DRAINAGE 



223 



neath the surface, receive the water, thoroughly filtered 

 and clear of sediment, and carry it to the drainage well 

 by which it is carried through the impervious layer and 





Figure 119. Elevating grader opening large ditch. 



enters the loose gravel or sand layer below. If the 

 water were allowed to run from the surface into the 

 drainage well, so much debris would be carried in that 

 the well would soon become clogged and water would 



Figure 120. Floating dredge. Longitudinal view showing scoop taking earth out of 

 the ditch in front of the boat. Cross-section showing scoop la position to deposit 

 earth on the bank of the canal. 



no longer sink freely through it. However, in some 

 instances, where the impervious layer is near the sur- 

 face and is not thick, drainage wells may be left open 



