in the abandoned mines was finally constructed by three-level coding using the method of the “Grounded Theory”. The parameter system contains a total of three main categories, eight sub-categories, and 26 parameters. The parameters developed in the system can support all aspects of the near-natural terrain design processes, and they can cover the parameter needs in multiple aspects of the topographic reconstruction of the mining sites. This work can provide support for further research on the methods of near-natural topographic reconstruction and improve its technical system.

Key words： abandoned mines; ecological rehabilitation; near-natural; topographic reconstruction

Cite this article

YANG Gang , SONG Tongtong , ZHANG Chengliang . Construction of a Parameter System for the Near-Natural Topographic Reconstruction of Abandoned Mines in the Arid Desert Region of Northwest China[J]. Journal of Resources and Ecology, 2023 , 14(4) : 733 -743 . DOI: 10.5814/j.issn.1674-764x.2023.04.006

1 Introduction

Topographic reconstruction is the first and most crucial part of land reclamation and ecological rehabilitation in abandoned mines, it is directly related to the redistribution and reuse of precipitation and it has a close connection with geological safety. It is also the prerequisite and basis for vegetation restoration. Unreasonable topographic construction will lead to severe soil erosion and affect later revegetation, resulting in significant maintenance and conservation costs.
At present, the designing of topographic reconstruction in abandoned mining sites lacks the quantitative assessment and targeted consideration of local watershed hydrology, surface morphology, water and wind erosion, and other factors. The overall concept of the watershed is also absent, and the plan of the scheme is not thorough enough. In the topographic design of dumps and other areas, the terrain is mostly designed as step-like artificially piled landforms, laid with artificially constructed straight drains for drainage. The slopes of such terrain are usually quite steep (i.e., the slope angles can reach 35%-45%), so precipitation cannot be infiltrated and is consequently lost rapidly, resulting in the failure to use it effectively. The slopes are also prone to erosion. Under extreme meteorological conditions, geological disasters such as landslides and mudslides are more likely to occur. In addition, these constructed landscapes are not coordinated and harmonious with the surrounding environment, and the habitat is singular, which is not conducive to biodiversity reconstruction and succession.
In recent years, the concept and method of near-natural topographic reconstruction have gradually been proposed and applied. Nature itself can upgrade and regenerate, and its original ecosystem characteristics provide the most important reference basis for ecological rehabilitation in abandoned mine sites. The goal of near-natural topographic reconstruction is to reconstruct the topography regarding adjacent, undisturbed natural features, terrains, and hydrological characteristics in stable areas, making the terrain of the abandoned mining sites reach or approach the natural state as much as possible. Therefore, it is possible to use the self-renewal and self-adjustment abilities of nature to achieve stabilization and regeneration (Yang, 2014).
Near-natural topographic reconstruction needs to be designed according to the characteristics of the local natural landscapes. Only by fully understanding the hydrology, geomorphology, geology, and water and wind erosion characteristics of the local undisturbed natural areas, is it possible to design the terrain that is most conducive to local natural ecological restoration. Therefore, it is necessary to construct a complete parameter system of near-natural topographic reconstruction by considering all aspects of the terrain design. The research on this issue is of great significance in the following three aspects.
(1) Providing reference targets and parameters for near-natural terrain design. In other words, what kind of features of the surrounding, undisturbed natural terrain need to be used as the reference targets for the near-natural terrain design? The parameter system can also provide quantitative parameters to support various aspects of the design.
(2) Providing quantitative criteria for the evaluation of the design results. The parameter system will contain a series of indexes that have significant impacts on the stability and naturalness of the terrain. These indexes can also provide reference standards for assessing the stability and near-naturalness of the designed terrain.
(3) Improving the technology of the near-natural topographic reconstruction. The near-natural topographic reconstruction technology is still in the process of development, where so many aspects need to be further explored and verified. The process of the parameter system construction itself, however, is an important part of the research on near-natural topographic reconstruction and plays a significant role in the improvement of its technology.
In the field of land reclamation and the ecological reconstruction of mining areas, some researchers have proposed certain evaluation indexes. However, these indexes either are designed for the measurement and evaluation of the soil and geology, etc., focus on water erosion landforms from the aspect of the watershed, or are too macroscopic to meet the specific design needs of the terrain. At present, there is no parameter system specifically oriented to the near-natural topographic reconstruction, which can comprehensively portray the near-natural terrain characteristics and meet the specific design needs.
The target region in this study is the arid desert region of Northwest China, which is typically fragile and ecologically vulnerable. This region is the area where wind erosion and water erosion work together, which not only has serious problems of water erosion but this area is also significantly affected by wind erosion. Therefore, these characteristics should be considered when we reconstruct the terrain and they should receive sufficient attention when we develop the parameter system. In this study, we develop a comprehensive parameter system for near-natural topographic reconstruction that is customized for the arid desert region of Northwest China by considering all the factors and providing support for further study of the near-natural topographic reconstruction technology.

