What runoff coefficient should I use for asphalt and roofs?

Asphalt streets are typically 0.83 (0.75 to 0.92) and roofs 0.85 (0.78 to 0.93). Use the upper end for new, well-jointed surfaces and for the less frequent design storms, when depression storage and losses are a smaller share of the rainfall.

What C value applies to grassland or forest?

Pasture and grassland on medium soils is typically 0.25 (0.15 to 0.35); forest and woodland 0.13 (0.07 to 0.20). Rural values depend strongly on soil, slope and the state of the vegetation, so read the row for your soil permeability and slope class rather than the category as a whole.

How do I calculate a composite runoff coefficient?

Weight each sub-area's coefficient by its area: C = Σ(Cᵢ Aᵢ) / ΣAᵢ. Measure the sub-areas from a land-use map or site plan and use one C per land-use and soil combination. The worked example on this page gives the arithmetic.

Should C increase for larger return periods?

Most design manuals say yes, because losses are a smaller fraction of a large storm. FHWA HEC-22 multiplies C by a frequency factor of 1.0 for return periods up to 10 years, 1.1 for 25 years, 1.2 for 50 years and 1.25 for 100 years, with the product capped at 1.0. Other manuals use their own factors or tabulate C by return period directly; follow the one your project is reviewed under.

Is the Rational method valid for large catchments?

It assumes uniform rainfall over the whole catchment for a duration equal to the time of concentration, which holds only for small areas. Most manuals limit it to somewhere between 0.8 km² (200 acres) for urban drainage and about 15 km² for rural catchments; beyond that, use a hydrograph method such as SCS/TR-55 or a unit hydrograph.

Runoff Coefficient Table (C Values) for the Rational Method

The Rational formula

The Rational method estimates the peak discharge from a small catchment as the product of a runoff coefficient, the rainfall intensity for a duration equal to the time of concentration, and the catchment area:

Q = \frac{C i A}{3.6} (SI: Q in m^{3} /s, i in mm/h, A in km^{2}) Q = C i A (US: cfs, in/h, acres)

C lumps together everything that separates rainfall from runoff at the peak: infiltration, depression storage, interception and the attenuation of the flow within the catchment. It is not a physical constant of the surface, which is why every table gives a range. Use the typical value as a starting point and move within the range according to the soil, the slope, the state of the surface and the return period.

How to read the tables

Slope class. Flat, moderate or steep terrain. Steeper ground sheds water faster with less time for infiltration, so C rises with slope. Rows without a slope variant apply to moderate slopes.
Soil permeability. Low (clays, shallow soils over rock), medium (loams) or high (sands, deep well-drained soils). Low permeability gives the highest C.
Basis. The 51 rows marked ASCE follow the land-use ranges of the ASCE manuals of practice. The rows marked Compiled (open space, agricultural and undeveloped land) are typical design ranges compiled from textbooks and design manuals rather than taken from one published table. Check both against the manual your project is reviewed under.

The values are for the frequent design storms, roughly 2 to 10 years. Apply your manual's frequency adjustment for rarer events (see the FAQ below). Use them with the Rational method calculator, which also computes the composite C for a catchment of mixed land use, and the time of concentration calculator, which sets the storm duration.

Tables

Jump to: Residential · Commercial · Industrial · Parks & Open Space · Agricultural · Paved Surfaces · Undeveloped

Residential

Surface	Slope	Soil permeability	C range	Typical C	Basis
Single-family detached residential, flat terrain	Flat	Low	0.35 – 0.55	0.45	ASCE
Single-family detached residential, flat terrain	Flat	Medium	0.25 – 0.45	0.35	ASCE
Single-family detached residential, flat terrain	Flat	High	0.20 – 0.35	0.28	ASCE
Single-family detached residential	Moderate	Low	0.40 – 0.55	0.48	ASCE
Single-family detached residential	Moderate	Medium	0.30 – 0.50	0.40	ASCE
Single-family detached residential	Moderate	High	0.25 – 0.40	0.32	ASCE
Single-family detached residential, steep terrain	Steep	Low	0.45 – 0.65	0.55	ASCE
Single-family detached residential, steep terrain	Steep	Medium	0.35 – 0.55	0.45	ASCE
Single-family detached residential, steep terrain	Steep	High	0.30 – 0.50	0.38	ASCE
Multi-family or townhouse residential	Moderate	Low	0.50 – 0.70	0.60	ASCE
Multi-family or townhouse residential	Moderate	Medium	0.40 – 0.60	0.50	ASCE
Multi-family or townhouse residential	Moderate	High	0.35 – 0.50	0.42	ASCE
Apartment or condominium complex	Moderate	Low	0.60 – 0.80	0.70	ASCE
Apartment or condominium complex	Moderate	Medium	0.50 – 0.70	0.60	ASCE
Apartment or condominium complex	Moderate	High	0.45 – 0.65	0.55	ASCE
High-density residential (row houses, flats)	Moderate	Low	0.70 – 0.85	0.78	ASCE
High-density residential (row houses, flats)	Moderate	Medium	0.60 – 0.80	0.70	ASCE
High-density residential (row houses, flats)	Moderate	High	0.55 – 0.75	0.65	ASCE

