586 PROTECTION AGAINST INUNDATIONS. 



SECTION II. INUNDATIONS. 

 1. Causes of Inundations. 



Inundations originate in valleys and plains owing to the 

 sudden thaw of masses of snow in the mountains or plateaux 

 above them, or to prolonged or heavy rainfall, or to the 

 interruption of watercourses hy landslips. 



The last of these causes is frequently due to clearing forests 

 from mountain sides, and to bad management of protection 

 forests. When, on hill-sides, the effects of rain and surface- 

 drainage are not reduced by a full leaf-canopy and by the 

 binding effects of the roots of the trees on the soil, as well 

 as the sponge-like action of the natural soil-covering of dead 

 leaves and moss on the surface-water, the latter runs down 

 unimpeded into the valleys, bringing with it quantities of 

 boulders, gravel, silt and mud ; this raises the beds of the 

 watercourses, and causes them to overflow and spread 

 destruction far and wide into the lower country, especially 

 by leaving cultivated lands covered with gravel and silt after 

 the floods have subsided. 



Most inundations in Central Europe occur in April, when 

 they are due to the general melting of the mountain snow, or 

 in August or November, owing to heavy rainfall. 



2. Damage done. 



Inundations carry away the soil-covering and humus from 

 forests, causing swamps and cold soil; they destroy young 

 plants, hinder the formation of coppice-shoots, interrupt 

 fellings and the export of forest produce, and often carry 

 away timber to great distances. The floating wood endangers 

 bridges, river-bank protection works, and works constructed to 

 facilitate the floating of timber. In spring-floods, trees 

 growing along the banks of streams may suffer from the 

 friction of the ice which is carried down. Many game-animals, 

 especially roe-deer, are drowned. 



The mud brought down by the flood, however, richly com- 

 pensates for the loss of soil-covering and humus. Eiver mud 

 contains not only nutritive mineral salts, but yields lime in 

 a fine state of division, which is therefore readily absorbed by 



