1. What is the Rational Method?

The Rational Method is a hydrological model used to estimate peak discharge rate from a drainage area during storm events. It's widely used in urban hydrology and stormwater management for designing drainage systems.

2. How Does the Calculator Work?

The calculator uses the Rational Method equation:

Where:

• \( Q \) — Peak discharge rate (cubic feet per second)
• \( C \) — Runoff coefficient (unitless, 0-1)
• \( I \) — Rainfall intensity (inches per hour)
• \( A \) — Drainage area (acres)

Explanation: The equation assumes that peak runoff occurs when the entire watershed is contributing to flow and rainfall intensity is constant throughout the storm duration.

3. Importance of Stormwater Discharge Calculation

Details: Accurate discharge rate calculation is crucial for designing adequate drainage systems, preventing flooding, meeting regulatory requirements, and protecting water quality in urban and suburban areas.

4. Using the Calculator

Tips: Enter runoff coefficient (typically 0.05-0.95 based on surface type), rainfall intensity for design storm, and drainage area in acres. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What are typical runoff coefficient values?
A: Pavement: 0.8-0.95, Lawns: 0.05-0.35, Forest: 0.05-0.25, depending on slope and soil type.

Q2: How is rainfall intensity determined?
A: Intensity is based on local rainfall frequency data for specific return periods (e.g., 10-year, 25-year, 100-year storm).

Q3: What is the maximum drainage area for Rational Method?
A: Typically used for areas less than 200 acres. Larger watersheds may require more complex hydrological models.

Q4: When is the Rational Method not appropriate?
A: Not suitable for areas with significant storage, complex topography, or where time of concentration exceeds storm duration.

Q5: How does time of concentration affect the calculation?
A: Rainfall intensity should correspond to the time of concentration, as this produces the highest runoff rate.