Lithologic and morphologic controls on debris flow dynamics in the Dolomites (Italian Alps, Italy)

In high-relief landscapes, debris flows represent the most efficient mean of erosion and sediment transport across spatial scales ranging from hectares to tens of square kilometers. In the Dolomite region of Italy, the landscape is dominated by steep massifs mainly made of limestone and dolostone rocks. Abundant talus and fan deposits connect the bottom of the valleys to the rocky massifs. Thick debris talus was deposited in post-glacial climatic conditions and is actively fed by steep dolomitic rock walls. Debris flows are widespread over the territory. They are commonly triggered by water discharge concentrated on steep headwater catchments and delivered to talus slopes. Headwater catchments are typically very steep (45°-60° on the average) and mostly consist of exposed bedrock with no vegetation and sparse to none soil cover. Debris flow fans are relatively steep (10 to 30°) Our study area extends over 250 km2 and includes about one-hundred debris flow catchments. We take advantage of the remarkable morphological similarities of the catchments to characterize their lithology, morphometry and investigate scale-relationships. We use LiDAR, field surveys and aerial photo interpretation to describe the principal topographic conditions associated with debris-flow initiation, transportation, and deposition. Debris flow-prone catchments associated to recent activity has been selected to investigate the role of geology and morphology as influencing factor at basin scale. They display a decreasing specific sediment yield with increasing size. Depending on the abundance of loose debris deposits, volumes of sediment delivered to the fan span about one order of magnitude. Dependence of deposited sediment volumes on the basin scale is clear and translates into an inverse proportionality when specific yield is considered.