1.2 and less than 2.0 if supercritical to prevent flow

capacities of channels that will satisfy the desired

instabilities and excessive wave heights. It should

conditions. The calculated values of discharge and

also be designed to have a depth-to-width ratio as

channel widths can be plotted on log-log paper as

near 0.5 (the most efficient hydraulic rectangular

shown in figure D-7 to determine the respective

cross section) as practical depending upon the local

relations for supercritical rectangular channels with

conditions of design discharge, maximum depth of

a depth-to-width ratio of 0.5, a slope of 0.01 foot

flow permissible, and commensuration of a slope

per foot, and a Manning's n of 0.015. Figure D-7

with that of the local terrain.

may then be used to select a channel width of 7.5

(12)

For example, assume that a paved

feet for conveyance of the design discharge of 400

rectangular channel is to be provided with a

cubic feet per second. The exact value of the

constraining parameter SB1/3/n2 can be calculated to

Manning's n = 0.015 and a slope of 0.01 foot per

foot (average slope of local terrain) for conveyance

be 87 and used in conjunction with a D/B ratio of

of the remaining constraining parameters, Q%gB5 =

of a design discharge of 400 cubic feet per second

0.5 and figure D-6 to obtain corresponding values

at supercritical conditions. A depth-to-width ratio

of 0.5 is desired for hydraulic efficiency and a

0.48 and F = 1.4, required to satisfy all of the

Froude number of flow between 1.2 and 2.0 is

dimensionless relations shown in figure D-6. The

desired for stable supercritical flow. The range of

actual discharge capacity of the selected 7.5-foot-

values of the parameter SB1/3]/n2(70-180) required

wide channel with a depth of flow equal to 3.75

to satisfy the desired D/B and range of Froude

feet can be calculated based on these relations to

number of supercritical flow can be determined

ensure the adequacy of the selected design. For

parameter %gB5 (0.44-0.68) can also be determined

from figure D-6. Corresponding values of the

example, based on the magnitude of a discharge

parameter equal to 0.48, the channel should convey

from figure D-6 for calculation of the discharge

419 cubic feet per second:

Similarly, based on the magnitude of a Froude

number of flow equal to 1.4, the channel should

convey a discharge of 432 cubic feet per second:

Obviously, the capacity of the 7.5-foot-wide chan-

nel is adequate for the design discharge of 400

cubic feet per second.

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