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Rational Method Formula:
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The Rational Method is a technique used in hydrology to estimate peak discharge and runoff volume from a drainage area. It's commonly used for urban drainage design and small watershed analysis.
The calculator uses the Rational Method formula:
Where:
Explanation: The equation calculates the total volume of runoff generated from a specific rainfall event over a given area and time period.
Details: Accurate runoff volume estimation is crucial for designing drainage systems, stormwater management facilities, flood control measures, and environmental impact assessments.
Tips: Enter the runoff coefficient (typically 0.05-0.95 based on surface type), rainfall intensity in inches per hour, drainage area in acres, and time duration in hours. All values must be positive numbers.
Q1: What is a typical runoff coefficient value?
A: Runoff coefficients vary by surface type: paved areas (0.8-0.95), lawns (0.05-0.35), forests (0.01-0.20). The value represents the fraction of rainfall that becomes surface runoff.
Q2: How is rainfall intensity determined?
A: Rainfall intensity is typically obtained from intensity-duration-frequency (IDF) curves specific to the geographic location, based on historical rainfall data.
Q3: What are the limitations of the Rational Method?
A: The method is most accurate for small drainage areas (typically < 200 acres) with homogeneous characteristics. It assumes uniform rainfall distribution and doesn't account for infiltration rates changing during a storm.
Q4: Can this method be used for large watersheds?
A: For larger watersheds, more complex hydrological models are recommended as the Rational Method becomes less accurate due to varying rainfall patterns and travel times.
Q5: How do I convert the result to other units?
A: 1 acre-inch = 3630 cubic feet = 27,154 gallons. Multiply the result by these conversion factors as needed.