Sediment transport has been restored after the demolition of Oroki Dam

Askoa Ibisate, a geographer specializing in river geomorphology, analyzed how the disappearance of the Oroki dam affects sediment transport. Ibisate concluded that the amount of pebbles mobilized by the demolition increased, and their journey expanded. The results are particularly important as surveillance work has been underway for seven years and the authorities have provided valuable information to predict the outcome of the dam demolition. This work has been published in the journal Geomorphology.

Thanks to the momentum of European Union policies, dams that were abolished with the aim of restoring the river are being removed. One example of this is the Orokidam (Gipuzkoa). This was the subject of a study by a river topography geographer from the River Functional Research Group. The dam was once located in Leitzaran and, above all, was demolished to improve the salmon habitat. In fact, the river terrain must function properly for the river to function properly and have good habitat.

In particular, renewing the riverbed is extremely important, and to do this, transporting sediment and water flows must be appropriate. “Rivers are transport systems and to meet ecological functions, they need to move sediments, nutrients and organisms.

In this context, to demonstrate the true effectiveness of dam removal, a study by UPV/EHU analyzed how demolition affected sediment transport throughout the process.

“The results show that the volume of transported sediment increased significantly, particularly along the stretches under the influence of the dam, and that the stones traveled longer than expected,” Ibisate explained.

Regarding the amount of mobilized sediment, this study shows that the number of displaced stones increased, especially after the dam was completely destroyed, but Ibisate was keen to make it clear that this had occurred gradually. In other words, these movements did not occur suddenly. “When it comes to dam destruction, the sediment accumulated over the years is suddenly replaced like a wave, resulting in damage to the use and infrastructure in the downstream flood plain.

The study emphasized that it would provide information that would provide great help to authorities in planning dam demolition. In both cases, specific characteristics of the river must be considered, but this study will provide a better understanding of the river and sediment flow regime. This will allow you to predict the topographical response to dam demolition and design specific strategies.

UPV/EHU researchers also emphasized the displacement of the stone. “There are stones that traveled 8.8 kilometers in a year, and that’s a lot. I didn’t think the sediments would move that much, and other experts in this field were surprised.”

1,800 stones were monitored for seven years

To obtain results, 1,800 stones (300 each year) were released at three points on the river during the seven years in which research was underway. “The control points allowed us to discover that the changes in sediment transport caused by dam demolition were in fact due to the demolition itself, and other factors, such as a significant increase,” Ibisate said.

The stones distributed around Leitzaran had codes embedded within them. So, to find out how much the sediment has moved, they used detectors to inspect the river (similar to metal detectors): “When the device detects the code, it tells you which stone is which stone and provides GPS coordinates.

Researchers said it was a monumental task as the river had to be scanned meter-by-meter after being monitored for seven years, at a distance of 11 kilometers from bank to bank. But the fact that it has been working for so long is exactly what makes UPV/EHU research so valuable. “Surveying usually takes place for a year or two shortly after the dam is demolished due to lack of funds.

“On the other hand, during the summer of 2018 and 2019, the wall was demolished in two phases, and then monitored the sediment movement until 2022. So we had the opportunity to gather information for three years after the entire dam was demolished.

Furthermore, in recent years there have been a variety of hydrological conditions, which further enriched the information collected. In fact, we could also measure the effect of flow on sediment transport after dam demolition.