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The transport of sediments from the Leitzaran River has been significantly restored with the demolition of the Olloki Dam

2025/02/27 Elhuyar Zientzia Iturria: Elhuyar aldizkaria

Researcher Askoa Ibisate in the laboratory. I'm talking about Arg. I'm talking about A. Ibisate / UPV

The launch of a dam on the Leitzaran River has increased the transport of torrents in number and distance, according to a study carried out by the UPV/EHU. They have also observed that sediments accumulated over the years have been transported gradually, not suddenly. Their findings, published in the journal Geomorphology, are useful for administrations in predicting the effects of dam demolitions.

Researchers from the River Operation research group have been studying the effects of the removal of the Olloki dam on sediment transport for seven years. It is concluded that after the demolition of the dam, more corrugations have been moved, and that the route traveled by the stones has also been prolonged.

Promoted by the policies of the European Union, unused dams are being eliminated with the aim of restoring rivers. An example is the Olloki Dam. It was demolished mainly for the purpose of improving the habitat of salmon. In fact, for the river to function properly and for the habitats to be in good condition, the geomorphology of the river must work well. Among other things, it is very important to renew the course, for which the transport of both sediments and flows must be adequate: “Rivers are a transport system and, in order to perform their ecological function properly, they must move water, sediments, nutrients and living beings. But the prey actually interferes with that. We say that they are like thrombi that originate in our veins”, explains the researcher Askoa Ibisate.

In this context, in order to show whether the removal of the dam has been really effective, the work of the University of the Basque Country has analyzed how the demolition has affected the transport of sediments throughout the process: “We knew it would come back, but we didn’t know how. Nor what effect it would have on the different points of the river. In this way, we have measured the movement of the corrugations in three specific locations before, during and after the ejection of the wall,” says Ibisat.

As for the amount of sediment mobilized, the study reports that the number of displaced torrents has increased especially after the full launch of the dam, but Ibisat tries to clarify that it has occurred in a progressive way. That is, that these movements have not occurred suddenly: “During the demolition of dams, one of the fears is that the sediments accumulated over the years start to explode like a wave and, as a result, cause damage to the uses and infrastructures located in the downstream flood plain. Instead, what we have seen is that the process is regulated and stones are gradually transported.”

Thus, they emphasize that the study provides useful information to the administrations for planning the demolition of dams. Although it is necessary to pay attention to the specific characteristics of the case streams in each case, the research allows us to better understand the functioning of the rivers and the sediment flow regime, predict the geomorphological responses to the demolitions of the dams and design our own strategy.

On the other hand, the researcher of the University of the Basque Country says that the displacement of stones is also noteworthy: “Some worms have traveled a distance of 8.8 kilometers in a single year, which is a lot. We didn’t think the sediments would move so much, and other experts in the field have been surprised.”

1,800 rocks marked in seven years of investigation have been deposited at three points of the river: two sites exposed to the dam (one upstream and one downstream) and a control point that was out of influence. “The control point has allowed us to know that the changes in the transport of sediments caused by the collapse of the wall have actually been due to demolition and not to other factors, such as a significant boom,” says Ibisat.

The stones had a code embedded in them. Thus, to know how much sediment has moved, they have traced the river using a detector (similar to the instruments used to find the metals): “When the device detects a code, it tells us what stone it is and gives us GPS coordinates. Thus, we know where we liberated him, where he has appeared and, therefore, how far he has traveled,” says Ibisat.

The fact that it has been going on for so long has given great value to the research: “Usually there is a 1-2 year follow-up due to lack of money; before and a little after the dam is demolished. We, on the other hand, placed the first marked stones in 2016 and for the next 6 years we looked at how far they had come. In between, in the summers of 2018 and 2019, the wall was demolished in two phases and then we followed the movement of sediments until 2022. Therefore, we have had the opportunity to gather information after the full expulsion of the dam, for three full years. It’s a long period of time and therefore significant results,” Ibisat elaborates. In addition, very different hydrological conditions occurred in these last years, which makes the information collected even more enriching. In fact, after the demolition of the dam, they have also been able to measure the effect of the flows on the transport of sediments.