The runoff coefficient (C) is a critical parameter in the Rational Method that represents the fraction of rainfall that becomes direct runoff. Selecting the right C value requires understanding how surface characteristics, soil properties, and slope affect runoff generation.
What Does the Runoff Coefficient Represent?
The runoff coefficient accounts for all rainfall losses:
Losses include:
- Infiltration into the soil
- Depression storage (puddles)
- Interception by vegetation
- Evaporation during the storm
A C value of 0.90 means 90% of rainfall becomes runoff (10% losses). A C value of 0.20 means only 20% becomes runoff (80% losses).
Runoff Coefficients by Surface Type
Developed/Impervious Surfaces
| Surface Type | C Range | Recommended Design Value |
|---|---|---|
| Paved Surfaces | ||
| Asphalt pavement | 0.70 - 0.95 | 0.90 |
| Concrete pavement | 0.80 - 0.95 | 0.90 |
| Brick/pavers (mortared) | 0.70 - 0.85 | 0.80 |
| Roofing | ||
| Conventional roofs | 0.75 - 0.95 | 0.90 |
| Gravel roofs | 0.70 - 0.80 | 0.75 |
| Green roofs (extensive) | 0.30 - 0.50 | 0.40 |
| Green roofs (intensive) | 0.20 - 0.40 | 0.30 |
| Semi-Pervious | ||
| Gravel roads/parking | 0.35 - 0.70 | 0.50 |
| Permeable pavers | 0.30 - 0.50 | 0.40 |
| Porous asphalt | 0.25 - 0.45 | 0.35 |
Undeveloped/Pervious Surfaces
Runoff coefficients for pervious surfaces depend significantly on soil type and slope.
Lawns and Turf
| Soil Type | 0-2% Slope | 2-7% Slope | >7% Slope |
|---|---|---|---|
| Sandy soil | 0.05-0.10 | 0.10-0.15 | 0.15-0.20 |
| Loam soil | 0.10-0.15 | 0.15-0.20 | 0.20-0.25 |
| Clay soil | 0.15-0.20 | 0.20-0.25 | 0.25-0.35 |
Agricultural Land
| Cover Type | 0-2% Slope | 2-7% Slope | >7% Slope |
|---|---|---|---|
| Cultivated (good) | 0.08-0.13 | 0.13-0.17 | 0.18-0.22 |
| Cultivated (poor) | 0.14-0.18 | 0.18-0.22 | 0.22-0.28 |
| Pasture/range (good) | 0.12-0.17 | 0.17-0.22 | 0.22-0.28 |
| Pasture/range (poor) | 0.20-0.25 | 0.25-0.30 | 0.30-0.35 |
Natural Vegetation
| Cover Type | 0-2% Slope | 2-7% Slope | >7% Slope |
|---|---|---|---|
| Forest/woods (good cover) | 0.05-0.08 | 0.08-0.11 | 0.11-0.14 |
| Forest/woods (poor cover) | 0.12-0.16 | 0.16-0.20 | 0.20-0.25 |
| Meadow | 0.10-0.15 | 0.15-0.20 | 0.20-0.25 |
Runoff Coefficients by Land Use Category
For preliminary design when detailed surface breakdown isn’t available:
| Land Use Category | C Range | Typical Value |
|---|---|---|
| Residential | ||
| Single-family (1/4 acre lots) | 0.30-0.50 | 0.40 |
| Single-family (1/2 acre lots) | 0.25-0.40 | 0.35 |
| Single-family (1+ acre lots) | 0.20-0.35 | 0.30 |
| Multi-family | 0.40-0.60 | 0.50 |
| Townhouses | 0.45-0.60 | 0.55 |
| Commercial | ||
| Business districts | 0.70-0.95 | 0.85 |
| Neighborhood commercial | 0.50-0.70 | 0.60 |
| Shopping centers | 0.70-0.90 | 0.80 |
| Industrial | ||
| Light industrial | 0.50-0.80 | 0.65 |
| Heavy industrial | 0.60-0.90 | 0.75 |
| Industrial parks | 0.50-0.70 | 0.60 |
| Institutional | ||
| Schools (with fields) | 0.35-0.55 | 0.45 |
| Churches (with parking) | 0.50-0.70 | 0.60 |
| Hospitals | 0.50-0.70 | 0.60 |
| Open Space | ||
| Parks (mostly grass) | 0.10-0.25 | 0.20 |
| Playgrounds | 0.20-0.35 | 0.30 |
| Cemeteries | 0.10-0.25 | 0.20 |
Factors Affecting Runoff Coefficient Selection
1. Soil Type
Soil is the primary factor for pervious surfaces:
| Hydrologic Soil Group | Infiltration Rate | C Adjustment |
|---|---|---|
| A (sand, gravel) | High (>0.3 in/hr) | Use lower C values |
| B (loam, silt loam) | Moderate (0.15-0.3 in/hr) | Use mid-range C values |
| C (sandy clay loam) | Slow (0.05-0.15 in/hr) | Use higher C values |
| D (clay) | Very slow (<0.05 in/hr) | Use highest C values |
2. Slope
Steeper slopes increase runoff:
- Flat (0-2%): Water has time to infiltrate
- Moderate (2-7%): Some increase in runoff
- Steep (>7%): Significant increase in runoff
General rule: Add 0.05-0.10 to C for every slope category increase.
