128 The Control of Mosquitoes 



second classification for varying periods of time 

 during the wet season, depending upon the rain 

 distribution in respect to time or continuity. 



Storm-water drains may fail to give satisfactory 

 results because of a tendency of the bottom or 

 sides to scour at times of heavy flow. The charac- 

 ter of soil at the bottom and sides of the ditch de- 

 termines its ability to remain standing with fixed 

 cross-section. It should be remembered that at 

 times of maximum rainfall the ditch water may 

 carry large quantities of gravel and stones, which 

 assist in causing more erosion than usual. Other 

 conditions being equal, the steeper the grade of 

 the ditch the greater will be the tendency to the 

 scouring of the sides and bottom. Soft spots or 

 places are often found along the line of the ditch 

 where the texture of soil lends itself to erosion; 

 and washouts may be expected at these points. 



Again, where the ditch becomes temporarily 

 obstructed by stone or otherwise, excessive local 

 scouring action may ensue, removing soil from 

 below the grade line of the ditch bottom, and 

 causing a hole in it. During subsequent storms 

 one or more stones may collect in this depression, 

 and travel with a circular grinding motion, that 

 enlarges the hole. These cavities are known as 

 pot-holes, and retain water long after the storm 



