TECHNIQUE OF DAMS 157 



erosion. In order to reduce the slope of the stream bed to approxi- 

 mately a 5 per cent grade, where the erosion was most severe, it was 

 necessary to construct twenty-six small and three large dams. Along 

 one stretch there was one dam every 85 feet. The catchment basin is 

 being grassed over and planted. In addition a very complete system 

 of rock drains has been built on the most dangerous slopes. A drain 

 13 feet deep and 5 feet wide cost $2.31 per running yard; one 5 feet deep 

 and 3.3 feet wide cost only 97 cents per yard. It was necessary to put 

 in a holding wall at the foot of the slope, where erosion was particularly 

 severe. The trees planted included alder, willow, Scotch pine, ash, oak, 

 maple, and poplar. It is interesting to note that for planting Scotch 

 pine, two-year seedlings are used in spots with two seedlings per spot. 

 The spots are 12 to 20 inches square and number about 2,800 per acre. 

 The cost of stopping the movement of the earth on Mont-Denis, together 

 with the system of trails that it was necessary to build, was $30,494, 

 and the improvements on this one torrent alone have cost, so far (1913), 

 over $129,310. 



This example affords an excellent illustration of the difficulty of cor- 

 recting erosion after it has once started, and shows the absolute necessity 

 of not letting it get a start. Much of the difficulty has been caused by 

 waiting too long before beginning corrective measures. 



Technique of Dams. The principal objects of dams are (1) to stop 

 material transported by the water, (2) to diminish the speed of the 

 water, (3) to prevent further erosion, (4) to prevent the enlargement 

 of the torrent bed and the erosion of the border talus, and (5) to hold up 

 unstable slopes. Dams are constructed in torrents and ravines that 

 are under active erosion. It is usually advisable to establish dams of 

 about the same size, that they may be homogeneous, and to avoid the 

 installation of secondary dams of doubtful stability. Of course, uni- 

 formity of construction cannot always be maintained. The dams must 

 be constructed to fit local conditions, and usually the torrents have a 

 double character. Their slopes are eroding and the material eroded is 

 carried along in the flood. In certain places it is sufficient to construct 

 a stone step across the bed to prevent further erosion. As the construc- 

 tion of these stone dams is entirely a work of engineering based on the 

 stress which they must withstand, no details of construction are given. 

 Above each dam it is customary to construct a rough paved area at the 

 level of the dam, 1\ to 5 feet in thickness. It is constructed with stones 

 which cannot be used in the masonry work. These paved areas are 

 made to diminish the drop upstream so as to decrease the pressure of 

 the water collected behind the dam. But there can be no ironclad pro- 

 cedure. So far as the work of correction is concerned, it is usually best 

 to construct only absolutely necessary dams at the base of the torrent 



