Ponti e dinamica fluviale. Effetti di reciproca interferenza

Vengono presentati alcuni risultati di uno studio condotto lungo il Torrente Orco (906 km² di bacino idrografico afferente al Fiume Po) nei suoi ultimi 36 km di percorso in pianura. Tale studio era stato in primo luogo finalizzato a ricostruire, attraverso un’analisi multitemporale di documenti cartografici ed aerofotografici, le principali variazioni avvenute nell’assetto planimetrico dell’alveo durante gli ultimi due secoli. In una fase successiva si era cercato di comprendere le possibili cause che avevano portato col tempo ad una generale semplificazione dell’originario e più articolato sistema di deflusso. Tra i numerosi dati emersi, era apparso significativo il ruolo svolto dall’attività antropica nelle sue varie forme e, in particolare, nelle costrizioni imposte alla naturale mobilità planimetrica dell’alveo con le strutture di quasi tutti i ponti realizzati sul corso d’acqua già a partire dalla metà del XIX secolo. Si portano ad esempio tre casi individuati nel tratto superiore, medio e inferiore del percorso fluviale indagato, rispettivamente a Rivarolo (ponte costruito nel 1850), a Feletto (ponte costruito nel 1935) e a Chivasso (ponte costruito nel 1932). Si descrive il tipo di condizionamento imposto dalle opere di attraversamento e si sottolinea la risposta fornita dal corso d’acqua per la presenza di questi ostacoli, in termini di ripetitività e gravità dei danni prodotti alle stesse strutture dei ponti, anche con ripercussioni a monte e a valle del sito di attraversamento. Recent decades have witnessed increasing damage to bridges caused by floods; therefore, besides conventional investigations into theoretical, experimental and practical problems of hydraulic disturbance induced by bridge construction, attention has shifted to the possible causes for recurrent bridge failure. An initial study in Piemonte looked into the efficiency of various types of bridges able to withstand severe bed load transport along Alpine water courses during extreme flood events. Later studies demonstrated connections between the type of bridge damage and different channel patterns; results showed that bridges crossing multi-thread channel (where the real width of the active channel and changes in its geometry are difficult to estimate) are more vulnerable to damage than those spanning a single-thread channel. Further studies following the severe November 1994 flood event in Piemonte revealed the relationship between the type of bridge damage (greater to bridge approaches than to the bridge) and the morphotopographic characteristics of the river crossing sites; this highlighted, moreover, the determinant role of abandoned branches near water courses. Recent investigation methods applied in Italy and abroad have provided specific criteria for evaluating the rating of stream channel stability near bridges. The bulk of these studies has focused on the impact of the river bed instability on the bridge, especially during extraordinary flood events; in contrast, ours focuses on the impact of the bridge on the river dynamics. The starting point was the results of an investigation into Piemonte’s rivers that provided an overview of the channel regulation works set over the past two centuries and the important role they have played in channel pattern changes. In several cases, bridge construction since 1800 has led to a simplification of complex channel systems, although this process has increasingly been facilitated by new river bank protections and, most recently, by excessive gravel and sand mining. The authors present three cases identified along the Orco River plain. The impact of these bridges on river dynamics has been recorded since their construction: repeated damage, especially to the abutments and the approaches has required continued repair and protection. Despite the defence works, some parts of the three bridge structures were destroyed by extraordinary floods, specifically those of 1977, 1993 and 2000.