3. Storm Intensity/Frequency
Higher intensity storms reduce infiltration capacity:
| Return Period | C Adjustment Factor |
|---|---|
| 2-10 year | 1.00 (base value) |
| 25 year | 1.10 |
| 50 year | 1.20 |
| 100 year | 1.25 |
4. Antecedent Moisture
For pervious surfaces, pre-storm soil moisture matters:
- Dry conditions: Use lower C values
- Average conditions: Use typical C values
- Wet/saturated conditions: Use higher C values
Most design applications assume average antecedent conditions.
Calculating Composite Runoff Coefficient
For mixed land use, calculate an area-weighted average:
Worked Example
Site breakdown:
| Surface | Area (acres) | C Value | C × A |
|---|---|---|---|
| Building roofs | 2.5 | 0.90 | 2.25 |
| Parking (asphalt) | 4.0 | 0.90 | 3.60 |
| Sidewalks | 0.5 | 0.90 | 0.45 |
| Lawn (clay soil, 2%) | 3.0 | 0.20 | 0.60 |
| Total | 10.0 | 6.90 |
Try the Runoff Coefficient Calculator →
Estimating C from Impervious Percentage
When you know the impervious percentage but not the detailed surface breakdown:
Typical values:
- C_imp = 0.90 (impervious surfaces)
- C_perv = 0.20 (pervious surfaces, varies by soil)
| Impervious % | Composite C (sandy soil) | Composite C (clay soil) |
|---|---|---|
| 20% | 0.26 | 0.34 |
| 40% | 0.44 | 0.52 |
| 60% | 0.62 | 0.70 |
| 80% | 0.80 | 0.88 |
| 100% | 0.90 | 0.90 |
Special Conditions
Frozen Ground
Frozen soil acts like impervious surface:
- Use C = 0.80-0.95 for pervious areas when frozen
- Critical for northern climates in spring
Green Infrastructure
Green infrastructure systems have lower C values:
| System Type | C Value |
|---|---|
| Bioretention (rain garden) | 0.20-0.40 |
| Infiltration trench | 0.20-0.30 |
| Permeable pavement | 0.30-0.50 |
| Vegetated swale | 0.15-0.30 |
| Green roof | 0.20-0.50 |
Compacted Soils
Construction traffic often compacts soils, reducing infiltration:
- Use C values 0.10-0.20 higher than native soil
- Consider soil decompaction during landscaping
- Compacted lawns can have C = 0.35-0.50
Regional Variations
Different agencies may specify different C values. Always check local standards:
- ASCE Manual of Practice 77: National reference values
- HEC-22: FHWA guidance for highway drainage
- Local jurisdiction manuals: May have site-specific requirements
Summary Table: Quick Reference
| Surface/Land Use | Low C | Typical C | High C |
|---|---|---|---|
| Asphalt/concrete | 0.80 | 0.90 | 0.95 |
| Rooftops | 0.75 | 0.90 | 0.95 |
| Gravel | 0.35 | 0.50 | 0.70 |
| Lawns (sandy) | 0.05 | 0.15 | 0.20 |
| Lawns (clay) | 0.15 | 0.25 | 0.35 |
| Woods | 0.05 | 0.15 | 0.25 |
| Single-family residential | 0.25 | 0.40 | 0.50 |
| Commercial | 0.60 | 0.80 | 0.95 |
| Industrial | 0.50 | 0.70 | 0.90 |
References
-
American Society of Civil Engineers. (2017). Design and construction of urban stormwater management systems (ASCE Manual of Practice No. 77). ASCE Press.
-
Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., Hydraulic Engineering Circular No. 22). U.S. Department of Transportation.
-
McCuen, R. H. (2016). Hydrologic analysis and design (4th ed.). Pearson.
-
Chow, V. T., Maidment, D. R., & Mays, L. W. (1988). Applied hydrology. McGraw-Hill.
-
American Iron and Steel Institute. (1999). Modern sewer design (4th ed.). AISI.
-
Urban Drainage and Flood Control District. (2016). Urban storm drainage criteria manual. Denver, CO.
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