5.1 UNIFORM FLOW
Uniform flow is strictly applicable only to a prismatic channel.
In uniform flow, flow depth, flow area, mean velocity, and discharge are constant along the channel.
For nonprismatic (natural)
channels of nearly uniform cross section, the term equilibrium flow is often used to describe
the flow condition that approximates or resembles the uniform flow of prismatic channels.
In uniform flow, all slopes, friction slope S_{f}, energy slope
S_{e}, watersurface slope S_{w}, and bottom slope S_{o},
are constant and equal
to one slope: S.
S_{f} = S_{e} = S_{w} = S_{o} = S
 (51)

There is no such thing as unsteady uniform flow. If the flow is unsteady, it is simply not uniform.
However, under a Vedernikov number V = 1,
uniform flow becomes neutrally stable and this is conducive to the development of roll waves
(Section 1.3). This is the "instability of uniform flow"
described by Chow (1959). At V < 1, flow disturbances are attenuated, and roll waves do not develop.
Establishment of uniform flow
From a mechanical standpoint, uniform flow occurs in a control volume
when the frictional force is equal to the gravitational force.
In the absence of section controls (Section 4.3), all openchannel flows
tend toward
uniform flow. In essence, Mother Nature "likes uniform flow."
In uniform flow, the property of uniqueness of the dischargeflow area rating (or dischargeflow depth rating) qualifies uniform flow as a channel
control.
Thus, uniform critical flow is a very strong type of openchannel control.
The depth of uniform flow is referred to as normal depth.
Figure 51 shows the establishment of uniform flow in
a sufficiently long channel. The top figure depicts subcritical normal flow, with upstream and downstream section controls.
The center figure depicts critical flow, with upstream and downstream section controls.
The bottom figure depicts supercritical normal flow, with upstream section control only.
Fig. 51 Establishment of uniform flow (Chow, 1959).


Velocity of uniform flow
In general, the mean velocity of uniform flow is described by the following formula:
V = C R^{ x} S^{ y}
 (52) 
in which C = friction coefficient, R = hydraulic radius, defined as R = A/P, and x and y
are exponents of R and S, respectively. The exponents vary with type of roughness (laminar, turbulent, transitional, or mixed laminarturbulent)
and crosssectional shape (arbitrary, hydraulically wide,
rectangular, trapezoidal, triangular, or inherently stable).
In practice, there are two established uniform flow formulas:
(1) the Chézy formula, and (2) the Manning formula.
Variations of these formulas are in current use (the dimensionless Chézy
and the ManningStrickler).
5.2 CHÉZY FORMULA
To derive the Chézy formula, the shear stress τ_{b} developed along the
channel bottom is modeled as a quadratic friction law:
in which ρ = mass density, f = a type of friction factor (drag coefficient), and V = mean velocity. This equation is dimensionless; therefore, it has a strong theoretical basis.
The shear force developed along the wetted perimeter of a control volume of length L is (Fig. 52):
F_{s} = τ_{b} PL = ρ f V^{ 2} PL
 (54) 
Fig. 52 The control volume for uniform flow (Chow, 1959).


The weight of the water in the control volume is W. This gravitational force is resolved along the direction
of motion to give:
For a channel of small slope: sin θ ≅ tan θ = S. Therefore:
F_{g} = W tan θ = W S = γ A L S
 (56) 
Equating frictional (Eq. 54) and gravitational forces (Eq. 56):
ρ f V^{ 2} P = γ A S
 (57) 
which reduces to:
f V^{ 2} = g (A /P ) S = g R S
 (58) 
where R = hydraulic radius.
Solving for V:
V = (g/f )^{1/2} (R S ) ^{1/2}
 (59) 
V = C (R S )^{1/2}
 (510) 
in which C = Chézy coefficient, defined as follows:
Thus, the friction factor f in Eq. 53 is:
g
f = ^{______}
C^{ 2}
 (512) 
Equation 510 is the Chézy formula.
A variation of the Chézy formula may be derived by solving for bottom slope S from Eq. 58:
V^{ 2}
S = f ^{ _____ }
g R
 (513) 
which is equivalent to:
D V^{ 2}
S = f ^{ _____ } ^{ _____ }
R
g D
 (514) 
Given Eq. 46, Eq. 514 reduces to:
D
S = f ^{ _____ } F^{ 2}
R
 (515) 
Equation 515 is basically the same as Eq. 45, which was
derived from the DarcyWeisbach equation applied to openchannel flow.
Thus, the dimensionless Chézy equation (Eq. 515)
and the modified DarcyWeisbach equation (Eq. 45) are
the same.
For a hydraulically wide channel, D ≅ R, and Eq. 515 reduces to:
which is the same as Eq. 48.
Table 51 shows corresponding values of f, DarcyWeisbach friction factor f, and Chézy coefficients.
Table 51 Corresponding values of f, f, and C.

Friction factor f 
DarcyWeisbach f 
Chézy C 
SI units 
U.S. Customary units 
0.002 
0.016 
70.02 
126.83

0.003 
0.024 
57.17 
103.55 
0.004 
0.032 
49.51 
89.68 
0.005 
0.040 
44.29 
80.21 

History of the Chézy Formula




Antoine Chézy was born at ChalonsurMarne, France, on September 1, 1718, and died on
October 4, 1798.
In 1749, working in Amsterdam,
Cornelius Velsen stated:
"The velocity should be proportional to the square root of the slope."
In 1757, in Hannover, Germany, Albert Brahms wrote:
"The decelerative action of the bed in uniform flow was not only equal to the
accelerative action of gravity but
also proportional to the square of the velocity."
Velsen and Brahms were working on the general laws and theories of Torricelli and Bernoulli.
Chézy used some of these ideas to develop his formula.
Chézy was given the task to determine the cross section and the related
discharge for a proposed canal on the river Yvette, which is close to Paris,
but at a higher elevation.
Since 1769, he was collecting experimental data from the canal of
Courpalet and from the river Seine.
His studies and conclusions are contained in a report to
Mr. Perronet dated October 21, 1775.
The original document, written in French, is titled
"Thesis on the velocity of the flow in a given ditch,"
and it is signed by Mr. Chézy, General Inspector of
des Ponts et Chaussées. It resides in file
No. 847, Ms. 1915 of the collection of manuscripts in the
library of the École.
In 1776, Chézy wrote another paper, entitled:
"Formula to find the uniform velocity that the water will have in a ditch or in a
canal of which the slope is known."
This document resides in the same file [No. 847, Ms. 1915].
It contains the famous Chézy
formula:
V = 272 (ah/p)^{1/2}
in which h is the slope, a is the area,
and p is the wetted perimeter.
The coefficient 272 is given for the canal of
Courpalet in an old system of units.
In the metric system, the equivalent value is:
V = 31 (ah/p)^{1/2}
For the river Seine, the value of the coefficient is 44.
Clemens translated into English the two Chézy papers.
Riche de Prony, one of Chézy's former students,
was the first to use the Chézy formula.
Later, in 1801, in Germany,
Eytelwein used Chézy's and De Prony's ideas to further the development
of the formula.

