1. What is Stormwater Runoff?

Stormwater runoff is rainfall that flows over the ground surface when it can't infiltrate into the soil. The Rational Method (Q = CiA) is commonly used to estimate peak discharge for small drainage areas.

2. How Does the Calculator Work?

The calculator uses the Rational Method equation:

Where:

• \( Q \) — Peak discharge (cfs)
• \( C \) — Runoff coefficient (0-1)
• \( i \) — Rainfall intensity (in/hr)
• \( A \) — Drainage area (acres)

Explanation: The equation estimates the peak rate of runoff during a storm event based on land cover characteristics, rainfall intensity, and drainage area.

3. Importance of Runoff Calculation

Details: Accurate runoff estimation is crucial for designing drainage systems, culverts, storm sewers, and other hydraulic structures to prevent flooding.

4. Using the Calculator

Tips: Enter the runoff coefficient (typically 0.3-0.95 based on surface type), rainfall intensity for the design storm, and drainage area in acres.

5. Frequently Asked Questions (FAQ)

Q1: What are typical runoff coefficients?
A: Pavement (0.95), roofs (0.95), lawns (0.3-0.5), forests (0.1-0.3). Values vary based on soil type and slope.

Q2: How is rainfall intensity determined?
A: From Intensity-Duration-Frequency (IDF) curves for your location and design return period (e.g., 10-year storm).

Q3: What's the maximum area for this method?
A: Typically limited to 200 acres or less. Larger areas may require more complex hydrologic models.

Q4: Does this account for time of concentration?
A: No, the Rational Method assumes peak runoff occurs when the entire watershed is contributing.

Q5: What units does this calculator use?
A: Uses US customary units (inches, acres, cfs). For metric, convert to mm/hr and hectares.