Manning’s roughness coefficient (n) is one of the most important—and uncertain—parameters in hydraulic calculations. This guide provides comprehensive tables and guidance for selecting appropriate n values for various conduit materials and channel conditions.
What Manning’s n Represents
Manning’s n is an empirical coefficient that accounts for energy losses due to:
- Surface roughness of the boundary
- Irregularities in channel cross-section
- Variations in size and shape along the channel
- Obstructions
- Vegetation
- Channel alignment (meanders and bends)
Manning’s n for Closed Conduits
Concrete Pipes
| Pipe Type | n Value Range | Recommended Design Value |
|---|---|---|
| Precast concrete pipe (good joints) | 0.011 - 0.013 | 0.013 |
| Cast-in-place concrete | 0.012 - 0.014 | 0.013 |
| Concrete culvert (smooth) | 0.011 - 0.013 | 0.012 |
| Concrete culvert (rough) | 0.013 - 0.017 | 0.015 |
| Corrugated concrete | 0.022 - 0.026 | 0.024 |
Metal Pipes
| Pipe Type | n Value Range | Recommended Design Value |
|---|---|---|
| Corrugated Metal Pipe (CMP) | ||
| 2-2/3” x 1/2” corrugation | 0.022 - 0.027 | 0.024 |
| 3” x 1” corrugation | 0.027 - 0.032 | 0.028 |
| 6” x 2” corrugation | 0.033 - 0.038 | 0.035 |
| Spiral rib (smooth interior) | 0.012 - 0.013 | 0.012 |
| Steel Pipe | ||
| Welded steel (new) | 0.010 - 0.012 | 0.011 |
| Riveted steel | 0.013 - 0.017 | 0.015 |
| Cast Iron | ||
| Coated cast iron | 0.012 - 0.014 | 0.013 |
| Uncoated cast iron | 0.013 - 0.015 | 0.014 |
| Ductile Iron | ||
| Cement-lined | 0.011 - 0.013 | 0.012 |
| Unlined | 0.012 - 0.015 | 0.013 |
Plastic Pipes
| Pipe Type | n Value Range | Recommended Design Value |
|---|---|---|
| PVC (smooth wall) | 0.009 - 0.011 | 0.010 |
| HDPE (smooth interior) | 0.009 - 0.011 | 0.010 |
| HDPE (corrugated exterior, smooth interior) | 0.010 - 0.013 | 0.012 |
| HDPE (corrugated throughout) | 0.018 - 0.025 | 0.022 |
| ABS | 0.009 - 0.010 | 0.010 |
| Fiberglass | 0.009 - 0.011 | 0.010 |
Other Conduits
| Type | n Value Range | Recommended Design Value |
|---|---|---|
| Vitrified clay | 0.011 - 0.015 | 0.013 |
| Brick sewers | 0.013 - 0.017 | 0.015 |
| Stone masonry | 0.017 - 0.030 | 0.025 |
| Box culvert (concrete) | 0.012 - 0.018 | 0.015 |
Manning’s n for Open Channels
Excavated/Constructed Channels
| Channel Type | n Value Range | Recommended Design Value |
|---|---|---|
| Earth (straight, uniform) | ||
| Clean, recently completed | 0.016 - 0.020 | 0.018 |
| Clean, after weathering | 0.018 - 0.025 | 0.022 |
| Gravel, uniform | 0.022 - 0.030 | 0.025 |
| Short grass, few weeds | 0.022 - 0.033 | 0.027 |
| Earth (winding, sluggish) | ||
| Grass, some weeds | 0.025 - 0.033 | 0.030 |
| Dense weeds, deep | 0.030 - 0.050 | 0.035 |
| Rock cut | ||
| Smooth and uniform | 0.025 - 0.035 | 0.030 |
| Jagged and irregular | 0.035 - 0.050 | 0.040 |
Lined Channels
| Lining Type | n Value Range | Recommended Design Value |
|---|---|---|
| Concrete | ||
| Trowel finish | 0.011 - 0.015 | 0.013 |
| Float finish | 0.013 - 0.016 | 0.015 |
| Formed, no finish | 0.014 - 0.018 | 0.017 |
| Gunite (good section) | 0.016 - 0.022 | 0.019 |
| Gunite (irregular) | 0.022 - 0.027 | 0.024 |
| Asphalt | ||
