28 
BULLETIN 181, U. S. DEPARTMENT OF AGEICULTURE. 
METHODS OF COMPUTATION. 
In computing the sizes of ditches and levees and the capacities 
of the floodways, the Chezy formula, v = c^rs, was used. In this 
formula c is a coefficient dependmg 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 floodway 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 wiU flow in a direction 
opposite to that in the floodway. 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 floodway; hence it 
should be safe to com- 
pute the discharge of 
the floodway as though 
the channel did not 
exist, adding thereto the 
discharge of the channel to obtain the total capacity of the flood- 
way. The capacity of a: 3| 
the section efgJi (fig. 8) 
was computed by using 
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 vaUey, and 
n was taken as 0.040. By adding the two results the total capacity 
of the floodway was obtained. 
Fig. 
50 60 
Drainage Area in Sq Miles 
-Discharge curve used in design of ditches, Big Black 
River, Miss. 
11 
Fig. 7.— Sketch showing directions of flow in floodway, Big 
Black River, Miss. 