5.3 MANNING FORMULA
The Manning formula, in SI units, is:
1
V = ^{ ____ } R^{ 2/3} S^{ 1/2}
n
 (517) 
where n = Manning's friction coefficient, friction factor, or simply Manning's n.
In U.S. Customary units, the Manning formula is:
1.486
V = ^{ ________ } R^{ 2/3} S^{ 1/2}
n
 (518) 
The quantity 1.486 is a conversion factor arising from the equivalence
1.486 = 1/(0.3048)^{1/3}. The factor is required to express
the original Manning equation (Eq. 517) in U.S. Customary units.
In order to compare with the Chézy formula, the Manning equation is expressed as follows:
1.486
V = ^{ ________ } R^{ 1/6} R^{ 1/2} S^{ 1/2}
n
 (519) 
Comparing Eqs. 510 and 519,
the relation between Manning and Chézy coefficients is obtained:
1.486
C = ^{ ________ } R^{ 1/6}
n
 (520) 
Equation 520 implies that while C varies with the hydraulic radius, the value of
n does not. This may be approximately true for (artificial) prismatic channels,
but it is generally not true for natural channels (Barnes, 1967). In natural channels, the value of n may vary with stage and flow depth. This is attributed to: Natural variations in channel roughness
with increasing stage, including the effect of overbank flows (Fig. 215), or Morphological changes in total bottom friction,
composed of skin and form friction, as the flow
rises from low stage, through intermediate stage, to high stage (Simons and Richardson, 1966).
Empirical formulas for Manning's n
Several correlations between Manning's n and particle size (grain diameter) have been
developed.
Williamson (1951) correlated the DarcyWeisbach friction factor f with relative roughness to yield the following relation (Henderson, 1966):
k_{s}
f = 0.113 ( ^{ ____ } ) ^{1/3}
R
 (521) 
in which k_{s} = grain roughness, in length units,
and R = hydraulic radius.
Since f = 8 (g /C^{ 2}), Eq. 521 reduces to:
8g R
C = ( ^{ ________ } )^{1/2} (^{ _____ } ) ^{1/6}
0.113 k_{s}
 (522) 
In U.S. Customary units, Eq. 522 may be conveniently reduced to:
1.486 R^{1/6}
C = ^{ ________________ }
0.0311 k_{s}^{1/6}
 (523) 
Comparing Eq. 523 with Eq. 520, n can be expressed in terms of boundary roughness as follows (k_{s} in ft):
n = 0.0311 k_{s}^{1/6}
 (524) 
A general expression for Manning's n in terms of relative roughness and absolute roughness is (Chow, 1959):
n = [f (R/k_{s})] k_{s}^{1/6}
 (525) 
which implies that in Eq. 524 the relative roughness is a constant (0.0311).
Assuming that boundary roughness may be represented by the d_{84} particle size, i.e., that for which 84% of the
grains (by weight) are finer, Eq. 524 converts to:
n = 0.0311 d_{84}^{1/6}
 (526) 
Strickler used a constant (0.0342) for the function of relative roughness f(R/k_{s}),
and the median particle size d_{50} as the representative grain diameter, to yield:
n = 0.0342 d_{50}^{1/6}
 (527) 
Since d_{84} > d_{50}, it is seen that the Strickler and Williamson equations are mutually consistent.
Table 52 shows values of Manning's n calculated with the Strickler formula (Eq. 527).
Table 52 Values of Manning's n calculated with the Strickler formula (Eq. 527).

Mean particle size d_{50} (ft) 
Manning's n 
0.0001 
0.007 
0.001 
0.011 
0.01 
0.016 
0.1 
0.023 
1 
0.034 

History of the Manning Formula




Robert Manning was born in Normandy, France, in 1816, and died in 1897. In 1826, he moved to Waterford, Ireland, and worked as an accountant.
In 1846, during the year of the great famine, Manning
was recruited into the Arterial Drainage Division of the Irish
Office of Public Works. After working as a draftsman for a while, he was promoted to
assistant engineer. In 1848, he became district engineer, a position he held until 1855.
As a district engineer, he read "Traité d'Hydraulique" by d'Aubisson des Voissons, after which he
developed a great interest in hydraulics.
From 1855 to 1869, Manning was employed by the Marquis of Downshire, while he supervised the
construction of the Dundrum Bay Harbor in Ireland and designed a water supply system for Belfast.
After the Marquis' death in 1869, Manning returned to the Irish Office of Public Works as
assistant to the chief engineer. He became chief engineer in 1874, a position he held it until his retirement in 1891.
Manning did not receive any education or formal training in fluid mechanics or engineering.
His accounting background and pragmatism
influenced his work and drove him to reduce problems to their simplest form.
He compared and evaluated seven best known formulas of the time: Du Buat (1786), Eyelwein (1814),
Weisbach (1845), St. Venant (1851), Neville (1860), Darcy and Bazin (1865), and Ganguillet and
Kutter (1869). He calculated the velocity obtained from each formula for a given slope and for
hydraulic radius varying from 0.25 m to 30 m. Then, for each condition, he found the mean value
of the seven velocities and developed a formula that best fitted the data.
Manning's original bestfit formula was the following:
V = 32 [RS (1 + R^{1/3})]^{1/2 }
which he later simplified to:
V = C R^{x } S^{1/2 }
In 1885, Manning gave x the value of 2/3 and wrote his formula as follows:
V = C R^{2/3 } S^{1/2 }
In a letter to Flamant, Manning stated: "The reciprocal of C corresponds closely with that of n, as
determined by Ganguillet and Kutter; both C and n being constant for the same channel."
On December 4, 1889, at the age of 73, Manning first proposed his formula to the Institution of Civil Engineers (Ireland).
This formula saw the light in 1891, in a paper written by him entitled "On the flow of water
in open channels and pipes," published in the Transactions of the Institution of Civil Engineers (Ireland).
Manning did not like his own equation for two reasons: First, it was difficult in those days to
determine the cube root of a number and then square it to arrive at a number to the 2/3 power. In addition,
the equation was dimensionally incorrect, and so to obtain dimensional correctness he developed the following equation:
V = C (gS)^{1/2 } [R^{1/2
} + (0.22/m^{1/2 })(R 
0.15 m)]
where m = "height of a column of mercury which balances the atmosphere," and C was a dimensionless number "which
varies with the nature of the surface."
However, in late 19th century textbooks, the Manning formula was written as follows:
V = (1/n) R^{2/3 } S^{1/2 }
Through his "Handbook of Hydraulics," King (1918) led to the widespread use of the Manning formula as it is
known today, as well as to the acceptance that the Manning coefficient
C should be the reciprocal of Kutter's n.
In the United States, n is referred to as Manning's friction factor, or simply
Manning's n. In Europe,
the Strickler K is the same as Manning's C, i.e., the reciprocal of n.
When K is used in lieu of n,
the Manning equation is referred to as the ManningStrickler or Strickler equation.