| Smooth | 0.013 - 0.016 | 0.014 |
| Rough | 0.016 - 0.020 | 0.018 |
| Riprap | ||
| D50 = 6” | 0.030 - 0.040 | 0.035 |
| D50 = 12” | 0.035 - 0.050 | 0.040 |
| D50 = 24” | 0.040 - 0.060 | 0.050 |
| Gabion | 0.025 - 0.035 | 0.030 |
Vegetated Channels
| Vegetation Type | n Value Range | Notes |
|---|---|---|
| Short grass (<2”) | 0.025 - 0.035 | Regularly mowed |
| Tall grass (6-12”) | 0.030 - 0.050 | Meadow, hay |
| Dense grass | 0.040 - 0.070 | Dense, tall, unmowed |
| Light brush | 0.035 - 0.070 | Sparse vegetation |
| Heavy brush | 0.070 - 0.160 | Dense willows, etc. |
Natural Streams
| Stream Type | n Value Range | Typical Value |
|---|---|---|
| Minor streams (width < 100 ft) | ||
| Clean, straight, no deep pools | 0.025 - 0.033 | 0.030 |
| Same, with stones/weeds | 0.030 - 0.040 | 0.035 |
| Winding, some pools/shoals | 0.033 - 0.050 | 0.040 |
| Sluggish, weedy, deep pools | 0.050 - 0.080 | 0.070 |
| Very weedy, heavy timber | 0.075 - 0.150 | 0.100 |
| Major streams (width > 100 ft) | ||
| Regular section, no debris | 0.025 - 0.060 | 0.035 |
| Irregular, rough section | 0.035 - 0.100 | 0.050 |
Floodplains
| Condition | n Value Range | Typical Value |
|---|---|---|
| Short grass (pasture) | 0.025 - 0.035 | 0.030 |
| High grass | 0.030 - 0.050 | 0.035 |
| Cultivated areas | 0.020 - 0.050 | 0.035 |
| Light brush/weeds | 0.035 - 0.070 | 0.050 |
| Medium brush | 0.045 - 0.110 | 0.070 |
| Heavy timber | 0.100 - 0.200 | 0.120 |
Factors Affecting Manning’s n Selection
1. Surface Roughness
The base roughness depends on material:
- Smooth: PVC, steel, finished concrete
- Moderate: CMP, unfinished concrete, gravel
- Rough: Riprap, gabions, natural channels
2. Vegetation
Vegetation effects depend on:
- Type: Grass vs. brush vs. trees
- Density: Sparse vs. dense
- Flexibility: Rigid vs. flexible (affects submergence behavior)
- Submergence: Relative to flow depth
- Season: Dormant vs. growing
3. Channel Irregularity
Non-uniform cross-sections increase roughness:
- Smooth: Uniform section throughout
- Minor: Slightly varying bottom/sides
- Moderate: Varying, eroded banks
- Severe: Heavily eroded, irregular shape
4. Channel Alignment
Meandering increases resistance:
- Straight channels: Use base n
- Gradual curves: Add 5-10%
- Severe meanders: Add 15-30%
5. Obstruction Effects
Debris, bridge piers, log jams:
- Negligible: Isolated small obstructions
- Minor: Sparse obstructions, <15% of area
- Appreciable: 15-50% of area affected
- Severe: >50% blocked
6. Stage and Discharge
n values can change with depth:
- In vegetated channels, n decreases as flow submerges vegetation
- In alluvial channels, n can increase with bedform development at higher stages
Composite Manning’s n
For channels with varying roughness across the perimeter, calculate a composite n:
Horton-Einstein Method
Weighted Perimeter Method
Worked Example
A trapezoidal channel has:
- Concrete bottom: P₁ = 10 ft, n₁ = 0.013
- Grass side slopes: P₂ = 15 ft (total both sides), n₂ = 0.030
Weighted perimeter method:
Horton-Einstein method:
Aging Effects on Pipe n Values
Pipes age and roughness typically increases:
| Material | New n | After 10 years | After 30+ years |
|---|---|---|---|
