Rainfall¶
Rainfall inflows apply a spatially uniform rainfall intensity over a polygon within the domain.
Properties¶
A rainfall polygon carries a title and a Data value. There is no type field to set: a rainfall is a rainfall because you drew it in the Rainfalls layer.
| Field | Description |
|---|---|
| Title | A name for this polygon |
| Data | Either a Constant intensity in mm/hr, or a saved Hyetograph / Design Storm |
Constant rainfall¶
Choose Constant and enter the intensity. The form shows mm/hr next to the field.
A constant of 50 applies 50 mm/hr for the whole simulation. In the exported scenario package that value appears as:
Time-varying rainfall¶
Choose Hyetograph / Design Storm and pick a saved series. The dropdown lists the hyetographs in your project, and Design storms below covers how to build one.
In the exported scenario package, a time-varying rainfall appears as an array of timestamp/value pairs:
[
{"timestamp": "2024-01-01T00:00:00Z", "value": 0},
{"timestamp": "2024-01-01T00:30:00Z", "value": 50},
{"timestamp": "2024-01-01T01:00:00Z", "value": 0}
]
The series is applied as a step function on a per-second index: each sample's rate is forward-filled until the next sample replaces it. It is not interpolated into a smooth curve, so the shape you supply is the shape the model sees.
Outside the series' own range the rate is zero, not the nearest sample. If a series starts later than the run does, it contributes nothing over the lead-in rather than holding its opening value there. A series with no overlap at all with the model window is a harder error: the Run is aborted rather than run as a dry model.
A rainfall layer also has to be attached to the scenario to have any effect. Hydrata warns you when one is drawn but not attached, and again when an attached layer's features carry no rainfall data, because in both cases the Run proceeds as though no rainfall were there.
Design storms¶
A design storm is a rainfall hyetograph for a chosen return period (for example a 100 year ARI / 1% AEP event) over a set duration. Hydrata applies a design storm the same way as any other time-varying rainfall: the timestamp/value array above is the hyetograph, with each value being the storm's rainfall intensity (mm/hr) at that point in the event.
Hydrata builds design storms for you, in the Hydrology panel. You do not need your own tooling or a spreadsheet for this. The panel holds intensity-duration-frequency tables, temporal patterns, and the design storms combined from the two, and a saved design storm is selectable directly on a rainfall polygon: set its Data field to Hyetograph / Design Storm rather than Constant.
The walkthrough is Build a Design Storm. In outline:
- Enter your jurisdiction's IDF statistics as an IDF table (rows are durations in minutes, columns are return periods, values are intensities in mm/hr).
- Choose a temporal pattern: the Alternating Block method, which is computed from your own table, one of the fixed SCS or Huff curves, or your own dimensionless curve.
- On the Design Storms page, pick that table and that pattern, tick the return period and duration you need, and save the storm.
The mesh-area total rainfall is whatever the resulting hyetograph integrates to, so the choice of return period and temporal pattern is yours to make and document. Hydrata does not pick a design standard for you, and it ships no rainfall statistics for your catchment.
How it works¶
Internally, rainfall polygons are applied as Polygonal_rate_operator instances. Your intensities are given in mm/hr and ANUGA works in m/s, so each rate is multiplied by a conversion factor of 2.7778e-7.
That factor is exactly 1 / (1000 x 3600): 1 mm/hr is one thousandth of a metre per hour, and an hour is 3600 seconds. If you are reproducing a run outside Hydrata, use that derivation rather than a rounded constant.