Experimental study on the discharge released by Bric Zerbino Dam spillways

The original project of the Ortiglieto reservoir dates to 1899. It was enhanced several times during its construction period to increase storage capacity. The final configuration, which constrained the Orba valley by two concrete gravity dams, entered into operation in 1925. The Orba valley was barred by two concrete gravity dams. The main dam, Bric Zerbino, was equipped by flood spillways which consisted of: 12 siphon spillways (Heyn), a side spillway and a pressure outlet regulated by a bell valve. The secondary dam, Sella Zerbino, had no spillway. (Anonymous 1925) On August 13th 1935, an heavy storm settled over the Orba river watershed: the siphon and the lateral spillways of the Bric Zerbino dam were unable to release the inflowing flood and both dams were overtopped. The pressure outlet was not opened over vibrations concerns leading to dam failure and the bell valve became ineffective within 15 minutes of operation due to mud blocking. The main dam (Bric Zerbino) was not damaged but the secondary dam (Sella Zerbino) collapsed, inundating the valley downstream and the small towns of Molare and Ovada (Natale et al. 2008; Natale and Petaccia 2013) This research investigated the flood outlets operating during the 1935 event through the development of a 1:30 physical model of one of the siphon spillways and a 1:40 physical model of the side spillway. A suite of modelling experiments were conducted in order to establish the stage-discharge relationship and the maximum discharge released during the 1935 event. Results obtained were compared against discharge estimates made in the original design plans and independently calculated with first principles hydraulic analysis.