DrainageCalculators

Modified Rational Method Calculator

Calculate preliminary detention storage requirements using the Modified Rational Method. Analyze multiple storm durations to find critical duration and maximum required storage. Professional hydrology tool.

What This Solves

Calculates preliminary detention storage volume by analyzing multiple storm durations to find the critical duration that maximizes required storage.

Best Used When

  • You need a quick preliminary estimate of detention storage for a small site (under 20-50 acres)
  • You want to evaluate how storage requirements change across different storm durations
  • You are screening detention options before detailed hydrograph routing analysis

Do NOT Use When

Key Assumptions

  • The Rational Method is valid for the site (drainage area under 200 acres, fairly uniform land use)
  • Inflow hydrograph is trapezoidal with duration equal to the storm duration
  • Allowable release rate is constant and independent of storage volume
  • Storage is the area between the inflow and outflow hydrographs
  • The critical storm duration produces the maximum required storage volume

Input Quality Notes

IDF curve data must cover a range of durations from the time of concentration up to several hours. Results are preliminary — always verify with full hydrograph routing for final design.

Input Parameters

Watershed Parameters

Enter watershed characteristics

Rational method C value (0 to 1)

acres

Contributing watershed area

min

Watershed Tc in minutes

Design storm frequency

Detention Design

Specify allowable release rate

cfs

Maximum permitted outflow (e.g., pre-development peak)

Leave blank to calculate peak discharge only without detention sizing.

IDF Curve Data

Intensity-Duration-Frequency relationship

IDF Equation: i = A / (t + B)C

where i = intensity (in/hr), t = duration (min)

Numerator constant

Time shift constant

Exponent

Modified Rational Method

The Modified Rational Method extends the standard Rational Method to estimate runoff volume and size detention facilities. It analyzes multiple storm durations to find the critical storm that requires the most storage.

Key concepts:

  • Peak Discharge: Q = C * i * A (Rational Formula)
  • Critical Duration: Storm length producing maximum storage requirement
  • Storage: Difference between inflow and allowable outflow volumes

The method is suitable for preliminary sizing of small detention facilities (typically < 200 acres).

Typical Runoff Coefficients

Land UseC Value
Asphalt/concrete0.90 - 0.95
Roofs0.85 - 0.95
Commercial areas0.70 - 0.95
Residential (1/4 acre lots)0.40 - 0.55
Residential (1 acre lots)0.30 - 0.45
Parks/open space0.10 - 0.25

Source: FHWA HEC-22 (2009), Table 3-1.

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

This disclaimer applies to the Modified Rational Method Calculator calculator and all associated calculations.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.