Debris-Flow (Torrent) Cones Triggered by Extreme Rainfall During the June 2020 Flash-Flood Event in the Eastern Polish Flysch Carpathians

The immediate cause of debris-flow cones formation was the short-lived but intense rainfall on 26 June 2020, which affected catchments of small rivers (ranging from several dozen to 200 km²) located in the Carpathian Foothills (300–500 m a.s.l.). The issue of debris-flow cones formation is presented in the context of the geomorphological effects of flash floods in selected catchments of the middle San River basin during the June 2020 events. Torrential cones were formed by debris-mud flows in forested, steeply inclined (>10%) V-shaped valleys (with catchment areas below 1.5 km⁻²). Valley incision reached approximately 1.0–1.5 m. Based on the type and grain size of the deposited material, three cone types were distinguished: 1) larger debris cones containing rock blocks up to 90 cm in diameter, 2) smaller silty-debris cones, composed mainly of clasts and soil-derived sands with a minor share of coarse debris, and 3) silty-sandy fan deltas, whose accumulation resulted from a temporary blockage of flow. The triggering of debris flows and the formation of torrential cones require hourly rainfall intensities of at least 40–50 mm (daily total around 80 mm). An important factor in the initiation of debris and mud flows is the antecedent phase preceding the main rainfall event. In this case, 11 days with rainfall before the downpour on 26 June 2020, led to the saturation of surface deposits, which facilitated the activation of flows and the transport of debris-water masses within the valleys.