Surface coal mining involves disturbing overlying rock layers (overburden) to expose and extract coal reserves using the area mining method wherein the overburden above the uppermost coal seam and the innerburden between the lower coal seams are removed in parallel strips (pits) across the coalfield until the area is mined. Area mining occurs progressively, beginning with removal of large vegetation (trees and shrubs) ahead of the pit prior to salvaging suitable topsoil in advance of the active pit. After being drilled and blasted, overburden material covering the shallowest coal seam is removed. As mining progresses, the overburden and innerburden are placed in piles in previously mined pits where the bottom seam has been completely removed, using draglines and auxiliary excavating equipment. This process is repeated in sequential fashion as the pit advances through the coalfield, resulting in several rows of removed overburden (spoil) piles and complete removal of the natural drainage network. As mining progresses and several rows of spoil piles are created, grading of the piles begins, using large equipment (bulldozers) to create a postmining topography.
The Surface Mining Control and Reclamation Act of 1977 demands that surface coal mine operators reclaim mined lands to meet a number of requirements:
The reclamation activities include the regrading of spoil piles to create an approved postmining topography, including restoring drainage networks, replacing salvaged topsoil, reestablishing vegetation cover, and implementing erosion-control measures.
The Office of Surface Mining Reclamation and Enforcement (OSM) is the US government agency responsible for regulating surface coal mining and reclamation activities. OSM assesses the success of reclamation activities based on performance standards, which include the requirement to minimize erosion and loss of topsoil and limit the contribution of additional suspended solids to receiving streams. The reclamation is considered to meet applicable performance standards if the postmining runoff is comparable to the premining runoff and the postmining sediment yield is less than or equal to the premining sediment yield.
Since the late 1980s, Ayres Associates has been using the erosion and sedimentation impacts (EASI) watershed erosion model. Because geographic characteristics had to be input manually, building a watershed model was onerous. In addition, analysis was lengthy, and modeling results could not be directly visualized on a map. Peabody Western Coal Company's (PWCC) early stability demonstrations required the use of CAD maps and laborious data manipulation to provide numerous inputs to the EASI model.
Watershed erosion modeling faces two major challenges: One is the organization of multiple datasets, and the other is the automation of the complicated workflow. Most physically based watershed erosion models, including EASI, require numerous datasets: aerial photography, topography, rainfall, vegetation, soil, erosion-control measures, and the drainage network. These datasets can be viewed as vertical layers within a watershed. The workflow of watershed erosion modeling involves
To overcome these labor-intensive and time-consuming processes and to help Peabody Western Coal Company comply with the Surface Mining Control and Reclamation Act of 1977, Ayers Associates developed easiTool with ArcGIS.