Ruolo svolto da piene straordinarie nel rimodellare la geometria di un alveo fluviale

Abstract

: Turitto O., Audisio C. & Agangi A., The role of severe floods in the adjustment of river channel geometry (IT ISSN 0394-
3356, 2008).
Numerous studies on river channel adjustments have focused on the role played by human activities in the processes of channel narrowing and incision which took place in several alluvial channels of the Italian rivers. The influence of embankments, bank protections, bridge structures and sediment mining appeared remarkable from the 1950s until the 1980s. Whereas a more or less evident process of channel widening was observed in some of these Italian rivers after the 1980s, leading to an opposite trend.
Through this note the authors mean to provide a representative example. The study case refers to the Orco River, an Alpine tributary of the Po River in the Turin territory (North-Western Italy); it drains 906 km2 and its channel is 83 km long. In the study reach, corresponding to the lower 36 km of the river course (mean channel slope 0.7%), the channel is incised in the cobble and gravel deposits of the plain.
Data collected come from different sources: a) published and unpublished information derived above all from Ministero dei Lavori
Pubblici official publications about Italian rivers, from technical reports and designs of fluvial defence or bridge construction (mainly
produced by Genio Civile) and from Regione Piemonte Archives (as flood levels and discharges recorded at a historical hydrometric
gauging station); b) historical and most recent maps dating from 1881 to 1991 (by Istituto Geografico Militare Italiano, Magistrato per il
Fiume Po, Regione Piemonte) at scales ranging between 1:25.000 and 1:5.000; c) aerial photographs taken every decade since 1945
until 2003 (by Istituto Geografico Militare Italiano, CNR-IRPI di Torino, Regione Piemonte and Provincia di Torino) at scales ranging
between 1:45.000 and 1:5.000. Through aerial photographs one could also verify the hydrographical configuration of the river on the
maps drawn at the same date.
Through Geographical Information System (GIS) procedures, the planimetrical situations of the river channel at various dates were
superimposed over the Regional Technical Map of the Regione Piemonte on a 1:10.000 scale; the different channel configurations
were compared and measured.
Investigations demonstrated that during the 19th century the river course showed almost everywhere a multi-thread channel with forms and numbers of ramifications variable both in space and time. However a slow and progressive simplification of fluvial configuration was observed; it was mainly produced through the abandoning of secondary ramifications and the islands joining the surrounding floodplain. This process was partly due to natural phenomena, persisting until the first half of the 20th century. In the last 50 years, however, the process was heavily accelerated by human activity, above all by channel regulation works and excessive sediment mining. These activities led to a rapid change in channel patterns, from multi-thread to wandering or single-thread configuration, favoured by a deepening of the river bed (1÷2 m on an average and a local maximum of 3.5 m).
Severe floods which occurred during the last years, in September 1993 (1500 m3/s) and in October 2000 (1650 m3/s), modified, however, the river bed geometry. In particular, the 2000 flood (the largest in the 80-year recording period) induced a remarkable increase of the mean channel width, also due to the renewed availability of sediments in the river bed due to the stop of excessive sediment mining after the 1980s.
The aim of this work is to illustrate some morphological effects triggered by these two severe floods. It also provides some examples of the effects triggered by previous severe floods similar to each other in peak flow discharge which, however, occurred in different
morphological conditions and sediment availability: with plenty of sediments the September 1947 flood (1410 m3/s, the largest before
the 1993) induced a widening of the braiding/wandering channel; with lack of sediments the October 1977 flood (about 1300 m3
/s) did not produce a widening of the channel (which meanwhile had turned into a narrow, incised and sinuous channel), but a bend growth.
The selected cases shown in the figures point out the most frequently observed processes: reactivation of old branches naturally abandoned or artificially closed off; severe bank erosion; opening of new peripheral branches by overflowing; variation of river bed elevation; bend growth; formation of new islands dissected from the floodplain by channel avulsion; adjustment of channel geometry proposing again (maybe only temporarily) the old multi-thread morphological configuration.
The study points out that the channel widening which occurred during the 2000 flood suggests the necessity to assign to the river a
large space of mobility to explain fluvial dynamics. Moreover, for a more natural future river management, an undisturbed wide space
of freedom will allow a restoration of both morphological and biological components of the fluvial system.