The Pyrenees, Spain-France

Erosion and rock fall at St. Elena, Spain

At this site, landslides and rockfall from a steep, ca. 35 m high slope consisting of loose glacial till, threaten an important road between France and Spain, the A-136 (RD-934 in France). There are often fallen rocks and debris on the road, and the visibility of the cars coming from both directions is low due to the curves, and the speed is often high. The location has been identified as a high-risk spot on this road. Increased frequency of heavy rain episodes may increase the hazard.

The implemented measures at St. Elena consist of reshaping the slope through terracing techniques, with the establishment of vegetation to stabilize the sediments. The terraces are formed by a 5 m high dry masonry wall at the base, followed by 10 terraces constructed of logs. The log constructions are in the form of timber gabions, filled with the local sediment, and with a 10 cm layer of organic soil on top for planting bush vegetation on the terraces. All plants to be used are local, with a preference for species with deep root systems. In addition, ca. 1 m deep holes are filled with organic soil at 3 m intervals for the planting of larger trees. The width of the construction is 32 m at the base and narrows off to ca. 20 m at the upper, 10th terrace.

The implemented measures at St. Elena are expected to provide several co-benefits:

• less frequent need for maintenance and repair, probably leading to reduced costs over time
• increased sense of safety for users of the road, particularly regarding the transport of goods between the two countries
• aesthetic benefits, and combining the measure with the system of foot-paths in the area

Rockfall at Artouste, France

At this site, the road, RD-934, bends around a sharp hairpin that turns below a steep, forested slope. The hazard at Artouste is caused by rockfall sourced from exposed rock ledges and loose blocks resting on the till surface in the steep slope. The forest has a protective effect for smaller blocks, but rocks often hit the road. In the worst case, the same rockfall event may hit the road three times. In 2013, a fatal accident occurred when a passing car was hit by a falling rock.

The implemented NBS measures consist of wooden structures designed to prevent the release of rocks and to stop released rocks before they reach too high a velocity. All release zones and individual loose blocks have been mapped and marked with flags indicating the type of measure to be implemented.

All materials were then lifted in with a helicopter, and the work was performed by hand. Construction of access roads and the use of heavy machinery, which would have been necessary for traditional measures, were avoided.

As an immediate action, the municipality has mounted rockfall nets in one steep section above the road. This will provide a good site for comparison between the effect of the NBS interventions and the traditional ‘grey’ structures (the nets) over time.

The structures have been constructed close to the potential release areas and immediately below. Only logs from the region are used, and the measures are to be combined with the preservation and proper maintenance of the existing forest.

An additional action related to the Artouste site is the establishment of test facilities for wooden protective structures, in which different tree types and structures can be tested. These facilities are established in both La Peña Estación, Spain, on the premises of the timber company, and at an open-field site, Gourzy, France, with similar characteristics to that of Artouste, both of which will be active long beyond the PHUSICOS project period.

As for other NBS, the interventions at Artouste are expected to provide several co-benefits in addition to the risk reduction:

• economic gain, through less need for repair of the road and less need for maintenance compared to traditional engineered measures
• local jobs with construction of the measures, with more to come regarding the potential for upscaling
• reduced fear among the users of the road, and more secure transport of goods and people, also crucial for the tourist industry of the area

The interventions are also assumed to have a sizeable upscaling potential, in particular when optimized through the experimental activities in the laboratory and full-scale test site.

Snow avalanches at Capet Forest, a village of Barèges, France

The village of Barèges, an important tourist destination, has been hit by snow avalanches from the mountainside to the north of the village several times. The village, which is part of the Grand Tourmalet ski destination and also has warm health baths, was evacuated because of avalanche hazards as late as 2013 and 2015.

There are several avalanche paths on the valley side, and a number of existing measures exist in the form of fences, stone- and concrete barriers, and other structures, some as old as the 19th century. These do function, but in extreme winters, the existing fences are buried in snow, and avalanches can be released from the snowpack above the fences. This was the situation during the last avalanche that reached the village. In addition, they require extensive maintenance, which must be carried out on foot and is, therefore, very laborious.

The implemented NBS measure consists of planted trees of 5 different species, all of which should be adapted to the climate and elevation (1800-2200 m asl.) of the site. The trees are meant to withstand the anticipated climate change and to be robust enough not to need extensive maintenance. The plants are either protected by 88 newly built wooden tripods, by existing grey structures, or by existing groups of trees. The plants are planted in polygons, downslope of the protective structure. There are 150 polygons, with 30-50 plants in each polygon.

About 5000 trees were planted over four years but with the exact age of plants in each polygon. A certain loss of plants is expected and even required since the plants in each collection (polygon) are planted too dense for the significant growth of all. The wooden tripods are also meant to protect against avalanche release as long as they are standing and until the planted trees grow high enough and develop a stem thickness to act as adequate protection against avalanche release within ca. 10-15 years.

The NBS interventions at the Capet Forest have several co-benefits:

less costly maintenance operations; less human pressure and probably economic savings

• probable economic savings through less need for frequent, costly maintenance, as well as less pressure on maintenance personnel
• environmental benefits through less need for helicopters to lift heavy material for maintenance – this is also of benefit for wildlife in the forest (it also helps preserve several of the species and biotopes in this high-elevation forest)
• increased sense of safety among the villagers, both regarding the village itself, but also regarding road closures, etc.
• less avalanche threat is vital for tourism in the area, which is essential (alpine skiing, health baths, etc.)
• great upscaling potential

Debris flows at Erill-La-Vall, Spain

The village of Erill-la-Vall in the Spanish Pyrenees is heavily affected by debris flow problems, where sediments are fed from tributary creeks and into a main debris flow channel leading to the village. The source area consists of thick (>50 m) deposits of glacial till. The annual precipitation of the area is about 1100 mm/year, mainly in the fall and spring. However, intensive showers in July-August seem to have the worst effect on the debris flow potential.

The implemented measures consist of a combination of revegetation to limit erosion and of terraces constructed by dry stone walls and timber (Krainer Walls) to minimize energy in the creeks and to avoid further lowering of the erosional level, all to reduce the feeding of sediments to the main debris flow channel, and thereby reduce the hazard posed to the village. The terraces are constructed in the lower parts of the steepest part of the two main gullies and are to be covered with organic soil and planted with local vegetation: grass, bushes, and trees. Soil and turf from the area are used, and natural fertilizers from local grazing animals are used on the terraces. About 2500 plants will be used, all local species. The terraces are constructed starting from below and as high up as possible with an excavator.

The site has been instrumentally monitored since 2007, including a weather station, micro-seismic network, flow- and turbidity measurements, and borehole through the entire till section behind the scarp, instrumented with piezometers. Hence, there are abundant available data and a good understanding of the relationship between precipitation and instability events.

Co-benefits expected from the interventions comprise:

• improved sense of safety among the population
• less need for costly maintenance
• positive effect on the terrestrial plant ecology of the area
• positive effect on tourism in the area, i.e., an economic co-benefit

Contact persons:

Didier Vergès d.verges@ctp.org
Eva Garcia e.garcia@ctp.org