Alluvial fan of Glazne river, Bansko town, SW Bulgaria

Abstract

At the place where Banderitsa and Demyanitsa rivers and their tributaries ceased to be confined to their narrow valleys one of the most impressive alluvial fans in South Bulgaria is formed – the one of Glazne river. The river valley morphology, as well as the evolution and the position of the Glazne fan, are controlled by the active normal faulting in the NE slopes and foot of Pirin Mountain. Тhe Quaternary glaciations produced large volumes of debris in the river valleys. There is an agreement that the processes of sediment transfer from the mountain to the Razlog graben have been highly active at the time and immediately after the Pleistocene glaciations. As a result, in the Bulgarian geological and geomorphological reports and scientific papers, the age of the alluvial fans at the NE foot of Pirin Mountain is assumed to be Late Pleistocene–Early Holocene. The results of our studies require a re-evaluation of these ideas. Using widely accepted methods for natural hazard assessment, our field and historical research and analysis allow us to claim that the Glazne fan is active. Parts of Bansko, that are situated in the upper-middle parts of the alluvial fan, have been affected by at least two significant debris floods during the XX century. The conducted measures to control river behavior have an important effect on risk reduction, yet they lead to significant modification of the zones of active aggradation and this means that new actions against future events must be taken.