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.
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.
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.
| Aspect | Log-Pearson Type III | Gumbel (EV1) |
|---|---|---|
| Parameters | 3 (mean, std dev, skew) | 2 (location, scale) |
| Skewness | Variable (data-driven) | Fixed (1.14) |
| Standard | USGS, FEMA (US) | International |
| Best for | Most US watersheds | Symmetric distributions |
Required:
Recommended:
Data Source: Annual peak flow data can be obtained from the USGS National Water Information System (NWIS).
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