Flood Frequency Analysis
Estimate the design flood discharge for a given return period using a simplified log-Pearson Type III frequency analysis.
Inputs
Results
Design Discharge
293 m³/s
Annual Exceedance Probability
1%
How to Use This Calculator
- Enter Mean Annual Peak Flow, Standard Deviation, and Return Period.
- Set Skew Coefficient.
- Review Design Discharge (m³/s) and Annual Exceedance Probability (%).
- Use Non-Exceedance Probability (%) and Frequency Factor (K) to inform your decision.
- Use the chart to visualize the results and explore different scenarios by adjusting inputs.
How the result changes with Standard Deviation
| Standard Deviation | Design Discharge | Annual Exceedance Probability |
|---|---|---|
| 5,000 | 561,529.6 m³/s | 1% |
| 17,500 | 9,864,097.1 m³/s | 1% |
| 32,500 | 41,269,691.7 m³/s | 1% |
| 45,000 | 87,737,130.6 m³/s | 1% |
What each input means
- Mean Annual Peak Flow
- Average of the recorded annual peak flow values in m³/s
- Standard Deviation
- Standard deviation of the annual peak flow series
- Return Period
- Average recurrence interval for the design event (e.g. 100 = 1% annual chance flood)
- Skew Coefficient
- Log-space skewness of the peak flow distribution; 0 = symmetric (log-normal)
How this is calculated
Formula
log Q = log μ + K × log σ (Log-Pearson Type III)Worked example, using the default values
- Identify Input Parameters4 parametersMean Annual Peak Flow = 150, Standard Deviation = 50, Return Period = 100, Skew Coefficient = 0 = 4 input(s) provided
- Calculate Design DischargeDesign Discharge293 = 293
- Calculate Annual Exceedance ProbabilityAnnual Exceedance Probability1 = 1
- Calculate Non-Exceedance ProbabilityNon-Exceedance Probability99 = 99
- Calculate Frequency FactorFrequency Factor = k2.327 = 2.327
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