1. What is the Rational Method?

The Rational Method is a technique used to estimate peak discharge (Q) for small drainage areas. It's commonly used in urban hydrology and for designing stormwater drainage systems.

2. How Does the Calculator Work?

The calculator uses the Rational Method equation:

Where:

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

Explanation: The equation assumes rainfall intensity is uniform over the entire drainage area and that the peak runoff occurs when the entire watershed is contributing to flow.

3. Importance of Peak Runoff Calculation

Details: Accurate peak runoff estimation is crucial for designing stormwater management systems, culverts, and drainage structures to prevent flooding and erosion.

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: What are typical values for C?
A: C values range from 0.05 (forests) to 0.95 (paved areas). Common values: 0.3-0.4 for lawns, 0.7-0.95 for roofs/pavement.

Q2: How do I determine rainfall intensity (I)?
A: Use local rainfall intensity-duration-frequency curves or NOAA Atlas 14 data for your location and design storm duration/return period.

Q3: What's the maximum area for Rational Method?
A: Typically used for areas < 200 acres, though some jurisdictions limit to smaller areas (e.g., 20 acres).

Q4: What are limitations of the Rational Method?
A: Doesn't account for storage effects, infiltration, or timing of runoff. Assumes steady rainfall and uniform conditions.

Q5: How to account for different land covers?
A: Use a weighted average C value based on the proportion of each land cover type in the drainage area.