160 CONTROL OF EROSION IN THE MOUNTAINS 



serves as a support for dry stone barricades that jut out about 6^ feet. 

 If the walls or benches are in a forested region the plantations are made 

 under their protection; here they are considered only temporary for 

 when the trees take hold protection is no longer necessary. Sometimes 

 it has been found advisable to establish ordinary snow walls to turn snow 

 slides from their course; where pockets of snow are heaped up by the 

 wind it is often necessary to build walls 3.3 to 5 feet high back of the 

 drifts to prevent the snow from blowing over. Wooden snow fences 

 and wind shields, so common in the western United States, are not 

 used in France. The tendency in France is ever toward simplification 

 in the protection against avalanches; sometimes chunks of earth are cut 

 out of a steep slope 13 to 16 feet in length and 3.3 feet in width at the 

 bottom. Stakes are then set at the lower limit of the cut and are bound 

 together with branches. With a large number of these steps cut in the 

 slope avalanches can often be prevented, for the pits or steps act as 

 catchment areas for loose material that slides down. These pits are 

 often placed 7.5 to 10 feet apart vertically and 6.5 feet apart horizontally. 

 Where there is danger from shdes in partly forested areas, or where the 

 timber has been killed by fire, dead and dying trees in improvement 

 fellings are cut, and their stumps, 2 to 5 feet high, used to anchor the 

 snow. 



Rock Drains. — Saturated soil loses all cohesion and flows on a steep 

 slope; it even slides on underlying strata that are less permeable. These 

 earth movements are due to the infiltration of water coming from pro- 

 longed rains, from the melting of snows, from deep springs, and from 

 irrigating canals that are not leak-proof. Such movements are combated 

 by drainage, which makes the soil cohere and thus prevents slides. In 

 order to accomplish this work canals or drains are built to conduct the 

 water into the valley bottoms. (See Fig. 12.) Where considerable 

 ground is in movement it is not possible to open trenches sufficiently 

 deep for drainage, but by digging a large number of drains the soil can be 

 dried up and a sufficiently resistant surface formed to prevent it from 

 disintegrating. The average depth of such drains is 6.5, 5, and 3.3 feet. 

 The deepest drains, called "collectors," ordinarily have a width at the 

 bottom of 28 inches and an increase in width toward the top of 1 in 5. 

 The best drains have a concave base, built on a radius of 10 or 20 per 

 cent, and are so constructed as to form a canal of triangular construction. 

 Above this base the ditch is filled with stones of all sizes, the largest 

 being placed at the bottom. Drains 6.5 feet deep for the chief drainage 

 and 3.3 feet deep for the lateral drainage generally suffice. The triangular 

 section is sometimes replaced by a half-circular section or by a rectangular 

 section, which is easier to construct. Second-class or third-class drains 

 have a depth of 3.3 to 5 feet and a width at the bottom of 16 to 24 inches. 



