1. What Is The Rational Method?

The Rational Method is a widely used empirical equation for estimating peak runoff flow rates from small drainage areas. It's commonly applied in urban hydrology and stormwater management design.

2. How Does The Calculator Work?

The calculator uses the Rational Method equation:

Where:

• \( Q \) — Peak runoff flow rate (m³/s)
• \( C \) — Runoff coefficient (unitless)
• \( i \) — Rainfall intensity (m/s)
• \( A \) — Drainage area (m²)

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

3. Importance Of Runoff Calculation

Details: Accurate runoff calculation is essential for designing stormwater drainage systems, culverts, detention basins, and flood control structures to prevent urban flooding and erosion.

4. Using The Calculator

Tips: Enter runoff coefficient (0-1), rainfall intensity in meters per second, and catchment area in square meters. 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), forests (0.05-0.25), depending on soil type and slope.

Q2: How is rainfall intensity determined?
A: Intensity is typically obtained from rainfall frequency-duration curves for specific return periods (e.g., 10-year storm).

Q3: What is the maximum area for Rational Method application?
A: Generally recommended for areas less than 200 acres (80 hectares), though this varies by jurisdiction.

Q4: When is the Rational Method not appropriate?
A: For large watersheds, complex topography, or when storage effects are significant. Other methods like NRCS methods may be better.

Q5: How do time of concentration and storm duration relate?
A: Storm duration should equal or exceed the time of concentration for the Rational Method to be valid.