| Year | Annual max 24h rainfall (mm) | Site notes |
|---|---|---|
| 2017 | 64 | Short burst, minor ponding |
| 2018 | 72 | Localized street runoff |
| 2019 | 81 | Drain inlets near capacity |
| 2020 | 67 | Moderate event, stable grading |
| 2021 | 95 | Overflow at one low point |
| 2022 | 88 | Temporary surcharge observed |
| 2023 | 103 | Peak event, strong inflow |
| 2024 | 76 | Good drainage performance |
- AEP = 1 / T
- Risk = 1 − (1 − AEP)L, where L is design life.
- β = s √6 / π and μ = x̄ − γβ (γ ≈ 0.5772).
- yT = −ln(−ln(1 − 1/T))
- xT = μ + β yT
- Intensity = xT / duration
- I = a · Tm / (D + b)c
- Depth = I · duration (consistent time units).
- Select a method: annual maxima or IDF parameters.
- Set return period and storm duration for your design.
- Enter a data list, or enter mean and deviation.
- Provide the expected design life to estimate risk.
- Press Calculate and review depth, intensity, and risk.
- Download CSV for records, or PDF for sharing.
Return period and annual probability
Return period T expresses how often a storm magnitude is expected to be exceeded in any single year. The calculator converts it to annual exceedance probability using AEP = 1/T. For drainage design, this helps align inlets, culverts, detention storage, and finished floor levels with local criteria. A 10‑year event implies about 10% chance of exceedance each year, not once every ten years.
Lifetime exceedance risk for assets
Long‑lived assets experience multiple opportunities for exceedance, so risk accumulates over the design life. The calculator uses Risk = 1 − (1 − AEP)L, where L is years in service. Example: for T = 25, AEP = 4%. Over 50 years the chance of at least one exceedance is about 87%, which is often higher than stakeholders expect.
Choosing storm duration for site drainage
Duration controls whether the design is driven by short, intense bursts or longer accumulation. The calculator reports both depth and intensity for the chosen duration to support sizing of conveyance and storage. Short durations are commonly checked for inlets and surface flow paths, while longer durations inform basin routing and downstream checks. Keep the duration consistent with your rainfall record or IDF source.
Using annual maxima versus IDF parameters
Annual maxima analysis is best when you have a consistent annual maximum series for the same duration and location. The Gumbel method estimates distribution parameters from the series mean and deviation, then computes the return level. With a short record, results can be sensitive to one extreme year, so review the sorted plotting positions table. IDF parameters are useful when agencies publish coefficients instead of raw records.
Example design scenario and outputs
The table below shows an example 24‑hour design using the IDF equation with sample coefficients. Use it to sanity‑check ranges before final selection, and always verify against your approved IDF or gauge record. For a 30‑year design life, the lifetime risk remains substantial even at higher return periods.
| T (years) | AEP | Risk over 30 years | Intensity (mm/hr) | Depth for 24h (mm) |
|---|---|---|---|---|
| 10 | 10% | 95.76% | 4.78 | 114.83 |
| 25 | 4% | 70.61% | 5.49 | 131.74 |
| 50 | 2% | 45.45% | 6.09 | 146.18 |
1) What is a return period in simple terms?
It is a statistical way to describe rarity. A T‑year storm has an annual exceedance probability of about 1/T, so it can occur in back‑to‑back years.
2) Why does the calculator show risk over design life?
Because projects operate for many years. Even “rare” storms may occur during service life, and the risk metric helps communicate expected exposure to owners and reviewers.
3) When should I use the annual maxima method?
Use it when you have a consistent annual maximum rainfall record for the same duration and location. More years generally improves stability of the fitted parameters.
4) When should I use IDF parameters?
Use it when your authority provides IDF coefficients or curves. It is efficient for multiple return periods and durations without maintaining a full historical series.
5) Does intensity equal depth divided by duration?
Yes, in this tool intensity is reported as average intensity over the selected duration. Peak sub‑hour intensities may be higher and should be checked using shorter durations.
6) How many years of data should I enter?
More is better, but practical records vary. As a rule, avoid fitting from fewer than 10 years if possible, and always review outliers and measurement consistency.
7) Can I export results for submittals?
Yes. Export CSV for design logs and calculations, and export PDF for quick sharing of the on‑screen result panel during reviews or client communication.