5.4 MANNING ROUGHNESS
Given Eq. 517 (or 518), once three of the variables are known, the fourth one can be calculated. Typically, R and S are known, and n is estimated, from which V can be calculated.
This is the direct method, the most typical way of using the Manning equation.
When increased accuracy is required, or else, when n cannot
be estimated with complete certainty, a measurement of velocity V, together with the measurement of hydraulic radius R and channel slope S,
is recommended to calculate n. This procedure is referred
to as the inverse method, or the calibration method. In practice, most
applications have used the direct method.
Estimation of Manning's n
There is no exact method or procedure to estimate Manning's n.
A proven set of recommendations is given below.
Recommendations for the estimation of Manning's n
To understand the factors that affect the
value of Manning's n and proceed accordingly.
To consult a table of typical values, and to base the estimation on judgment and experience.
To consult several pictorial collections for which the value of Manning's n has been
documented with sufficient accuracy.
To become acquainted with the appearance of typical channels for which the Manning's n values
are known.

Chow (1959) presented a
pictorial collection of twentyfour (24) typical channels for which the Manning's n has been established.
The values documented by Chow range from n = 0.012 (a canal lined with concrete slabs, with very smooth surface)
to n = 0.150 (a natural river in sand clay soil, irregular sides slopes and uneven bottom). Chow (1959) listed values of Manning's
coefficient as low as n = 0.008 (lucite, acryclic plastic) to as
high as n = 0.200 (flood plains of natural streams, with dense willows, in the summer)
(Table 54).
These values are applicable to
channel flow in the turbulent regime.
Barnes (1967) presented a fullcolor pictorial collection of fifty (50) typical stream channels across the United States, for which the Manning's n had been calculated by calibration. The Barnes collection can be viewed online at Roughness Characteristics of Natural Channels. The lowest value of Manning's n documented by Barnes is n = 0.024, for the Columbia River
at Vernita, Washington (Fig. 53). The highest value of Manning's n is n = 0.075,
for Rock Creek near Darby, Montana (Fig. 54).
Fig. 53 The Columbia River at Vernita, Washington.


Fig. 54 Rock Creek near Darby, Montana.


Arcement and Schneider (1989) presented a fullcolor pictorial collection of fifteen (15) typical flood plains in the Southestern United States,
for which the Manning's n had been calculated by calibration. The Arcement and Schneider collection can be viewed online at Manning's
Roughness Coefficients for Natural Channels and Flood Plains.
The lowest value of Manning's n documented by Arcement and Schneider is n = 0.100,
corresponding to Cypress Creek near Downsville, Louisiana
(Fig. 55). The highest value is n = 0.200,
corresponding to Thompson Creek near Clara, Mississippi (Fig. 56).
Fig. 55 Cypress Creek near Downsville, Louisiana.


Fig. 55 Thompson Creek near Clara, Mississippi.


Factors affecting Manning's n
In practice, the value of Manning's n is highly variable. In natural stream channels it can range from slightly lower than 0.020 for some very large rivers featuring a
relatively smooth boundary (Fig. 57), to higher than 0.200 for
small creeks in steep mountain streams (Fig. 58).
The various factors affecting Manning's
roughness coefficient are listed in Table 53.
Fig. 57 Paraguay River at Forte Coimbra, Mato Grosso do Sul, Brazil.


Fig. 58 Rachichuela Creek, La Leche river basin, Lambayeque, Peru.


Table 53 Factors affecting Manning's roughness coefficient.

Factor 
Description 
Surface roughness 
Fine grain sizes lead to low values, while coarse grain sizes lead to high values.

Vegetation 
Type, height, density, and
spatial distribution of vegetation have a definite role in affecting flow velocity. Values of n in vegetated channels may exceed 0.250, and in some cases, rise to 0.400 or greater. 
Channel irregularities 
Sand bars, ridges and depressions, and holes/humps
in the channel bed create additional roughness in the form of local energy losses.

Channel alignment 
Generally,
a straight channel will feature a lower n, while a sinous channel will have a larger n.
Sinuosity may increase channel roughness by as much as 30% (Chow, 1959). 
Aggradation and degradation 
Changes in channel morphology
will increase/decrease roughness in unpredictable ways. The effect will depend on the type of material forming
the bed, the widthtodepth ratio (aspect ratio), and the quantity of sediment being transported (sediment load). 
Channel obstructions 
Log jams, bridge piers, and other obtructions tend to increase channel roughness. The effect will depend on the
type of obstructions, their relative size, shape, number,
and spatial distribution.

Size and shape of the channel 
Generally, smaller channels have larger roughness, while larger channels have smaller roughness (compare
Fig. 57 with Fig. 58 above). The
typically higher aspect ratio of larger channels tends to decrease roughness. 
Stage and discharge 
Roughness varies with stage and discharge in largely unpredictable ways. Mean velocities vary from very low stage to very high stage in
complex patterns. A typical sketch is shown in Fig. 215. 
Season of the year 
For vegetated channels, or channels lined with vegetation, surface roughness
increases during the growing season, and decreases
during the dormant season, subject to a latitudinal effect. 
Suspended load and bedload 
Sediment transport, either as suspended load or bed load, will consume additional energy
and lead to increases in overall channel friction. 

