It is well known that floods in the Mediterranean area are usually flash floods (Gaume et al., 2009; Braud et al., 2014; Llasat et al., 2016), associated with very heavy rains with a short duration. In general, they cause local damage in coastal populations or mountainous regions, which can sometimes be very serious. Other times they can affect large regions, as happened with the Gard floods (France) in 2002 (Braud et al., 2010). Most of these episodes do not appear in the best-known flood databases such as EM-DAT (https://www.emdat.be/, last access: 21 July 2024) or Munich Re's NatCatSERVICE (https://www.munichre.com/en/solutions/for-industry-clients/natcatservice.html, last access: 21 July 2024), because these databases are often based on indirect information (i.e. from the insurers that Munich Reinsurance reinsures), so many events are not included, either because the insurers are not aware of the events or because they are not considered to have had sufficient impact (Llasat et al., 2013a; Wirtz et al., 2014). For example, there are very few such episodes that have affected the Pyrenees mountain region in these databases. More systematic and precise studies are therefore necessary, but the high workload required to elaborate these studies means that they are only available for few regions. This is the case of INUNGAMA (Llasat et al., 2014), which includes all the flood events that have affected Catalonia (NE Spain) between 1900 and 2020 (partial information available in the Flood Observatory of Catalonia, https://experience.arcgis.com/experience/484172e12fae4cbb934441203ee04e36/, last access: 21 July 2024), and FloodHYMEX (Llasat et al., 2013b, 2020), which currently includes all the catastrophic flood episodes (following the criteria of “catastrophic” introduced in Barriendos et al., 2003) that have affected Catalonia, the Valencian Community and the Balearic Islands, in Spain; the former Languedoc-Roussillon region, Midi-Pyrénées and PACA (Provence-Alpes-Côte d'Azur) in France; Calabria, in Italy; and all of Greece (available at https://mistrals.sedoo.fr/catalogue/, last access: 21 July 2024).

On the other hand, concern about the impact of climate change in mountainous regions, including natural hazards, has grown significantly in recent decades. Proof of this is the increase in publications on this topic (i.e. Beniston, 2003; Beniston and Stoffel, 2014; Zimmermann and Keiler, 2015; Steiger et al., 2022) including a cross-chapter devoted to mountains in the Sixth Assessment Report of the IPCC (2022). There are, however, few studies that address mountain massifs in their entirety from a climatic or meteorological perspective. This is the case of the Pyrenees, a cross-border mountainous region between Spain, France and Andorra. To facilitate the international management of a massif distributed between three countries, the Pyrenees Working Community (CTP) was created, which in turn founded the Pyrenean Climate Change Observatory (OPCC, https://www.opcc-ctp.org/en, last access: 21 July 2024), which promotes the observation and research on climate change from a multidisciplinary approach. Given that the Pyrenees are key in the generation of water resources in the surrounding regions, where more than 20 million people live, as well as in the production of hydroelectric energy, the OPCC promoted the PIRAGUA project financed by the European call for projects POCTEFA (https://www.opcc-ctp.org/en/piragua, last access: 21 July 2024) and whose results are available to the public in the OPCC Geoportal (https://www.opcc-ctp.org/en/geoportal, last access: 21 July 2024) and the publications of Beguería et al. (2023a, b). Among these results, noteworthy are those obtained from trend analysis. Clavera-Gispert et al. (2023) show that in autumn there is a predominance of significant negative trends throughout the mountain range, mainly in September, for low (P10) average (P50) and high flows (P90), for the period 1980–2013. A similar predominance is observed in summer, while only a relevant significant positive trend is observed in the western part (Basque Country, Spain), in the month of March. The decrease in flow throughout the mountain range is more clearly manifested when the period is reduced to 1990–2013. This negative trend becomes more pronounced, even on an annual scale, when analysing projections of annual precipitation for the middle and end of the century, especially in the western part of the Pyrenees, while in the eastern part, especially in Catalonia (Spain), an increase is observed, which will also be reflected in the average annual flows (Beguería et al., 2023a). CLIMPY, another project from the OPCC, concluded that the projections for the next 70 years do not show a significant trend in the heavy precipitation index (CP95) at the mountain range scale; only a certain decrease could be detected in the south-eastern slope (Catalonia, Spain) and a certain increase in the northern and western parts (French basins and Navarre and the Basque Country in Spain) (Amblar-Francés et al., 2020). In this context the question arises about extremes, specifically, about floods.

