1. What is the Rational Method?

The Rational Method is a technique used in hydrology to estimate peak discharge or runoff rate from a drainage area. It's commonly used for urban drainage design and small watershed analysis.

2. How Does the Calculator Work?

The calculator uses the Rational Method equation:

Where:

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

Explanation: The runoff coefficient represents the fraction of rainfall that becomes surface runoff, varying with surface type and soil conditions.

3. Importance of Drainage Calculation

Details: Accurate drainage calculation is crucial for designing effective stormwater management systems, preventing flooding, and ensuring proper infrastructure design.

4. Using the Calculator

Tips: Enter appropriate runoff coefficient (0-1), rainfall intensity in inches per hour, 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 (0.01-0.20), depending on soil type and slope.

Q2: How is rainfall intensity determined?
A: Intensity is typically based on historical rainfall data for a specific location and storm return period.

Q3: What are the limitations of the Rational Method?
A: Best suited for small drainage areas (typically <200 acres), assumes uniform rainfall distribution, and doesn't account for storage effects.

Q4: When should more complex methods be used?
A: For larger watersheds, complex terrain, or when detailed hydrograph analysis is required.

Q5: How does soil type affect runoff coefficient?
A: Sandy soils have lower runoff coefficients than clay soils due to higher infiltration rates.