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CHAPTER IL 



DRAINAGE BY GRAVITATION. 



The drainage of every district will vary either with the 

 elevation of the ground, the nature of the soil, the distance of 

 the district from the outfall, and other circumstances. It is 

 frequently the case that a low fen or polder has not only the 

 rainfall due to its own area to contend with, but also that of 

 the adjacent higher land. When the water has to be pumped 

 this is a serious consideration ; and even when it flows away 

 by gravitation, the water coming from the higher land will 

 over-ride the low-land drainage and frequently be the cause 

 of flooding or of obstruction to the free discharge of the low- 

 land water. To obviate this, whenever practicable, a drain 

 surrounding the district is made, which collects the high-land 

 water and prevents it finding its way into the lower level. 

 These drains were termed by Mr. Rennie "catch-water 

 drains " ; and the same purpose is attained in Holland by a 

 dyke and canal termed the " ringvart" 



Rainfall. — If the district to be dealt with consists purely 

 of low land, provision must be made for the whole of the rain 

 which falls on the surface in winter, as from the low level of 

 the land none of the rain can soak away. In winter the 

 loss by evaporation and absorption by vegetation in wet 

 weather amounts to scarcely any perceptible quantity, and 

 with the outfall stream probably bank-full, and all the sur- 

 rounding land saturated, no deduction can be made for natural 

 soakage. If the district consists partly of high land, the pro- 

 portionate area of the drains can be made less, as some of the 

 rain of the higher districts will be absorbed by the soil on 



