What the maps did not show: dwellings inside the observed extent of the October 2024 DANA in Valencia, and the official flood maps

The question. How many dwellings lay inside the observed extent of the DANA of 29 October 2024 in the province of Valencia? How many of them lay outside the official flood maps? And how many were built after Real Decreto 638/2016 limited building in flood zones?

• The official maps. The State's hazard maps for the areas of significant potential flood risk (ARPSI, RD 903/2010; 10, 100 and 500 years) and the Generalitat's PATRICOVA.

The answer, as a range. About 39–66% of the dwellings inside the observed extent were outside every official flood zone, depending on which flood extent and which rule is used.

The rules.

• The zone envelope. The hazard map's water-depth grid, with the dry holes that it leaves at buildings closed up to 2 ha. PATRICOVA polygons are used as published.
• The ARPSI maps alone. 67.4% of the Copernicus-extent dwellings were outside them under the reference rule. 8,435 lay inside the 100-year zone.

The extent matters most.

• Coverage. The Copernicus extent was delineated from satellite and aerial images. The Generalitat's footprint covers only the Magro and the Poyo, Saleta and Picassent ravines, yet holds 1.6 times as many dwellings. With their union (158,044 dwellings) the reference share is 42.8%.
• Like for like. In the same 66 municipalities, the share outside every zone is 51.6% with the Copernicus extent and 40.8% with the Generalitat's footprint (footprint on the depth grid; 49.4% and 38.9% under the reference rule).
• Robustness. Copernicus product versions and ±10/±25 m edge buffers move the reference figure only within 50.1–53.9%. Leaving out one municipality gives 47.7–54.9%.
• Places. The figures differ a lot by town:
• 71.9% outside in Paiporta, 95.8% in Picanya and 100.0% in Sedaví;
• at least 98% inside an ARPSI zone in Beniparrell and Massanassa;
• l'Alcúdia mapped almost only by PATRICOVA's geomorphological level, which has no return period.

Built after 2016.

• How many. 1,776 dwellings in the Copernicus extent are in buildings that the Catastro dates to 2017–2024.
• Map status. 874 of them lay outside every official zone, and 735 inside an ARPSI zone (156 inside its 100-year zone).
• 2025 or later. Another 410 are dated 2025 or later: rebuilding and new building, without distinction.
• No illegality. These are counts of exposure, not of illegality. RDPH arts. 9 ter and 14 bis allow building in flood zones on land already urbanised on 30 December 2016, under conditions that open data do not show.

The pilot. For Paiporta we recompute the scouting pilot: 11,987 of 11,988 dwellings inside the first delineation (the pilot had 11,986) and 624 dwellings in 99 buildings dated 2017 or later.

Caveats.

• Map version. The ARPSI maps read are the version of 20 December 2024, after the flood. Internet Archive captures show that the version in force on the day was dated 5 July 2022. What changed is not known.
• Depth maps, not polygons. The SNCZI zone polygons and the preferential flow zone could not be obtained: their download sits behind an ALTCHA challenge, which we did not pass. The ARPSI depth maps were read as rendered 2 m masks.
• Prior counts not searched. Press and institutional counts of damaged dwellings could not be searched.

Prior work.

• Closest work. Camarasa-Belmonte et al. (2025) found 14.1% of the flooded area outside the combined official map. With the same Universitat de València flood map, which the Generalitat's footprint starts from, the share of dwellings outside is 40.8%. They also note that the Copernicus extent «presentaba importantes problemas en áreas urbanas».
• Possible overlap. A UPC master's thesis on exposed dwellings in the metropolitan area is under embargo until 2030.
• Novelty claim. It is limited to the academic literature and the sources listed in paper.md §3.