Spillway Collapse of the Montedoglio Dam on the Tiber River, Central Italy: Data Collection and Event Analysis

A large number of dams were built in Italy in the first decades of the last century to supply the needs of industrial, electric power, agricultural, and drinking purposes. Among them, the Montedoglio dam, an important reservoir on the Tiber River located in central Italy, with a drainage area of $276{\mathrm{km}}^2$ and a maximum storage volume of about $153{\mathrm{Mm}}^3$. The dam is an earth-fill structure with overfall spillway partly controlled by two sluice gates. On December 29, 2010, due to the partial sudden collapse of the spillway, a huge volume of water flooded the valley below the dam, severely damaging the territory, but luckily without causalities. Considering that scarce data are available for this type of event worldwide, this paper aims to illustrate the collected data of the studied event in terms of reservoir levels, using a discharge hydrograph observed at downstream gauged river sites and the flooded valley area. This study aims also to simulate the breach evolution and the downstream propagation of the outflow. Considering the Nash-Sutcliffe performance measure in simulation of the recorded reservoir levels, the time of breach formation and the corresponding discharge coefficient are obtained through an optimization procedure, yielding values of 0.02 h and 0.15, respectively. Finally, the Manning roughness coefficients for the downstream main channel and the floodplain areas are detected through a methodology based on one-dimensional hydraulic modeling by considering different flooding scenarios of valley.