Cowan (1956) has developed a rational
methodology
for estimating Manning's n. Cowan's equation is:
n = (n_{o} + n_{1} + n_{2} + n_{3} + n_{4} ) m_{5}
 (528) 
where:
n_{o} =
basic n value for a straight, uniform, smooth channel
n_{1} =
addition to account for surface irregularities
n_{2} =
addition to account for variations in the size and shape of the cross section
n_{3} =
addition to account for obstructions
n_{4} =
addition to account for the effect of vegetation on flow conditions
m_{5} =
factor to account for channel sinuosity (meandering).
Table 53 lists the appropriate values to be used in Eq. 528.
Table 53 Corrections to Manning's n (Eq. 521).

Channel conditions 
Values 
Type of material on channel boundary 
Earth 
Sand, silt and clay boundary 
n_{o} 
0.020 
Rock cut 
Rock outcrop or rock boundary 
0.025 
Fine gravel 
Gravel up to 8 mm diameter 
0.024 
Coarse gravel 
Gravel of more than 8 mm diameter 
0.028 
Degree of surface irregularities 
Smooth 
Best regular condition 
n_{1} 
0.000 
Minor 
Good dredged channels, slightly eroded side slopes 
0.005 
Moderate 
Fair to poor dredged channels,
moderately eroded side slopes 
0.010 
Severe 
Badly eroded canals and channels,
highly irregular or jagged surfaces of channels excavated in rock 
0.020 
Variations in shape and size of channel cross section 
Gradual 
Smooth, or small variations 
n_{2} 
0.000 
Alternating occasionally 
Large and small sections alternate occasionally, occasional shifting of main flow from side to side 
0.005 
Alternating frequently 
Large and small sections alternate frequently,
frequent shifting of main flow from side to side 
0.0100.015 
Effect of obstructions 
Negligible 
(a) The extent to which the obstructions
occupy or reduce the flow area, (b) the character of the obstructions (sharpedged or angular objects induce greater turbulence than curved, smoothsurface objects), and (c) the
positioning and spacing of the obstructions, transversally and longitudinally, in the channel reach under
consideration 
n_{3} 
0.000 
Minor 
0.0100.015 
Appreciable 
0.0150.030 
Severe 
0.0300.060 
Effect of vegetation 
Low 
Turf grasses or weeds, where the
flow depth is 2 to 3 times the height of the vegetation 
n_{4} 
0.0050.010 
Medium 
Turf grasses or weeds, where the
flow depth is 1 to 2 times the height of the vegetation 
0.0100.025 
High 
Turf grasses or weeds, where the
flow depth is about equal to the height of the vegetation 
0.0250.050 
Very high 
Turf grasses or weeds, where the
flow depth is less than onehalf (1/2) the height of the vegetation 
0.0500.100 
Channel sinuosity 
Low 
Sinuosity less than 1.2 
m_{5} 
1.00 
Medium 
Sinuosity between
1.2 and 1.5 
1.15 
High 
Sinuosity greater than 1.5 
1.30 

Table 54 lists values of Manning's n for channels of various kinds, compiled by Chow (1959).
For each kind of channel, the minimum, normal, and maximum values of n are shown. The normal values
are recommended only for channels with good maintenance. Values generally recommended for design are shown
in bold.
Table 54 Recommended range of values of Manning's n. ^{1}

1 
2 
3 
4 
Type of channel and description 
Minimum 
Normal 
Maximum 
A 
Closed conduits flowing partly full 

A1 
Metal 


a. 
Brass, smooth 
0.009 
0.010 
0.013 


b. 
Steel 



1. 
Lockbar and welded 
0.010 
0.012 
0.014 



2. 
Riveted and spiral 
0.013 
0.016 
0.017 


c. 
Cast iron 



1. 
Coated 
0.010 
0.013 
0.014 



2. 
Uncoated 
0.011 
0.014 
0.016 


d. 
Wrought iron 



1. 
Black 
0.012 
0.014 
0.015 



2. 
Galvanized 
0.013 
0.016 
0.017 


e. 
Corrugated metal 



1. 
Subdrain 
0.017 
0.019 
0.021 



2. 
Storm drain 
0.021 
0.024 
0.030 
^{1} Chow, V. T. 1959. Openchannel hydraulics. McGrawHill, New York.


Click here to display the complete Table 54.

5.5 COMPUTATION OF UNIFORM FLOW
From Eq. 52, the discharge in openchannel flow is:
Q = V A = C R^{ x} S^{ y} A
 (529) 
Equation 529 can be expressed as follows:
Q = K S^{ y} = K S^{ 1/2}
 (530) 
in which K = conveyance, defined as:
Or, alternatively:
Q
K = ^{______}
S^{ 1/2}
 (532) 
Following Chézy:
Following Manning, in SI units:
1
K = ^{____} A R^{ 2/3}
n
 (534) 
In U.S. Customary units:
1.486
K = ^{________} A R^{ 2/3}
n
 (535) 
The conveyance K contains information on friction and crosssectional size and shape, and is independent of the channel slope.
Channels with composite roughness
A channel that overflows its banks usually has more than one value of Manning's n, one for inbank flow, and two additional ones,
one
for left overbank and another for right overbank (Fig. 59). A composite value of Manning's n may be calculated under the assumption that the velocities for all three subsections, inbank, left overbank, and right overbank, remain the same. While this assumption is convenient,
it sidesteps the issue of possible flow nonuniformity across the composite cross section.
Assume a channel of varying roughness along its wetted perimeter, with N being the number of subsections. The wetted perimeters are:
P_{1}, P_{2}, P_{3}, ..., P_{N}.
The corresponding values of roughness are:
n_{1}, n_{2}, n_{3}, ..., n_{N}.
Assuming that all velocities are equal:
V_{1} = V_{2} = V_{3} = V_{N} = V
 (536) 
For any subsection i :
1 V_{i} = ^{ ____ } R_{i}^{ 2/3} S^{ 1/2}
n_{i}
 (537) 
1 V_{i} = ^{ ____ } (A_{i} / P_{i} )^{ 2/3} S^{ 1/2}
n_{i}
 (538) 
The flow area for subsection i is:
V_{i}^{ 3/2} n_{i}^{ 3/2} P_{i}
A_{i} = ^{ _________________ }
S^{ 3/4}
 (539) 
The total flow area of the channel is:
V^{ 3/2} n^{ 3/2} P
A = ^{ _________________ }
S^{ 3/4}
 (540) 
The total flow area is equal to the sum of the subareas. Therefore:
V^{ 3/2} n^{ 3/2} P = ∑ (V_{i}^{ 3/2} n_{i}^{ 3/2} P_{i} )
 (541) 
^{N} 
From Eq. 536, all the
velocities are equal. Thus, Eq. 541 reduces to:
n^{ 3/2} P = ∑ (n_{i}^{ 3/2} P_{i} )
 (542) 
^{N} 
Thus, the value of Manning's n for a channel of composite cross section is:
∑ (n_{i}^{ 3/2}
P_{i} ) N
n = [ ^{ _________________ } ] ^{2/3}
P
 (543) 
Fig. 59 A composite channel cross section.


