Fenomeni impulsivi di erosione e sedimentazione dei corsi d' acqua nella morfodinamica attuale del territorio

Abstract

The parossistic nature of bedload transport phenomena in hydro-graphic systems is highlighted in the paper on the basis of a critical review of case histories in the Alps and River Po valley territories. The review is based on the direct observation of types of propulsive deposition and erosion produced by events of flood and debris flow which occurred in the past decades and are comparable to historical examples that might have had similar evolution. Case histories including urban areas are also considered to stress the damage a propulsive phenomenon can cause to inhabitants. The paper considers in detail the following aspects: a) size of accumulation and erosion forms, b) sedimentary character of deposits, c) form distribution versus hydrographic system and d) environmental implications ensuing from the hazard caused by the modelling action of rivers and torrents. Depositional forms are typical of mountain stream interconnections where cone-shaped deposits are repeat-edly activated. Deposits related to debris flow processes in many cases are some decametre thick (such as the August 1878 deposit at Rio Rosso in the River Isarco basin and the September 1950 at Val Sissone in the river Adda basin, for instance) with mobilised boulders that may be more than 1 metre in diameter in a very heterogeneous matrix. Due to the damming of receiving streams, sedimentary bodies are often connected with temporary sedimentation basins upstream (such as it happened in the River Isarco basin at Rio Gonder in August 1891 and at Rio Tina in August 1921, and in the River Adda basin at Val Pola in July 1987). Gravelly and sandy torrent and river overflows onto alluvial plains and valley floors, which are associated with bedload and suspended transport show a marked sizevariability being often no more than 1 metre thick (cfr. the River Po in November 1951 and September 1992 and the River Adda in July 1987). Cases of instantaneous erosion include both neoformation of streams and river head retreat; instantaneous erosion has caused also an increase in development of lower order streams into high energy basins (cfr. the Sesia basin of the Strona torrent at Cossato in November 1968). Other forms were more directly governed by morphodynamics of hydrographic systems, and involved changes of longitudinal profile and planimetry; such forms have been seen in zones of overcutting of quaternary deposits, a process which is sometimes quite marked in mountainous areas - cfr. the basin of the river Avisio, a tributary of River Casare (near Trento, N Italy) in September 1882 and the torrent Bramafam in the river Stura basin in September 1993. Cases of propulsive erosions occurred in plains, with meander cuts along river axis and embankment breaches at the ground level (such as at the Po River in November 1994).