Geomorphological study of the debris flow events in the Harsovska River (Blagoevgradska Bistritsa catchment), observed within 2019-2023

Debris flow events in the Blagoevgradska Bistritsa catchment (SW Bulgaria) become more frequent in the past few years. To localize the debris source along the tributary system, the Harsovska River is chosen in this study. During the observation period a torrential event was registered in June 2023. The synoptic situation, which caused the flood, is re-analyzed and also geomorphological research is made for depositional landforms in the river channel for the period 2019-2023. In the study, the alluvial sediments are defined by grain size and also by morphoscopic study of pebble-sized grains. During the field research, two critical flood-risk sites are pointed, located in the Harsovo hamlet. The data about past debris flow events are collected from residents of Harsovo with interviews regarding their experience in this issue. In the period of the study, the main part of debris nourishment in Harsovska catchment is provided by the tributary Marulevska River, where active mass movement processes are registered. However, the alluvial profiles of both rivers – Marulevska and Harsovska, showed an alternation of coarser and finer materials, resulting of turbulent and calm discharge. Such periodical change in the sedimentary environment of the two compound rivers demonstrates their unsteady torrential character. Regarding the fortification activities in this area, at the moment some new forestation has been noticed. Still, the species, used to fortify the river banks are not suitable for the climate and hydrological regime of the region. The channel fortifications, built in the middle of the 20th century, currently are not maintained properly, which puts under threat of future debris flow events the settlements and infrastructure along this catchment.