Design Storm - [source code]
Builds a design-storm hyetograph from an IDF (Intensity-Duration-Frequency) curve, with an optional climate allowance. Feed it to Stormwater Run. Give it EITHER the three coefficients of an i = a/(t+b)^c fit, OR a published depth-duration table (the form NOAA Atlas 14, KOSTRA and FEH actually distribute). The alternating-block and Chicago patterns reproduce the IDF depth at every sub-duration, which is what makes the result a design event rather than a shape.
Input
| Name | Nickname | Description | Type |
|---|---|---|---|
| Return Period | T | Design return period in years — the 1-in-T event. Local guidance sets this; 10, 30, 50 and 100 are the usual choices, with the higher values reserved for infrastructure whose failure matters most. Reported on the result; it does not itself change the rainfall, which comes from the curve you supply. | Number |
| Duration | Dur | Storm duration in MINUTES. Worth sweeping rather than guessing: a small, sealed catchment usually peaks under a short intense burst, a large or absorbent one under a longer storm, and the critical duration is whichever produces the worst result here. | Number |
| IDF Coefficients | abc | Three numbers a, b, c of an intensity fit i = a / (t + b)^c, with t in minutes and i in mm/h. Ignored when a depth-duration table is wired. | Number |
| Durations | Dmin | Durations (minutes) of a published depth-duration table — the form NOAA Atlas 14, KOSTRA and FEH distribute. Pair with Depths; wiring these overrides the coefficients. | Number |
| Depths | Dmm | Total rainfall depths (mm) matching Durations, for the return period being designed against. Interpolated in log-log space so intermediate durations stay on the published curve. | Number |
| Pattern | How the depth is distributed through the storm. Alternating Block (default): peak centred, and every sub-duration matches the IDF curve. Chicago: the same increments arranged around an earlier peak — the harsher test for a design leaning on depression storage. Uniform: constant intensity. Simplest, and understates peak ponding for the same total depth. | Text | |
| Block Length | Block | Length of each hyetograph block in minutes. A real modelling choice, not a discretisation detail: the finer it is, the higher the peak block. 5 minutes is the usual default for urban work. | Number |
| Climate Allowance | Climate | A published percentage uplift on design rainfall. These are ALLOWANCES, not projections — nothing here downscales a climate model — and the source travels onto the Summary output so a report can cite it. Guidance is revised; check a value more than a couple of years old against your own regulator. | Text |
Output
| Name | Nickname | Description | Type |
|---|---|---|---|
| Storm | The design storm, for Stormwater Run. | Generic Data | |
| Intensities | I | Rainfall intensity of each block (mm/h), in order. | Number |
| Total Depth | Depth | Total rainfall over the storm (mm). | Number |
| Summary | The storm in words, including the climate allowance and its source. | Text |