Seven decades of debris flow activity. Spatio-temporal observations at connected and disconnected debris flow fans to the Lake Plansee (AT).

Debris flow activity at Lake Plansee, Austria, is evident through numerous debris cones continuously transferring sediment to the lake. Lacustrine sediment records of fan deltas were used to analyze the debris flow activity since 2120 BCE with Kiefer et al. (2021) identifying a drastic increase in debris flow activity since 1920. Furthermore, the photointerpretation of historical aerial imagery combined with modeling of debris flow volumes at the northern slope of Lake Plansee since 1947 suggests an increased trend since the 1980s (Dietrich et al., 2016). Despite the lithological and climatic similarities between the slopes that surround Lake Plansee, debris cones at the northern slope are primarily connected to the lake, while those on the southern slope remain highly active but disconnected.

This contribution aims to advance our understanding of spatio-temporal dynamics on debris flow fans and factors influencing sediment connectivity to the lake. We revise the historical aerial imagery since 1952 to automatically detect ‚active‘ debris channels using image processing and derive time series of photogrammetric Digital Surface Models (DSMs) for change detection.We identified 34 debris catchments with debris flow activity since 1952. Our objectives include (i) analysis of the spatio-temporal patterns of erosion and deposition at each fan to trace their evolution, (ii) quantifying sediment transfer rates from connected fans to lake Plansee in the last 70 years, (iii) identifying the role of vegetation changes in debris fan evolution and (iii) refining our understanding of precipitation and temperature as controlling factors influencing debris flow activity and connectivity or dis-connectivity of active debris channels to lake Plansee. The presented results intend to comprehend the intricate patterns that lead to debris flow exhaustion and increased or decreased activity.

 

_{Dietrich, A., & Krautblatter, M. (2017). Evidence for enhanced debris-flow activity in the Northern Calcareous Alps since the 1980s (Plansee, Austria). Geomorphology, 287, 144-158.}

_{Kiefer, C., Oswald, P., Moernaut, J., Fabbri, S. C., Mayr, C., Strasser, M., & Krautblatter, M. (2021). A 4000-year debris flow record based on amphibious investigations of fan delta activity in Plansee (Austria, Eastern Alps). Earth Surface Dynamics, 9(6), 1481-1503.}