2 Related work

Many researchers have conducted relevant studies on the characteristic factors and evaluation indexes involved in ecological rehabilitation in mining areas. For example, Bai et al. (1998) studied the characteristics of land disturbance and the applicable technical system of ecological reconstruction in open dumps through the analysis of various index factors, such as slope angle, water and wind erosion modulus, soil bulk density, and the material composition of the surface. Yang et al. (2005) selected landforms, slope, effective soil thickness, texture condition, and soil type as the factors for evaluating the land resources in abandoned mines. Bi et al. (2007) applied the 3S technology to landscape analysis and found that landscape indexes such as number of patches, average patch area, number of sub-dimensions, diversity, etc., can better reflect the landscape structures of reclaimed land and their changes. Wang et al. (2011) systematically analyzed the evaluation indexes and methods of environmental impact brought by land revitalization. However, these studies mainly focus on the measurement and evaluation of the soil, geology, and landscape diversity in the mining areas. Thus, they cannot meet the parameter needs of near-natural topographic reconstruction.
Among studies more specific for topographic reconstruction, Toy and Chuse (2005) took the catchment as the unit and focused on the selection of parameters such as channel density, average slope, elevation difference, and watershed area to elaborate the topographic reconstruction methods. Yang et al. (2014, 2017) systematically discussed the theoretical system and technical methods of near-natural topographic reconstruction and constructed an index system by selecting a total of seven indexes at two levels from among many possible indexes such as geology, spatial geography, and watershed hydrology. Toy and Chuse (2005) and Yang et al. (2017) mainly considered the effect of hydraulic erosion in shaping the geomorphology and their chosen parameters were focused on the portrayal of watershed geomorphology, while they did not give enough consideration to other topographic features, such as local surface morphology, geology, and wind erosion. In addition, these parameters are insufficient for the specific design of the channel and slope.
Bugosh proposed the topographic reconstruction model GeoFluvTM based on the theory of watershed geomorphology (Bugosh, 2004; Bugosh, 2009; Bugosh and Epp, 2019). The model takes the watershed as the unit and realizes the dynamic equilibrium by calculating the relatively stable slopes and channels between the flow production and sink. GeoFluvTM lists a series of calculated parameters, such as channel density, channel sinuosity, topographic slope, etc. This model requires that these parameters should be taken from the adjacent, undisturbed mining sites so that they can be used as a reference for calculating the channel morphology and topographic skeleton in the reconstruction regions. However, the GeoFluvTM model is mainly focused on the calculation of the specific channels and topographic skeleton, so its parameters are insufficient for describing the overall topographic features in the reconstruction areas. In addition, similar to the work conducted by Yang et al. (2014, 2017), the GeoFluvTM model is entirely based on the water erosion that shapes the terrain. Therefore, these parameters are not complete enough to describe the arid desert region in Northwest China, which often suffers from the combined effects of wind and water erosion.
The relevant research shows that the technology of near-natural topographic reconstruction still needs to be explored further, and the construction of its parameter system has still not been systematically discussed so far. Therefore, this study plays a significant role in the improvement of the near-natural topographic reconstruction technology.

