Acta Musei Moraviae

Surface depresssions above the exploration area of the mined lignite deposit Ivanka in Hovorany, Moravia, Czech Republic

Based on the analysis of the DMR5G digital model of relief and historical aerial orthophotos, a total of 157 depression structures were identified above the excavated mining area of the Ivanka lignite deposit in Hovorany (South Moravia, Czech Republic). Historical aerial orthophotos were used from the 1960s to identify these depressions on fields where their traces had already been obliterated. Depressions with a minimum dimension of approx. 2 to 3 m were identified, the maximum dimension found was 16 m. The studied depressions are approximately circular in shape and have a depth of up to 5 m. In some cases, multiple generations of these structures can be observed, when a smaller depression is located inside a larger depression. This is an anomalous phenomenon, because nowhere else in the Czech Republic can such a large number of depressed structures be found on mined areas of shallow lignite deposits. A number of individual inbreaks above wastes are known, but these are not open pits above concrete mine corridors. The origin of these depressions is primarily anthropogenic, when after the end of lignite mining at the end of 1959, the surface was connected to the collector in the underlying bed of the lignite bank. Rainwater and water from melting snow reached the originally dry deposit with an almost horizontally placed bank of lignite. The underground water was drained towards the former dry watercourses, where several rotational landslides occurred along the SW–NE fault due to irrigation. Most of the depressions are located at the boundary of the bearing area with a slightly decreasing slope of the relief, modeled by the main scarps of the landslides. We assume that, from the point of view of genesis, the process of formation of depressions was involved, in addition to anthropogenic mining activity, by piping processes, when fine clasts were washed out due to the influence of soaked waters and moved to mined areas. In the last about 40 years, new depressions have not occurred in this area. The process of their formation has been significantly slowed down and their occurrence is limited only to already existing depressions.