values of SB1/3/n2 and Q%gB5 are determined as 30

Froude number of flow of 0.8, the corresponding

for S and Q based on n = 0.015 and B = 1 foot

yields

and 0.275, respectively. Solving these regulations

Q%gB5 to be from 3 to 30 and 0.085 to 0.275,

Greater widths of hydraulically efficient rectangular

channels would convey greater discharges, but

respectively. The relations between discharge and

slopes flatter than 0.00675 foot per foot would be

channel width for subcritical rectangular channels

required to prevent the Froude number of flow

with a depth-to- width ratio of 0.5, a slope of 0.001

from exceeding 0.8. Therefore, a rectangular chan-

foot per foot, and a Manning's n of 0.015 can be

nel of the most efficient cross section and a slope as

plotted as shown in figure D-7 to select the 11.5-

steep as 0.01 foot per foot are not practical for

foot-width of channel required to convey the design

subcritical conveyance of the design discharge and

discharge of 400 cubic feet per second.

(17) As a check, the exact value of SB1/ 3/n2

the example problem. A similar analysis for any

shape of channel would result in the same conclu-

can be calculated to be 10.1 and used in conjunc-

sion; stable subcritical conveyance of the design

tion with a D/B ratio of 0.5 and figure D-6 to

straining parameters, Q%gB5 = 0.16 and F = 0.47,

discharge on a slope of 0.01 foot per foot is not

obtain corresponding values of the remaining con-

feasible.

(15)

Assuming that the average slope of

required to satisfy all of the dimensionless relations

the local terrain was about 0.001 foot per foot for

for rectangular channels. The actual discharge

the example problem, practical subcritical paved

capacity of the selected 11.5-foot-wide channel

channels could be designed as discussed in

with a depth of 5.75 feet can be calculated based on

paragraphs (16) through (19) below.

these relations to ensure the adequacy of the

(16)

Based on the desired range of Froude

selected design. For example, based on the

numbers of flow (0.25 to 0.8) in a rectangular

magnitude of the discharge parameter (0.16), the

channel of efficient cross section (D/B = 0.5),

channel should convey 407 cubit feet per second:

figure D-6 indicates the corresponding range of

values of the restraining parameters SB1/3/n2 and

Similarly, based on the Froude number of flow to

0.47, the channel should convey a discharge of 422

cubic feet per second:

Therefore, the 11 .5-foot-wide channel is sufficient

(18) A similar procedure would be followed

for subcritical conveyance of the design discharge

to design a trapezoidal channel with a depth-to-

of 400 cubic feet per second and, based on figure

width ratio of 0.3, a slope of 0.001 foot per foot,

D-1, is sufficient for transporting materials as large

and a Manning's n of 0.015 utilizing figure D-3.

as average size gravel.

For example, in order to maintain a Froude number