Scenario: Runoff Estimate for a Mixed-Use Site

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 = 80)

The curve number represents the runoff potential of the site. For a mixed-use development, you need to calculate a weighted CN based on the different land covers and their areas:

We round to CN = 80 — wait, let me recalculate. The sum of the weighted values is 14.7 + 24.5 + 16.6 + 9.8 + 14.8 + 7.4 = 87.8. So the composite CN is approximately 88. Let me use 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:

Try This Scenario

Next Steps

1. Use the Pond Sizing Calculator to determine specific detention facility dimensions
2. Check the Stormwater Compliance Checklist for permit requirements
3. Run the Time of Concentration Calculator to determine the critical storm duration for peak flow analysis