Significant Topographic Changes in the United States
According to the label assigned from the NLCD to each change polygon, mining is the predominant land use/land cover represented in the topographic change inventory. This is not surprising, as surface mining operations have been previously identified in the literature as the largest direct anthropogenic process in terms of the amount of material moved. Further evidence of the dominance of mining as the primary human geomorphic activity is seen in its contribution to the total volume of material moved as calculated from the entire topographic change dataset.
The polygons labeled with mining as the majority land cover account for 57.5 percent of the total number of change polygons, but they contribute 74.9 percent of the total volume of material moved. It is likely that this contribution from mining is even higher, as many of the polygons included in the change mask because of their close proximity to mine locations also are probably mines that were not yet in operation at the time of NLCD source data collection.
Some of the most notable examples of the topographic surface changes resulting from mining are seen in the Appalachian coalfields where mountaintop mining is a commonly used approach for coal extraction. Although the practice of mountaintop mining has been used since the 1970s, it continues to be a controversial issue, with ample documentation both supporting and criticizing it. Regardless of the arguments for and against mountaintop mining, it is without dispute that a significantly altered landscape is the result of the practice.
Remote sensing and other geospatial data, including multitemporal elevation data, have been used to successfully map and describe landform features associated with mountaintop mining. In mountaintop mining operations the ridges are removed to expose the coal seam, and the overburden from the excavation is deposited into the heads of adjacent valleys, graded, and stabilized. In terms of topographic change detection through DEM differencing, the area of ridge removal is indicated by a significant decrease in elevation, while the adjacent valley fills appear as areas of significantly increased elevation. In the context of movement of materials by geomorphic processes, the mountaintop removal is the initiation of motion (erosion), the agent of motion (transportation) is the dragline excavator and truck, and the cessation of motion (deposition) is the valley fill.
The figure below shows a mountaintop mining area in Perry County in eastern Kentucky. In the middle panel, the blue areas represent significant elevation decreases (mountaintop removal), and the red areas represent significant elevation increases (valley fills). The spatial arrangement of the adjacent blue and red polygons (cuts and fills, respectively) separated by a thin area is the characteristic signature of mountaintop mining in the highly dissected topography of the central Appalachians. Clearly, the areas between the leveled ridges and filled valleys have also been disturbed, although the observed vertical differences in those areas were not large enough to exceed the significant change threshold bounds. This illustrates one of the limitations of using just elevation change to detect human geomorphic activity: the individual change polygons may not delineate the entire disturbed area. Although the elevations between adjacent cut and fill polygons have not changed enough to be detected, mining has generally flattened the entire area, so the distribution of local slope and aspect values has been significantly altered.
The Landsat image (bottom panel) in the figure above illustrates an important aspect of topographic change due to mining in Appalachia. There is a corresponding change in land cover as the forest is removed prior to mining operations. Monitoring of the land cover trends in this region indicates that coal mining is the dominant driver of land cover change, especially forest conversion, in the years from 1973 to 2000. Analysis of the polygons included in the topographic change mask due to their close proximity to mine locations shows that the majority (more than 54 percent) are labeled as forest according to the NLCD. Many of these polygons are likely located in the eastern United States coalfields because of the high density of mines in central Appalachia. In contrast, shrubland, agriculture, or grassland was identified as the majority land cover category for 16 percent, 14 percent, and 10 percent, respectively, of the polygons located within 500 meters of known mines. It is likely that many of these polygons are located near expanding mines in the western United States.
The Landsat image in the bottom panel of the figure above also includes a feature that illustrates an interesting issue associated with mountaintop mining, that of postmining land use. Active mining operations coincide with many of the change polygons, although the image indicates that some of the disturbed area has already been reclaimed and revegetated. In the right central portion of the image, an airport runway has been built on newly available flat ground that resulted from mining operations. Such postmining development has been touted as one of the advantages of mountaintop mining.