The runoff coefficient (C) represents the fraction of rainfall that becomes runoff. Values range from 0 to 1, with higher values indicating more impervious surfaces and greater runoff. These coefficients are used with the Rational Method for peak flow estimation in small drainage areas (typically < 200 acres).
Rational Method Equation
Peak Flow (Q):
Q = C i A
Where:
- Q = peak runoff rate (cfs)
- C = runoff coefficient (dimensionless)
- i = rainfall intensity (in/hr)
- A = drainage area (acres)
Note: The formula gives Q in cfs when i is in in/hr and A is in acres (conversion factor 1.008 is approximately 1).
Showing 21 of 21 surface types
| Surface Type ▲ | C Min | C Typical | C Max | Category | Source |
|---|---|---|---|---|---|
Asphalt Driveway/Walks Asphalt driveways, sidewalks, and parking lots | 0.75 | 0.85 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Asphalt Street Asphalt paved street surface | 0.70 | 0.85 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Brick Pavement Brick or stone block pavement | 0.70 | 0.78 | 0.85 | Impervious Surfaces | HEC-22, Table 3-1 |
Built-up Roof Built-up or membrane roofing | 0.85 | 0.90 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Cemeteries Cemeteries with maintained grass | 0.10 | 0.18 | 0.25 | Parks and Open Space | HEC-22, Table 3-1 |
Concrete Driveway/Walks Concrete driveways, sidewalks, and parking lots | 0.80 | 0.90 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Concrete Street Concrete paved street surface | 0.80 | 0.90 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Gravel Roof Built-up roof with gravel cover | 0.85 | 0.88 | 0.90 | Impervious Surfaces | HEC-22, Table 3-1 |
Gravel Surface Gravel roads, driveways, and parking areas | 0.25 | 0.40 | 0.60 | Impervious Surfaces | ASCE MOP 77, Table 5-1 |
Lawn (Clay Soil, Flat) Lawn with clay soil, flat slope (0-2%) | 0.13 | 0.15 | 0.17 | Lawns | HEC-22, Table 3-1 |
Lawn (Clay Soil, Moderate) Lawn with clay soil, moderate slope (2-7%) | 0.18 | 0.20 | 0.22 | Lawns | HEC-22, Table 3-1 |
Lawn (Clay Soil, Steep) Lawn with clay soil, steep slope (>7%) | 0.25 | 0.30 | 0.35 | Lawns | HEC-22, Table 3-1 |
Lawn (Sandy Soil, Flat) Lawn with sandy soil, flat slope (0-2%) | 0.05 | 0.08 | 0.10 | Lawns | HEC-22, Table 3-1 |
Lawn (Sandy Soil, Moderate) Lawn with sandy soil, moderate slope (2-7%) | 0.10 | 0.13 | 0.15 | Lawns | HEC-22, Table 3-1 |
Lawn (Sandy Soil, Steep) Lawn with sandy soil, steep slope (>7%) | 0.15 | 0.18 | 0.20 | Lawns | HEC-22, Table 3-1 |
Metal Roof Metal roofing surfaces | 0.90 | 0.95 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Parks (Grass Cover) Public parks with grass cover | 0.10 | 0.18 | 0.25 | Parks and Open Space | HEC-22, Table 3-1 |
Playgrounds Playgrounds with mixed surfaces | 0.20 | 0.28 | 0.35 | Parks and Open Space | HEC-22, Table 3-1 |
Railroad Yard Railroad yard areas with ballast and gravel | 0.20 | 0.30 | 0.40 | Unimproved Areas | HEC-22, Table 3-1 |
Shingle Roof Composition shingle roofing | 0.75 | 0.85 | 0.90 | Impervious Surfaces | HEC-22, Table 3-1 |
Unimproved/Vacant Land Unimproved or vacant land with natural cover | 0.10 | 0.20 | 0.30 | Unimproved Areas | HEC-22, Table 3-1 |
Frequency Adjustment Factors (Cf)
For return periods greater than 10 years, multiply the runoff coefficient by the frequency factor below. The adjusted C value should not exceed 1.0.
| Return Period (years) | 2 | 5 | 10 | 25 | 50 | 100 |
|---|---|---|---|---|---|---|
| Cf | 1.00 | 1.00 | 1.00 | 1.10 | 1.20 | 1.25 |
Source: HEC-22, Table 3-2. Adjusted C = C x Cf (maximum 1.0)
Slope Adjustment
For pervious surfaces, slope affects runoff coefficients. The lawn values in the table are categorized by slope:
Flat (0-2%)
Lowest C values - maximum infiltration opportunity
Moderate (2-7%)
Intermediate C values - typical suburban conditions
Steep (>7%)
Highest C values - reduced infiltration time
Design Guidance
Composite Runoff Coefficient
For drainage areas with multiple surface types, calculate an area-weighted composite C:
Ccomposite = sum(Ci x Ai) / Atotal
Range Selection
- Use typical values for general design
- Use maximum values for conservative design or critical infrastructure
- Consider soil type, slope, and development intensity when selecting within the range
Method Limitations
- Rational Method is best for small watersheds (< 200 acres)
- Assumes uniform rainfall over the drainage area
- Assumes steady-state conditions (rainfall duration >= time of concentration)
- Does not account for detention/storage effects
Soil Type Considerations
For lawn and vegetated areas, the table provides separate values for sandy (well-drained) and clay (poorly-drained) soils. Select based on the predominant soil type or HSG:
- Sandy: HSG A and B soils
- Clay: HSG C and D soils
Primary Sources
- FHWA HEC-22 (2009). Urban Drainage Design Manual, 3rd Ed. Tables 3-1, 3-2.
- ASCE MOP 77 (2006). Design and Construction of Urban Stormwater Management Systems. Table 5-1.
- McCuen, R.H. (2005). Hydrologic Analysis and Design, 3rd Ed. Prentice Hall.