Attack by Drainage 



131 



area. The extent of the seepage outcrop line 

 may vary with the duration of a rainy season. 

 Some seepage outcrops may be permanent, others 

 intermittent, and yet others may be active only 

 for a short time, during periods of excessive rain, 

 and may not become active every year. 



The character of the surface soil below the 



LongLtadinal Seot'ioa 

 Intercepting Tile Drain 



OroBS Section 

 Intercepting 



Xiie Stain- 

 Longitudinal section and cross section of intercepting tile 

 drain. 



seepage area may vary. If more or less imper- 

 vious, water remains on the ground surface, and 

 Anopheles axe produced. In places where seepage 

 water is commonly found, it is well to examine all 

 sharp changes of grade in surface topography, and 

 the ground near the toes of abrupt slopes. As the 

 rainy season advances, the line of seepage outcrop 

 often moves up the slope, and in controlling this 

 water the highest points of the outcrop must be 

 determined. Under the conditions existing at 

 Panama it was found best, as a general rule, to 



