140 The Control of Mosquitoes 



water temporarily diverted until the concrete has 

 set. A thin layer of concrete about one inch thick 

 should be laid on the prepared ditch bottom, the 



Betsinine Wall 



ueed at Sharp 



Curve of Concrete 



Lined Ditch 



Jtetalulng Wai; 



Key Walls used at 



Outside Edge of Sharp 



Curre of Concrete 



Lined Ditch 



To Prevent Storm Water 

 LeavlnB Ditch 



To prevent water leaving lined ditches at bends or sharp 

 curves, key waUs are used to break the current, or 

 the outside wall is made higher. 



wire mesh pinned down on it, and the second layer 

 of concrete installed, leaving the wire mesh 

 embedded in the concrete. River gravel is as good 

 as broken stone or screenings and sand for this 

 purpose. In larger ditches the sides are sloped, 

 and lined for about a foot above the floor line. 