Channels of compound cross section
The cross section of a channel may be composed of several distinct subsections, and each of them may have a different roughness.
For example, an alluvial channel subject to seasonal flooding generally consists of a main channel and two side channels (Fig. 59).
The side channels are usually rougher than the main channel; therefore, the mean velocity in the main channel
is usually greater than those of the side channels.
The Manning equation may be applied separately to each subsection, and the total discharge is equal to the sum of the subsection discharges.
For the channel as a whole, the mean velocity is equal to the total discharge divided by the total area.
The velocity distribution coefficient applicable to the whole channel is different from the velocity distribution coefficients applicable to each
subsection. The total velocity distribution coefficient may be computed as explained below.
Assume a total number of subsections N and several subsections i,
where i varies from 1 to N. From continuity (Eq. 24):
Q_{i} = V_{i} A_{i}
 (544) 
where Q_{i} = discharge through subsection i,
and V_{i} = mean velocity through subsection i, of flow area A_{i}.
Furthermore, from Eq. 530:
Q_{i} = K_{i} S^{ 1/2}
 (545) 
where K_{i} = conveyance through subsection i (Eq. 533 or 534).
Combining Eqs. 544 and 545:
V_{i} = (K_{i} / A_{i}) S^{ 1/2}
 (546) 
The total discharge is:
N
Q = ∑ Q_{ i}
i = 1
 (547) 
Using Eq. 545 in Eq. 547:
N
Q = ( Σ K_{i} ) S^{ 1/2}
i = 1
 (548) 
From Eq. 24, Q = V A. Thus:
N
V = ( Σ K_{i} ) ( S^{ 1/2} / A )
^{i = 1}
 (549) 
From Eq. 224, the energy coefficient is restated as follows:
N
Σ V_{i}^{ 3} A_{i}
^{i = 1}
α = ^{_____________}
V^{ 3}A
 (550) 
Likewise, from Eq. 231, the momentum coefficient is:
N
Σ V_{i}^{ 2} A_{i}
^{i = 1}
β = ^{_____________}
V^{ 2}A
 (551) 
Replacing Eqs. 546 and 549 into 550, the composite energy coefficient α, applicable to the entire
cross section, is obtained:
N A^{2} [
Σ
(α_{i} K_{i}^{ 3} / A_{i}^{ 2} ) ]
^{i = 1}
α = ^{____________________________}
N
( Σ K_{i} ) ^{3}
^{i = 1}
 (552) 
Likewise, replacing Eqs. 546 and 549 into 551, the composite value of β is:
N A [
Σ
(β_{i} K_{i}^{ 2} / A_{i} ) ]
^{i = 1}
β = ^{____________________________}
N
( Σ K_{i} ) ^{2}
^{i = 1}
 (553) 
Computation of uniform flow
With reference to Fig. 510, the following proportion holds: z /1 = x /y.
Then, the top width T is:
T = b + 2x = b + 2zy
 (554) 
The flow area A is:
A = (b + x ) y = (b + zy ) y
 (555) 
Fig. 510 Definition
sketch for a trapezoidal cross section.


The wetted perimeter P is:
P = b + 2 (y^{ 2} + z^{ 2}y^{ 2} )^{1/2}
 (556) 
Simplifying:
P = b + 2 y ( 1 + z^{ 2} )^{1/2}
 (557) 
From the Manning equation, the discharge Q is:
k
Q = ^{ _____ } A R^{ 2/3} S^{ 1/2}
n
 (558) 
in which k = 1 in SI units, and k = 1.486 in U.S. Customary units.
Since R = A /P, Eq. 558 reduces to:
Q n A^{ 5/3}
^{ _________ } = ^{ _______}
k S^{ 1/2} P^{ 2/3}
 (559) 
Replacing Eqs. 555 and 557 into Eq. 559:
Q n [ (b + zy ) y ] ^{ 5/3}
^{ _________ } = ^{ _____________________________}
k S^{ 1/2} [b + 2 y ( 1 + z^{ 2} )^{1/2}] ^{ 2/3}
 (560) 
Simplifying:
Q n Q n [ (b + zy ) y ] ^{ 5/2}  ( ^{ ________ } ) ^{3/2} [ 2 y ( 1 + z^{ 2} )^{1/2} ]  ( ^{ ________} )^{ 3/2} b = 0
k S^{ 1/2} k S^{ 1/2}
 (561) 
The input data consisting of: (1) discharge Q, (2) bottom width b, (3)
size slope z [z:H to 1:V, Fig. 510], (4) bottom slope S, and (5) Manning's n. With given input data,
Eq. 561 is solved for the normal depth y . Then, with Eqs. 554 and 555:
Equation 561 is the general formula for uniform or normal flow,
applicable to prismatic channels of trapezoidal cross section.
For a rectangular channel: z = 0. Likewise, for a
triangular channel of symmetrical section: b = 0.
To solve Eq. 561, it is expressed in the following form:
Q n Q n f (y) = [ (b + zy ) y ] ^{ 5/2}  ( ^{ ________ } ) ^{3/2} [ 2 y ( 1 + z^{ 2} )^{1/2} ]  ( ^{ ________} )^{ 3/2} b
k S^{ 1/2} k S^{ 1/2}
 (564) 
Making the change of variable x = y for simplicity:
Q n Q n f (x) = [ (b + zx ) x ] ^{ 5/2}  ( ^{ ________ } ) ^{3/2} [ 2 x ( 1 + z^{ 2} )^{1/2} ]  ( ^{ ________} )^{ 3/2} b
k S^{ 1/2} k S^{ 1/2}
 (565) 
The solution of Eq. 565 is accomplished by a
trialanderror procedure.
An iterative algorithm based on function value is described below. An example of normal flow is shown in Fig. 511.
Normal depth algorithm based on function value
Assume an initial value of x = 0.
Then: f (0) = 
[ ( Qn ) / (k S_{o} ) ] ^{3/2} b which is a large negative number.
It is confirmed that the initial value of the function is smaller than zero.
Assume an initial value of the trial interval Δx = 1.
Set x = x + Δx
Calculate f (x)
Stop
when Δx < Δx_{ TOL}. A typical value of Δx_{ TOL} is 0.0001.
If f (x) < 0, return to Step 3.
if f (x) > 0,
set Δx =
0.1 Δx
Set x = x  9 Δx
Return to Step 4.

