What floods and earthquakes cost

What we use

• Flood areas of the official plans (ISPRA, national map of flood hazard): how often water can arrive. Area P3 about once every 30 years, P2 every 150, P1 every 500. Licence CC BY-SA 4.0.
• European river flood maps from the European Commission's Joint Research Centre (Copernicus): how much water reaches each spot, for nine different frequencies. Licence CC BY 4.0.
• JRC damage curves for European homes: how much of the building is ruined by 50 centimetres of water, by one metre, by two. Licence CC BY 4.0.
• MPS04 seismic hazard curves from INGV: how strongly the ground can shake at every spot in Italy, and how often. Licence CC BY 4.0.
• European seismic risk model ESRM20 (EFEHR): which kinds of building each municipality has (masonry, reinforced concrete, how many storeys, from which period), how much each one is damaged by each level of shaking, and what the ground is like. Licence CC BY 4.0.
• Istat construction cost index for a residential building: brings the cost of rebuilding a home up to today. Licence CC BY 4.0.

How we use them

The report works out the average damage a year for a reference home of 100 square metres: the sum of every possible damage, each weighted by how likely it is. In almost every year nothing happens; now and then a lot happens. The average is the figure to set beside the premium of a natural disaster insurance policy.

• Floods: if the address is in a flood area, the frequency comes from the official map and the depth of water from the European model. The European model only follows large rivers: where it has no data, the report gives a range between 20 centimetres and one metre of water and says so. Outside the flood areas the flood damage is zero, even where the European model, which does not know about embankments, would have water.
• Earthquakes: for each kind of building in the municipality we add up the damage from every possible level of shaking, with the official hazard at the spot and the ground amplification of the 2018 Italian building code. The range goes from the sturdiest to the most fragile homes in the municipality, because open data do not say how a single home is built.
• Cost to rebuild: the one from the European model (1,300 euros per square metre in Milan and Rome, 1,000 in towns over 10,000 people, 900 elsewhere, 2020 prices) brought up to date with the latest Istat index.

The method is that of the European model ESRM20. We compared it region by region with the results ESRM20 publishes: the order of magnitude matches, and in Emilia-Romagna the average is the same (1.74 per thousand against 1.72). The differences come from the hazard: we use the official Italian one, ESRM20 the European one.

Limits

• It is an average over many years, not a forecast: it does not say whether damage will come, or when.
• Only the building counts, not furniture, cars or businesses, and not people.
• The kinds of building are those of the municipality (2011 census): a new earthquake-resistant home sits towards the bottom of the range, old masonry towards the top. For a single home you need the year it was built and a check by a qualified engineer.
• Landslides are not in the figure: no open model says what they cost on average. The report says whether the ground is in a landslide risk area in its own section.
• Where the national earthquake model has no data (Sardinia and a few small islands, where strong shaking is very rare) the report does not work out the earthquake part and says so.

Official pages: ESRM20, JRC flood maps, JRC damage curves, INGV's MPS04, Istat construction cost index. See also Landslides and floods and Earthquakes.