Contrasting impacts of heavy rainfall in neighbouring catchments: the 2nd September 2022 event in the western part of the Karlovo graben

Post-flood field studies along the northern slope of the Sredna Gora Mountain indicate a drastically uneven response of the river channels to the extreme rain. Northwest of the village of Slatina four streams merge to join the Stryama River floodplain. The largest drainage basin is the westernmost one (0.3 km2), the other two are smaller (0.16–0.12 km2). Because these basins are adjacent, it is highly unlikely that during rainstorms, they will receive different amounts of precipitation. However, a six-hour extreme rainfall event generated significantly different impacts across the four catchments. Contrary to expectations, smaller watersheds were most severely affected, and channel changes were catastrophic. In contrast, the largest one was almost unaffected, with no evidence of transportation of significant amounts of clasts, as only vegetation stripping was observed in the vicinity of the channel. Field data, combined with analysis of post-event aerial imagery, indicate that the variability of the geomorphological response was conditioned by the occurrence of shallow landslides, which in turn transformed into debris flows.