3 Principles and methods

3.1 Principles for constructing the parameter system

(1) Principle of completeness
The parameter system needs to fully consider all the factors that affect the near-natural topographic reconstruction. It should not only include the geomorphic characteristics of the watershed, but also local surface features, channel morphology, and other more detailed topographic parameters. The influences of wind and water erosion should also be considered. In addition, the parameter system needs to provide the local environmental factors that directly affect the topographic reconstruction, such as extreme weather precipitation and geological conditions. A comprehensive parameter system will provide effective support for the terrain design.
(2) Principle of scientific validity
The parameter system includes a variety of parameters, so the settings of the index in each aspect should follow the scientific theory of that field. For example, when characterizing watershed geomorphology, it is necessary to conform to the theory of watershed geomorphology; when characterizing local topography, it is necessary to conform to the basic theory of digital terrain analysis; and when setting the parameters relevant to water erosion, wind erosion, and geology, it is necessary to conform to the theory of erosion geomorphology.
(3) Principle of operability
The availability of these parameters should be given certain consideration in constructing the parameter system. Only those parameters which are representative, easy to obtain and calculate, and have clear meaning should be selected under the principle of completeness.
(4) Principle of multi-scale analysis
The parameter system is to characterize the topographic features based on two scales. One is the geomorphic characteristics of the watershed at the watershed scale. The other is the local surface morphology and channel characteristics at a smaller scale. This is necessary for several reasons.
a) From a large scale, the space of topographic reconstruction in abandoned mines can range from a few hectares to several tens or even hundreds of hectares. Such a broad area range can have a significant impact on the local natural watershed system. Therefore, it is necessary to take the watershed as the unit and analyze the landform features within the watershed scale of the local natural terrain.
b) From a small scale, the small local topographic features, which often range on scales from a few dozen to several hundred square meters, also play an important role in terrain stability, water retention, and even biodiversity. Thus, these features must be considered in terrain design. Moreover, when designing channels, it is necessary to consider the local natural channel features, which are also a kind of small-scale characteristic parameter.

3.2 Methods for constructing the parameter system

In constructing the parameter system, conventional methods include the literature frequency method, expert selection method, theoretical derivation method, and principal component analysis method (Li et al. 1998).
Our analysis shows that researchers have conducted many studies on ecological restoration in mining areas and topographic factors, and even proposed hundreds of factors. However, the required parameters for near-natural topographic reconstruction have not been systematically discussed. Moreover, the method of near-natural topographic reconstruction itself is still in the research stage and it is not well developed. Under these circumstances, analyzing and summarizing the required parameters based on the existing literature is difficult, so it is hard to draw scientific conclusions by conventional inference. Consequently, this study adopts the theoretical tool of “Grounded Theory” to summarize the concepts, clarify the understanding, and construct the parameter system of the near-natural topographic reconstruction.
Grounded Theory is an inductive method proposed by Corbin and Strauss (1998). Its method conducts three stages of the coding process based on systematic data collection, namely: Open coding, Axial coding, and Selective coding, which is finally integrated into a theoretical system or model. The coding process based on Grounded Theory helps researchers to systematically analyze and summarize the sophisticated data and eventually form the theoretical model. This method is considered one of the most scientifically valid qualitative research methods. It is very applicable to studies that lack theoretical explanations or for which the existing theories are not sufficient.
By introducing quantitative research tools (e.g., multi-level coding) into the qualitative oriented research, the Grounded Theory can provide systematic solutions to complex problems that lack quantitative models and have many influencing factors. This approach has already been widely applied to many sociological studies and the evaluation of parameters (Luo and Wang, 2022). The problem of “the near-natural topographic reconstruction” faced in this study still lacks a clear model for calculation, evaluation, and the construction of its parameter system. In this regard, we can adopt the “Grounded Theory” to summarize and analyze the parameters so as to make the parameter system systematic and scientifically valid.
To construct the parameter system of near-natural topographic reconstruction, this study strictly conforms to the process of Grounded Theory and adopts the following steps: literature research→design of interview outline→conduct interviews→open coding→axial coding→selective coding.
In the first step, a great deal of literature relevant to the topographic reconstruction of the mines was assessed, the main aspects of the near-natural topographic reconstruction were determined, and various parameters that may be involved in near-natural topographic reconstruction were summarized. On this basis, the outline for the expert interview was designed.
In the interview phase, 12 experts were selected for face-to-face or online interviews. Ten of the 12 experts were randomly selected for coding analysis, and the remaining two experts were targeted for the theoretical saturation test. The interviews used a semi-structured format.
Open coding refers to the process of decomposing, comparing, and initially classifying the acquired documents. In this study, key expressions and 105 suggested parameters were extracted from the records of the expert interviews. Then they were coded, organized, and categorized. Finally, a total of nine categories and 26 parameters were obtained.
Axial coding refers to the further summarization of the categories obtained through open coding, in which concepts of the same nature are grouped into higher-level categories. Thus, the connections between main categories and subcategories are established. Throughout the axial coding, a three-level parameter structure is constructed in this study, which contains three main categories, eight subcategories, and 26 parameters.
Selective coding is the process of logical testing and supplementation. This includes adjusting the categories, supplementing the underdeveloped categories, and conducting the sampling and interviews again to achieve theoretical saturation. At this stage, if there is a new discovery, it will return to the open coding stage and the process will cycle until no new concept is found. Eventually, the final parameter system of the near-natural topographic reconstruction is formed.