Fig. 511 Normal flow at WelltonMohawk canal, Wellton, Arizona.


Newton's approximation to the root
The above iteration uses solely the value of the function to approximate to the root.
A faster algorithm makes use of Newton's approximation,
which is based on the tangent.
Note that for Newton's iteration to work well,
it is first
necessary get close to the root using the function iteration described above.
Otherwise, Newton's tangent method may not converge.
With reference to Fig. 512, the value of the tangent at x_{o} is:
f(x_{o})
f '(x_{o}) = ^{_________}
x_{o}  x_{r}
 (566) 
where x_{o} = current trial value of x, f(x_{o}) = value of the function at x_{o}, x_{r} =
new value of x, which gets closer to the root.
Fig. 512 Definition sketch for Newton's iteration.


From Eq. 566, solving for x_{r} :
f (x_{o})
x_{r} = x_{o}  ^{________}
f '(x_{o})
 (567) 
As shown in Fig. 512, when f (x_{o}) increases with x_{o}
(as is the case for Eq. 565), as the root is passed,
the function value and the tangent value are positive; therefore, the denominator of Eq. 566 is also positive, and x_{r} lies to the left of x_{o}. With each subsequent iteration, the root is approximated in a sawtooth fashion,
until the specified tolerance is satisfied.
It is readily shown that Eq. 567 applies also when f (x_{o}) decreases as x_{o} increases, i.e.,
as in the case of critical flow, see Section 4.2.
The value of f '(x) is:
Q n f ' (x) = x^{ 5/2} (5/2) (b + zx )^{ 3/2} z + (b + zx )^{ 5/2} (5/2) x^{ 3/2}  ( ^{ ________ } ) ^{3/2} [ 2 ( 1 + z^{ 2} )^{1/2} ]
k S^{ 1/2}
 (568) 
Simplifying Eq. 568:
Q n f ' (x) = 5 z x^{ 5/2} (b + zx )^{ 3/2} + 5 x^{ 3/2} (b + zx )^{ 5/2}  ( ^{ ________ } ) ^{3/2} ( 1 + z^{ 2} )^{1/2}
k S^{ 1/2}
 (569) 
The procedure for Newton's approximation to the root of Eq. 565 is described below.
Normal depth algorithm: Newton's approximation
Assume an initial value of x_{o} = 0.
Assume an initial value of the trial interval Δx = 1.
Set x_{o} = x_{o} + Δx
Calculate f (x_{o})
If f (x_{o}) < 0, return to Step 3.
If f (x_{o}) > 0,
calculate the root x_{r} using Eqs. 565 and 569:
f (x_{o})
x_{r} = x_{o}  ^{________}
f '(x_{o})

Stop
when (x_{r}  x_{o})
is small enough. A typical value of the difference is 0.0001.
Otherwise, set x_{o} = x_{r} and return to Step 6.

Example 51.
Using ONLINE CHANNEL 01,
calculate the critical flow depth for the
following flow conditions: Q = 3 m^{3}/s;
b = 5 m; z = 1; S = 0.001; n = 0.015.
 
ONLINE CALCULATION. Using the
ONLINE CHANNEL 01 calculator,
the normal depth is y_{n} = 0.473 m;
the normal velocity is v_{n} = 1.16 m/s.
the normal Froude number is F_{n} = 0.562.



Example 52.
Using ONLINE CHANNEL 01,
calculate the critical flow depth for the
following flow conditions: Q = 20 ft^{3}/s;
b = 13 ft; z = 2; S = 0.0008; n = 0.013.
 
ONLINE CALCULATION. Using the
ONLINE CHANNEL 01 calculator,
the normal depth is y_{n} = 0.631 ft;
the normal velocity is v_{n} = 2.221 ft/s.
the normal Froude number is F_{n} = 0.514.



5.6 COMPUTATION OF FLOOD DISCHARGE
The high stages and swift currents that prevail during floods combine to increase the risk of
accident and bodily harm (Fig. 513). Therefore, it is generally not possible to measure discharge during the
passage of a flood. An estimate of peak discharge can be obtained indirectly by the use of open
channel flow formulas. This is the basis of the slopearea method.
Fig. 513 Flood stage in a tropical river.


To apply the slopearea method for a given river reach, the following data are required:
The reach length,
The fall, i.e., the mean change in water surface elevation through the reach,
The flow area, wetted perimeter, and velocity head coefficients at upstream and downstream cross sections, and
The average value of Manning's n for the reach.
The following guidelines are used in selecting a suitable reach:  Highwater marks
should be readily recognizable (Fig. 514).
The reach should be sufficiently long so that fall
can be measured accurately.
The crosssectional shape and channel dimensions should
be relatively constant.
The reach should be relatively straight, although a
contracting reach is preferred over an expanding reach.
Bridges, channel bends,
waterfalls, and other features causing flow nonuniformity should be avoided.
Fig. 514 Local man showing water level reached by flood, Karnataka, India (1991).


