DRAINAGE 



227 



tile, receives its water only after it has filtered through 

 a few feet of soil and carries very little solid sediment. 

 In cold countries, the sewer will sometimes allow the 

 water to flow through 

 much earlier in the 

 spring than will the 

 deep open drain under 

 the conditions just men- 

 tioned, since the ice and 

 snow that will accumu- 



1 4. ji,^ j^ /K+^t, Figure 129. Drainage well beside pond; B, 



late in the Cleep dltCn tile drain discharging into drainage well; C, 



, 1 1 -i f porous earth; D, impervious stratum through 



mtlSt be melted beiOre which the water cannot sink; E, layer of gravel 



1 into which the water entering the well will sink. 



the accumulated water 



can begin to flow. This difference often makes it wise 

 to use the sewer rather than the open drain in carrying 

 surface water through higher portions of land. The cost, 



however, must be very care- 

 fully calculated because large 

 tiles and the deep excavations 

 for such sewers are expensive. 

 Stone and board drains. 

 In the earlier history of drain- 

 age, before earthen tiles were 

 used, stone and wood, and 

 even pieces of sod and peat, 

 were used in the construction 

 of underdrains. In Figure 

 131 are shown drains made 

 of stone laid in different ways. 

 In Figure 132 is shown the V- 

 shaped drain in the bottom of 

 the ditch covered with a plank 

 laid on shoulders of earth, this 

 plank sustaining the weight of 

 Figure 130. vertical outlet for tile the earth thrown back into 



drains through impervious stratum 



wa\er s SoS m the wh tiie ms receive the t " e ditch, ^ so other methods 



