La bassa valle del fiume Musone (Marche): geomorfologia e fattori antropivi nell’evoluzione della pianura alluvionale

Musone River Valley (Marche): geomorphology and human impact on the evolution of the alluvial plain

Abstract

It is possible to attribute the growth of a barrier beach to the first important woodland clearings which occurred from the Bronze to the Iron age. Landward, this barrier created swamps and lagoons where the sedimentation concentrated up to the medioeval times. Until 1400 the channel pattern is of meander type and, in the lower part of the valley, the fleeds spread all ever the alluvial plain. Upvalley it cuts Upper Pleistocene alluvial sediments creating several terraces hanging on the valley finer. After the Middle Age the channel pattern changes progressively to a braided type, testifying a cospicous increase in sediment load; this alteration in the pattern is due to the soil erosion following the progressive agricultural reoccupation of the slopes. The coastline remains almost in the same position from Roman to Medioeval times, where a progradation which received a considerable acceleration after the 1400 is documented. During this period the Musone River is artificially diverted towards the Aspio River and delimitated by artificial level while the coastal plain is reclaimed and cultivated. The alluvial plain keeps an aggradational phase until the end of 1800 as well as the coastline which advances for more than 100 in comparison with the medioeval one. A strong downcutting started, following quarrying and the progressive building of artificial levees, after the ’40-50. Several bridges were destabilized and fell down or refoundated. The building of trasversal dams locally stabilizes the erosion, but on a larger scale, creates more intense erosion and generates a step-like longitudinal profile. The downcutting reached valures higher than 10 m in 50 years and interested at first the alluvial sediments then the bedrock. As a consequence of the reduced sediment lead, the coast underwent erosion; in order to prevent this phenomena a lot of artificial longitudinal barriers were erected. Nowadays along the coastline, relatively stable parts alternate with tracts in a state of erosion.