The accuracy of the slopearea method improves as the reach length increases (Fig. 515). A suitable
reach should satisfy one or more of the following criteria:
The ratio of reach length
to hydraulic depth should be greater than 75,
The fall should be greater than or
equal to 0.15 m: F ≥ 0.15 m, and
The fall should be greater than either of the velocity heads
computed at the upstream and downstream cross sections.
Fig. 515 Slopearea method schematic.


The procedure consists of the following steps:
Calculate the conveyance K at upstream and downstream sections:
1
K_{1} = ( ^{__} )
A_{1} R_{1}^{ 2/3}
n
 (570a) 
1
K_{2} = ( ^{__} )
A_{2} R_{2}^{ 2/3}
n
 (570b) 
in which K = conveyance; A = flow area; R = hydraulic radius;
n = average reach Manning's n; and 1 and 2 denote the upstream and downstream sections, respectively
(Eq. 570 is in SI units).
Calculate the reach conveyance, equal to the geometric mean of upstream and
downstream conveyances:
K = ( K_{1} K_{2} )^{1/2}
 (571) 
in which K = reach conveyance.
Calculate the first approximation to the energy slope:
in which S = first approximation to the energy slope; F = fall; and L = reach length.
Calculate the first approximation to the peak discharge:
Q_{i} = K S^{ 1/2}
 (573) 
in which Q_{i} = first approximation to the peak discharge.
Calculate the velocity heads:
α_{1} ( Q_{i} /A_{1} )^{ 2}
h_{v1} = ^{______________}
2g
 (574a) 
α_{2} ( Q_{i} /A_{2} )^{ 2}
h_{v2} = ^{______________}
2g
 (574b) 
in which h_{v1} and h_{v2} = velocity heads at
upstream and downtream sections, respectively; α_{1} and α_{2}
= velocity head coefficients at upstream and downstream cross sections,
respectively; and g = gravitational acceleration.
Calculate an updated value of energy slope:
F + k ( h_{v1}  h_{v2} )
S_{i} = ^{___________________}
L
 (575) 
in which S_{i} = updated value of energy slope, and k = loss coefficient.
For expanding flow, i.e., A_{2} > A_{1}, k = 0.5;
for contracting flow , i.e., A_{1} > A_{2}, k = 1.
Calculate an updated value of peak discharge:
Q_{i} = K S_{i}^{ 1/2}
 (576) 

Return to step 5 and repeat steps 5 to 7. The procedure is terminated when the difference between two successive values of peak discharge Q obtained in step 7
is negligible. In practice, this is usually accomplished within three iterations.
5.7 UNIFORM SURFACE FLOW
Overland flow over a catchment surface is referred to as surface flow. Typically, in overland flow,
the depth of flow is very small compared to the width. Under these conditions, the flow may be either laminar or turbulent,
depending on the absolute and relative roughnesses. If velocities and depths of flow are sufficiently small, the flow may be laminar;
otherwise, the flow may be transitional or turbulent, depending on the Reynolds number (Section 1.4).
In overland flow, a mixed laminarturbulent regime is commonly present.
This type of flow is characterized by changes from laminar to turbulent regime
under the spatially varying flow conditions normally encountered in catchment/watershed/basin surface flow.
Under laminar flow conditions, the exponent of the rating for surface flow is β = 3.
Under turbulent flow conditions, the exponent is β = 3/2 for Chézy friction, and β = 5/3 for Manning friction.
Mixed laminarturbulent flow regimes feature values of β varying between laminar and turbulent.
Laminar surface flow
With reference to Fig. 516, the acting shear stress at level P is:
τ_{a} = γ (y_{m}  y ) S
 (577) 
According to Newton's law of viscosity, the resisting shear stress at P is proportional to the vertical velocity gradient:
dv
τ_{r} = μ ^{ _____ }
dy
 (578) 
in which μ = a constant of proportionality referred to as the dynamic viscosity.
Fig. 516 Definition sketch for uniform surface flow.


Equating acting and resisting stresses:
dv
μ ^{ _____ } = γ (y_{m}  y ) S
dy
 (579) 
In differential form:
μ dv = γ (y_{m}  y ) S dy
 (580) 
The mass density γ = ρg, and
the dynamic viscosity μ = ρν, in which ν = kinematic viscosity.
Thus, Eq. 580 reduces to:
gS
dv = ^{ _____ } (y_{m}  y ) dy
ν
 (581) 
Integrating Eq. 581:
gS
v = ∫ ^{ _____ } (y_{m}  y ) dy
ν
 (582) 
gS y^{ 2}
v = ^{ _____ } [ y_{m} y  ^{ _____ }] + C
ν
2
 (583) 
where C is a constant of integration. For v = 0: y = 0; therefore: C = 0, and the mean velocityflow depth relation is:
gS y^{ 2}
v = ^{ _____ } [ y_{m} y  ^{ _____ } ]
ν
2
 (584) 
Equation 584 reveals that the velocity profile of uniform surface flow has a parabolic distribution.
The dischargedepth rating is obtained by integrating Eq. 584 between the limits of 0 and y_{m}, i.e., from bottom to surface, to yield:
gS y^{ 2}
q = ∫ v dy = ^{ _____ } ∫ [ y_{m} y  ^{ _____ } ] dy
ν
2
 (585) 
gS y_{m}^{ 2} y_{m}^{ 3}
q = ^{ _____ } [ ^{ _____ }  ^{ _____ } ]
ν
2 6
 (586) 
which reduces to:
gS
q = ^{ ______ } y_{m}^{ 3}
3ν
 (587) 
Or:
q = C_{L} y_{m}^{ 3}
 (588) 
where C_{L} = laminar discharge rating coefficient, defined as:
gS
C_{L} = ^{ ______ }
3ν
 (589) 
Note that under laminar flow, the exponent of the discharge rating is β = 3 (Eq. 588), and the rating is a function of the
internal friction, or internal viscosity, represented by the kinematic viscosity ν. Thus, laminar flow is a function of temperature.
Given Eq. 577, the mean velocity in laminar flow, v = q /y_{m}, is:
v = C_{L} y_{m}^{ 2}
 (590) 
The turbulent Chézy discharge rating is:
q = C S^{ 1/2} y_{m}^{ 3/2}
 (591) 
The turbulent Manning discharge rating in SI units is:
q = (1/n) S^{ 1/2} y_{m}^{ 5/3}
 (592) 
Likewise, in U.S. Customary units is:
q = (1.486 / n) S^{ 1/2} y_{m}^{ 5/3}
 (593) 
It is seen that the exponent of the rating varies from β = 3 for laminar flow (Eq. 588), to β = 3/2 for turbulent Chézy friction (Eq. 591),
or β = 5/3 for turbulent Manning friction (Eq. 592). In uniform surface flow, values of β in the range between laminar and turbulent
represent the condition of mixed laminarturbulent flow (Section 1.3).
The Vedernikov number is:
(β  1) v
V = ^{ ____________ }
(g y )^{1/2}
 (594) 
Under V = 1, the flow is neutrally stable, promoting
the development of roll waves (Fig. 517). The relation between the exponent β and the Vedernikov number V is described below.
Relation between exponent β and Vedernikov number

Under laminar conditions: β = 3. Thus, V = 2 F.
Therefore, under laminar conditions, the flow becomes unstable when F = 0.5.

