Scientists model sediment deposits carried by the Naviscence River in the Swiss Valais canton. They are simulating possible scenarios for the future of the Zinal ski area, which could be threatened by the river. These research results could be applied elsewhere in the Valais.
How long will it take until Switzerland’s picturesque Anniviers Valley is filled in with silt and gravel? More quickly than one might think, if global warming indeed leads to a predicted increase in sediment deposits during the course of the coming century. Of course, a lot of water will flow down the mountain valleys before they’ll be buried in sediment. For some villages, the threat of rocks and torrential flooding is always a serious problem, even on the short term.
Mountain streams transport all kinds and sizes of sediment loads. Depending on where it piles up, this primarily rocky debris can be seriously dangerous. Lakes can form upstream, flooding buildings. If natural dams break in flood situations, the mix of water and gravel can cause devastating damage downstream.
Two thousand truckloads of rocks
This is the case for Zinal, a popular ski resort situated on the Naviscence River. The river originates in the Zinal glacier at the southern end of the Anniviers Valley, and is fed as it winds its way down towards the village by several other, smaller streams. A series of small retaining dams were built along the river to mitigate the risks associated with stones and other glacial sediments. Today, these dams hold around 10,000 cubic meters of material and are reaching their saturation point. “It’s almost inconceivable to evacuate these materials out of the valley,” explains professor Christophe Ancey, head of EPFL’s Environmental Hydraulics Laboratory. “The road going up the valley is one of the most narrow and winding roads in the entire canton, and to get the right rhythm going, you’d have to run 1,250 to 2,00 trucks back and forth every year!”
What can be done with these impressive pileups of rock and gravel, whose quantity is predicted to increase as glaciers and permafrost melt under the effects of global warming? In collaboration with CREALP (The Valais Center for Research on the Alpine Environment), EPFL is studying the possibility of leaving them where they are in the valley, while still not adversely impacting protected zones. The research is being done both in the laboratory and in the field.
Upstream of Zinal, the meandering Naviscence has formed the “Plats de la Lé,” a zone that is flat enough to be used as a cross-country ski trail in winter. Cantonal authorities want to know if it would be possible to retain sediments here, or even to deposit thin layers of sediment over this large surface area. “The challenge is to find a way of holding the sediments back,” says Ancey. Do dams need to be built, or can natural slopes be used‘ But in that case, what would happen in the event of a flood’
150 years of evolution
A lot of research has been done at EPFL and with other partners to study this pressing situation that is in need of appropriate solutions. In particular, projects have been undertaken to better understand the mechanisms at work in sediment transport and deposit. Currently, sensors installed under the riverbed are continuously “listening” to the passage of stones and measuring the flow rate. Many research projects were carried out in the valley this summer. The data obtained will be correlated with temperature and rainfall measurements to try and establish a model. “We observed in several cases that there is not necessarily a direct correlation between climatic phenomena and sediment transport,” Ancey says.
To try and get a better understanding of the situation, a spherical sensor about 10cm in diameter that can be thrown into the river is being developed at EPFL. As if flows down the river like a real stone, the device will measure the accelerations, trajectories and forces involved in this kind of movement. The sensors could also be used to study flows in avalanches. A historical study, using old aerial photographs and topographical measurements, is also making it possible to retrace how the situation has evolved since 1950, in order to help develop a simulation that will be able to make forecasts up to the year 2100.
Melting glaciers will clearly generate more problems than just a lack of water and the reduction in the electricity that can be generated by hydropower installations – although these are unquestionably important issues. “The Anniviers Valley is where we’re working, but the entire canton of Valais is concerned,” warns Ancey.