| Concrete | 0.012 | 0.013 | 0.014-0.015 |
| CMP | 0.024 | 0.027 | 0.030-0.035 |
| PVC | 0.010 | 0.011 | 0.012-0.013 |
Causes of increased roughness:
- Joint separation
- Corrosion/erosion
- Sediment deposits
- Root intrusion
- Tuberculation (iron pipes)
- Biofilm growth
Cowan’s Method for Estimating n
For natural channels, Cowan (1956) proposed:
Where:
- n₀ = Base value for straight, uniform channel
- n₁ = Correction for surface irregularity
- n₂ = Correction for channel variation
- n₃ = Correction for obstructions
- n₄ = Correction for vegetation
- m = Correction for meandering
Cowan’s n-Value Adjustments
| Factor | Condition | Value |
|---|---|---|
| n₀ (Base) | Earth | 0.020 |
| Rock cut | 0.025 | |
| Fine gravel | 0.024 | |
| Coarse gravel | 0.028 | |
| n₁ (Irregularity) | Smooth | 0.000 |
| Minor | 0.005 | |
| Moderate | 0.010 | |
| Severe | 0.020 | |
| n₂ (Section variation) | Gradual | 0.000 |
| Alternating | 0.005 | |
| Alternating frequently | 0.010-0.015 | |
| n₃ (Obstructions) | Negligible | 0.000 |
| Minor | 0.010-0.015 | |
| Appreciable | 0.020-0.030 | |
| Severe | 0.040-0.060 | |
| n₄ (Vegetation) | Low | 0.005-0.010 |
| Medium | 0.010-0.025 | |
| High | 0.025-0.050 | |
| Very high | 0.050-0.100 | |
| m (Meandering) | Minor | 1.00 |
| Appreciable | 1.15 | |
| Severe | 1.30 |
Common Mistakes in n Selection
1. Using Catalog Values Blindly
Published values are starting points. Consider site-specific conditions.
2. Ignoring Variation with Stage
n may vary significantly with depth, especially in vegetated channels.
3. Over-Precise Values
Specifying n = 0.0134 implies false precision. Round appropriately (typically to 3 decimal places).
4. Ignoring Maintenance
Design capacity assumes maintained condition. Unmaintained channels may have much higher n.
5. Forgetting Safety Factors
For critical applications, consider using a range of n values to assess sensitivity.
Summary
For pipes:
- Concrete: 0.012-0.015
- CMP: 0.022-0.035
- Plastic: 0.010-0.012
For constructed channels:
- Concrete lined: 0.013-0.019
- Grass lined: 0.025-0.050
- Riprap: 0.030-0.060
For natural channels:
- Clean, straight: 0.025-0.040
- Winding, with vegetation: 0.040-0.070
- Heavy vegetation/debris: 0.070-0.150
References
-
Chow, V. T. (1959). Open-channel hydraulics. McGraw-Hill.
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Cowan, W. L. (1956). Estimating hydraulic roughness coefficients. Agricultural Engineering, 37(7), 473-475.
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Arcement, G. J., & Schneider, V. R. (1989). Guide for selecting Manning’s roughness coefficients for natural channels and flood plains (Water Supply Paper 2339). U.S. Geological Survey.
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Federal Highway Administration. (2012). Hydraulic design of highway culverts (3rd ed., Hydraulic Design Series No. 5). U.S. Department of Transportation.
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Barnes, H. H. (1967). Roughness characteristics of natural channels (Water Supply Paper 1849). U.S. Geological Survey.
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American Concrete Pipe Association. (2011). Concrete pipe design manual. ACPA.