Key Geomorphological Parameters of Torrential Streams in the Middle Struma Valley, Bulgaria

The tributary network of the middle course of the Struma River is characterized by a torrential flow regime driven by both regional climatic characteristics and their variability, as well as anthropogenic activity. This study summarizes observations of specific torrential flood events over the past 20 years, encompassing geomorphological descriptions and analyses of landforms and deposits, as well as synoptic analyses of the conditions that triggered these events. Five left- and right-bank tributaries of the Struma River between the Blagoevgrad and Sandanski Basins were analyzed. Landforms were documented through field observations, while deposits were examined with respect to grain-size distribution and by macroscopic analysis for the coarser fractions. The synoptic analysis was conducted using data from several nearby meteorological stations. The results indicate that the observed flows are of a debris unconsolidated type and generate ephemeral landforms. Their occurrence is in conjunction with the development of convective cumulonimbus cloud cells. A critical threshold is established between 25–32 mm of precipitation over a period of about two hours. The increasing frequency of problems related to torrential flooding in the area is attributed to the inability of existing mitigation structures (e.g., concrete check dams, afforestation measures, etc.) to constrain effectively flow activity, as well as to their inadequate maintenance. Furthermore, the construction of buildings and roads has not been adapted to the geomorphic and hydrological characteristics of these flows.