1. Roughness & Mannings n-value
Channels and Floodplains
Culverts
Extra Session on n-values is provided because:
It is difficult to visualize channel roughness compared to other variables in the Manning’s Equation
Manning’s n value has as great an affect on channel hydraulics as other easily visualized variables like area, slope, hydraulic radius
We adjust Manning’s n to calibrate models, check channel stability and channel capacity
Manning’s n changes with time (e.g. trees and brush growing in a diversion channel will increase n)

2. Teaching Objective Understand that:
resistance to flow depends on roughness
Manning’s n value is simply a parameter used by hydraulic engineers to represent roughness
Roughness changes with time (e.g. brush growing in channels, culverts aging & deteriorating)
Learn how “n” values affect hydraulic parameters
Obtain basic information on choosing or calculating n values

3. Application
From an analytical standpoint, Mannings n value is a coefficient that needs to be chosen to calculate or model flow
Used for both culverts and open channels

4. Significance
From a practical standpoint, roughness affects all of the characteristics of flowing water (flow, velocity, water surface elevation) and therefore affects sediment transport, flooding, navigation, ecosystem restoration, etc…..
The significance of roughness becomes more apparent, perhaps, when we compare a cross section plotted on an exaggerated scale to the same cross section plotted on a true scale.

6. Channel with Vegetation
Riparian vegetation has a significant effect on roughness values for this channel during a flood

7. Manning Equation for Velocity v = 1. 49 R0. 67 S0. 5
Manning Equation for Velocity v = R0.67 S n where, v = velocity, ft/sec n = roughness, s/ft1/3 R = hydraulic radius, ft S = hydraulic slope, ft/ft Note: If R increases, v increases If s increases, v increases If n increases, v decreases
1. This is the Manning’s equation written for velocity. We can use the continuity equation Q = vA to convert this to discharge.
2. An increasing n value represents greater roughness and greater resistance to flow, so velocity decreases
3. The hydraulic radius is equal to the cross sectional area divided by the wetted perimeter. For channels of a given width, the hydraulic radius is greater for the deeper channels. For wide channels the hydraulic radius can be approximated by the flow depth.

8. Example of what happens to velocity if we change variables in Mannings equation v = 1.49R2/3 s1/2 / n R s n v 5
.0001
.03
1.46 10
2.32
.0002
2.06
.06
.73
Example >>>
If R is doubled
If s is doubled
If n is doubled
As hydraulic radius increases, velocity increases……. But doubling hydraulic radius does not double velocity.
As hydraulic slope increases, velocity increases……. But doubling hydraulic slope does not double velocity.
As roughness increases, velocity decreases…… And doubling roughness reduces velocity by half.
The point is that while roughness may be not be easily visualized, it is important to understand. If trees grow in you diversion channel, thus increasing roughness, its ability to convey water is reduced.

9. Guidance exists for choosing n values
USGS, Water Supply Paper 1849, Barnes
Hydraulics Handbooks & Textbooks The table below is from Vennard & Street, pg 470
Corrugated Pipes
.024
Concrete pipes, open channels
.013
Small channels, clean
.03
Large channels (width > 100’
.025
Floodplains (natural vegetation)
1. There are hydraulic handbooks, text books, and many other references that provide n values.
The numbers in Vennard & Street are typical values and one should always assume that there is a range of conditions depending on age and condition of culverts and pipes, the amount of vegetation in channels, etc.
The USGS (Barnes, Water Supply Paper 1849) provides many examples of n values for rivers and stream in the United States. See

10. Structures have been used to change roughness in rivers
Pile Dikes, Missouri River
Structures have been used to
change roughness in rivers
Jetty Jacks, Rio Grande floodplain,
Albuquerque, NM
Wing Dams, Mississippi River

11. Culverts

12. Old pipe with new extension
Galvanized Steel
Old pipe with new extension
n = .024

13. Concrete Pipe
n = .013

14. Wooden Pipe
District 1 in Duluth
State Highway MN 23

15. Plastic Pipe
“Smooth Plastic” dual wall HDPE has slight corrugations. PVC (no photo available) would also be “Smooth Plastic”

16. Channels & Floodplains

17. Riprap in Open Channels
n-value is based on a representative size of the substrate gradation,such as the D50. D50 is the sediment diameter at which 50% of the weight of a sediment sample is made up of particles of smaller diameter
the bigger the representative size, the greater the n-value
The Strickler relation between Manning n and
mean particle size d50 (feet). (From Chow, 1959):
n = d50 1/6
The Strickler equation was developed for streams in the NE United States and the Mississippi River.

18. Riprap Roughness D50 = 0. 5' n = 0. 035 D50 = 1. 0' n = 0. 040 D50 = 2
Riprap Roughness D50 = 0.5' n = D50 = 1.0' n = D50 = 2.0' n = doubling the representative riprap size does not double the n-Value
The main point is that there is guidance for choosing n values like the table shown earlier, the Strickler equation for rivers, and riprap roughness values which can be used for different size riprap.