4 Construction of the parameter system based on the Grounded Theory

In this section, the key steps mentioned in section 3.2 are discussed in detail.

4.1 Literature research

Firstly, research was conducted on the factors and evalua- tion indexes involved in the ecological rehabilitation of mining sites. As mentioned in the previous section, various researchers have proposed a series of indexes specific for the ecological rehabilitation in the mining areas, but the current indexes mainly focus on the measurement and eval uation of the soil, geology, and landscape diversity. Thus, they cannot directly meet the design needs of near-natural topographic reconstruction. Some of these parameters, such as the geological indexes for the material composition of the surface, the type of soil, as well as slope angles, have potential value for the design of the terrain. Therefore, they are selected as candidate parameters in the outline of the expert interviews.
Moreover, the extensive literature concerning the topographic reconstruction of the mining sites was consulted. As mentioned in the previous section, Toy and Chuse (2005) and Yang (2014) both proposed several indexes for the near-natural topographic reconstruction. They focused on the shaping effect of watershed hydraulic erosion on landforms, and thus most of the indexes they chose were related to the watershed landform morphology. In this study, we referred to these indexes in considering the watershed geomorphologic features. In addition, the GeoFluvTM model proposed by Bugosh listed a series of specifically calculated parameters, such as channel density, channel sinuosity, channel gradient, and maximum 1-hour precipitation once in 2 years (Bugosh, 2004). These parameters are all supplemented in the outline of the interview in this study.
Meanwhile, regarding the natural terrain features, previous researchers have proposed hundreds of indexes. Various researchers have conducted many studies on the classification of these indexes and proposed dozens of classification methods based on different perspectives and application needs (Wood, 1996; Florinsky, 1998; Weiss, 2001; Tang et al., 2017). In this study, the main focus was on those factors that can assist in near-natural topographic reconstruction. For example, specific terrain elements such as channels and slopes need to be designed, and some characteristics of natural watersheds need to be referred to when the near-natural terrain is constructed. Therefore, the object- oriented classification approach has the most valuable reference for this study (Zhang, 2013). Based on the classification of the terrain factors using the object-oriented method, 25 candidate terrain factors for channels, slopes, and watersheds were selected from among the hundreds of potential terrain factors.
Through adequate literature research, all the parameters that might be relevant to the near-natural design were selected, and they provided the basis for the design of the interview outline.

4.2 The design of the interview outline and expert interviews

Twelve interview questions were designed to inspire the experts’ thinking and answers. The questions in the interview are:
(1) What is the most important factor that we should consider in the near-natural topographic reconstruction?
(2) Which parameters related to watershed geomorphic characteristics have reference values for the near-natural topographic reconstruction?
(3) The channels need to be designed when we reconstruct the near-natural topography. Thus, which parameters related to channels need to be considered?
(4) Which parameters related to slope characteristics need to be considered?
(5) Which parameters related to terrain complexity, or terrain statistic features need to be considered?
(6) Wind erosion is significant in the arid desert regions of Northwest China. Thus, which parameters related to wind erosion need to be considered?
(7) What is the significance of parameters like precipitation for the design of the channels?
(8) Which parameters related to geology and soil need to be considered?
(9) Which parameters related to the engineering need to be considered?
(10) Which parameters related to erosion need to be considered?
(11) What other important parameters need to be supplemented?
(12) What are other suggestions for constructing the parameter system of the near-natural topographic reconstruction of the abandoned mines in the arid desert regions of Northwest China?
Among these 12 questions, the first question aims to provide the basis for the later selection of the parameters and also to inspire the experts. Then, the most basic aspects of the system are classified into nine categories (corresponding to the nine questions) to consult the experts about the parameters that have reference values for the topographic reconstruction. The last two questions were designed for supplemental suggestions from the experts.
During the interviews, the following semi-structured interview strategy was adopted. After the questions were posed, the experts were first allowed to discuss them freely. Then pre-prepared parameters were given to the experts for their comments, and the parameter list was supplemented according to their answers after the free discussion. This kind of approach will not limit the experts’ thinking. Instead, it can provide opportunities for the experts to revise and supplement their answers. Two of the 12 experts (both from research institutes) were selected to test and supplement the parameters in the selective coding stage. The remaining experts were interviewees for the open coding.

