The retreat of the Llobregat delta

Translation into English of an article from the Taula del Llobregat

The Llobregat delta has retreated 800 metres in a century. A study by the Universitat Politècnica de Catalunya reveals that the width of the river at its mouth was 275 metres in 1846 and is currently 36 metres.

Scientists from the Universitat Politècnica de Catalunya reveal surprising data on the Llobregat river and its delta, in a study published in the journal Natural Hazards and Earth System Sciences. The most significant data is the quantification of the delta's retreat in just over 100 years. Using geolocated historical maps, they determined that the Llobregat delta, at its mouth, retreated 800 metres between 1891 and 2000, before the diversion of the mouth for the expansion of the Port of Barcelona.

These data are relevant because it has never been calculated with this level of precision. "This retreat is very important. We knew that sand was being added to the Llobregat delta because it was retreating, but the extent of this retreat was unknown," explains Arnau Prats, a researcher at the UPC and one of the co-authors of the study.

The scientists started with a hypothesis that was not subsequently confirmed. "The hypothesis we put forward was that infrastructures, civil engineering works, the AVE railway, dams, but above all the motorway and the expressway, were responsible for this modification of the lower course of the Llobregat", explains Carles Ferrer, professor and researcher at the UPC and one of the three authors of the study.

Les universitaires ont eu une surprise : "La voie rapide et l'autoroute ont été construites il y a 50 ans et l'échelle de temps du recul du delta est de plus d'un siècle", souligne Juan P. Martín Vide, également enseignant et chercheur à l'UPC et troisième co-auteur de l'étude. Il fallait donc chercher les causes ailleurs.

The Llobregat lock dams and the erosive action of the sea, main causes of the delta's retreat

The authors of the study consider that the reservoir dams of La Baells, La Llosa del Caballo and Santo Ponç are not the main cause of the delta's retreat because they are located several kilometres from the mouth and "the transport of sediment is slow", according to researcher Arnau Prats. He explains that "the transport of coarse sediments from the bottom of the river can take decades to arrive from Baells to the mouth".

The three authors of the study -Juan P. Martín Vide, Carles Ferrer and Arnau Prats- point to the lock dams as the main cause of the delta's retreat. They were built throughout the 19th century with the aim of exploiting hydraulic energy. These infrastructures, as well as the different channels in the last river section, have reduced the contribution of coarse sediments to Prat de Llobregat, which are the ones that make the delta move forward. In addition, the action of the waves has eroded the land that the river had gained on the sea.

On the other hand, the researchers found that when there is a flood at the headwaters of the river, the marshes reduce the supply of sediment because they reduce the water flow by 20%. And this reduction has an effect on the transport of sand and gravel. "We have found that the reservoir dams cause, on average, a 20% reduction in the peak flow of the Llobregat at the river mouth, and that this 20% reduction leads to a 40% reduction in the transport of sediments from the river. Nevertheless, the impact of this inertia is still limited in the delta. According to Professor Juan P. Martin Vide, the effect "will be more important in a few decades".

Eliminating channelling to reverse the trend

Scientists do not foresee miracle solutions but a change in the trend. "We can't talk about solutions like in other areas of life or engineering, where everything is solved once the remedy is known and applied. Here, we should rather think of changes in trends", explains Juan P. Martín Vide.

They believe that the measures that can be taken will take a long time to become visible. "We have to keep in mind that these lock dams are located several kilometres upstream from the delta. Therefore, any action taken now would take a long time to be noticed," adds Carles Ferrer. He suggests: "In addition to the lock dams, we could also think about the small streams, tributaries, etc. that are channelled through concrete dikes, which also limit the sediment supply to the delta and are closer to the mouth. We could think about removing these channels which, in the long term, could help to increase the sediment supply from the Llobregat to the delta.

The removal of these channels and lock dams is the long-term solution proposed by the study, while the traditional provision of sand on the beaches of the delta is a short-term and ephemeral measure.

The new river mouth, a sediment trap

The Llobregat is one of the rivers that have been intensively exploited since the industrial revolution in Catalonia.

From its source in Castellar de n'Hug to its mouth in Prat de Llobregat, there are 170 kilometres of reservoir dams at La Baells, La Llosa del Caballo and Santo Ponç, as well as numerous lock dams and channels.

In 2004, the diversion of the mouth 2 km further south than the original mouth was inaugurated, due to the expansion of the port of Barcelona. However, the infrastructure did not foresee what would happen to the sediment. "The new mouth is twice as wide as the old one. And this poses a problem for the transport of sediment from the river to the beach. During floods, when there is more sediment transport, the riverbed is wider and the water velocity is therefore lower. As a result, the material transported by the river is trapped at the bottom and reaches the coast less. And this fact is not without consequences: "What happens is that this bottom increases because of the accumulation of sand that comes from upstream and is not deposited on the coast. And this, in the long term, can be a problem because the hydraulic capacity of this channel can decrease due to the rise in the bottom," concludes researcher Arnau Prats.

The "hardest working" river

Since the beginning of the 19th century, the Llobregat River has been equipped with lock dams and channels to supply the textile factories born of the industrial revolution. So much so that the French geographer Pierre Deffontaines (1894-1978) said: "Perhaps no river in the world has been so intensively exploited as the indigent Llobregat".

Sources :

https://beteve.cat/medi-ambient/retroces-delta-riu-llobregat/

https://www.sostenible.cat/reportatge/el-preu-de-la-revolucio-industrial-al-llobregat