THE PERTURBATION OF CATCHMENT WIDE DENUDATION RATES IN DEBRIS FLOW DOMINATED CATCHMENTS

Threshold dominated mountainous regions are commonly related with landscape process subjected to debris flows and landslides and their associated hazards. Quantifying process rates, and specifically meaningful denudation rates, in such environments is complicated by the episodic nature of the occurrence of debris flows and landslides. While commonly sediment yield studies are referred to as underestimating denudation rates, cosmogenic nuclide (¹⁰Be) based mean catchment wide denudation rates are integrating such events if appropriate spatial catchment scales with respect to frequency and magnitude of events are considered. However, catchment wide denudation rates require geomorphic and isotopic steady state in all compartments of the catchment and a well mixing of these compartments at the sampling site. We have tested these assumptions in a catchment prone to episodic debris flows activity in Central Switzerland. Nuclide concentrations obtained over a time-series of 3 years vary by a factor of two at the same sampling site. Reduction in nuclide concentrations correlates with the occurrence of debris flows just prior to sampling. Significant erosion of the debris flow track and debris flow cone (>15 m) with abrupt aggradation in the trunk stream (>10 m) occurred – a fact that is not immediately evident at the sampling site farther 6 km downstream. Common area weighted mass balances and mixing studies may not be appropriate in such catchments since debris flow fans directly deposited at the trunk stream do not allow sufficient mixing.