592 PROTECTION AGAINST INUNDATIONS. 



Fig. 268 shows how a valley is protected by a series of 

 barrages. 



(d) Wattle-fences. These consist in simple wattle-fences 

 made in curves and almost horizontal, the central point being 

 somewhat lower than the sides ; they are afterwards raised 

 as the bed of the torrent rises (Fig. 264, A and B). When 

 further denudation is no longer feared, the middle of the 

 ravine is paved with stones as shown in Fig. 264, C. The first 

 fejices are erected at distances apart of three meters ; they 

 are commenced from below and continued up-stream. Their 

 erection is continued until the bed of the ravine is raised high 

 enough for no more denudation to be feared. 



This simple plan, devised in 1838 by Jenny, is advisable 



A B c 



Fig. 264. Jenny's Wattle-Fences. 



where only mud free from large stones comes down, and for 

 lateral ravines leading into the main torrent. 



(e) Addenda. All the above-mentioned works serve to 

 secure the bed of the torrent. The question of maintaining 

 its sides will be discussed further on. 



Such works are useful in districts with granite or other 

 rocks, other than limestone. Torrents coming from limestone 

 districts or from glaciers cannot be properly regulated. 



In connection with the above works, the slopes leading 

 down to the torrent must be properly graded, and a system 

 of horizontal protective trenches laid out, so as to cut off the 

 soil-water and distribute it over a larger area. These trenches 

 have proved extremely useful in preventing floods. They 

 are beneficial to forest-vegetation, facilitate plantations, and 

 also revi e the original springs of water. 



The local forest-staff should carry put the above works and 



