What Is the Rational Method Peak Stormwater Runoff Calculator?
The Rational Method was developed by Thomas Mulvaney in 1851 to estimate peak runoff discharge from small rainfall catchments.
How Does the Rational Method Peak Stormwater Runoff Calculator Work?
Assumes that rainfall occurs at uniform intensity over the entire watershed for a duration at least equal to time of concentration.
Rational Method Peak Stormwater Runoff Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Q = C \cdot I \cdot A \text{ (CFS)}
Peak runoff Q equals runoff coefficient C times rainfall intensity I times drainage area A.
How to Use the Rational Method Peak Stormwater Runoff Calculator
- Select land-use runoff coefficient C.
- Input design storm rainfall intensity I in inches/hour.
- Specify total drainage area in acres.
Step-by-Step Example Calculation
Suburban Commercial Development
Input Values:
runoffCoefficient:0.65
rainfallIntensity:2.5
drainageArea:15.0
Worked Steps: A 15-acre commercial development under a 2.5 in/hr storm produces a peak stormwater runoff of 24.38 CFS (over 656,000 gallons/hour).
Understanding Your Result
Peak Discharge (Q): Peak flood runoff rate in cubic feet per second (CFS).
Runoff Volume Per Hour: Volumetric water generation rate in gallons/hour.
Factors That Affect the Result
- Soil permeability, vegetative canopy, impervious surface percentage, and design storm return period.
When Should You Use This Calculator?
- Storm sewer pipe sizing, culvert design, street gutter spacing, and retention basin overflow weirs.
Assumptions & Limitations
- Valid for small drainage basins under 200 acres where channel storage routing is negligible.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Standard civil engineering hydrology formulation.