4.3 Open coding

In this study, the information obtained from the interviews of 10 experts was analyzed and organized sentence by sentence, and a total of 105 suggested parameters were obtained. Each piece of information was coded in the form of ZJ01-01, ZJ01-02, etc. For example, ZJ05-10 represented the 10th suggested parameter from the 5th respondent. Ten categories with a total of 33 parameters were initially summarized by combining and analyzing these suggested parameters. On this basis, some of these parameters were adopted, but others were discarded based on the principles of completeness, scientific validity, and operability. Parameters with conflicting opinions (e.g., parameters related to erosion intensity), parameters not directly related to terrain design (e.g., funding, the willingness of Party A, etc.), and parameters suggested less than five times (i.e., less than five experts suggested setting up that parameter) were removed. As shown in Table 1, eventually nine categories with a total of 26 parameters were obtained. The number in parentheses after each parameter indicates the number of experts who recommend the parameter.
Among the parameters in Table 1, the minimum distance from the watershed to the source of the channel (Bugosh, 2004) means that when precipitation occurs, the upper part of the slope (the part near the watershed) will not form an erosion channel immediately. Rather, it will take a certain distance for the flow to gather enough erosion force before the channels form. This parameter is used to reflect this phenomenon. It is related to precipitation, precipitation intensity, surface roughness, the material composition of the surface, soil texture and thickness of the soil layer (Sun, 2021), etc., and it is an important reference for near-natural channel design.
Table 1 Parameters obtained from open coding
Category Parameters
Influence of the reconstruction regions Disturbance degree of reconstruction region to the watershed (abbr. DDRRW) (6)
Geomorphological features of the watershed Drainage density (10); Elevation difference of watershed (10); Average watershed slope (10); Main channel length (8); Watershed area (7); Watershed roundness (7)
Channel characteristics Channel gradient (10); Channel sinuosity (10); Width of the bottom channel (10); Width-to-depth ratio of the channel (10); Minimum distance from the watershed to the source of the channel (9)
Slope characteristics Slope (10); Slope length (6)
Parameters related to terrain complexity Relief amplitude (6); Terrain roughness (5)
Soil-related parameters Material composition of the surface (10); Soil texture (8); Thickness of the soil layer (7)
Precipitation-related parameters 1-hour maximum precipitation once in 2 years (2-yr, 1-hr) (8); 6-hour maximum precipitation once in 50 years (50-yr, 6-hr) (8)
Wind-related parameters Dominant wind direction (8); Dominant wind speed (8); Duration of dominant wind (7)
Engineering-related parameters The natural angle of repose (7); Maximum angle of construction safety (9)

Note: The number in parentheses indicates the number of experts who recommend the parameter.

