Dendrogeomorphic approaches for identifying the probable occurrence of debris flows and related torrential processes in steep headwater catchments: The Hrubý Jeseník Mountains, Czech Republic

Abstract

Culminating parts of mid-mountain ranges in Central Europe are interwoven by a network of high-gradient streams. These parts, together with the surrounding slopes, form a coupling system in which hazardous geomorphic processes, such as landslides, debris flows or flash floods, can occur. In the case of the Hrubý Jeseník Mountains (Eastern Sudetes, Czech Republic), we present an evaluation of the debris flows and related torrential processes in the high-gradient streams of the selected catchment based on dendrogeomorphic methods. In addition to the classical dendrogeomorphic approaches we refined the procedure of identifying certain and probable events using new weighted indexes for event reconstruction. Additionally, because of the steep narrow channels without a characteristic accumulation cone, a new spatial approach is defined based on the spacing of disturbed trees for each event (random distribution or clustering) and location of disturbed trees in an individual section of the valley floor. Using Kernel Density analysis for each event and calculating the particular grids in ArcGIS software, we are able to describe the spatial reach and probable nature of an event (debris flow or hyperconcentrated flow). Dendrogeomorphic analysis of 397 sampled trees (predominantly coniferous trees) revealed 24 torrential events (15 certain, nine probable) since 1928 with peaks in 1991, 1997 and 2010 according to index values. In addition, the spatial distribution of disturbed trees (grouped in clusters) in these years indicates debris flow events in the upper parts of the catchment. It seems that new dendrogeomorphic approaches should be valuable in remote, steep headwater catchments where several types of processes affect the valley floors.