13th August 1935: a catastrophic dam failure in the Orba Valley (Piedmont, Italy)

Between 1919 and 1925 the “Officine Elettriche Genovesi” (O.E.G.), a large industrial company, built a large hydroelectric plant in the municipality of Molare (Piedmont, northern Italy). The reservoir created by the barrage of the Torrent Orba (hydrographic catchment of the River Po) by means of two dams had a capacity of 18 million cubic metres. On the morning of 13th August 1935, after nearly 10 years of operation, the rock sill on which one of the two dams had been constructed – named Secondary Dam of Sella Zerbino –collapsed following a very heavy downpour. The failure caused the sudden emptying of the reservoir and, as a consequence, a large amount of water poured into the underlying valley, wreaking havoc along the whole course of the Torrent Orba as far as its confluence with the River Bormida, some 50 km away, near the town of Alessandria. At least 111 people lost their lives in the disaster. The criminal trial held by the Turin Courts in 1938 returned a verdict of acquittal for the managers of the O.E.G. and the designer of the plant. This article pinpoints the multiple causes which led to the disaster and which can be mainly ascribed to the lack of geological investigation within the hydroelectric plant area. In addition, thorough hydrological and hydraulic studies, which could have accurately determined the adequacy of the dam’s discharge system, were not carried out. Finally, the original project was changed several times, in order to reduce the building costs and increase the volume of water stored without ever considering the proper safety of the plant. The Molare disaster, which occurred nearly 30 years before the Vaiont catastrophe, shows disturbing analogies with the latter, which will be discussed. The other dam, named “Main Dam of Bric Zerbino”, still stands within a remote, abandoned meander of the Torrent Orba which, after the 1935 collapse, found a new course in correspondence with the rock sill where the failed dam had stood. The disasters of Molare and Vaiont represent two unfortunate examples of the serious consequences which can be brought about by underestimating or neglecting the geological and environmental setting of sites affected by important engineering works.