The human factor in the frequency of torrential flooding

Torrential flooding is a phenomenon with wide-reaching social and economic effects. Understanding the process which causes them and the periods of recurrence can be crucial for preventing the risks they entail, and also for the integral management and care of the water basins, particularly the small ones in the mountains about which there is little or no information available.

The Portainé water basin, located in the central part of the Parque Natural de l’AltPirineu (Lleida), at the top of which is the Port Ainé ski resort, shows active signs of erosion and deposition caused by torrential flooding. As is the case for most mountain basins, there are no metering stations and no thorough documents about gauging.  We do have the data from the building work on the ski resort, which started in 1970 with the building of the first stretch of the access road to a height of 1,600m. The station was opened in 1986 and in 1995 the access road was finished, up to a height of 2,000m. We have consulted different reports on other infrastructure works (canals under the road and drainage systems), which were insufficient to prevent the damage arising from 2006 onwards due to the build-up of floods on the road, which crosses the gorge at several points, and the resulting economic losses.

With a view to finding out whether there have been recent changes in the basin´s flood dynamic, and also the possible link with the different infrastructure work, we carried out multi-discipline research into different geological, geomorphic, hydrological, documentary and dendrogeomorphological aspects. Following an intensive examination of the basin, we chose the alluvial fan formed by the Portainé gorge where it meets the river Romadriu for dendrogeomorphological analysis.  This kind of analysis, which combines dendrochronological dating using tree rings with geomorphic data, allows us, among other things, to date the damage produced by torrential flooding to the vegetation on the banks.

Between 2014 and 2015 several samples and dendrochronological data were taken in the mixed, deciduous forest of many different species which live on the alluvial fan; dominant species include poplars (Populus tremula L. and Populus nigra L.) and mountain ash (Fraxinus excelsior L.). Other common species are cherry (Prunus avium L.), sessile oaks (Quercus petraea (Matt.) Liebl.), limes (Tilia platyphyllos Scop.) and walnut (Juglans regia L.). Here the trees acted as an obstacle to the flood currents which overflowed from the main channel, and had debarking and damage from different dates, the result of the impact of materials dragged along by the water (rocks, tree trunks, sand and others…).

We analysed a hundred and sixty-six dendrochronological samples (wood cores which contain the growth rings) from sixty-seven carefully geo-referenced trees of ten different species, in order to identify and date the effects of damage caused by flooding over the past 50 years. By measuring and dating the temporal sequence of the width of the rings, we can use these samples, along with qualitative and quantitative analysis of this data, to date the damage and identify anomalies in growth due to the damage, such as, for example, sudden changes in the width of the rings or a lack of rings. Other aspects like the age of the trees or the date of the loss of the main branch («decapitation») are also significant evidence of previous flooding. Using all this data, we estimated the distribution over time and space of different torrents in the area under study.

We were able to date reliably at least ten years, in the period 1969-2010, with evidence of damage to the trees due to the effects of torrential floods. The spatial distribution of the damage was almost uniform along the stretch of the river under study, except in certain years when there was a relatively significant increase in the lower part of the alluvial fan. The average frequency estimated for the flooding during this period was 4.5 years. However, as from 2006, both the dendrogeomorphologic evidence and the documented damage to the road there was a much shorter recurrence interval between floods (ten torrential floods in nine years, at an average frequency of less than one year), and this was not due to an increase in the intensity and frequency of rainfall in the area according to the sources consulted.

All of this supports the hypothesis that the human factor (in this case the anthropic activities stemming from the operation of the ski resort) has affected the geomorphologic stability of the basin and, thus, has modified the hydrological response, changing the frequency and magnitude of the torrential floods. Recent human intervention has taken place at the head of the gorge, in the area where the skiing pistes are, and in particular, the most recent ones have favoured superficial flow at the expense of infiltration causing an increase in the concentrated flow volume reaching the gorge. This conclusion should have important implications for territorial planning and the design of future work in the hydrographical basins in the mountains.

Our research has opened a new source of knowledge about the torrential dynamics in mountain water basins and the link to human activity. The analysis of damage to trees may be the only palaeo-hydrological source of information about previous torrents and floods. Furthermore, as regards methodology, the use of a large number of different species and different dendrogeomorphological evidence has proved to be applicable, efficient and reliable. However, it has been the combination and integration of different sources of data, together with the dendrogeomorphological and, in particular, geomorphologic and documented data, which has offered the best method and allowed us to find reliable interpretations of the recent changes in the Portainé hydrographical basin.

Mar Génova Fuster is full Professor at the Universidad Politécnica de Madrid. As well as teaching and researching at the Escuela Técnica Superior de Ingeniería de Montes, Forestal y del Medio Natural, focused on Botany and Geobotany, she has taken part in different research teams analysing and evaluating the information provided by the ring widths in trees.

Further reading:

Furdada G, Victoriano A, Díez-Herrero A, Génova M, Guinau M, De las Heras A, Palau RM, Hürlimann M, Khazaradze G, Casas, JM, Margalef A, Pinyol J, González M. 2020. Flood consequences of land-use changes at a ski resort: Overcoming a geomorphologic threshold (Portainé, Eastern Pyrenees, Iberian Peninsula). Water12: 368. 

García–Oteyza J, Génova M, Calvet J, Furdada G, Guinau M, Díez-Herrero A. 2015. Datación de avenidas torrenciales y flujos de derrubios mediante metodologías dendrogeomorfológicas (barranco de Portainé, Lleida, España). Ecosistemas 24: 43-00.

GénovaM, Díez-HerreroA, Furdada G, GuinauM,  Victoriano A. 2018. Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé basin, Spanish Pyrenees). Tree-Ring Research 74: 144-161. 

Génova M, Furdada G, Guinau M. 2023. Dendrogeomorphological evidence of flood events in the upper catchment of the Noguera Pallaresa river (Pyrenees, Spain). Pirineos, 178, e080.