1. What is the Rational Method?

The Rational Method is a technique used to estimate the peak discharge from a drainage area. It's commonly used in urban hydrology for small drainage areas (typically less than 200 acres).

2. How Does the Calculator Work?

The calculator uses the Rational Method equation:

Where:

• \( Q \) — Peak flow rate (cubic feet per second, cfs)
• \( C \) — Runoff coefficient (unitless)
• \( I \) — Rainfall intensity (inches per hour, in/hr)
• \( A \) — Drainage area (acres)

Explanation: The equation estimates the maximum rate of runoff during a storm event by considering land use characteristics (C), rainfall intensity (I), and the size of the drainage area (A).

3. Importance of Peak Flow Calculation

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

4. Using the Calculator

Tips: Enter the runoff coefficient (between 0 and 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 values for the runoff coefficient (C)?
A: C values range from 0.05 for forested areas to 0.95 for paved areas. The value depends on land use and soil type.

Q2: How is rainfall intensity (I) determined?
A: Rainfall intensity is typically obtained from Intensity-Duration-Frequency (IDF) curves for the specific location and design storm.

Q3: What are the limitations of the Rational Method?
A: It's best for small watersheds (<200 acres), assumes uniform rainfall, and doesn't account for storage effects or timing of runoff.

Q4: What time of concentration should be used?
A: The rainfall intensity should correspond to a duration equal to the time of concentration for the watershed.

Q5: Can this method be used for large watersheds?
A: For larger watersheds, more complex hydrologic models like HEC-HMS or SWMM are recommended.