158 CONTROL OF EROSION IN THE MOUNTAINS 



until the small dams at the head have been built. The base dam must 

 be faced upstream on a part of the stream bed which is absolutely stable. 

 It is necessary to avoid building so-called "suspended" dams, which 

 must necessarily disappear as the stream erodes upward or downward. 

 It often happens that a torrent is intersected by a rocky barrier. This 

 barrier or ledge is an excellent point for the base dam. 



Many of the dams in the Alps are simple, some of them consisting of 

 a single log staked in position across the bed of a ravine. The general 

 tendency is to avoid building expensive masonry dams and to build 

 simple stone dams without mortar. Formerly it was often the practice 

 to curve the face of the dam, but now they are built straight, for ex- 

 perience has proved that the curved dam is not necessary to withstand 

 the average pressure. 



Walls and Protection Against Avalanches. The main protective 

 measures against avalanches are walls, benches, or steps with wattle 

 work or high stumps. (See Fig. 11, a to c.) Avalanches are very 

 frequent in the high mountains, and most of them follow the depres- 

 sions or the lines of least resistance, where they do not cause a great 

 deal of damage. But it is often necessary to prevent them, especially 

 if they menace villages or carry great quantities of eroded material or 

 if they damage forested areas. Walls (or benches) are usually built 

 where the snow begins to slide. The walls are built of dry masonry and 

 usually have a total height of 6 feet and a minimum length of 50 feet. 

 The width at the top is from 24 to 31 inches, depending on the height. 

 The depth of the foundation varies with the ground and is sometimes as 

 much as 5 feet. So far as possible, large stones are used, preferably of 

 the same width as the wall. Masonry is used when dry stone of the 

 proper size and quantity is not available. On a slope that has a practi- 

 cally uniform grade walls are run on the level at equal distances apart. 

 Where the snow slides in regular runways, the walls are really dams, 

 some with a clear height of 10 feet and a width of 31 inches at the top, 

 with an increase of one in five inches to the base. In Savoie, where 

 there is no resistant material at hand for such construction and where 

 consequently it is not possible to build regular barriers, it is necessary 

 to build benches, a bench being a flat pocket in the rock 3.3 feet wide 

 and about 33 feet long. Where possible, this is inclosed with a small 

 wall on the outside of the cut to collect debris. The benches constructed 

 in the Pyrenees are of two different types: (1) Horizontal platforms 

 about 6^ feet wide, slopes slightly downward toward the mountain. 

 The talus is held by a dry stone wall with a decrease in width of 1 in 5 

 toward the top. Its length is 16 to 49 feet, 33 feet being the average. 

 These platforms are 26 to 33 feet apart horizontally and about 33 feet 

 below one another. (2) The platform cut into the side of the mountain 



