162 CONTROL OF EROSION IN THE MOUNTAINS 



as soon as the drainage has made the soil sufficiently stable. Drainage 

 has always been satisfactory where it has been used to stop superficial 

 slides over a small area or to dry up saturated soil. Where earth is 

 sliding over a nonpermeable surface the timber still standing is almost 

 invariably cut so as not to retain surplus water. 



Paving Channels. The typical torrent is almost dry except during a 

 period of storms. To prevent erosion, enlargement, and changes in the 

 main channel bed it is necessary to pave the bed. This facilitates the 

 passing of the detritus during floods, especially below the base dam. 



Tunnels and Aqueducts. In a few places it is necessary to use tunnels 

 or aqueducts to conduct excess water through ridges or over artificial 

 obstacles, such as roads or railways. For example, near Thonon (Haute- 

 Savoie), where the limestone soil is badly eroded, a road is protected by 

 carrying the wash from a ravine over it on an aqueduct. 



Wattle Work (Garnissage). On steep slopes (more than 60 per cent) 

 and in small ravines wattle work is often necessary. (See Fig. 13, a and 

 6.) A common method is to stick willow shoots in the ground 1 to 3 

 feet apart and to weave willow branches in between the shoots to hold 

 the rocks and shifting earth. The shoots take root and when established 

 assist in holding the sod, when forestation becomes possible. Another 

 system of building wattle work (now largely abandoned) was to lay the 

 brush straight up and down the stream bed and pin it in position with 

 cross pieces every 6.5 feet. The latest method is to lay the brush as 

 formerly but to hold it in place with two stakes driven in the soil in the 

 shape of a V, the head of the V pointing down stream. According to 

 Demontzey: 



"The 'garnissage' correction of little dry ravines and the consolidation of unstable 

 slopes is as follows: 



"In the dry ravines of the Alps 'garnissage' is often used in the bed of the stream; 

 stems of branches are laid on the bottom so that the ends of the branches may be toward 

 the top. By dry ravines is meant those that carry no water during normal times or 

 those in which there is a very small trickle of water. The branches are so placed as 

 to form a slightly concave surface, and are held in place here and there by cross pieces 

 fixed in place by stakes. The most common form of 'garnissage' employed during the 

 past years has been to place brush on the bottom of the ravine and then to mat it down 

 by interwoven branches. When completed, the branches form squares on top of the 

 debris. Winter willow or poplar branches are used, and the ends are covered with 

 earth, commencing from the top of the ravine and extending downward, so that they 

 can take root and form a living protection. Most 'garnissage' eventually results in 

 vegetation that forms a permanent protection against erosion. In the Maritime Alpes 

 where branches cannot be had at a reasonable expense, the flow of water in the ravines 

 is controlled by a series of little dams formed of balls of sod, used alone or combined 

 with stone. When the water falls it is stopped at each dam so that it cannot attain 

 sufficient velocity to be dangerous. The debris backs up behind each little dam and 

 further lessens the velocity of w r ater. . . . The dams must become larger and larger 

 down stream in order to resist the floods . . . until these dams reach a height of 3.3 