19. Variation in n-value
As depth increases, channel n-values usually decrease, though there could be exceptions to this (see Chow, pg 104).
n-values in the floodplain and along channel banks may increase during the growing season and decrease during the dormant season
Floodplain, Dormant Season
Floodplain, Growing Season
Water Elevation
The main point here is that n values can vary with flow depth, season, and even temperature.
Greater depth usually results in lower n values since the boundary roughness has less overall influence on the flow characteristics of the cross section.
Seasonal changes in aquatic vegetation (e.g. the growth of emergent vegetation) or terrestrial vegetation (e.g. Leaf growth) can increase n-values.
Theoretically, temperature can affect n-values since viscosity decreases with increasing temperature, but generally prototype data doesn’t consistently show a winter to summer change in n-values. The PDT for the Atchafalaya River flowline study tried to identify this effect by comparing January-February stage discharge relationships to those in May-June, but no consistent shift was found. Other factors such as channel depth or vegetation conditions are more important.
Bankfull Depth
Channel
0.05 .1
Manning’s n-value

20. Change in floodplain features and Manning’s n with time (Upper Mississippi River)
Trees, Shrubs, Grass in 1900
n = .1
v = R0.67 S n
As n decreased, v increased
resulting in more flow in the
Floodplain over time
As the floodplain was converted from terrestrial flooplain, to marsh, to open water, the roughness of the floodplain decreased causing them to convey more water during floods, and resulting in secondary channel erosion, which resulted in even more floodplain flow.
Open Water in 1992
n = .03
Marsh in 1956
n = .05

21. Composite n Values
Complex channels may have several different n-values
Horton Method: Applies to a single cross section, which represents a reach’s 6 components (listed below). Used in HEC-RAS (see Ch. 2, Pg 2-6 HEC-RAS users manual, version 3.1, Nov 2002)
1. earthen material 2. regularity of a given section 3. regularity among sections 4. obstacles 5. vegetation 6. sinuosity
Middle Rio Grande near Albuquerque, NM
The vegetated sand bar on the right side of the channel has a higher n value than the open channel to the left side.
The equation above is equation 2-6, pg 2-7 from the HEC-RAS, Version 4, March 2008 users manual.
n=0.025
n=0.050
N /3
nc = (Pini1.5)
i=1 P

22. Stability/Capacity Design in Open Channels
Stability can be assessed by using an n-value slightly lower than the estimated n-value.
calculated velocity will be greater, area will be less, the flowline will be lower, and there will be a greater tendency for erosion
Capacity can be assessed by using an n-value slightly greater than the estimated n-value
calculated velocity will be less, area will be greater, and flowlines will be higher
Assumed n-values affect calculated area and velocity If flow is held constant the sensitivity of our calculations can be tested by raising or lowering n values
Stability can be tested by lowering n-values. Can the channel withstand the attack of the faster moving flow and not erode?
Capacity can be tested by raising n-values. Can the channel convey the design flood without induced damages?

23. Columbia River at Vernita, Wash.
Indian Fork below Atwood Dam,
near New Cumberland, Ohio
n = 0.024
n = 0.026
The Manning n-value is dependent on many factors including surface roughness and sinuosity. When field inspection is not possible, the best method to determine n is to use photographs of river channels where n has been determined using the Manning formula. The USGS Water Supply Paper 1849 is one of the best sources of photographs with calculated n values. These n values were determined for river reaches with existing gauge data supplemented by additional surveys to determine Q, A, R, and s so that the n-value could be calculated.
Source of Information:
Roughness Characteristics of Natural Channels
U.S. Geological Survey Water Supply Paper 1849
By Harry H. Barnes, Jr.

24. Champlin Creek near Colorado City, Tex. Clark Fork at St. Regis, Mont.
Source of Information:
Roughness Characteristics of Natural Channels
U.S. Geological Survey Water Supply Paper 1849
By Harry H. Barnes, Jr.

25. Esopus Creek at Coldbrook, N.Y.
Salt Creek at Roca, Nebr.
n = 0.030
Source of Information:
Roughness Characteristics of Natural Channels
U.S. Geological Survey Water Supply Paper 1849
By Harry H. Barnes, Jr.

26. Salt river below Stewart Mountain Dam, Ariz.
Yakima river at Umtanum, Wash.
n = 0.032
n = 0.036
Source of Information:
Roughness Characteristics of Natural Channels
U.S. Geological Survey Water Supply Paper 1849
By Harry H. Barnes, Jr.

27. Wenatchee River at Plain, Wash.
Deep River at Ramseur, N.C. .
n = 0.049
Source of Information:
Roughness Characteristics of Natural Channels
U.S. Geological Survey Water Supply Paper 1849
By Harry H. Barnes, Jr.

28. n = 0.097 Rolling fork near Boston, Ky. Rolling fork near
Boston, Ky. Looking through
Right overbank.
n = 0.097
n = 0.046
Source of Information:
Roughness Characteristics of Natural Channels
U.S. Geological Survey Water Supply Paper 1849
By Harry H. Barnes, Jr.

29. Summary Hydraulic characteristics are affected by n
n values change with time
There is guidance on choosing n values
Can verify n values by calibrating to data
Computer models rely on user input on n values but also employ methods to vary n with depth
Can adjust n values to do sensitivity analysis