Dendrogeomorphic reconstruction of past debris-flood activity along a torrential channel: an example from Negoiul basin (Apuseni Mountains, Romanian Carpathians)

Debris floods commonly affect remote mountain areas by damaging the existing infrastructure. It is therefore important to identify and map the hazards related to this hydrogeomorphic process, to include it in the planning policies and decision-making process. Because remote mountain areas are rarely long-term monitored, the data regarding past debris flood events are scarce, therefore indirect methods might be a solution to identify the areas affected. As veritable natural archives, trees are able to record the signs of past debris flood activity in their annual ring structure. Here, we reconstruct the past hydrogeomorphic activity in a torrential channel within a small mountain basin located in the Apuseni Mountains (Romanian Carpathians). The peak-flow discharge magnitude of an extreme debris flood event occurred in 2006 on the main channel was estimated based on 16 cross-sections. Using the marks left by the flow on trees located within measured cross-sections, an average flow discharge of 90 m(3)/s was back-calculated based on two hydraulic methods, namely the product of cross-sectional area and the average velocity of the flow. A dendrogeomorphic approach was then undertaken, in order to date other events that affect in the past similar areas along the main channel. Tree-ring analysis revealed that at least two debris floods events of the magnitude comparable to that of the 2006 event occurred along the torrential channel during the last 8 decades (1940-2010). Dendrogeomorphic methods are useful to estimate the debris-flood frequency and the results can improve the hydrogeomorphic hazard zonation in the studied area, but also in other mountain basins.