1935 Sella Zerbino Dam-Break Case Revisited: A New Hydrologic and Hydraulic Analysis

Abstract

The Bric Zerbino and Sella Zerbino barrages dammed two branches of the Orba Valley in Italy to create the Ortiglieto reservoir and supply the Molare hydropower plant. The secondary dam, the Sella Zerbino, was erected over a fault, whose geotechnical characteristics were deemed extremely poor. On August 13, 1935, a very intense and localized storm produced a flood that overtopped both dams. On that tragic occasion the Sella Zerbino Dam collapsed at 1:27 p.m. resulting in catastrophic destruction and 111 victims. Nevertheless, the Sella Zerbino tragedy was soon forgotten. The designers of the dam and the directors of the electric company [Officine Elettriche Genovesi (OEG)] managing the power plant were acquitted in the subsequent trial. This paper aims at revisiting and studying the historical case with current hydraulic and hydrologic knowledge to show that the flaws in the design process infringed even upon the Italian regulations on dam design and construction in place at that time. The rainfall event has been reconstructed to evaluate its return period. The flood wave entering the Ortiglieto reservoir has been evaluated. The dam-break wave propagation has been simulated with a two-dimensional (2D) model based on shallow water equations written in conservative form. A finite volume first-order numerical solver on an unstructured grid has been used to integrate the equations.