106 



MOSQUITO ERADICATION 



ally coming out of the ground, the remainder of the swamp lower down 

 the hillside became completely dry. 



"In each of these cases, collection of the water depended upon the 

 ditches being constructed primarily as intercepting rather than con- 

 ducting ditches. In the case of swamp No. 1, the ditches happened to 

 be intercepting and conducting at the same time; but more often 

 separate conducting ditches must be constructed to carry off the water 

 after it has collected in the intercepting ditches." 





>0r • jfcfr 





m*\ '/W, !>W 



Fig. 69. — Seepage outcrop, Case No. 3. (After Metz.) 



DITCH CONSTRUCTION IN GENERAL 



The open ditch is the type of ditching most frequently 

 employed in anti-mosquito drainage. While the first cost is 

 undoubtedly less, it is believed, however, that, over a period 

 of years, some more permanent type of ditch is cheaper, owing 

 to the more or less continuous maintenance expense that open 

 ditches require. Open ditches must be kept free from grass, 

 weeds, debris, etc., and the scouring action of the water in 

 flood-times must also be repaired. 



Ditching may be required on firm, open land, where the expense 

 is at a minimum, or in wooded swamp areas, where the work 

 becomes costly and tedious. The methods, of course, will vary 

 with conditions; thus, on firm ground, machinery may be em- 

 ployed to great advantage; in swamps, dynamite may be cheapest. 



In all cases, however, the following principles are applicable: 



1. The ditches should be just as few as will answer the purpose. 



2. Wherever possible, they should have clean-cut, sloping- 

 edges, narrow bottoms and straight courses. 