Under turbulent Chézy conditions: β = 1.5. Thus, V = 0.5 F.
Therefore, under turbulent Chézy conditions, the flow becomes unstable when F = 2.

Under turbulent Manning conditions: β = 5/3. Thus, V = (2/3) F.
Therefore, under turbulent Manning conditions, the flow becomes unstable when F = 3/2, i.e., F = 1.5.

Fig. 517 Roll waves on the spillway at Turner reservoir, San Diego County, California.


QUESTIONS
When does uniform flow become unstable?
What is the Chezy formula based on?
What is the difference between the Manning
and Chezy formulas?
What is the minimum value of Manning's n
that can be achieved in practice?
What is the range of values of Manning's n
measured by Barnes?
What is the range of values of Manning's n
measured by Arcement and Schneider for flood plains?
Why is the calculation of composite roughness
using Eq. 543 only an approximation?
What five input variables are used in the
computation of uniform flow in a trapezoidal channel?
Why is it better to use Newton's approximation
to the root rather than relying solely on the function approximation to solve the normal depth problem?
What is the minimum ratio of reach length
to hydraulic depth in the slopearea method?
What is the exponent of the dischargedepth
rating under laminar flow
conditions?
What is the exponent of the dischargedepth
rating under turbulent Chezy friction in hydraulically wide channels?
What is the exponent of the dischargedepth
rating under turbulent Manning friction in hydraulically wide channels?
Under what value of Froude number is the flow
likely to become unstable under laminar flow conditions?
PROBLEMS
Prove that the DarcyWeisbach
friction factor is related to Manning's n by the following relation:
f_{D} = 8 g n^{ 2} / (k^{ 2} R^{ 1/3})
in which f_{D}
= DarcyWeisbach friction factor, g = gravitational acceleration,
R = hydraulic radius,
and k = constant specific for the system of units,
equal to 1 in SI units and 1.486 in U.S. Customary Units.
Express the relation in SI and U.S. Customary units.
Calculate the discharge Q using the Manning
equation, given: flow area A = 23.5 ft^{2}; hydraulic radius R = 5.6 ft;
channel slope S = 0.0025; Manning's n = 0.035.
Calculate the discharge Q using the Manning
equation, given: flow area A = 45 m^{2}; hydraulic radius R = 6 m;
channel slope S = 0.003; Manning's n = 0.04.
Given f = 0.0025,
calculate the discharge Q for a flow area A = 12.4 m^{2}, hydraulic radius R = 2.1 m;
and channel slope S = 0.0015.
Given f = 0.0035,
calculate the discharge Q for a flow area A = 18 ft^{2}, hydraulic radius R = 4.5 ft;
and channel slope S = 0.0018.
Use
ONLINE CHANNEL 01 to calculate normal depth, velocity, and Froude number
for the following case:
Q = 150 m^{3}/s, b = 10 m, z = 2, S_{o} = 0.0005, n = 0.025.
Use
ONLINE CHANNEL 01 to calculate normal depth, velocity, and Froude number
for the following case: Q = 250 cfs, b = 20 ft, z = 1, S_{o} = 0.001,
n = 0.030.
Using ONLINE CHANNEL 15,
calculate the discharge for a prismatic channel with b = 20 ft, y = 3 ft, z = 2, n = 0.025, S = 0.0016.
Fig. 518 Definition sketch for a trapezoidal channel.


Using ONLINE CHANNEL 15,
calculate the discharge for a prismatic channel with b = 6 m, y = 1 m, z = 1.5, n = 0.015, S = 0.0002.
A recent
flood on Clearwater Creek has left observable water marks on a certain river reach.
To estimate the flood magnitude, hydraulic data has been measured
at two cross sections A and B, a distance of 1,850 ft apart.
The reach fall between the cross sections is 9.1 ft and
the average Manning's n is 0.035.
The upstream flow area, wetted perimeter, and Coriolis α coefficient are
550 ft^{2}, 55 ft, and 1.17;
the downstream flow area, wetted perimeter, and α coefficient are
620 ft^{2}, 52 ft, and 1.10.
Use SLOPE AREA to calculate the flood discharge.
Calculate the unitwidth discharge in an overland flow plane, under laminar flow, with mean depth of 1.5 cm and slope of 0.001.
Assume water temperature T = 20^{o}C. Report discharge in L/s/m.
REFERENCES
Arcement, G. J. and V. R. Schneider. 1989. Guide for selecting Manning's roughness coefficients for natural channels and flood plains. U.S. Geological Survey WaterSupply Paper 2339, Washington, D.C.
Barnes, H. A. 1967. Roughness characteristics of natural channels. U.S. Geological Survey WaterSupply Paper 1849, Washington, D.C.
Cowan, W. L. 1956. Estimating hydraulic roughness coefficients. Agricultural Engineering, Vol. 37, No. 7, pp. 473475, July.
Chow, V. T. 1959. Openchannel hydraulics. McGrawHill, New York.
Henderson, M. H. 1966. Openchannel flow. Macmillan, New York.
Simons, D. B., and E. V. Richardson. 1966. Resistance to flow in alluvial channels. U.S. Geological Survey Professional Paper 422J, Washington, D.C.
Williamson, J. 1951. The laws of flow in rough pipes. La Houille Blanche, Vol. 6, No. 5, SeptemberOctober, p. 738.
http://openchannelhydraulics.sdsu.edu 

150710 10:30 
