What it means, without formulas
A 100-year flood is one that, averaged over a very long time, is exceeded once every hundred years. It does not mean it arrives punctually once a century: it may not happen for generations, or it may happen two years running. The longer the return period, the rarer the flood and the more ground the water covers on the map.
So the zones are read from the inside out: the 10-year zone is closest to the river and floods most often; the 500-year zone contains the other two and reaches further. A home inside the 10-year zone is far more exposed than one that only falls inside the 500-year zone.
Where these periods appear
Spain's Ministry for the Ecological Transition (MITECO) draws the river flood hazard maps for those three scenarios, under the EU Floods Directive 2007/60 and Royal Decree 903/2010. For each scenario they show the expected extent of the flood and the depth of the water. They can be viewed in the National Flood Zone Mapping System (SNCZI), which also publishes a 50-year zone.
Return periods are used in other technical standards with other values: the Spanish National Geographic Institute's seismic hazard map, for example, is calculated for a return period of 475 years.
What the HOUSINGFAX report tells you
Check 10, «Flood risk», crosses the building's location with the official SNCZI maps and tells you whether it lies inside the 10, 50, 100 or 500-year zones, whether it is in the preferential flow zone and, if it is outside, how far away the nearest 500-year zone is. In Alicante, Castellón and Valencia it adds the hazard level from PATRICOVA, the Valencian regional flood plan.
All with a map on aerial imagery, the source and the date of the query. Look through the sample report and check your home for free.
What the return period does not tell you
It does not say when the next flood will come or whether water would get into your flat: that depends on the floor, the access points and any flood defences. Being outside every zone does not mean there is no risk either, because that stretch of river may not have been studied. The report sums up what the official maps say and does not replace an on-site technical inspection of the building.
Short answers
What is a T100 zone?
The area a flood with a 100-year return period would cover, the medium-probability scenario on the official maps. It includes the 10-year zone and lies within the 500-year zone.
If there has been a 100-year flood, will it take another 100 years to happen again?
No. The return period is a long-term average frequency: the same flood can happen again the following year.
Where can I see which zone a building is in?
In the viewer of the National Flood Zone Mapping System (SNCZI) run by the Ministry for the Ecological Transition. The HOUSINGFAX report sums it up with a map and the distance to the nearest zone.
Is being in the 500-year zone serious?
It is the low-probability or extreme-event scenario. It does not stop you buying, but ask the owners' association about water getting into garages and ground floors and get an insurance quote for the exact address.
Official sources you can consult
The links let you review the primary source. Their inclusion does not widen their purpose or turn a piece of context into a diagnosis.