530 



IOWA DEPARTMENT OF AGRICULTURE. 



TABLE 4-NUMBEtl OF ACRES DRAINED BY OPEN DITCHES. 

 Depth of Water 7 feet. Depth of Ditch at least 9 feet. 



TABLE 5— NUMBER OF ACRES DRALNED BY OPEN DITCHES. 

 Depth of Water 9 feet. Depth of Ditch at least 1 U feet . 



Average Width of Water. 



The tables for open ditches are calculated by the well known Kutter's formula, using 

 a ' 'coefficient of roughness " equal to 0. 03. This coefficient of roughness is the value rec- 

 ommended by gutter for channels in moderately good condition having stones and weeds 

 occasionally. For ditches in first-class condition, the number of acres may be increased 

 25% . The tables have been calculated for ditches having sides with slopes of one foot 

 horizontal to one toot vertical but are approximately correct for other slopes. 



The capacity of the ditches has been made as recommended by C. G. Elliott, U. S. 

 Agricultural Department drainage expert, as follows, the ditches to run not more than 

 8-10 full for the capacities mentioned : 



Above the upper heavy line (tables 2, 3, and 4>, % in. depth of water per 24 hours. 



Between the heavy lines, y^ in. depth of water per 24 hours. 



Below the lower heavy line J^ in. depth of water per 24 hours. 



Local conditions may vary the size needed, and it is necessary to consult a drainage 

 engineer in each case. 



Example 4. What width of ditch, having a fall of 5 feet per mile, and a depth of 

 water of 3 feet, will be required to drain an area of 5 square miles (3200 acres) ? Answer. 

 About 12 feet. 



Example 5. What sized ditch having a fall of 3 feet per mile, and 9 feet depth of 

 water, will drain an area of three townships (69120 acres)? Answer. About 22 feet. 



