Experimental assessment of the stage–discharge relationship of the Heyn siphons of Bric Zerbino dam

• The stage–discharge relation of the siphons of the Bric Zerbino dam was determined.
• Five operating phases were identified for the Heyn siphons.
• An hysteretic behavior was highlighted in the stage–discharge relationship.
• Tests showed that the siphons performed less efficiently than originally expected.

The Ortiglieto reservoir, in the Piedmont region of Northern Italy, was created barring the Orba River with two dams: the main one, Bric Zerbino, having spilling facilities, the secondary one, Sella Zerbino, without spillways. On 13th August 1935, a heavy storm hit the Orba river basin. The water level in the reservoir increased by more than 10 m, as the spillways were unable to release the inflowing discharge, overtopping both dams. The Bric Zerbino dam was not damaged, while the Sella Zerbino dam collapsed, flooding the downstream valley and the small towns of Molare and Ovada. A 1:30 scale model of one of the siphon spillways of the Bric Zerbino dam was built to estimate its stage–discharge relationship and the maximum discharge released during the 1935 tragic event. Three piezoelectric pressure transducers and an ultrasonic level gauge were used to determine the hydraulic performances of the siphon.