Alla Sella Zerbino
Coordinates: 44 ° 34 ′ 7 ″ N , 8 ° 36 ′ 55 ″ E
The Alla Sella Zerbino dam (Diga dello Zerbino, Diga di Molare, Sella Zerbino Dam) near Ovada in Piedmont in the Ligurian Apennines , Province of Alessandria / Italy broke on August 13, 1935 at 1:15 p.m. during a flood. The tidal wave caused great damage in Molare and Ovada; there were over 100 dead.
The two dams
The dam was a curved gravity dam . It dammed the Orba River to Lake Ortiglieto, was built in 1917–1923 and put into operation in 1924. The dam was used to generate electricity from hydropower and was operated by the "Officine Elettriche Genovesi" (OEG) company. The storage volume was probably around 18 million m³.
The shape of the terrain made a second dam wall necessary at another point of the reservoir, 500 m away on the left bank, after the height of the main dam had been increased by 13 m compared to the original plan. During construction there were geological problems with the subsoil.
Data of the main dam Bric Zerbino (Diga di Bric Zerbino):
- Height: 47 m
- Length: 70 m
- Radius of curvature: 200 m
Dates of the Sella Zerbino secondary dam (Diga secondaria di Sella Zerbino):
- Height: 14 or 16 m
- Length: 110 m
The valleys behind the two locking points unite behind the Saccone mountain, around which the Orba flows in a meander , to form the Orba valley.
The catastrophe
On August 13, 1935, within eight hours there was 364 mm of precipitation in the Orba catchment area , in places even more: in Lavagnina 554 mm (182 in 2 hours), in Pianpaludo 453 mm, in Masone 377 mm. 30% of the mean annual precipitation fell in 24 hours. The discharge of 15 m³ per second and km² led to a flood discharge of 2,200–2,300 m³ per second - a millennium flood. However, the Bric-Zerbino dam only had a discharge capacity of 800 to 900 m³ per second. That's why the water level kept rising. At 1 p.m. the situation became critical, and at 1.15 p.m. the Sella Zerbino dam broke.
The tidal wave of around 18 million m³ of water first destroyed a hotel and the power station building 3 km below. 15 minutes later it reached Molare, where, among other things, an 18 m high and 120 m long bridge was destroyed. Then the floods reached the city of Ovada, which is about 3.5 km northeast of Molare. The total runoff is estimated at 30 to 50 million m³. In total, more than 100 people were killed.
The Bric Zerbino dam is still standing today, but is not in use.