l6o ENGINEERING FOR LAND DRAINAGE. 



sheds, as well as experience with artificial ditches, does 

 not sustain this opinion. The larger the area the 

 greater the proportion of drainage-water which is held 

 back by obstructions, so that the discharge of the out- 

 let ditch for a given time is not so great, neither is the 

 proportion which is finally discharged as great for large 

 as for small areas. The quantity of water to be carried 

 per acre is greater for open ditches than for tile drains, 

 since we have assumed that much of the flood water is 

 to be carried by surface drains in the case of large tile- 

 drained areas, and hence we must make provision for 

 carrying the final surplus through the main channels 

 which are now under consideration. 



From a large number of observations upon areas 

 which have been drained by ditches whose discharging 

 capacities were known, it has been found that calcula- 

 tions should be made upon the following basis, in which 

 it is assumed that at flood time the main ditch is to run 

 eight tenths full. 



Areas from looo to 3000 acres, with lateral fall of 

 land toward the main at the rate of 2 to 3 feet per mile, 

 remove | inch in depth of water in twenty-four hours. 



Areas 3000 to 8000 acres remove ^ inch in depth 

 in twenty-four hours. 



Areas 8000 to 30,000 acres remove J inch in depth 

 in twenty-four hours. 



If the land is rolling so that considerable areas have 

 slopes of I to 3 or more feet per 100, the above figures 

 should be increased proportionally. For ordinary 

 drainage districts the ^-inch standard will usually ap- 

 ply, understanding that very heavy rainfall will very 

 nearly fill the ditch* 



