Mountain torrent alluvial fans in the French Alps: Morphometry and solid flow processes

The paper examines certain aspects of the morphometry of alluvial fans in the French Alps. The relationships between fan area and gradient, drainage basin area, basin ruggedness, and flow processes (bedload transport versus debris flows) are analysed. Alluvial fan area shows very little correlation with drainage basin area, but it is strongly controlled by the available width of the receiving valley floor. A closer relationship is found between alluvial fan gradient and basin ruggedness. The study also shows clear differences between high-gradient fans formed at the exit from small, rugged basins, where debris flows dominate the flow process, and low-gradient fans formed by bedload torrents from larger, less rugged basins. These results, which confirm those obtained from other geographical regions, are used to classify the flow processes affecting alluvial fans on the basis of the simple morphometric parameters of drainage basins and alluvial fans.