Rational Method Runoff Calculator
Estimate peak runoff using the Rational Method with rainfall and catchment inputs.
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Data Source and Methodology
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte: "Hydrology Handbook", ASCE Manual No. 28. Read more
The Formula Explained
The rational method formula: Q = C \times i \times A, where:
- C is the runoff coefficient.
- i is the rainfall intensity in inches per hour.
- A is the area in acres.
Glossary of Variables
- Runoff Coefficient (C): A dimensionless coefficient representing the fraction of rainfall that becomes surface runoff.
- Rainfall Intensity (i): The rate of rainfall, typically in inches per hour.
- Area (A): The size of the area being analyzed, typically in acres.
- Runoff (Q): The flow rate of runoff, typically expressed in cubic feet per second (cfs).
Example Calculation
Given: C = 0.75, i = 2 in/hr, and A = 10 acres. The runoff is calculated as: Q = 0.75 \times 2 \times 10 = 15 cfs.
Frequently Asked Questions (FAQ)
What is the Rational Method used for?
The Rational Method is used to estimate the peak discharge of runoff from a watershed during a storm event, particularly for small watersheds.
How do I determine the runoff coefficient?
The runoff coefficient depends on land use, soil type, and surface conditions. Tables are available in engineering handbooks to guide selection.
Can the Rational Method be used for large watersheds?
The Rational Method is typically only used for small watersheds (less than 200 acres) because it assumes uniform rainfall intensity across the watershed.
What units should I use in the formula?
The formula uses C as a dimensionless number, i in inches per hour, A in acres, and Q will be in cubic feet per second.
Is the Rational Method applicable for urban areas?
Yes, the Rational Method is particularly useful for urban drainage systems where rapid runoff estimation is required.
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https://www.lmnoeng.com/Hydrology/rational.php
Last code update: 2026-01-19
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