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BULLETIN 181, U. S, DEPABTMENT OF AGEICULTUEE. 



METHODS OF COMPUTATION. 



In computing the sizes of ditches and levees and the capacities 

 of the floodways, the Chezy formula, v = c-yjrs, was used. In this 

 formula c is a coefficient depending upon channel 

 conditions and determined by Kutter's formula, in 

 which the coefficient of roughness, n, was taken at 

 0.030 for ditches, 0.035 for cleared channels, and 

 0.040 for floodways. 



To provide a margin of safety, ditches were 

 given a depth of 1 foot greater than that computed 

 as necessary to handle the discharge. The tops 

 of the levees were taken at 3 feet above the high- 

 water line as computed. 



In determining the capacity of the fioodway it 

 was necessary to consider its cross section in two 

 parts, owing to the fact that in many of the bends 

 of the channel the water will flow in a direction 

 opposite to that in the fioodway. Such a condition 

 is shown at a in figure 7. The friction existing be- 

 tween the two bodies of 

 water flowing in oppo- 

 site directions is with- 

 out doubt less than that 

 between the water and 

 the ground surface in 

 the fioodway; hence it 

 should be safe to com- 

 pute the discharge of 

 the fioodway as though 

 the channel did not 

 exist, adding thereto the 



Drainage Area in Sq Miles 



Fig. 6.— Discharge curve used in design of ditches, Big Black 

 River, Miss. 



U 

 ui 

 > 



■J 3? 



3£ 



^11 



discharge of the channel to obtain the total capacity of the fiood- 

 way. The capacity of 

 the section ejgh (fig. 8) 

 was computed by asing 

 the slope of the river 

 channel, and taking n 

 equal to 0.035; whereas 

 in determining the ca- 

 pacity of the section 

 ahcd the slope used was 

 that of the valley, and 

 n was taken as 0.040. By adding the two results the total capacity 

 of the fioodway was obtained. 



Fig. 7.- 



j.Gii,del 

 -Sketch showing directions of flow In fioodway. Big 

 Black River, Miss. 



