DrainageCalculators

Flood Frequency Analysis Calculator

Perform flood frequency analysis using Log-Pearson Type III and Gumbel distributions per USGS Bulletin 17C. Estimate flood magnitudes for various return periods from annual peak flow data. Professional-grade statistical hydrology tool.

What This Solves

Performs statistical flood frequency analysis using Log-Pearson Type III and Gumbel distributions to estimate flood magnitudes for various return periods from annual peak flow data.

Best Used When

  • You have a record of annual peak flows at a gauged site and need design flood estimates
  • You need to estimate the 10-, 25-, 50-, or 100-year flood at a stream gauge location
  • You want to fit a statistical distribution to observed flood data per USGS Bulletin 17C guidelines

Do NOT Use When

Key Assumptions

  • Annual peak flows are independent and identically distributed random variables
  • The record is stationary (no significant trends from land use change, climate change, or regulation)
  • Log-Pearson Type III is the standard distribution per USGS Bulletin 17C
  • Low outliers are handled using the Grubbs-Beck test or conditional probability adjustment
  • The skew coefficient can be weighted with regional skew per Bulletin 17C guidelines

Input Quality Notes

Longer records produce more reliable estimates. Records under 10 years should be supplemented with regional data. Check for regulation changes, diversions, or land use shifts that may make older data non-representative.

Expert Analysis Tool

This calculator performs statistical flood frequency analysis per USGS Bulletin 17C. Requires historical annual peak flow data. Results should be verified by a qualified hydrologist for regulatory applications.

Input Parameters

Annual Peak Flow Data

Enter historical annual maximum instantaneous peak flows

Enter values separated by commas, spaces, or new lines (cfs). Minimum 10 values required.

Values entered: 0

Enter corresponding years for each peak flow (for plotting position labels)

Analysis Options

Configure the statistical analysis method

Select the probability distribution for analysis

Method for calculating empirical exceedance probabilities

Calculate upper and lower bounds for flood estimates

Regional Skew Adjustment (Optional)

Weighted skew improves estimates when regional data is available

From USGS regional skew map or study (typically -0.5 to 0.5)

Mean square error of regional skew (default: 0.302)

Tip: Regional skew values can be found in USGS Bulletin 17C (Appendix 8) or state-specific USGS publications. If not provided, only station skew will be used.

About Flood Frequency Analysis

Flood frequency analysis uses historical annual peak flow data to estimate the magnitude of floods for various return periods (recurrence intervals). The analysis fits a probability distribution to the observed data and extrapolates to estimate flows with lower probabilities of occurrence.

Log-Pearson Type III is the standard method recommended by the U.S. Geological Survey (USGS) and FEMA for flood frequency analysis in the United States. The Gumbel distribution is a simpler alternative often used internationally.

Distribution Comparison

AspectLog-Pearson Type IIIGumbel (EV1)
Parameters3 (mean, std dev, skew)2 (location, scale)
SkewnessVariable (data-driven)Fixed (1.14)
StandardUSGS, FEMA (US)International
Best forMost US watershedsSymmetric distributions

Data Requirements

Required:

  • Minimum 10 annual peak flow values
  • Annual maximum instantaneous peaks
  • Consistent measurement method
  • Representative of current conditions

Recommended:

  • 20+ years for reliable skew estimate
  • Regional skew for weighted analysis
  • Outlier screening before analysis
  • Verification with nearby gages

Data Source: Annual peak flow data can be obtained from the USGS National Water Information System (NWIS).

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 Flood Frequency Analysis 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.