One of the objectives of PIRAGUA was the analysis of floods in the entire Pyrenean region, where flash floods can have a great direct impact on both the fixed and floating population, as well as on water services and energy resources. Some examples are the catastrophe at the Las Nieves campsite, in Biescas (Aragon, Spain), on 7 August 1996, in which 87 people drowned (García-Ruiz et al., 1996); the June 2013 floods with catastrophic damage in Spain and France (https://hepex.org.au/flash-floods-in-the-french-western-and-central-pyrenees-17-19-june-2013/, last access: 21 July 2024); and the floods produced on 7 November 1982 (Trapero et al., 2013) that affected the three Pyrenean countries. The recent death of two people who were canyoning in the Pyrenees of Aragon (Spain) on 2 September 2023, when a flash flood event occurred, is another example of this type of event that most people are unaware of. However, until now there has been no database or flood catalogue that specifically covers the Pyrenean regions, no less the Pyrenees massif as a whole. For this reason, in the same way that the final objective of FloodHYMEX (Llasat et al., 2013a, b) was to cover the entire Mediterranean region, it was decided to create a similar database for the Pyrenees but one that includes all types of flood episodes. The aim of this article is to present the first systematic database of flood episodes covering the entire Pyrenees massif, for the period 1981–2015, analyse the trends in light of the most recent articles (Clavera-Gispert et al., 2023) and characterise the weather types favourable to these flood events. This database is available at http://hdl.handle.net/10261/270351 (last access: 21 July 2024) (Llasat et al., 2022). Following the introduction of the study area, the structure of the base, the criteria followed for its construction and the sources of information used are shown. The spatial and temporal distributions of flood episodes, both in the massif and the administrative regions, as well as the weather types, are then analysed. The paper ends with conclusions and discussion, where some adaptation measures are also commented.

Figure  summarises the monthly distribution of the number of flood events for each of the regions. In the case of the Catalan Pyrenees, most events are concentrated between September and November, while in Occitanie the October–November period stands out. In the Aragon Pyrenees, the August maximum stands out, extending the flood period until November. The summer season is also very significant in Andorra, with maxima in July and August. In Navarre, the events are distributed throughout the October–July period, with no remarkable month. This distribution differs from that of Nouvelle-Aquitaine, with a greater concentration from May to June, and from that of the Basque Country, with two different periods: June–August and January–March. This distribution leads us to believe that the regions with maxima in autumn are due to Mediterranean disturbances, while the maxima in spring and winter are associated with cold fronts, and summer encompasses convective events. In the same figure the solid line shows how the monthly evolution in the total number of episodes (an episode that affects several regions is considered as a single episode) is unimodal, with a maximum in summer (42 % of the total number of annual events) that would be associated with flooding by very intense and local convective rains. This is confirmed by the intra-annual evolution of the number of flood episodes, excluding ordinary ones (see the dashed line in Fig. ), in which the summer maximum disappears to give way to a bimodal distribution with maxima in June and October. The unimodal distribution bears a considerable resemblance to the distribution of the percentage of convective precipitation throughout the year obtained for the Spanish Mediterranean region (Llasat, 2001; Llasat et al., 2021). In these 36 years, the highest number of victims occurred in August. Generally, these are individuals who engage in risky sports, although the high number of deaths is due to the Biescas disaster. The number is also relatively high in November and is associated with longer-duration events.

The evolution of the flood events that have produced notable damage (extraordinary and catastrophic ones) in the different regions of the Pyrenees shows high variability. The trend analysis on a regional scale is only significant at 95 % according to the Mann–Kendall test for Nouvelle-Aquitaine, which shows an increase of 0.34 events per decade that is due to the increase of extraordinary events. For informational purposes, although they are not significant at 95 %, positive trends were also observed in Catalonia, Navarre, Occitanie and Andorra, although 0.1 events per decade is only exceeded in Catalonia and Occitanie. On the contrary, in Aragon and the Basque Country, the trend is negative, but it does not reach -0.1 events per decade. Taking the Pyrenees massif in its entirety, the trend of notable flood events is positive and equal to 0.5 events per decade, but it is not significant at 95 %. On the other hand, if ordinary episodes are also considered, the trend of flood events for the entire Pyrenees increases until 0.84 events per decade, but it is only significant at 90 % (Fig. ).

