Barcelona Field Studies Centre

Extreme weather changes the Ebro Delta

23 January 2020

The Ebro Delta has suffered one of its most serious floods in recent history. Storm Gloria, with storm waves of up to 8 metres in height and rainfall of 200 litres per square metre flooded much of the land, its lagoons and rice fields. The survival of the delta was already threatened by the loss of sediment contributed from the River Ebro and subsidence of the land through soil compaction. Approximately 47% of the delta lies only 50 centimetres above sea level with several cultivated areas lying below the level of the sea. A changing climate with more extreme weather is increasing the vulnerability and appears a growing and serious threat.

Extreme weather changes the geography of the Ebro Delta: after Storm Gloria After Storm Gloria
Extreme weather changes the geography of the Ebro Delta: before Storm Gloria Before Storm Gloria

Ebro Delta before and after Storm Gloria
Source: Sentinel satellite images processed by Copernicus EMS/Annamaria Luongo

The Ebro Delta normally covers an area of 320 km² and extends 22 kilometres into the sea but Storm Gloria has blurred the image. A total of 30 km² were buried under saline water when the sea reached 2 to 3 kilometres inland. The flood waters will gradually recede in coming weeks but the concern remains that these floods are becoming more frequent and beginning to acquire a chronic character. There are areas of the delta where the annual retreat is between 6 and 7 metres per year, and the problem is getting worse.

The IRTA, a research centre of the Ministry of Agriculture, Livestock, Fisheries and Food of the Government of Catalonia, considers that assessing storm damage caused in the delta is complex.[1] Rice fields are damaged by seawater. The salinity of this flood, however, fluctuates from seawater to freshwater depending on the proximity of the fields to the sea or the river. Many families live by mollusc harvesting and nine nurseries have been destroyed and 50 nursery owners badly affected.

Flora and fauna are also in danger. The entry of sea water into the fresh water lagoons has drastically changed the salinity conditions. Some species will adapt, but many others may disappear. The flooding of the rice paddies will take its toll on very specific bird communities with a predilection for the mouths of coastal rivers. Beyond the increased salinity of the lagoons and rice paddies, the regression of the beaches is also a problem for birds.

The lack of sediment contributed from the River Ebro is behind the erosion of the delta. There are now 187 dams on the river, and most sediments settle in front of the dams instead of reaching the delta and the sea. Regular winter river floods used to cover the land in new sediment, compensating for natural subsidence caused by compaction of the soil. These winter floods no longer occur as the waters of the Ebro River have been managed and diverted for summer irrigation of the rice fields.

Dams are not the only reason for less sediment carried by the river. Reafforestation and rural depopulation also play an important role. The period of most accelerated growth of the delta was in the 15th and 16th centuries when large-scale deforestation of the Ebro river basin for agriculture and ship building occurred. This caused widespread soil erosion which contributed large amounts of sediment to the river and its delta. The delta evolution diagram below shows how the rapid rate of growth led to the old town of Amposta, a seaport in the 4th century, becoming landlocked 25 km from the river mouth. Today, forests are spreading in almost all Western countries, protecting the soils from erosion and starving the rivers of sediments. Spain has seen one of the largest increases in tree area, gaining 33% of forest cover since 1990. In 1990 28% of Spain was forested; in 2015 the proportion was 37%.[2]

The rapid rate of growth of the Ebro Delta led to the old town of Amposta, a seaport in the 4th century, becoming landlocked 25 km from the river mouth
The evolution of the Ebro Delta

Rural depopulation has contributed to the growth of forests and reduced sediment carried by the Ebro river to the delta. Half of Spain's towns and villages carry a very high to moderate risk of being completely depopulated in the medium and long term.[3] They are losing population at an average rate of five per hour as their inhabitants continue to age and young people leave to find work and more services elsewhere. Abandoned farmland quickly reverts to scrub and woodland, protecting the soil from erosion.

In the longer term, warming oceans caused by climate change increase the vulnerability of the delta. Sea level is estimated to be rising a few millimetres annually through warming of the sea and coupled with natural subsidence, adds to the threats.

The calculations of IRTA show that it would take at least 1.2 million tons per year of sediments for the delta to stop receding.[4] The reservoirs upstream are left with the sediments that naturally created the delta and that could now feed the area. In the mid-twentieth century, before the construction of the dams, the delta received approximately 20 million tons per year while now only 90,000 tons arrive. The reservoirs retain almost 99% of the sediments that the river would naturally provide.

The high costs of artificial sediment transfer (by truck or pipeline) are likely to rule this out as a solution. With no sediment supply, adaptation may be the only viable option for managing the Ebro delta given a relative sea level rise and more extreme weather scenario. This implies managed retreat of the coastline. Surface area losses or land use changes in the lower parts of the delta would be permitted, with natural regeneration of salt marsh on abandoned rice fields. Agriculture would be concentrated in the higher parts of the deltaic plain.

The Ebro delta is a relatively recent feature on the Mediterranean coastline and was one of the first to benefit from human impacts. Land use changes since the Romans, particularly deforestation, contributed to its rapid growth. Humans, who indirectly drove the growth of the delta, are today contributing to its destruction. Past disagreements over the level of threat of coastal erosion have been replaced by a general consensus that the very existence of the delta in its current form is in danger.

Foot Notes

2 The Growth of Forests in Spain 1990-2015 Barcelona Field Studies Centre (31.1.2020)

3 Rural depopulation in Spain 2016 Barcelona Field Studies Centre (25.1.2020)

The Evolution of the Ebro Delta