For the precipitation-related parameters, all experts indicated that such parameters are necessary, but none of them specified which frequency and duration of rainfall parameters to count. For this reason, we selected two parameters regarding the GeoFluvTM model: 1-hour maximum precipitation once in 2 years (abbr. 2-yr, 1-hr) and 6-hour maximum precipitation once in 50 years (abbr. 50-yr, 6-hr). Most experts agreed with these two parameters. The published research shows that we can use 2-yr, 1-hr to calculate the flow of the bankfull because the maximum 2-yr, 1-hr is equivalent to the flow of the flat bank during the period of full water in the river (Bugosh, 2004), by which the width and cross-section of the channels can be determined. For the rivers with a tendency to flood, the dominant channel morphology is associated with a 50-year storm event. Thus, the 50-yr, 6-hr can be used to calculate the maximum flood tolerance value of the channel, by which the channel morphology and cross-section can be calculated.
For the wind-related parameters, the three parameters of dominant wind direction, dominant wind speed, and dominant wind duration were set to measure the dominant wind conditions in this region during the year. These parameters can be used to adjust the direction of the secondary ridge of the feather-like terrain to form terrain that is perpendicular to the main wind direction, thus increasing the terrain roughness and reducing wind erosion.
In addition, according to the experts’ answers to the first question in the interview outline, “ensuring the local water system is unobstructed” was given as the most important factor in topographic reconstruction, which was agreed by all the experts. Furthermore, six experts indicated that the level of disturbance of the reconstruction region to the local natural watershed should be evaluated before the topographic design because the level of disturbance can be used to determine the urgency of the near-natural terrain design and may be helpful for the later plan stages. According to this suggestion, a new index was added, i.e., “Disturbance degree of reconstruction region to the watershed (abbr. DDRRW)”. The definition and value of this index are shown in Table 2.
Table 2 Disturbance degree of the reconstruction region to the watershed
Description of classification Degree
The reconstruction region lies in the mainstem of the watershed, or at the confluence of tributaries with the mainstem 4
The reconstruction region lies in multiple non-order Ⅰ channels of the watershed, exclusive to the confluence of the mainstem 3
The reconstruction region lies in the non-order Ⅰ channel of the watershed or the confluence of an order Ⅰ channel with another tributary 2
The reconstruction region lies in the order Ⅰ channel of the watershed 1
The channel order in Table 2 is classified according to the Strahler classification method (Strahler, 1952), with the order I channel being the lowest-ranking tributary and the mainstream being the highest-ranking channel. A graphic representation of the four cases is given in Fig. 1. The degree is assigned by judging the cases in the order of priority from top to bottom in Table 2. If there are only order I channels in the watershed, i.e., the mainstem is the order I channel, and the reconstruction regions lie in its channel, then the degree is 4 instead of 1. The higher the degree of this index, the greater the influence brought by the reconstruction regions on the watershed. Thus, the channel route of the topographic reconstruction needs to be designed prudently so that it can be harmonious with the surrounding natural water system.
Fig. 1 Disturbance degree of the reconstruction region to the watershed

4.4 Axial coding

Logical relationships were found among the nine categories, which can be further classified. The nine categories can be divided into natural terrain characteristics (watershed scale, local terrain scale, terrain complexity); environmental characteristics (soil, water, wind); and parameters related to the reconstruction region (influence degree, engineering). Thus, the parameters were further grouped into a three-level parameter structure of three main categories, eight sub- categories, and 26 parameters.

4.5 Selective coding

Two experts were selected for a discussion to check the completeness and rationality of the parameter system constructed in the axial coding. Both of them were researchers engaged in ecological restoration in research institutes, and even participated in the national project of the ecological restoration in the mining areas. The two experts did not propose any new additional indexes and they approved of the completeness and rationality of the parameter system shown in Table 3.
Table 3 Parameter system constructed by axial coding
Main category Sub-category Parameters Remarks
Natural terrain
characteristics		 Watershed scale Watershed area; watershed roundness; drainage density Surface-domain characteristics
Main channel length Line-domain characteristics
An elevation difference of the watershed; average slope Relief characteristics
Local terrain unit scale Sinuosity; minimum distance from the watershed to the source of the channel Channel planar characteristics
Channel gradient Characteristics of channel longitudinal section
Width of the bottom channel; the width-to-depth ratio of the channel Characteristics of channel cross-section
Slope, slope length Slope characteristics
Terrain complexity Relief amplitude Vertical characteristics
Terrain roughness Horizontal characteristics
Environmental
characteristics		 Soil Material composition of the surface; soil texture;
thickness of the soil layer
Precipitation 2-yr, 1-hr Determination of the width and cross-section of the channel bank
50-yr, 6-hr Determination of the morphology and cross-section of the dominant channel
Wind Dominant wind direction; dominant wind speed; duration of dominant wind
Parameters related to the reconstruction region Influence of the reconstruction regions Disturbance degree of reconstruction region to the
watershed (abbr. DDRRW)
Engineering-related Natural angle of repose
Maximum angle of construction safety

Note: 1-hour maximum precipitation once in 2 years (abbr. 2-yr, 1-hr); 6-hour maximum precipitation once in 50 years (abbr. 50-yr, 6-hr).