Of all the recorded flood events, 41 affected two or more Pyrenean regions (Table ). Of these, 34 were international cross-border, highlighting the episodes that affected Spain and France (71 %), while 24 % affected Spain and Andorra simultaneously, and 5 % affected all three countries (Table ). The most frequent are episodes that simultaneously affect two or more of the regions of Catalonia, Aragon, Occitanie and Andorra: 41 % of the transregional episodes took place between Catalonia and Occitanie, while 24 % were between Catalonia and Aragon and another equal percentage between Occitanie and Nouvelle-Aquitaine (Table ). They occur mainly in autumn (Fig. ), which is consistent with the fact that it is in this season when the entry of organised Mediterranean disturbances is produced, usually associated with the presence of a depression in the Mediterranean and favoured by a warm sea (Llasat, 2009). On the contrary, between January and April practically no transnational episodes are recorded, and none has been recorded in February, which points to more localised events.

Table  shows the list of cross-border flood events. As an example, Fig.  shows the municipalities that were affected by the floods of November 1982. The heavy rains began on 6 November in Spain and extended to Andorra and France on 7 November, ending the following day. They had a particular impact on the Eastern Pyrenees. The maximum total rainfall was 610 mm in Py (France), 556 mm in La Molina (Spain) and 203 mm in Soldeu (Andorra) (Fig. ). As a result of the floods, 14 fatalities were recorded in Catalonia, with damage valued at EUR 994 million (2015) (Catalonia), EUR 88 million (2015) (Aragon) and EUR 132 million (2015) (Andorra). This flood event is one of the most extensive and intense of the 20th century in southwestern Europe. In it, the mountains played a very relevant role, favouring the orographic ascent and the release of instability (Trapero et al., 2013). The large amount of precipitation that occurred was due to the feeding of water vapour from remote sources, such as the subtropical Atlantic (Insua-Costa et al., 2019).

Figure  shows the 12 weather types (WTs) obtained from the combination of mslp and z500 and explaining practically 80 % of the variance, corresponding to the flood episodes that affected the Pyrenees between 1981 and 2015. First of all, it should be noted that there is no significant trend in any of the WTs. The WT 1-, characterised by a marked depression to the northwest of the Iberian Peninsula and a talweg, is the most characteristic of the episodes affecting the Eastern and central Pyrenees (Fig. a) in autumn (Fig. b). This favours the advection of warm and humid air from the Mediterranean in low levels and feeds humidity from remote sources. This was the case during the November 1982 episode (Llasat, 1987) and is the case in a large number of the episodes of intense rainfall that take place in the western Mediterranean (Insua-Costa et al., 2022; Miró et al., 2022). Note in Fig. b that this type of weather shows its maximum frequency in the month of October, the month with the most episodes of flooding in the Pyrenees. The WT 1+ favours the entry of Atlantic air over the western Pyrenees and, like the WT 1-, can lead to the passage of highly organised disturbances typical of winter and autumn (Fig. b), with notable rainfall accumulations in the western and northern part of the Pyrenees (Lemus-Canovas et al., 2019b) where most of the floods are recorded (Fig. a). In fact, the highest frequency is recorded in January (Fig. b). On the other hand, the dominant weather type in summer is WT 2- (Fig. b), which shows in the slight wave over the Iberian Peninsula, while on the surface the situation is relatively unclear. It would therefore be a situation favourable to isolated convection or poorly organised weather systems, typical of that time of year but which can also result in some episodes of very intense rainfall, as also described in Lemus-Canovas et al. (2021). This is the situation that characterised the episode of Biescas (Aragon), which occurred on 7 August 1996, and the floods that affected the Basque Country and Nouvelle-Aquitaine in August 1983, mainly concentrated in the southern half of the Pyrenees (Fig. a). The WT 3+ shows the formation of a mesoscale depression off the coast of Catalonia that results in instability and the E–SE air flow over the Eastern Pyrenees (Fig. a). The highest frequency is recorded in spring, specifically in June, a month that, as we have already seen, comes after October in terms of flood frequency (Fig. b). An example of this would be the floods of June 2013. These northern movements are usually associated with summer floods and can affect any area in the Pyrenees. Finally, it can be observed that in November the WT 4+ and WT 5+ dominate (Fig. b), both with a very marked groove over the Iberian Peninsula that will favour the vorticity to the east and the contribution of air flow from the Atlantic that feeds intense rainfall, especially in the easternmost sector of the mountain range (Lemus-Canovas et al., 2019c). These WTs are mainly associated with floods in the eastern part of the Pyrenees (Fig. a). An example is the episode of November 2005, which had a serious impact on Catalonia.

