Dendrogeomorphic assessment and sediment transfer of natural vs. mining-induced debris-flow activity in Călimani Mountains, Eastern Carpathians, Romania

During the second half of the twentieth century, in the central part of the Călimani Mountains, the mining activities for sulfur-rich ore extraction and processing produced significant landscape changes. Sediment transfers across the affected catchments started during the mining period and are still ongoing today, more than two decades after the cessation of the mining activities in 1997. Subsequently, neither rehabilitation works nor erosion control measures were undertaken to stabilize the spoil heap deposits and natural reforestation. Also, material consolidation was impeded by the highly unstable talus slopes. Moreover, little is known about the frequency of the hydrogeomorphic processes responsible for sediment transfers and their consequences. Dendrogeomorphic investigations, grain-size analyses, and topographic surveys were carried out to document the history of hydrogeomorphic activity, the volume of material stored behind the retention dams, and the rate of sediment accumulation. Tree-ring analysis allowed us to reconstruct ten debris flow events in the natural environment, as well as in mining disturbed environment for the period 1931–2017 and 1970–2017, respectively. Triggering of debris flows appears related to spring and summer precipitation originating from short duration convective systems or long-lasting rainfalls. In these remote mining areas, the -reconstructed frequency of debris flows (return intervals of 8.1 and 4.7 years) might be explained by the rainwater accumulation on the spoil heap platform and the availability of sediments, which flow down the talus and then follow the stream channels to reach one of the sediment retention reservoirs (7.4–8.4 cm y−1).