Lithologic and glacially-conditioned controls on debris-flow sediment flux in Alto Adige, Italy

Debris flows are efficient processes of sediment transfer from slope base to alluvial fans in mountain drainage basins. To advance current understanding of debris-flow sediment dynamics at the regional scale we analyze a historical (1998-2009) database of debris flows in 82 selected basins of the Alto Adige Province, north-eastern Italy. By combining historical information on event-based debris-flow volumetric deposition, high-resolution (LiDAR-derived) digital topography, and mapping of Quaternary sediment stores we are able to (i) characterize the principal topographic conditions at which debris-flow initiation, transportation, and deposition operate; (ii) detail debris-flow sedimentary signatures across spatial scales; and (iii) link debris-flow sediment flux to morphometry, lithologic variability, and sediment availability. We show that basin-wide specific yield obeys a negative scaling relation with basin area, which in turn is strongly controlled by dominant bedrock geology and abundance of Quaternary deposits. When the event-based sediment flux is subsumed across spatial scales, a more complex sedimentary signature is apparent, regardless of sediment availability. We hypothesize that this signature is characteristic of most debris-flow dominated settings.