The types of weather associated with cross-border episodes depend on the time of year in which they occur. Of the eight cases recorded between June and August (Fig. ), seven have been characterised by WT 2-. In September, types WT 2- and WT 3- dominate. In October and November, which have recorded 16 common episodes, there is no dominant type of weather, and it is even the case that unusual types appear, such as WT 4-, characterised by a deep depression to the west of the British Isles that extends up to 500 hPa and even more, with a talweg that crosses the Iberian Peninsula from northwest to southeast.

In this article, the first flood database integrating the entire Pyrenees massif and available to the public has been presented (http://hdl.handle.net/10261/270351, last access: 21 July 2024). This database, PIRAGUA_flood, includes, at a municipal scale, all flood events that have affected each of the 1803 municipalities comprising this cross-border region between 1981 and 2015, of which 609 are Spanish, 1187 are French and 7 are Andorran. Each event is characterised based on a scale that considers the impacts, providing information on these, including the number of victims, and in the Spanish case compensation paid by the Insurance Compensation Consortium. Hydrometeorological information is also included. The database has been constructed using information from press sources, official lists provided by various public agencies, scientific studies and rainfall analyses. In the case of Catalonia and Andorra, systematic daily-scale information on all types of flood events, including those with lesser impact, has been available. For other regions, it is possible that some of these minor events may have gone unnoticed. However, since this information is typically difficult to obtain from any other source, it has been decided to maintain it. Additionally, these types of floods are becoming more common due to urbanisation of the territory.

If we take into account that some events can affect more than one region, the number of events that have produced floods in the Pyrenees between 1981 and 2015 has been 181, of which 128 affected Spain, 43 affected France and 46 affected Andorra. Of these, 34 were cross-border. The November 1982 catastrophic flood event was the most extensive and affected the three countries. The other very extensive episode took place in June 2013 and affected Spain and France. The region with the greatest number of flood events has been Catalonia (66 episodes), although only 4.5 % produced catastrophic damage to the massif. Andorra follows, with a total of 46 events, of which 4.3 % were catastrophic. However, the one that records the most serious events in terms of percentage is Nouvelle-Aquitaine, since, of the 17 events recorded, 35 % were catastrophic. Note that, if the ordinary events are not taken into account, Catalonia is still the one with the most events (29), followed by Aragon (26) and Occitanie (24). It follows that the eastern part of the Pyrenees was the most affected by flood episodes, some which had catastrophic impacts.

The economic impacts are not available in France and Andorra, and in the case of Spain, only the compensation payments by the Insurance Compensation Consortium between 1996 and 2015 are known. In total, the CCS paid EUR 142.5 million (2015) for flood damage in the Spanish Pyrenees. For this period, and despite being in the third position in terms of the number of events, Navarre is the region that received the most compensation in terms of damage for events, with a total of EUR 65.8 million (2015). This high amount compared to Catalonia, which recorded more than twice as many events, is explained as Pamplona (the capital of Navarre) is within the Pyrenees region and received payments worth EUR 18 million (2015) in 2015. In this case, the risk of flooding is greater due to the high exposure, despite a moderate hazard compared to other Pyrenees regions. Something similar could take place in Catalonia on a regional and municipal scale. Although Val d'Aran was not as affected by floods as other Catalan regions, it is a very rich region, with a municipality, Vielha, to which the CCS paid more than EUR 5 million (2015) for damage caused by floods from 17–20 June 2013. The latter was the most expensive episode since 1996, with an amount worth EUR 27 million (2015) claimed in the Spanish Pyrenees. However, including the non-Pyrenees part of Aragon and Catalonia, this amount rose to EUR 31 million (2015), and in France (mainly Occitanie) it rose to EUR 240 million (2015) in insurance compensation alone (public works are considered to have exceeded this amount). The most expensive episode in Spain since 1981, however, if we take into account all of the autonomous communities affected, was 25–28 August 1983, which resulted in a total of EUR 2.9 billion (2015) in compensation paid out in the Basque Country, Navarre and Cantabria. This high amount is a consequence of the serious damage suffered in the Basque Country, mainly in industrial areas.

