156 CONTROL OF EROSION IN THE MOUNTAINS 



One of the first problems to arise — a prolDlem of great importance 

 practically — was : How much ground should the work of restoration 

 cover? According to Surrel and his followers it is necessary to include the 

 entire receiving basin of a torrent and the slopes of the gorge. To-day 

 it is still felt to be best to include as large an area as possible to take in 

 the torrent basins and their slopes. The communes usually, with their nar- 

 row self-interest, argue for withdrawing the minimum area; the Forest Ser- 

 vice, on the other hand, realizes the technical requirements and desires to 

 set aside the area already eroded and the area immediately threatened. 



The correction of the effects of a "torrent" comprises a number of 

 operations, all closely related to their effect upon the surface of the 

 slopes and the bed of the stream: Dams (barrages), walls, rock drains, 

 protection against avalanches, the paving of channels, the building of 

 tunnels and aqueducts, wattle work (garnissage), and the introduction of 

 grass, shrubs, or trees. 



Concisely stated, a theoretical torrent above a rich village might be 

 controlled somewhat as follows: Where a ledge of rock crosses a stream 

 bed, and where there are good foundations, a base masonry dam is con- 

 structed; at the same time dry stone dams are built higher up the main 

 bed to prevent deeper erosion of the stream bottom. A slope in danger of 

 sliding into the main bed is fixed by a masonry retaining wall. Small, 

 incipient gullies are rocked over as drains to prevent further erosion and 

 to act as permanent drains. Small ravines are held in place by wattle 

 work and garnissage (p. 162). Beginning at the top of slopes that 

 are liable to erosion selected areas are first sodded; then, as soon as the 

 soil is stable enough, shrubs are planted on the slopes and in the stream 

 beds. After shrubs are established and the soil is held in place trees are 

 planted. Wattle work may be necessary here and there where the soil 

 is crumbling away, and the main bed of the torrent perhaps must be paved 

 to prevent further washing below the base dam. 



An illustration of the corrective methods employed (see also p. 168) 

 in full swing is afforded by the torrent of St. Julien, in Savoie. Here the 

 schist is easily eroded, for it is very friable. The torrent begins at an 

 altitude of 9,186 feet, is 6.2 miles long, and covers some 4,942 acres of 

 land, of which 1,866 acres belong to the State. The village of St. Julien 

 had been damaged, parts of a railroad and wagon road had been washed 

 away, and portions of Mont-Denis were gradually sliding and were being 

 washed away by the torrent. The corrective measures employed were 

 as follows : 



Where there was danger that the stream might undermine a promi- 

 nent ridge, and to reduce the flow of water in the main channel, it was 

 conducted 843 feet through a 32 per cent tunnel cut through solid rock. 

 Where the stream passed the village it was paved to prevent further 



