1 6 The Drainage of Fens and Low Lands. 



is increased to 3 to i in light soils, easily washed away, or 

 where the substratum is soft and will not bear the weight 

 of the sides after the drain is excavated. 



In excavating drains where the substratum consists of soft 

 alluvial soil, frequently overlain by mateiial of a denser and 

 more stable character, the bottom will spring up and let the 

 sides down. When ground of this character has to be 

 encountered, the soil excavated must be moved to a sufficient 

 distance to prevent its weight forcing the sides into the 

 cutting, and should not be placed nearer than six feet. It 

 may even be necessary in very bad places to build the sides 

 up with fascines to prevent them slipping in. 



As the drains diminish in size the slopes may decrease to 

 I ^ to I J to I for the second class drains, down to | to i for 

 the main ditches. 



Area of Land occupied by Drains. — The area of land 

 occupied by drains varies considerably with the position and 

 chaiacter of the land and its means of discharge. In the 

 low polders in Holland, drained by windmills, it is estimated 

 that one-tenth of the land is occupied by water. If drained 

 by steam power, one-twentieth is frequently occupied. The 

 following proportions of the area of drains to the total area of 

 land of the principal polders in Holland is given in Huet's 

 ' Stoombemaling van Polders en Boezems ' : — 



Polders. «Z1?* Proportion. 



De Beemster 18,824 



Haarlem Meer 44>7oo 



Rijnland 305,000 



Kennemerland 16,000 



Gedeete van Vijf i8,ooo 



Amstelland 74,100 



DejQand 74,100 



Wateiscliap de Rotte in Scliieland . . 27, 200 



13-4 

 t 



J 



27 

 1 

 38 



X 



_ 



I 



59 



% 



77 

 1 



8s 



* * Stoombemaling van Poldeis en Boezems,' doox A. Huet, C.E h Gravenhage, 



iSS5 