The number of victims is closely linked to specific cases, such as the floods of August 1996 in Biescas and the flooding of November 1982, in which the majority of the deceased were swept away by water in their home or summer residence. The Biescas case is an example where all components contributing to the risk were high. It involved a camping site located in a debris cone at the foot of a dam in an area prone to storms (high hazard), with numerous people (high exposure) who took shelter in tents or trailers, and there was no early warning system (high vulnerability). Apart from these cases, fatalities are normally people who tried to cross a river, usually in a vehicle, or people doing adventure sports. The total number of victims in the Spanish Pyrenees was 121, with 13 in Andorra and 20 in the French Pyrenees.

The flood season starts in May in the Atlantic region and progresses to reach a maximum in autumn in the Mediterranean regions. Catalonia, Aragon, Navarre and Occitanie all share the month of October among the months that record the highest number of events. The Basque Country and Nouvelle-Aquitaine share the month of June in common, while in Andorra is summer. If we only take into account events that caused notable damage (extraordinary and catastrophic episodes according to the nomenclature agreed here), a positive trend can be observed across the entire massif of 0.5 events per decade, although it is not significant. However, if the ordinary episodes are included, the trend becomes 0.84 events per decade, and it is significant at 90 %. When analysing in terms of communities, Nouvelle-Aquitaine is the only region with a significant positive trend at 95 % (0.34 events per decade). This positive trend in Nouvelle-Aquitaine cannot be justified by the trend in the 90th percentile discharge, nor the lower percentiles, as shown in the study by Clavera-Gispert et al. (2023). To attribute this trend accurately, it would be necessary to conduct a study on more extreme discharge events and land use to associate it with an increase in vulnerability, exposure, or hazard. On average, projections also do not indicate an increase in intense precipitation in the Pyrenees (Amblar-Francés et al., 2020), but a recent study (Poncet et al., 2024) including the Mediterranean part of Occitanie shows that the magnitude of the most intense floods will intensify.

From the classification of weather types, it can be concluded that most are dominated by southern component flow over the Pyrenees region, with a talweg on the Iberian Peninsula. There is a frequent depression in the vicinity, either in the Atlantic or in the Mediterranean, which is in line with the studies carried out on intense rains in the Mediterranean region (according to the IPCC classification (2022), the Pyrenees massif would be included in the region) (Jansa et al., 2014). This type of configuration favours vorticity advection at the medium and upper part of the troposphere and the consequent instability at low levels, the advection of warm and humid air, and, occasionally, the contribution of humidity from remote sources. No specific weather type is observed for cross-border episodes.

This study also has an important social objective. The aim is to make the population living in the Pyrenees and who practise hiking or any other mountain sport aware that floods are also very important in this region and cause serious damage and even fatalities. As has been shown in this paper, many flood episodes are transnational, which calls for cooperation in the prevention and mitigation of flood risk between regions and countries. This is more relevant considering the United Nations call for an “Early Warning for All” to ensure that everyone on Earth is protected from natural hazards, including flood events through life-saving early warning systems by the end of 2027. Furthermore, in Europe there is a strong collaboration framework, thanks to the ESA Copernicus observation programme, MeteoAlarm from EUMETNET (European Network of National Meteorological Services) and Euromed regional Programme for Prevention, Preparedness and Response to Natural and Man-made Disasters (PPRD), in which all European civil protection agencies participate. However, to achieve the full potential of these efforts, it is important to enhance their implementation in specific cross-border areas, such as the Pyrenees. The Life PYRENEES4CLIMA Project (https://www.pirineos-pyrenees.eu/en/pyrenees4clima-projet-life/, last access: 12 September 2024) is designed to meet this challenge.