What to know - Large dams
A dam is a hydraulic structure designed to bar a watercourse that creates a reservoir by blocking its natural course.
In Italy, there are currently more than 500 large dams, which are structures with a height of more than 15 meters or can contain a volume of water of more than 1 million cubic meters (for example, the Cantoniera dam in the Province of Oristano). In addition, in Italy, there are thousands of smaller dams.
Dams are crucial to the management and sustainable use of water resources. In Italy, dams are primarily used to produce hydroelectric energy, irrigate farmland, and as drinking water reservoirs.
They also fulfill another important function: they store water during rainfall and then release it progressively over time to control river levels, limiting downstream flooding.
Dams are growing in importance as we experience more frequent alternating periods of drought and heavy rainfall due to climate change.
The operators of Italy's large dams are public or private subjects that have obtained concessions from Regions and Autonomous Provinces.
Large dams are under state supervision through the Directorate-General for Dams and Water Infrastructure (DG Dams) of the Ministry of infrastructures and transport (MIT) for the safety and protection of public security.
Large dams may be affected by intense natural events (such as rainfall, landslides, earthquakes, etc.) or structural problems.
Some events, such as heavy and prolonged rainfall, may require the activation of spillway operations: these are common scenarios that are part of the ordinary management of the reservoir. Other events, such as large landslides or structural damage, may affect the safety of all or part of the structure; these are rare situations but of considerable impact.
In both cases, flooding of downstream areas bordering the watercourse may occur. Such flooding can vary in severity: it is generally more rapid and intense in areas near the dam and becomes less severe as one moves further away.
Following the introduction of legislation concerning safety measures in facilities in the 1920s, catastrophic events in the territories downstream of dams have become increasingly rare in our country. The last significant event was in 1963, involving the Vajont dam at the border between Friuli-Venezia Giulia and Veneto.
To guarantee the safety of the dam and downstream territories, specific procedures are established that require the activation of the National Civil Protection Service. In particular, different alert phases are foreseen to signal increasing severity that may result in the flooding of downstream areas.
The most severe phase is the "collapse" phase, which is declared when there is evidence of structural damage to the dam, landslides resulting in uncontrolled water discharge, or other phenomena that can lead to a catastrophic event, with loss of life or significant damage.
The dam operator is in charge of promptly informing all authorities involved for the activation of the National Civil Protection Service. The Region coordinates what needs to be done, but the Civil Protection Department will intervene if the scenario is extraordinary. The mayors activate municipal plans, inform citizens about risk scenarios, and decide on the actions to protect the population.
A Civil Protection Document and a Dam Emergency Plan must be drafted for each large dam.
The Civil Protection Document addresses the actions to be implemented by the operator to activate the civil protection system. It is prepared by the branch of DG Dams of the Ministry of infrastructures and transport, in cooperation with the competent hydraulic authority, the regional civil protection, and the operator, and is approved by the competent Prefecture.
The Dam Emergency Plan (DEP) is drafted and approved by the Region or Autonomous Province in cooperation with the Prefectures territorially concerned. The Plan outlines procedures for managing flooding caused by spillway operations and the eventual collapse of the dam.
Municipal Civil Protection Plans acknowledge the information in the Dam Emergency Plans and include the areas at risk of flooding.
Near the watercourses downstream of the dams, signs generally indicate the areas most affected by the danger of flooding, including flash floods, due to spillway operations. Each dam is also equipped with an acoustic siren activated during discharge operations to warn people who may be in the area of the approaching flood.
The Directorate-General for Dams of the Ministry of infrastructure and transport (MIT) monitors the safety of dams, starting with the engineering, construction, testing, and commissioning phases of new facilities, and periodically performs tests to re-evaluate their stability from a hydrogeological-hydraulic and seismic perspective.
Non-structural prevention activities, such as warning and planning, support these structural activities.
The IT-alert public warning system is activated upon declaration of the "collapse" phase to warn the population of areas downstream of a dam that a flash flood may occur. IT-alert message is then sent to the mobile phones of the potentially affected population to inform them of the possible danger. Therefore, it is essential to know right away what to do.
The downstream flooding of a dam is preceded by specific alert phases during which the National Civil Protection Service is activated.
In some cases, the period for warnings may be so narrow that it is impossible to warn the population. Therefore, it is essential to know:
- if the municipality where you live, work, or stay is downstream of a large dam and, in particular, if the area where you are located is at risk of flooding
- in the proximity of watercourses, downstream of large dams, signs are generally placed warning of areas potentially exposed to the danger of flooding due to spillway operations
- in the event of a large dam collapse, an IT-alert message will be sent to the population residing in municipalities downstream of the structure that could be affected by the flood. Remember that the IT-alert system has some limitations. For more information, go to www.it-alert.gov.it/en
- each large dam is equipped with an acoustic siren activated to warn people of the occurrence of a flood wave, following the activation of discharge operations
- during a flood, water can rise suddenly, even by one or two meters in a few minutes
- some areas flood earlier than others due to their proximity to the dam and the watercourse
- at home, the most dangerous areas are basements and ground floors; outdoors, underpasses, areas near embankments and bridges, highly steep roads, and generally all areas lower than the surrounding area are most at risk
- the power of water can also damage buildings and infrastructure (bridges, rorckfill platforms, embankments), and the most vulnerable ones could suddenly fail or collapse
- the safety of a building depends on many factors, for example, the type and quality of materials used in construction, the altitude at which it is located, the distance from the large dam and watercourse, the number of floors, and the more or less direct exposure to the impact of the wave.