590 PROTECTION AGAINST INUNDATIONS. 



drops wash away only earth and sand, and loosen the larger 

 rocky masses, but the brooks carry down stones and gravel. 

 The masses descending on all sides are received by the 

 channel course. After heavy rain, the raging torrent rushes 

 down its narrow bed, weighted with earth, sand and stones. 

 It tears away and undermines fresh material from the bed 

 and sides of the stream, and the projecting banks give way 

 and add to the moving debris. 



When the torrent emerges from its narrow bed into the 

 level plain, its force diminishes. The rubbish is heaped up 

 into a cone of debris, or may be washed away by a larger 

 stream or river. 



The most striking phenomenon in each torrent is the 

 varying amount of water. The Ehine at Basel varies 

 between its highest and lowest level, as 1 : 20. The torrent 

 of Faucon, in the Lower Alps, once carried off 70 per cent, 

 of the rainfall, or 60,000 cub. m. of water, in twenty minutes, 

 and at the same time, 180,000 cub. m. of rubbish. 



By regulating torrents and their feeders the formation of 

 the products of denudation is reduced, the velocity of the 

 water is slackened, earth, gravel and boulders are retained 

 in the mountains, while, if possible, a steady and continuous 

 flow of water is maintained. 



Works of the following nature should be designed in 

 accordance with the nature of the locality, the characters 

 of torrents, the area of the collecting ground, and the funds 

 available : 



(a) Barricades of trees with their entire crowns thrown across 

 torrents, or stones strongly joined together, with a tree in 

 front of them. These protect the base of the torrent from 

 deepening. Large masonry barricades are best made bow- 

 shaped, with the bend up-stream. The nearer they are to 

 one another, the better protection they afford. 



(b) Paving in masonry the bed of the torrent. This not only 

 prevents deepening, but also obstructs the carrying down of 

 silt, etc. These linings are made as in Fig. 262, A and B. The 

 bow-shaped form A is preferable, but the polygonal form is 

 easier and cheaper to construct, and suffices when the debris 

 is not composed of very large pieces, 



