The flood and landslide event of November 4–6 1994 in Piedmont Region (Northwestern Italy): Causes and related effects in Tanaro Valley

Abstract

During the first week of November 1994, the persistence of a wide depression over North-West Europe caused heavy rains on a large part of Piedmont region Precipitations of about 200 mm/day at various rain gauges were recorded. In different Piedmontese areas 92 pluviometric stations, with long-term series (60–80 years), recorded values which were over the previous maxima: in 18 cases for 1 day and in 20 cases for 2 days in succession. The Tanaro Valley (area of 8500 km2), located in southern Piedmont, was the basin mostly affected by these extreme rainfalls. Particularly on November 4 and 5, over 200 mm of rain were recorded in the upper and middle part of the valley and in the upper portions of its tributaries: the Belbo, Bormida and Orba rivers. Precipitations reached a maximum hourly intensity of 55 mm, with maximum cumulative value of 264.4 mm over 24 hours. Rainfalls produced exceptional discharges of the rivers characterized by peaks never recorded before (unit discharge 0.6–7.3 m3s−1 km2). Along the rivers large floods caused widespread damage to 38 urbanized areas. The effect was catastrophical: 44 victims, 2000 homeless, over 10 billion US $ of damage in all. If we consider the event on the whole, we must go back to 1879, May 26–27, to find a comparable case in the Tanaro Valley: therefore we can consider the 1994 event on a secular scale.