INUNDATIONS. 



591 



(c) Valley -revetments. (German, Thalspcrren ; French, 

 Barrages.} These are constructions that reduce the gradient 

 of the torrent bed and the destructive power of the water. 



Fig. 262. 



They protect not only the bed of the torrent below them, but 

 raise its level behind them. 



These revetments may be made of wood or of masonry, 

 the latter owing to its superior durability is far preferable. 



Fig. 263. 



Their mode of construction varies, according to circumstances ; 

 they are erected at regular intervals and collect silt and 

 stones behind them. For small torrents and where rapid 

 cheap work is required, mere, wattle-fences may suffice, 