Commercial

Surface	Slope	Soil permeability	C range	Typical C	Basis
Downtown or central business district	Moderate	Low	0.80 – 0.95	0.88	ASCE
Downtown or central business district	Moderate	Medium	0.70 – 0.95	0.82	ASCE
Downtown or central business district	Moderate	High	0.65 – 0.90	0.78	ASCE
Neighbourhood commercial strip or retail	Moderate	Low	0.60 – 0.75	0.68	ASCE
Neighbourhood commercial strip or retail	Moderate	Medium	0.50 – 0.70	0.60	ASCE
Neighbourhood commercial strip or retail	Moderate	High	0.45 – 0.65	0.55	ASCE
Shopping centre or mall with parking	Moderate	Low	0.70 – 0.90	0.80	ASCE
Shopping centre or mall with parking	Moderate	Medium	0.60 – 0.80	0.70	ASCE
Shopping centre or mall with parking	Moderate	High	0.55 – 0.75	0.65	ASCE

Industrial

Surface	Slope	Soil permeability	C range	Typical C	Basis
Light manufacturing, assembly plants	Moderate	Low	0.65 – 0.85	0.75	ASCE
Light manufacturing, assembly plants	Moderate	Medium	0.55 – 0.80	0.67	ASCE
Light manufacturing, assembly plants	Moderate	High	0.50 – 0.70	0.60	ASCE
Heavy manufacturing, processing plants	Moderate	Low	0.75 – 0.95	0.85	ASCE
Heavy manufacturing, processing plants	Moderate	Medium	0.65 – 0.90	0.77	ASCE
Heavy manufacturing, processing plants	Moderate	High	0.60 – 0.80	0.70	ASCE
Warehousing and distribution centres	Moderate	Low	0.70 – 0.85	0.77	ASCE
Warehousing and distribution centres	Moderate	Medium	0.60 – 0.80	0.70	ASCE
Warehousing and distribution centres	Moderate	High	0.55 – 0.70	0.62	ASCE

Parks & Open Space

Surface	Slope	Soil permeability	C range	Typical C	Basis
Parks and landscaped gardens	Moderate	Low	0.15 – 0.30	0.22	Compiled
Parks and landscaped gardens	Moderate	Medium	0.10 – 0.25	0.17	Compiled
Parks and landscaped gardens	Moderate	High	0.08 – 0.18	0.13	Compiled
Recreational playgrounds and sports fields	Moderate	Low	0.25 – 0.40	0.32	Compiled
Recreational playgrounds and sports fields	Moderate	Medium	0.20 – 0.35	0.27	Compiled
Recreational playgrounds and sports fields	Moderate	High	0.15 – 0.28	0.22	Compiled
Cemeteries and memorial parks	Moderate	Low	0.15 – 0.30	0.22	Compiled
Cemeteries and memorial parks	Moderate	Medium	0.10 – 0.25	0.18	Compiled
Cemeteries and memorial parks	Moderate	High	0.08 – 0.18	0.13	Compiled

Agricultural

Surface	Slope	Soil permeability	C range	Typical C	Basis
Cultivated row crops, flat terrain	Flat	Low	0.30 – 0.50	0.40	Compiled
Cultivated row crops, flat terrain	Flat	Medium	0.20 – 0.40	0.30	Compiled
Cultivated row crops, flat terrain	Flat	High	0.10 – 0.30	0.20	Compiled
Cultivated row crops, moderate slope	Moderate	Low	0.35 – 0.55	0.45	Compiled
Cultivated row crops, moderate slope	Moderate	Medium	0.25 – 0.45	0.35	Compiled
Cultivated row crops, moderate slope	Moderate	High	0.15 – 0.35	0.25	Compiled
Cultivated row crops, steep terrain	Steep	Low	0.40 – 0.60	0.50	Compiled
Cultivated row crops, steep terrain	Steep	Medium	0.30 – 0.50	0.40	Compiled
Cultivated row crops, steep terrain	Steep	High	0.20 – 0.40	0.30	Compiled
Small grain crops (wheat, maize)	Moderate	Low	0.25 – 0.45	0.35	Compiled
Small grain crops (wheat, maize)	Moderate	Medium	0.15 – 0.40	0.28	Compiled
Small grain crops (wheat, maize)	Moderate	High	0.10 – 0.30	0.20	Compiled
Pasture and grassland	Moderate	Low	0.20 – 0.45	0.32	Compiled
Pasture and grassland	Moderate	Medium	0.15 – 0.35	0.25	Compiled
Pasture and grassland	Moderate	High	0.10 – 0.25	0.18	Compiled
Forest and woodland	Moderate	Low	0.10 – 0.30	0.20	Compiled
Forest and woodland	Moderate	Medium	0.07 – 0.20	0.13	Compiled
Forest and woodland	Moderate	High	0.05 – 0.15	0.10	Compiled

