270 



STATE BOARD OF AGRICULTUKE. 



No. 2. Improper form for opeu ditches. 



When the ground will admit the use of teams, open ditches can be con- 

 structed very cheaply, and given a very flat slope, with the plow and scraper. 

 The practice of leaving the waste earth close to the banks of open ditches is a 

 very pernicious one. This earth should be thrown well back from the ditch, 

 and so leveled off or distributed as not to prevent the surface water from 

 reaching the ditch. If it is left on the bank of the ditch its weight tends to 

 cave the ditch in, it is also liable to be worked in by rains. 



The construction of open ditches is less affected by variations from grade 

 than a line of tile under-drains since the ditch is permanently open, obstruc- 

 tions to the flow of water can be seen and removed at any time. In practical 

 work of construction, it has been found that good results are secured if stakes 

 showing the depth are placed only once in twenty rods. In long lines of open 

 ditches it is essential that a level of the line be taken, as in most cases the fall 

 that can be secured is slight. This level is required in all town or county 

 drains. 



LEAST FALL 02s OPEN" DRAINS. 



The least fall cf large bodies of flowing water may be very small indeed. 

 The average fall for the Amazon river is only six inches per mile. The aver- 

 age fall of the Lower Nile, seven inches per mile ; the Lower Ganges, four 

 inches ; the average for the whole Mississippi, seventeen inches, below Cairo, 

 twenty-eight inches ; the Ohio, six inches. The Ehine is the most rapid 

 large river in the world, its fall from Geneva to Lyon being eighty inches per 

 mile, and from Lyon to its mouth, thirty-two inches per mile. 



The flow of water, or strength of current, depends as well upon the volume 

 and depth, as upon the slope. This fact has probably been noticed by all, 

 that the flow through the same stream is much stronger at times of high water 

 or freshets, than at low water. From observation of the action of the laws 

 of nature in rivers, much can be learned in regard to the form and slope of 

 open ditches. 



From the fact that the strength of current depends upon the volume as 

 well as the depth, this general law is observed, that the larger the drain the 

 smaller the grade necessary to secure efiicient working. The even, regular 

 banks and direct course are also of advantage to the artificial, as compared 

 with the natural stream. The least slope for ditches three or four feet wide 

 across the bottom, should not average less than one foot per mile, although 

 occasional stretches may be much less, provided a good body of water is flow- 

 ing. Changes in the grade line, however, should be as slight as possible, 

 otherwise material taken up by the water where the current is strong, will be 

 deposited where it is slack. The action of a stream, itself, tends to a uniform 

 grade line. There is also a maximum fall as well as a least fall. It is evident 

 that the current should not be so great as to corrode and eat away the banks. 



The open ditch serves its purpose completely as an outlet for large swamps 



