356 PREVENTION OF TORRENTS AMONG THE ALPS. 



Thus are formed immense excavations which discharge incalculable 

 quantities of materials into the torrents which tbej border. It is 

 estimated that twelve millions of cubic meters per annum have been 

 passed at Perthius upon the Durance, near the Ebone. 



Means employed to prevent this degradation. — The means practiced in 

 the Alps to prevent this degradation consist in — 



1. The prohibition of pasturage. 



2. Banquettes, or general dams. 



3. Partial dams. 



4. Mule-paths. 



5. Sustaining-walls of stone laid dry. 



6. Drains. 



7. The opening of parallel horizontal belts. 



8. Coverings of wood, straw, &c. 



9. Plantations of various kinds. 



10. Sowing of forest seeds and sodding. 



The prohibition of pastiirage. — Pasturage, particularly that by sheep 

 and goats, being the principal cause of these injuries^ should be wholly 

 forbidden within the district to be regenerated. 



Banquettes, or general dams; partial dams. — In one or two years after 

 taking the work in hand, (this time being necessary to ascertain the 

 points where nature would be powerless of itself to consolidate the 

 ground), they portion off the area to be artificially restored, into two or 

 three great zones by horizontal banquettes, about 1.5 meters wide, with 

 the view of catching all the waters, and of preventing them from acquir- 

 ing a velocity. After these general dams, partial ones are also made. 

 These dams are of variable dimensions, and of a certain form and construc- 

 tion I causing the depositabove them of large accumulations of sediment, 

 which fill the bottoms of the ravines, brace up against the ma5?s, con- 

 solidate the two opposite slopes, which thus mutually support one another, 

 extend the water-level along the banks, and deaden by successive falls 

 the velocity of the current. They are generally constructed as dry walls, 

 are slightly curved upward, and are considerably higher at the ends 

 so as to turn the water from the banks and force it to pass in the middle. 

 But too much importance should not be given to the curve, which pre- 

 sents the great inconvenience of throwing the current to the sides, and 

 causing whirlpools, which wear cavities and endanger the solidity. It 

 is not even proven that these curves are indispensable, for in one of the 

 strongest torrents of the Alps, several barriers very much curved down- 

 ward resisted perfectly, which appears to indicate a feeble pressure. 

 But it is prudent to arch them a little, both above and below, to prevent 

 the ends from being undermined or turned, which often happens when 

 the wall is straight on the other side. 



According to the resistance they have to support, the floor of the bar- 

 riers is fastened to sills of walnut, in concrete (this is best), or simply 

 formed by large stones placed together in a bed of concrete that extends 

 one or two meters, and is protected by a slope- wall, which they take 

 pains to carry up to the level of the flow-line, as is done around the 

 piles of bridges, with the view of preventing eddies. It is also prudent 

 when there is reason to expect the passage of heavy substances, to clamp 

 the stones forming the crown of the barrier and also the platform. 



The height of these barriers ought to be sufiBcient to cause a tilling up 

 above them so as to obliterate the slope of erosion. Their thickness 

 will vary between a third and a quarter of their height, with a slope of 

 els'" to 0.20™ to each meter in height, according to length of materials, 

 with the lower side vertical and the slope above. 



