Flood Frequency Analysis

Annual Exceedance Probability is defined purely by your chosen Return Period — a 100-year return period always corresponds to a 1% annual exceedance probability by definition, regardless of the flow statistics. Mean Annual Peak Flow and Standard Deviation only affect Design Discharge (how large that 1%-probability event actually is in m³/s), not the probability itself.

A longer return period represents a rarer, more extreme event, which raises the statistical frequency factor used in the log-Pearson calculation and pushes Design Discharge upward. A 500-year design discharge will always come out higher than a 100-year design discharge for the same flow statistics, since it targets a less frequent, larger event.

It captures how asymmetric your peak-flow distribution is in log space — zero means a purely log-normal assumption, while a nonzero value applies the fuller log-Pearson Type III correction. A zero default is a reasonable starting assumption when you don't have a computed station skew value; real flood studies typically calculate skew from the actual data or blend it with regional skew estimates.

This calculator uses a simplified normal-distribution approximation for the frequency factor and asks you to supply mean, standard deviation, and skew directly. A certified study computes these statistics from an actual annual peak-flow record, uses a proper Pearson Type III frequency-factor table (not a normal approximation), and typically weights station skew against regional skew per USGS Bulletin 17C guidance — treat this tool as a planning estimate, not a design-grade analysis.

Mean Annual Peak Flow sets the central tendency of your flow distribution in log space, and Design Discharge is built directly from that log-space mean plus a statistical adjustment for return period and variability. A higher average peak flow shifts the whole distribution upward, so the estimated discharge for any given return period increases correspondingly.