Peculiarities of the materials involved in the debris flows in Kresna Gorge, Southwest Bulgaria

Abstract

Debris flow events periodically occur in the northern part of the Kresna Gorge, SW Bulgaria. Most of them damage the infrastructure and block the traffic along the National Route E79. In the present paper, we analyze the physical properties of the materials involved in debris flow transportation and sedimentation. Samples taken from events in 2017 and 2021 were studied. Plasticity properties were found in the materials deposited in and near the alluvial fan. The plasticity index is up to 23.4% according to BDS EN ISO 17892-12. The particle size composition of the samples shows a predominance of gravelly sand and sandy gravel materials according to the BDS EN ISO 14688-2 standard. The samples taken from the transport zone are defined as gravel and less commonly as sandy gravel. The sand content in them is lower – from 5.3 to 48.5% (average 21.5%) than in the samples taken from the source area and accumulation zone, where the average values are 43.8% and 47.4%, respectively. In the accumulation zone, the quantity of sand content also increases – the average value is 14.1%, while in the source and transport zones it is 6.6% and 8.2%, respectively. The debris flows materials are polydisperse, with dominantly angular and subangular roundness. The coarser grains are accumulated within the stream channel. The finer grains (silty and clayey) are more common in the periphery of the accumulation zone (alluvial fan).