The Situation
You are preparing a stormwater analysis for a 10-acre mixed-use development site. The site currently has a mix of agricultural land and some existing commercial buildings. The proposed development includes retail, parking, a small park, and residential townhomes. You need to calculate the runoff volume for the 25-year, 24-hour design storm to size detention facilities.
The site has Hydrologic Soil Group C (clay loam, moderate infiltration), and the 25-year, 24-hour rainfall depth for this location is 6.0 inches.
Which Calculator to Use
The SCS Curve Number Calculator is the standard method for estimating runoff volume from mixed-use sites. It accounts for different land cover types through the weighted curve number.
Walking Through the Inputs
Rainfall Depth (P = 6.0 inches)
This is the total 25-year, 24-hour precipitation for your location. You can look this up from NOAA Atlas 14 by entering your coordinates. For this example, we are using 6.0 inches, which is typical for much of the eastern and central United States.
Curve Number (CN = 88)
The curve number represents the runoff potential of the site. For a mixed-use development you calculate an area-weighted composite CN from the different land covers and their areas. Each contribution equals the land cover’s CN multiplied by its fraction of the total site:
| Land Cover | Area (acres) | % of Site | CN (HSG C) | Weighted Contribution |
|---|---|---|---|---|
| Commercial/retail roofs | 1.5 | 15% | 98 | 14.7 |
| Parking lots (asphalt) | 2.5 | 25% | 98 | 24.5 |
| Residential townhomes (1/8 acre lots) | 2.0 | 20% | 83 | 16.6 |
| Streets/sidewalks (paved) | 1.0 | 10% | 98 | 9.8 |
| Park/open space (grass, good condition) | 2.0 | 20% | 74 | 14.8 |
| Landscaped areas (grass, good condition) | 1.0 | 10% | 74 | 7.4 |
| Total | 10.0 | 100% | 87.8 |
Summing the weighted contributions gives 14.7 + 24.5 + 16.6 + 9.8 + 14.8 + 7.4 = 87.8, so the composite curve number rounds to CN = 88.
Drainage Area (A = 10 acres)
The total site area is 10 acres.
Antecedent Moisture Condition (AMC II - Normal)
AMC II (normal conditions) is the standard for design purposes. AMC I (dry) and AMC III (wet) are used for sensitivity analysis but not for the primary design.
Initial Abstraction Ratio (Ia/S = 0.20)
The traditional SCS method uses an initial abstraction ratio of 0.20 (meaning 20% of the maximum retention is abstracted before runoff begins). Some newer guidance recommends 0.05, but 0.20 is still the standard for most jurisdictions.
The Results
With CN = 88 and P = 6.0 inches:
- Maximum retention (S): S = (1000/88) - 10 = 1.36 inches
- Initial abstraction (Ia): Ia = 0.20 x 1.36 = 0.27 inches
- Runoff depth (Q): Q = (P - Ia)^2 / (P - Ia + S) = (6.0 - 0.27)^2 / (6.0 - 0.27 + 1.36) = 4.63 inches
- Runoff volume: 4.63 inches x 10 acres = 46.3 acre-inches = 3.86 acre-feet = 168,200 cubic feet
For comparison, the pre-development runoff:
- Pre-development CN = 72 (agricultural, good condition, HSG C)
- Pre-development runoff depth = 2.98 inches
- Pre-development volume = 29.8 acre-inches = 2.48 acre-feet
The development increases runoff by 55% (from 2.98 inches to 4.63 inches), requiring significant detention.
What This Means for Your Project
Detention Requirement
The difference between post- and pre-development runoff volume is approximately:
- (4.63 - 2.98) x 10 = 16.5 acre-inches = 1.38 acre-feet = 60,100 cubic feet
This is the minimum additional volume that must be stored on-site (in ponds, underground detention, or a combination) to reduce the post-development peak to the pre-development level.
Practical Context
- 60,100 cubic feet is equivalent to about 450,000 gallons of water
- A detention pond averaging 4 feet deep would need a surface area of about 15,000 sq ft (0.34 acres)
- That is roughly 3.4% of the site area — typical for this density of development
Sensitivity Check
Always run the calculation for multiple storm events:
| Storm Event | Rainfall (in) | Pre-Dev Runoff (in) | Post-Dev Runoff (in) | Difference (in) |
|---|---|---|---|---|
| 2-year | 3.2 | 0.96 | 1.85 | 0.89 |
| 10-year | 4.8 | 2.22 | 3.75 | 1.53 |
| 25-year | 6.0 | 2.98 | 4.63 | 1.65 |
| 100-year | 7.8 | 4.22 | 6.14 | 1.92 |
Try This Scenario
Try this scenario in the SCS Curve Number CalculatorFrequently Asked Questions
How do you calculate a weighted curve number for a mixed-use site?
Multiply each land cover’s curve number by its fraction of the total area, then sum the products. For this 10-acre site the area-weighted values total 87.8, which rounds to a composite CN of 88. This area-weighting is the standard TR-55 approach for sites with more than one cover type.
What curve number should I use for parking lots and roofs on HSG C soil?
Impervious surfaces such as asphalt parking, streets, sidewalks, and commercial roofs use CN = 98 on all hydrologic soil groups because almost no rainfall infiltrates. Only the pervious portions (lawn, park, landscaping) change with the soil group.
How much detention volume does this development need?
The post-development runoff (4.63 in) exceeds pre-development (2.98 in) by 1.65 in over 10 acres, or about 1.38 acre-feet (60,100 cubic feet). That is the minimum extra storage needed to hold the post-development peak down to the pre-development rate for the 25-year storm.
Should I use an initial abstraction ratio of 0.20 or 0.05?
The traditional SCS method uses Ia/S = 0.20, and most jurisdictions still require it. The newer Ia/S = 0.05 value produces slightly more runoff and is used in some updated state manuals. Always confirm which ratio your reviewing agency requires before finalizing detention sizing.
Next Steps
- Use the Pond Sizing Calculator to determine specific detention facility dimensions
- Check the Stormwater Compliance Checklist for permit requirements
- Run the Time of Concentration Calculator to determine the critical storm duration for peak flow analysis
- Compare land-cover values against the runoff coefficients reference if you also need a Rational Method peak flow for storm-drain sizing