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Avalanche and torrent control in the Spanish Pyrenees

by FAO staff

IN May 1955, foresters of the Sixth Division of Forest Hydrology of the Patrimonio Forestal del Estado (National Forests Organization) were hosts to foresters of the Alps and Pyrenees Divisions of the Direction des Eaux et Forêts (Directorate of Waters and Forests) who had come to see avalanche and torrent control work in a segment of the Spanish Pyrenees. A staff member of the Forestry Division of the Food and Agriculture Organization was invited to accompany the party.

At Canfranc near the French-Spanish frontier, where the Spanish railway system joins that of France, are marshalling yards and a railway station called the Estación Internacional. The station is in a deep valley surrounded by steep, rocky mountains and is annually threatened by avalanches. To protect the railroad installations, the Spanish foresters are constantly working to prevent the occurrence of avalanches. The lower slopes of the mountains were virtually denuded of forest cover prior to the initiation of this work 50 years ago but have since been almost entirely recovered with Pingus sylvestris, P. montana, Larix europea and several species of Populus. Higher on the mountains engineering structures have been sited, designed to contain or to break up avalanches before they reach the valley.

The most troublesome path of avalanches in this area, Torrente Estiviellas, begins at an elevation of 2,500 meters in a zone of bare granitic rock, having little or no cover of trees or other vegetation and where snow accumulates 12 meters deep during the winter. Slopes of greater than 100 percent are common. The engineering works in this torrente are designed either to prevent the snow from starting to move; or to stop avalanches which have already begun their descent; or to break up the avalanche by disintegrating it.

FIGURE 1. - Timber snow-bridge.

FIGURE 2. - Side view of the concrete snow-bridge.

FIGURE 3. - Concrete snow-bridge seen from above.

FIGURE 4. - Structures built on the slopes overlooking the railroad at Canfranc.

For the first purpose banquettes, wire nets or chains, poles of wood and iron and recently the so-called "snow bridges" have been built. The snow bridge, which is known as râtelier in Switzerland, is made of timber (Figure 13 and reinforced concrete (see Figures 2 and 3).

To stop avalanches which have already begun, large masonry dikes are used. These "hollow dikes," as they are called, have a large opening in the lower part to prevent waters from above from filling up the space behind the dike with solid debris. They are placed in strategic positions where the natural slopes will take the direct impact of an avalanche and reduce the force with which it subsequently falls on a dike.

The more modern structures tend to break up and disperse the avalanches so that the smaller fractions can be held by trees planted lower down on the slope. They may be either hollow dikes topped by a line of arches across which the avalanche passes and is broken up, or structures made of masonry piers bridged in between by logs. Use of such structures (Figure 4) to hold back avalanches which have already begun to move is a new feature and has not been tried out in any other constructions. The idea originated when it was noticed that an avalanche had been stopped in its descent down the mountain by two enormous pine trunks lying in a gorge in its path. Logs were first used only experimentally because it was thought that the impact of snow and rocks would easily break them, requiring- constant replacement. However, they have given satisfactory service so far and make this kind of structure much less costly than those made entirely of masonry or concrete.

No avalanches have reached the railroad installations since work was begun in this area, but the Spanish foresters emphasized that, to justify work of this character, the values being protected must be very high. Other parts of the Pyrenees are equally prone to avalanches but the low value of these localities does not give claim to structural protection. Instead, the people in the valleys must move to other areas in winter.

Torrent control

Torrent control is also a great problem to the forester but as in the case of avalanche control, the values to be protected below a torrent must be high to justify the very considerable costs involved.

In the mountains near the town of Biescas is a tremendous project of torrent control. It is in a drainage area where entire hillsides are sloughing away, causing tremendous volumes of silt and rock to be deposited in the streams below. The justification for this project is the prevention of excessive siltation of hydro-electric developments in the river below and to protect the only road into the large main valley. The headwaters rise from loose, granitic soils, highly erosive, on steep slopes. Above the main works there are 12,000 hectares of watershed.

There are three small villages in the watershed containing a total of thirty families, each family owning about 20 hectares of cultivated land. Approximately 1,000 head of sheep and 1,000 head of cattle graze on the watershed during the snow-free period. The sheep are trailed to lower country during the winter but the cattle are fed hay in the villages. These villages may ultimately have to be purchased as a protection measure under the scheme referred to later. In the meantime, in the immediate area of the landslides, a series of six or eight masonry dams have been constructed across the torrent (Figure 5), connected by large rock walls which prevent the soil which slips down the steep slopes from entering the stream. Even so, erosion rates are so severe that one of these dams has been completely filled with silt and debris in one year. When the dams cease to function as such, they form instead a series of steps over which the water passes.

