Short-term evolution of an active basin–fan system, Aspromonte, south Italy

Alluvial fans are widespread throughout Calabria (southern Italy), but only a minority of them show evidence ofcurrent activity. By means of interpretation of aerial photographs, orthophotos and satellite images, comparisonof topographic maps, reconstruction of major rainfall events, and geomorphologic and sedimentologic fieldobservations, we reconstructed the recent history of the geomorphic and sedimentation events and the consequentmorphological changes of an active basin–fan systemmerging with the bed of an ephemeral streamlocallynamed fiumara. The case study is located along the Ionian side of the Aspromonte massif, a sector of Calabriawhere mass movement, intensive erosion, and channel mass transport are the predominant morphodynamicprocesses.Evidence is available of large volumes of debris produced bymassmovements and erosion processes affecting theupper sector of the feeder basin. The aggressive climate with intense winter storms is the principal cause oferosion and mass movements. Largest rainfall events occurred in clusters of 4–6 events between the 1930sand the early 1970s, at roughly 20-year time intervals. The development of the fan appeared as incipient in1955 air photos. Activity continues today, with events of sediment output from the feeder basin. Large rainfallevents are reflected, with a delay of ca. eight years, by aggradation/dissection and progradation/retrogradationcycles in the feeder channel and the fan, respectively. Mutual interference between fan development andhuman activity exists, aswell as does the interaction between the fan progradation and the floods of the receivingfiumara stream.Based on the aforementioned survey tools, two almost 10-year cycles were recognised. Feeder channel aggradation,owing to the debris input of the feeder subbasin and to the presence of check dams built along the feederchannel, continues until the channel slope reaches a threshold value. This can also be caused by the trimmingof the fan toe by the fiumara. The stored alluvium is therefore released, resulting in progradation at the fan siteuntil the feeder channel slope decreases. Subsequently, a renewed cycle starts again in response to sedimentinput from the source area.The morphological and sedimentological evolution of the active basin–fan system indicates how short-termtrends and cyclic modifications of such systems can be important, especially where tributary basins join largerstream beds of a fiumara-type torrent.