Systematic-Record LP3 Screening Calculator

Fit basic Log-Pearson Type III or Gumbel distributions to complete positive systematic annual peaks for screening; EMA, MGBT, historical/censored data, and zero-flow handling are not included.

Systematic-record screening only

This calculator fits basic LP3 or Gumbel distributions to complete, positive annual peaks. It is not a complete Bulletin 17C analysis: EMA, MGBT, historical information, censored or interval data, and zero-flow handling are not implemented. Use USGS PeakFQ or an agency-approved workflow for regulatory or design work.

Input Parameters

Annual Peak Flow Data

Enter historical annual maximum instantaneous peak flows

Annual Peak Flows

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

Values entered: 0

Years (Optional)

If supplied, enter exactly one whole-number year for each peak, in the same order

Analysis Options

Configure the statistical analysis method

Distribution Type

Select the probability distribution for analysis

Plotting Position Formula

Method for calculating empirical exceedance probabilities

Show approximate 90% intervals (central-t/delta method)

Optional sensitivity bounds only; these are not Bulletin 17C noncentral-t/EMA confidence intervals

Regional Skew Adjustment (Optional)

Weighted skew improves estimates when regional data is available

Regional Skew Coefficient

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

Regional Skew MSE

Mean square error of regional skew (default: 0.302)

Tip: Enter both the regional-skew coefficient and its mean square error only from a current, published USGS or agency study that applies to this site. This screening tool does not determine regional applicability. If omitted, only station skew is used.

About this screening fit

A simple flood-frequency fit uses systematic 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 reference distribution in US federal guidance, but the full Bulletin 17C procedure includes data treatment that this tool omits. The Gumbel distribution is included only as a simpler comparison.

Distribution Comparison

Aspect	Log-Pearson Type III	Gumbel (EV1)
Parameters	3 (mean, std dev, skew)	2 (location, scale)
Skewness	Variable (data-driven)	Fixed (1.14)
Standard	Reference distribution in US guidance; this fit is not full B17C	International
Best for	Screening complete, positive systematic records	Method comparison and sensitivity screening

Data Requirements

Required:

Minimum 10 positive annual peak flow values
Annual maximum instantaneous peaks
No zero, censored, or interval-valued records
Consistent measurement method
Representative of current conditions

Recommended:

20+ years for reliable skew estimate
Regional skew for weighted analysis
PeakFQ/EMA and MGBT review before design use
Verification with nearby gages

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

For educational purposes only. Not a substitute for professional engineering judgment.