5 Analysis of the parameter system

5.1 Support of the parameter system for the design process of the near-natural topographic reconstruction

Referring to the GeoFluvTM model and the design method of Yang (2014), the design of the near-natural terrain can be divided into three stages: the stage of analyzing the impact of reconstruction on the environment, the stage of watershed design, and the stage of the local terrain design. The relationships between the parameters and each stage of the design process were sorted out, and the framework diagram shown in Fig. 2 was finally formed.
Fig. 2 Analysis of the parameter system based on the design process
(1) Stage of analyzing the impact of reconstruction on the environment
The impact on the environment, such as on the current hydrology and soil, is evaluated in this stage, and this allows the necessity of the near-natural topographic reconstruction to be assessed. The DDRRW in the parameter system can provide a crucial reference for the stage.
(2) Stage of watershed design
This stage is for outlining the design of the reconstruction region, mainly determining the trend of the main channels and the division of the watershed brought by the channels. This stage can be carried out by focusing on the watershed scale parameters in the parameter system.
(3) Stage of local terrain unit design
This stage is used to finish the design of channels and slopes in each watershed based on the overall watershed design. The channel and slope parameters in the local terrain scale of the parameter system can provide support for this stage. In addition, precipitation parameters need to be considered in the channel design; while in the slope design, parameters related to engineering can be referenced. The wind-related parameters in the parameter system can be referred to in the slop direction design. The soil parameters have reference values for the designs of both the channels and slopes.
As seen in Fig. 2, the eight categories of parameters in the parameter system cover the three main stages of near-natural terrain design. Therefore, the parameter system is complete and systematic in terms of the topographic reconstruction design process.

5.2 Multiple aspects of the parameter needs in the topographic reconstruction

The parameter system in this study was constructed mainly according to the near-natural design needs, but other aspects of the parameter need in the topographic reconstruction in the mining regions were also taken into account. The following categorization and analysis of the related parameters in the parameter system is based on several aspects. Note that the following categories of parameters are not mutually exclusive, but often overlap with each other.
Parameters related to topographic stability. The parameters “natural angle of repose” and “maximum angle of construction safety’’ give the geological safety guarantee indexes of the topographic reconstruction in the mining regions; while the slope angle, slope length, and other parameters describe the stable slope characteristics of the natural terrain in the adjacent undisturbed regions. Thus, the designed terrain will have a stable guarantee by referring to these parameters.
Parameters related to water and soil conservation. The channel density, parameters related to the channel morphology (channel sinuosity, channel gradient, etc.), and precipitation are closely connected to the soil and water conservation in local natural landforms. These parameters can assist in the design of channels and terrain with great soil and water conservation properties.
Hydrological parameters of the natural landforms. The six parameters related to watershed geomorphology provide the basic watershed conditions of the area, which can be used as a reference for selecting the reference areas for topography, and can also be used for basic watershed design.
Parameters for the channel and terrain design. In designing the channels and slopes, five channel morphology parameters, two slope parameters, as well as channel density and two precipitation parameters all can be used for calculating the channel morphology and slope shape.
Topographic stability evaluation indexes. Evaluating the stability of the reconstructed terrain is a necessary part of assessing the rationality of this topographic design scheme. Several indexes which are closely related to the stability of the terrain, such as channel density, elevation difference of the watershed, relief amplitude, and average slope of the watershed, are included in the parameter system. The comparisons of these indexes before and after the topographic reconstruction can be used for evaluating the effect of the topographic stability improvement. For example, a decrease in the average slope of the watershed indicates that the terrain is relatively flat, the erosion intensity tends to moderate and the terrain has a better stability.
Evaluation indexes of the near naturalness of the reconstructed terrain. The core of the near-natural topographic reconstruction is to make the reconstructed terrain as close as possible to the local stable natural terrain. Therefore, it is necessary to evaluate the similarity between the designed terrain and the local natural terrain. The similarity of the watershed between the designed terrain and the natural terrain can be assessed by indexes such as channel density, elevation difference, average slope, watershed area, and channel sinuosity; and the similarity of local topography can be assessed by indexes such as slope and terrain complexity.
Reference indexes related to wind erosion. If local wind erosion is considered in the terrain design, three wind-related parameters can be used to assess the conditions of the dominant wind in this region, and thus assist in the slope design.
The above categorization and analysis show that the parameter system covers all the aspects of the topographic reconstruction in the mining areas. Thus, it can provide a comprehensive parameter guarantee for the design and evaluation of the topography.

