Attack by Drainage 149 



washed away. The grades of pipe Hnes conveying 

 ditch water under roads or buildings should be 

 slightly more than that of the ditch leading to 

 them. Their entrances should be screened to 

 prevent debris entering the pipe without causing 

 water to be impounded above the pipe at time of 

 floods. A stone or board floor, or apron, at the pipe 

 outlet will prevent erosion. 



The stability of open ditches and the cost of 

 keeping them to true grade and uniform cross 

 section with freedom from larvae depends upon the 

 following factors: 



Character of the soil. 



Frequency of heavy rainfall. 



Grade of the ditch bottom. 



Change of grade with corresponding deposit of 



material carried in flood time. 

 Presence of seepage water in banks of ditch. 

 Natural angle of repose of ditch banks when wet. 

 Presence or absence of vegetable matter washed 



downstream. 

 Absence or presence of grass on the banks. 



Natural watercourses are classified as ditches, 

 and have to be trained to keep the water at maxi- 

 mum velocity during dry periods. 



The question of uniform cross section of ditches 

 has already been referred to, and variations of 

 width in a body of flowing water may not only 



