Haigh, Martin, and Svetlana Gentcheva-Kostadinova. 2007. “Geomorphological Impact of Erosion Control Measures on a Steep Coal-Spoil Embankment, Pernik, Bulgaria”. Geografia Fisica E Dinamica Quaternaria 30 (2): 179-83. https://www.gfdq.glaciologia.it/index.php/GFDQ/article/view/324.