164 CONTROL OF EROSION IN THE MOUNTAINS 



feet and a length of several yards. Where sod cannot be obtained layers of branches 

 alternating with layers of stone are used. Sometimes also these ravines are held in 

 place by small dry stone dams some 16 to 23 inches in height, soUdly anchored. In 

 the high mountains, in certain places, some of the ravines are filled with stones just as 

 if they were small drainage canals. This method of cover has given good results on 

 slopes situated at an altitude of 6,500 feet in the upper Verdon valley. In addition to 

 fixing the ravine bottom, a necessary precaution to prevent erosion from extending 

 farther up is to construct just below the summit a so-called consolidation wall about 

 20 inches in height, which if well anchored holds the ground. After this wall is built 

 the soil around it can often be sodded over. The methods used for fixing unstable 

 banks or slopes are extremely variable. If their instabiUty is very pronounced they 

 must be sustained at the bottom by transverse works very much like the wall that 

 holds up the side of the ridge. Some slopes can be held up by a superficial cover of 

 branches held in place by stakes or poles, such as are used in a ravine. Under the 

 shelter of this cover natural vegetation can easily be developed. Little horizontal 

 structures, wattle work 10 to 12 inches high, or fascines reinforced by. layers of shoots 

 between which are sown forage plants, are used everywhere. Where stones are abun- 

 dant little walls of dry stone are built, topped with sod." 



Forestation, with Examples.^ — The purpose of dams, walls, drains, 

 wattle work, and other artificial "dead works" is to stop the movement 

 of the surface soil, because until that is anchored forestation is impossible, 

 and before the final protective cover is planted the surface must often 

 be held in place by grass or shrubs. It is clear from the French literature 

 on reforestation that engineers have not always admitted or realized the 

 importance of vegetative cover for permanent reclamation: 



"Some geologists have expressed the view that a torrent is a phenomenon whose 

 development cannot be stopped. If this opinion is accepted there is nothing to do but 

 to let the destruction of the mountain go on and try to defend the valleys against the 

 results of torrents. Some feel that protection against torrents can be obtained in a 

 certain measure (when the hmit of slope erosion has approached) by installing a canal 

 to conduct the eroded material from the foot of the mountains to the river. But the 

 danger is not overcome. The detritus is merely transported from the torrential valley 

 to the main valley. Others believe that it is impossible to slow up the torrent by 

 these canals and use the water for commerce. They beheve that a dam can be built 

 high enough to retain the flood waters and that these waters, once stored, can be used 

 for commerce or for agriculture, thus making the flood a benefit rather than a damage. 

 Quite often this method is possible, provided the ground and the economic conditions 

 permit, but it cannot be considered a general answer to the problem. Foresters take 

 another point of view. We have already seen that the rapidity of erosion depends on 

 the fluid mass, on the slope, on the river bed, and on the resistance of the ground. Is 

 it not necessary, therefore, to try to retard the flow and diminish it and to retain the 

 rocky debris which is sliding on the slopes? This result can be obtained only by a 

 forest cover on the soil in and between the existing ravines.'' 



Surrel (quoted by Huffel) concluded that: " (1) Forests stop the forma- 

 tion of torrents; (2) deforestation delivers the soil as a prey to torrents; 



' The methods of forestation are described in Chapter V and the chief species used 

 in French reforestation work are given in the Appendix, p. 407. 



