Stormwater runoff calculator

Calculate peak runoff using the Rational Method

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The Stormwater Runoff Calculator helps civil engineers estimate peak stormwater runoff rates using the Rational Method. Calculate runoff flow rates based on drainage area, surface type, and rainfall intensity for stormwater management and drainage design.

The Rational Method (Q = C × I × A) is the most common approach for estimating peak runoff from small drainage areas. Proper runoff estimation ensures adequate drainage capacity and helps design stormwater management systems.

Enter drainage area, surface type (impervious vs. pervious), and design rainfall intensity to calculate peak runoff rate. The calculator uses standard runoff coefficients for different surface types and helps size drainage infrastructure including pipes, swales, and detention facilities.

Features

Everything you need

Rational Method calculations
Runoff coefficient database
Multiple surface type support
Peak flow rate determination
Design storm intensity inputs
Drainage area calculations
Imperial and metric units

How it works

Simple workflow

1
Enter drainage area
2
Select surface type (asphalt, grass, roof, etc.)
3
Input design rainfall intensity
4
Review calculated peak runoff rate
5
Size drainage infrastructure accordingly

Use cases

Built for real-world scenarios

Drainage design

Size drainage pipes, swales, and channels based on calculated runoff rates.

Stormwater management

Estimate runoff for detention pond and stormwater facility design.

Site development

Calculate runoff impacts of development and impervious surface increases.

Permit applications

Generate runoff calculations required for stormwater management permits.

FAQ

Frequently asked questions

Rational Method: Q = C × I × A, where Q is peak flow (cfs), C is runoff coefficient, I is rainfall intensity (in/hr), and A is drainage area (acres). It assumes uniform rainfall and is valid for areas under 200 acres.

C values: Asphalt/concrete 0.90-0.95, Roofs 0.95, Gravel 0.50, Grass/lawn 0.25-0.40, Forest 0.10-0.20. Higher values mean more runoff, less infiltration.

Common design storms: 10-year (2-3 in/hr), 25-year (3-4 in/hr), 100-year (4-6 in/hr). Intensity varies by location and duration. Check local drainage criteria.

Use pervious surfaces (grass, permeable pavement), install green infrastructure (rain gardens, bioswales), and implement detention/retention facilities to reduce peak flows.

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We're always looking to improve our calculators. If you have ideas for new features, improvements, or found something that could work better, we'd love to hear from you.

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