5.3 Application case

To test this system, the gangue dump in the first district of Ningdong Yangchangwan Coal Mine was used as a research case and the parameter measurements and calculations were carried out according to the parameter system constructed above. Based on these parameters, the topographic reconstruction in the gangue dump was conducted, thus verifying the completeness and effectiveness of this parameter system.
The Yangchangwan Coal Mine is located in Ningdong Town, Lingwu City, Ningxia Hui Autonomous Region. This area belongs to the arid desert region of Northwest China, which has a typical continental climate, with an average annual rainfall of 212 mm and evaporation of 2862.2 mm. The Northwest wind prevails throughout the whole year, with gusts greater than 17 m s-1 for more than 50 days and an average annual number of sandstorm days of about 25. The wind erosion is also significant in this region. The digital elevation data with 5.0 m resolution obtained from the Resource 3 satellite in August 2019 was used as the basic data for this study. Then, through the digital terrain analysis, together with the field surveys and literature review, the various aspects of the parameters were assessed.
In the parameter system of Table 4, the parameters of the natural terrain characteristics, including the watershed scale, local terrain scale and terrain complexity, were all obtained through the analysis of the digital terrain, and the specific calculation process was obtained from the literature (Cai et al. 2021). Precipitation-related and wind-related parameters were obtained by consulting the website of the China Meteorological Administration (http://data.cma.cn), and the soil data were obtained from field surveys. The two parameters regarding engineering were obtained by consulting the construction personnel and the mine design.
The analysis of the influence of the reconstruction regions showed that this region lies in the confluence of two order I channels, and thus the value of the degree of the reconstruction region is 2, indicating that this region has an ordinary impact on the current watershed environment. For the watershed design, the two channels in the reconstruction regions were designed with regard to the surrounding natural terrains based on the multiple channel parameters of the parameter system. Then in the stage of local terrain design, the GeoFluvTM model was mainly adopted to generate the reconstructed terrain based on the parameters of the local terrain scale and precipitation in the parameter system. As shown in the Fig. 3, the left graph shows the terrain before reconstruction, the middle shows the artificially designed channels, and the right shows the generated 3D reconstructed terrain.
Fig. 3 Topographic reconstruction of the gangue dump in the first district of Yangchangwan Coal Mine. (a) Contour map of the gangue dump before reconstruction; (b) Designed channels, where the orange lines represent the designed channels; (c) Generated topography after reconstruction based on the GeoFluvTM model
In this terrain design, the influence of the wind has not been considered. However, the wind erosion plays a significant role in shaping the terrains of the arid desert region in Northwest China and should be taken into account in the subsequent terrain designs. Therefore, it is necessary to include the wind-related parameters in the parameter system.
In addition, the parameters of “The natural angle of repose” and “Maximum angle of construction safety” were used as the references for setting the slope parameters in the GeoFluvTM model.
The results of the topographic reconstruction in the first district of Ningdong Yangchangwan Coal Mine have shown that the parameters of the parameter system proposed in this study are measurable and can cover the requirements of all stages of the actual topographic reconstruction. This success demonstrates that the parameter system is complete and operable for supporting the topographic reconstruction of abandoned mines in the arid desert region of Northwest China.

6 Conclusions

Focusing on the near-natural design needs, the parameter system of the near-natural topographic reconstruction of the abandoned mines was constructed by adopting the Grounded Theory approach. The parameter system specifies the key parameters in the near-natural topographic reconstruction and organizes them into multi-level and multi-category forms. These parameters can provide support for each stage of the near-natural topographic design process and can meet the needs of many aspects of the near-natural topographic design, such as stability, water and soil conservation, channel design, stability evaluation, and near-natural evaluation. This work represents an active exploration of the near-natural topographic reconstruction technology, which can provide strong support for further research on the method of near- natural topographic reconstruction and additional improvements of its technical system.
However, this work still has two main shortcomings.
(1) The parameters related to wind erosion are relatively simple. How can the effect of wind erosion be considered in the terrain design? This itself is an unclear question. Therefore, the parameters related to wind erosion in the current parameter system are not complete, and can only provide some starting reference for slope design.
(2) The influence of geological differences on the topographic characteristics is not considered. Mining reconstruction regions, such as waste dumps, gangue dumps, etc., are mostly material bodies composed of the accumulation of the debris, whose composition is often completely different from the surrounding undisturbed natural regions. Thus, one aspect which remains to be explored is whether the parameters of the terrain characteristics in the surrounding undisturbed regions are suitable as a reference for the terrain reconstruction. The geological differences in topography reconstruction have not been considered in the parameter system in this study.
These are not the only unresolved issues in the construction of the parameter system, but there are also additional issues that have yet to be explored in the current near-natural topographic reconstruction.

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