1. What is the Rational Method?

The Rational Method is a technique used to estimate peak discharge for small drainage areas. It's commonly used in urban hydrology and stormwater management to size drainage structures.

2. How Does the Calculator Work?

The calculator uses the Rational Method equation:

Where:

• \( Q \) — Peak discharge (m³/s)
• \( C \) — Runoff coefficient (unitless)
• \( I \) — Rainfall intensity (mm/hr)
• \( A \) — Drainage area (hectares)

Explanation: The equation estimates the maximum rate of runoff during a storm event based on the characteristics of the drainage area.

3. Importance of Rainfall Intensity Data

Details: Accurate rainfall intensity data is crucial for proper stormwater system design. Intensity-Duration-Frequency (IDF) curves are typically used to determine appropriate intensity values for different return periods.

4. Using the Calculator

Tips: Enter the runoff coefficient (typically 0.1-0.9), rainfall intensity from IDF curves, and drainage area in hectares. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What are typical runoff coefficient values?
A: Range from 0.1 (forests) to 0.9 (paved areas). Urban areas typically range 0.7-0.95.

Q2: How is rainfall intensity determined?
A: From IDF curves based on storm duration and return period (e.g., 10-year, 100-year storms).

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

Q4: How should drainage area be measured?
A: As the total area contributing runoff to the point of interest, including all impervious surfaces.

Q5: What time of concentration should be used?
A: The Rational Method assumes rainfall duration equals time of concentration for the drainage area.