INLAND DRAINAGE 



127 



< ^53S^ 



"In order to avoid scouring out of banks above the lining by storm 

 water, particularly at sharp curves or bends of ditches, the outer 

 wall lining should be raised to meet the condition, or the ditch may be 

 widened or key-walls installed. Key-walls . . . will prevent the 

 side-scour and under-scour of linings of straight ditches of heavy grades. 

 The key-wall should extend 6 inches to a foot or more into the ground 

 below the bottom ditch lining. Branch ditches should enter lined 

 ditches at an acute angle or on 

 a curve, with a sharp grade near 

 the junction. 



"In all cases, weep-holes or 

 seepage-holes, sloping toward the 

 center or bottom of the ditch 

 and located just above the key- 

 wall, should be provided. This 

 is required to prevent the effect 

 of currents which may be under 

 or behind the concrete ditch lining. Weep-holes should be made in 

 side walls before the concrete has set. They should be used wherever 

 water might be behind the lining." 



Fig. 87.- 



(MoJifieJ after U Prince) 



-Cross-section of key-wall. 



, S/one and qmve/ 



SUB-SURFACE TILE DRAINS 



Tile drainage within the last 25 years has become of great 

 importance as an agricultural measure to improve crops by 

 reducing excessive moisture in the soil. Such drainage is of great 

 value in anti-mosquito work, inasmuch as it 

 lowers the water-table, thus allowing pools of 

 surface water to be absorbed more rapidly. 

 Tile drains also may be used to intercept seepage 

 water and for other purposes in connection with 

 anti-mosquito drainage. 



Sometimes in an anti-mosquito campaign 

 it is possible to persuade owners of low-lying, 

 swampy lands about a town to tile drain them 

 for agricultural purposes. Where this can be done, a double 

 benefit is derived — the lands are made more productive and 

 the mosquito breeding-places thereon permanently eliminated. 



Again, in purely anti-mosquito work, it has been found that in 

 the long run it frequently pays to substitute tile drains for open 

 ditches. According to the report of the Panama Canal Zone 

 Health Department for 1919, substitution of tile drains for open 

 ditches in pasture lands resulted in a considerable saving by 



Fig. 88. — Cross- 

 section of fill over 

 intercepting tile 

 drain. 



