EPFL helps revitalize Sarine River habitats downstream of Rossens dam

The absence of natural flood events means that, downstream of dams, rivers always flow at the same rate. The channel bed silts up as time passes, and the lack of sediment replenishment degrades fish and invertebrate habitats and causes species diversity to decline.

Until recently, this very fate had befallen the Sarine River downstream of the Rossens arch dam in Fribourg Canton. The vast structure, standing 83 meters high and 320 meters wide, was built in 1948 to hold back the river and create Lake Gruyère.

Intervention in 2016

In 2016, ahead of the first artificial flood to be triggered since the dam was built, Groupe E energy supplier and Fribourg Canton invited researchers in to conduct an experiment. The team, comprising scientists from EPFL, the Swiss Federal Institute of Aquatic Science and Technology (Eawag), the University of Zurich, and Zurich University of Applied Sciences (ZHAW), tested out a hypothesis that had never been proven outside a laboratory: that depositing sediment along the banks of a river before a flood could enhance wildlife habitats. The findings, recently published in Scientific Reports, reveal a 36% improvement in habitat suitability – as measured by the Hydro-Morphological Index of Diversity (HMID) – when compared with the baseline situation. Outside the impact area, meanwhile, post-flood HMID increased by only 18%.

“Our research shows how sediment deposits can increase ecosystem diversity,” explains Severin Stähly, the paper’s lead author. Stähly recently completed his PhD at EPFL’s Laboratory of Hydraulic Constructions, where he was supervised by Honorary Professor Anton Schleiss. The team conducted the experiment just before the 2016 artificial flood, when more than nine million cubic meters of water were released from the lake into the river. They distributed four sediment deposits as alternate rectangular bars along the river banks. Each bar measured 22 meters long, 8 meters wide and 1.5 meters high, and contained 250 cubic meters of sediment. In laboratory tests, deposits of this type had been shown to produce more varied riverbed morphology following an artificial flood, with plant and wildlife thriving more as flow depth and velocity variability increased.

Following 500 stones
The researchers could have collected the 1,000 cubic meters of sediment they needed upstream of the dam and transported it down to the Sarine below. Instead, they excavated the material from a part of the river that had turned into forest, as this also served to partly restore the river’s original path.
Stähly waded into the water to measure riverbed morphology before and after the flood, using the HMID index to evaluate the quality of river habitats according to the variability of flow depth and velocity. He also fitted RFID tags to almost 500 similar-sized stones. Once the flood had passed, Stähly combed every inch of the river with a receiver, noting down the stones’ new locations to understand how they had moved under the effect of the sediment deposits.

The results clearly demonstrate that sediment deposits are effective at revitalizing river habitats. But other questions remain unanswered. “To further refine our research, we’ll need to conduct other, similar large-scale experiments and constantly monitor the river,” says Stähly. The EPFL team has presented its research at two international conferences, where the findings have sparked considerable interest.

The study was carried out as part of the Swiss National Science Foundation’s National Research Programme “Energy Turnaround” (NRP 70).