Paved Surfaces

Surface	Slope	Soil permeability	C range	Typical C	Basis
Asphalt-paved streets and roads	Moderate	Low	0.80 – 0.95	0.88	ASCE
Asphalt-paved streets and roads	Moderate	Medium	0.75 – 0.92	0.83	ASCE
Asphalt-paved streets and roads	Moderate	High	0.70 – 0.90	0.80	ASCE
Concrete paving and surfaces	Moderate	Low	0.85 – 0.98	0.92	ASCE
Concrete paving and surfaces	Moderate	Medium	0.82 – 0.95	0.88	ASCE
Concrete paving and surfaces	Moderate	High	0.80 – 0.92	0.86	ASCE
Brick paving (with sand joints)	Moderate	Low	0.75 – 0.90	0.82	ASCE
Brick paving (with sand joints)	Moderate	Medium	0.70 – 0.85	0.77	ASCE
Brick paving (with sand joints)	Moderate	High	0.65 – 0.80	0.72	ASCE
Rooftops (flat or pitched)	Moderate	Low	0.80 – 0.95	0.88	ASCE
Rooftops (flat or pitched)	Moderate	Medium	0.78 – 0.93	0.85	ASCE
Rooftops (flat or pitched)	Moderate	High	0.75 – 0.90	0.82	ASCE
Gravel roads and driveways	Moderate	Low	0.35 – 0.70	0.52	ASCE
Gravel roads and driveways	Moderate	Medium	0.25 – 0.60	0.42	ASCE
Gravel roads and driveways	Moderate	High	0.15 – 0.45	0.30	ASCE

Undeveloped

Surface	Slope	Soil permeability	C range	Typical C	Basis
Bare or disturbed soil, flat terrain	Flat	Low	0.30 – 0.60	0.45	Compiled
Bare or disturbed soil, flat terrain	Flat	Medium	0.20 – 0.50	0.35	Compiled
Bare or disturbed soil, flat terrain	Flat	High	0.10 – 0.35	0.22	Compiled
Bare or disturbed soil	Moderate	Low	0.40 – 0.75	0.57	Compiled
Bare or disturbed soil	Moderate	Medium	0.30 – 0.60	0.45	Compiled
Bare or disturbed soil	Moderate	High	0.20 – 0.45	0.32	Compiled
Bare or disturbed soil, steep terrain	Steep	Low	0.55 – 0.85	0.70	Compiled
Bare or disturbed soil, steep terrain	Steep	Medium	0.40 – 0.70	0.55	Compiled
Bare or disturbed soil, steep terrain	Steep	High	0.30 – 0.55	0.42	Compiled
Natural veld, scrubland, or bushveld	Moderate	Low	0.20 – 0.50	0.35	Compiled
Natural veld, scrubland, or bushveld	Moderate	Medium	0.12 – 0.40	0.26	Compiled
Natural veld, scrubland, or bushveld	Moderate	High	0.08 – 0.28	0.18	Compiled
Rocky or near-impervious terrain	Moderate	Low	0.70 – 0.95	0.82	Compiled
Rocky or near-impervious terrain	Moderate	Medium	0.65 – 0.90	0.77	Compiled
Rocky steep terrain, cliffs	Steep	Low	0.75 – 0.95	0.85	Compiled

Composite C for mixed land use

Where a catchment contains several land uses, weight each coefficient by its area:

C_{w} = \frac{\sum _{i} C _{i} A _{i}}{\sum _{i} A _{i}}

For example, a 5.0 ha catchment with 2.0 ha of asphalt streets (C = 0.83) and 3.0 ha of single-family housing on moderate slopes and medium soils (C = 0.40) has a composite coefficient of (0.83 × 2.0 + 0.40 × 3.0) / 5.0 = 0.57.

Area weighting is exact for the peak only when every part of the catchment contributes at the same time. Where a large paved area drains straight to the outlet and the pervious area lies far upstream, the paved area alone can give a higher peak than the whole catchment: check the impervious part on its own, with its own shorter time of concentration.

Limits of the method

The Rational method assumes the rainfall intensity is uniform over the catchment and constant for the time of concentration, and that the return period of the peak flow equals that of the rainfall. These assumptions hold for small, mostly urban catchments. It gives a peak only, with no hydrograph and no volume, so it cannot be used to size storage. For larger or rural catchments, or where storage matters, use the TR-55 calculator or a unit hydrograph.

Runoff coefficient table