The entire torrent course from the point of these structures to the valley below, a distance of 1.5 to 2 kilometers, is confined by such a series of masonry steps and walls. They reduce the velocity of the water and absolutely control its movements, but full control of this torrent has not yet been achieved. A tremendous effort has been made to establish trees on the areas where erosion is active, Populus sp., Salix sp., and Pinus sylvestris being used for this purpose.

In another area visited, the facilities of a tourist resort are under constant threat from avalanches. At the present time, this resort is used only during the summer months but it is hoped to develop it as a winter sports center, and considerable work is being planned to protect the facilities. The impressive thing about the whole project is the enormous amount of background engineering work which has gone into the planning phase. Extremely detailed maps have been drawn up showing contours, areas of greatest avalanche danger, types of structures which seem to be suited for particular conditions, placement of these structures, availability of structural materials for them and their costs of construction. The planning work for the entire basin from which avalanches threaten has been completed before construction work has been started.

On the other side of the valley is another landslide which has been largely controlled by a series of rock walls built across the slide and by the planting of trees in the spaces between. The torrent is channelled down a long series of masonry steps and carried beyond the fertile, irrigated farmland in the valley so that the danger of flooding is greatly reduced.

The soils of the Pyrenees appear to be highly unstable and this, coupled with runoff rates from snowmelt and torrential summer storms, results in widescale erosion. Tremendous volumes of silt, rock and other debris have been deposited in huge alluvial fans at the mouths of the small drainages.

Cultivation of so-called farm lands in the mountain areas has also contributed greatly to the high rates of erosion. Now, in order to gain better control of the mountain watersheds, the Patrimonio is undertaking a land purchase program. In some cases, the Forest Service is buying entire small villages along with the lands belonging to the villages and settling the people on irrigated farms being developed in the valleys of the main rivers of Spain. Villagers are not forced to sell their lands and many times tradition and years of family occupancy make them reluctant to settle elsewhere. The final persuasion is usually the relatively high prices which can sometimes be paid for these lands. The Government feels justified in this because purchase gives the foresters control of key areas on the watersheds for carrying out improvement programs while the villagers receive sufficient capital to begin operations on their new irrigated farms. It is the policy of the Government to increase agricultural production through the development of as much irrigated land as possible.

On one watershed area the Government has purchased seven villages and the surrounding land, an active program of tree planting and testing of adapted grasses is being carried out. Laborers who are doing the work are permitted to live with their families in the villages and are allowed to cultivate the small fields nearby which are judged suitable for this use. Incidentally, one of the Uniform Mediterranean Nurseries for testing forage plants, initiated by the FAO Working Party on Mediterranean Pasture and Fodder Development is established here by the Spanish foresters who, as a matter of fact, have established plots for growing seed of certain grasses and legumes which are useful both for grazing and rehabilitation of forest land.

Because of the high rates of siltation and the torrential nature of the runoff from the mountain watersheds, floods in the lower streams and rivers are often destructive and present serious control problems. To assist control, Spain has a law which assigns to the State all lands on the banks of the rivers which are subject to flood, unless other ownership can be proved. Even in cases where right of ownership can be established, the State has the power to plant trees on the banks of the river although the land owner is entitled to receive 40 percent of the income from the trees. This law appears to be a very useful weapon for the forester in getting more trees established along streams and obtaining better control of waters in the stream. In addition, some low masonry walls or other types of barriers may be built out from one bank of the stream to divert the water away from roads or other public works being threatened.

Watershed management

Altogether, Spanish foresters have an excellent appreciation of the problems existing on the watershed lands of their country and they are working hard to solve those problems. Although most of these men have had a strong engineering training, they realize that it is not possible or desirable to apply engineering methods to all the problems of land, water, and soil management. They are well aware that a good cover of trees and other vegetation is essential to control soil erosion and excessive runoff of snowmelt and rainfall. Because of this, Spain is planting new forests at a rate faster than any other country in Europe. Spanish foresters are preparing to plant grass on lands where it has been destroyed, they are attempting to withdraw from cultivation those lands not suited to such use, and they are planning a program of forest range improvement through reduced stocking and other good management practices. They recognize that watershed management involves sound farming and range management practices as well as forestry and engineering.

FIGURE 5. - Masonry dams for torrent control near Biescas. Poplar planting is shown in the foreground.

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