Prerequisites¶
To build a model in Hydrata you need three input data sets:
| Input | Required | Format | Notes |
|---|---|---|---|
| Elevation data | Yes | GeoTIFF (.tif) | Must be in a projected coordinate system |
| Inflow data | Yes | Drawn in interface | Rainfall polygons or surface flow lines |
| Boundary data | Yes | Drawn in interface | External boundary lines around the domain |
You will also need to decide on two Scenario settings:
- Mesh resolution: the target edge length of a mesh triangle, in metres. The field is labelled Base mesh size (m). Halving it roughly quadruples the triangle count, so it is the main driver of how long a Run takes and what it costs.
- Duration: how long a period the simulation covers. You enter it as hours and minutes, and Hydrata stores it in seconds.
Tutorial elevation data¶
The tutorial uses a 2.5 km square of drone LiDAR over the lower Msimbazi River in Dar es Salaam, Tanzania. It is real, messy, open survey data rather than a clean synthetic grid, which is the point: it triggers the same checks your own data will.
Download the tutorial elevation file (7 MB)
- 1 m GeoTIFF, 2500 by 2500 cells, EPSG:32737 (WGS 84 / UTM zone 37S), Float32, NoData -9999
- The survey covers the river corridor, about 40% of the square, in a diagonal band; everything outside it, and open water inside it, is NoData (59% of the cells). That is the survey, not a damaged file, and it is why the next step adds global terrain around it.
- Bridge and culvert openings are not cut into the surface. The LiDAR sees road decks.
These are ellipsoid heights, not heights above sea level
Values run from about -27 m to +22 m because they are WGS 84 ellipsoid heights and the geoid sits roughly 27.6 m below the ellipsoid here. The file is internally consistent, so a simulation built on it alone is valid. Hydrata will warn you about the vertical datum if you combine it with a terrain measured above sea level, such as Copernicus GLO-30. That warning is expected on this file, and it is a fair rehearsal for your own data.
Attribution and licence¶
This file is open data under the Open Database License (ODbL) 1.0, which requires attribution and share-alike. If you publish the data, or a terrain, mesh or scenario package built from it, publish that under ODbL too and carry this credit:
Aerial survey data, Resilience Academy (Tanzania Urban Resilience Program / Ramani Huria); survey by CDR International & Shore Monitoring for the World Bank, Feb 2019
Publishing a map image or a flood figure made from it is a Produced Work: keep the credit, but share-alike is not triggered. The full notice, the licence link and this clip's provenance are in ATTRIBUTION-ODbL.txt.
Tutorial Scenario settings¶
Use these two values for the tutorial. Every later step assumes them.
| Setting | Tutorial value |
|---|---|
| Mesh resolution (Base mesh size) | 5 metres |
| Duration | 3 hours (Hours 3, Minutes 0) |
At 5 m the built mesh runs to roughly 120,000 triangles per square kilometre, so a boundary ring around the whole 2.5 km square comes out near 750,000 triangles, which predicts a Run of roughly ten minutes on Hydrata's cloud GPUs; a ring around just the floodplain is a third of that. Hydrata's pre-build estimate reads lower than the mesh it then builds, so read the figure on the built Scenario. Hydrata quotes a Run before it starts, and a quote at or below the free threshold costs nothing, so the tutorial Run is quoted free. Free Runs are capped per day, and the Billing tab in your account shows the threshold, the cap and what you have used today.
Going finer costs more, quickly: at 3 m the same area is about three times as many triangles and at 2 m about six times. Refine locally with a mesh region instead of lowering the base mesh size for the whole domain.
Tutorial rainfall: the Dar es Salaam IDF table¶
The tutorial storm is a one in one hundred year, three hour design event, which fits inside the three hour Run above with its peak in the middle. To build it in Hydrology you first enter an intensity-duration-frequency (IDF) table. One power-law curve covers Dar es Salaam, taken from the World Bank 2017 Msimbazi flood risk report:
h(d, T) = K(T) * 36.44 * d^0.25 millimetres, d in hours
K(5 yr) = 1.23 K(10 yr) = 1.42 K(50 yr) = 1.83 K(100 yr) = 2.01
The IDF grid stores intensity in mm/hr and is filled cell by cell from the keyboard, so the table below is the type-from reference. Entering the 100 yr column alone is enough to derive the tutorial storm; the other three columns only make the curve chart look like a family.
Dar es Salaam IDF table, intensity in mm/hr
| Duration (min) | 5 yr | 10 yr | 50 yr | 100 yr | Note |
|---|---|---|---|---|---|
| 5 | 289.0 | 333.6 | 429.9 | 472.2 | extrapolated |
| 10 | 171.8 | 198.4 | 255.6 | 280.8 | extrapolated |
| 15 | 126.8 | 146.4 | 188.6 | 207.2 | extrapolated |
| 20 | 102.2 | 118.0 | 152.0 | 167.0 | extrapolated |
| 30 | 75.4 | 87.0 | 112.2 | 123.2 | extrapolated |
| 45 | 55.6 | 64.2 | 82.7 | 90.9 | extrapolated |
| 60 | 44.8 | 51.7 | 66.7 | 73.2 | |
| 120 | 26.7 | 30.8 | 39.7 | 43.6 | |
| 180 | 19.7 | 22.7 | 29.3 | 32.1 | |
| 240 | 15.8 | 18.3 | 23.6 | 25.9 | |
| 300 | 13.4 | 15.5 | 19.9 | 21.9 | |
| 360 | 11.7 | 13.5 | 17.4 | 19.1 | |
| 540 | 8.6 | 10.0 | 12.8 | 14.1 | |
| 720 | 7.0 | 8.0 | 10.3 | 11.4 | |
| 900 | 5.9 | 6.8 | 8.7 | 9.6 | |
| 1080 | 5.1 | 5.9 | 7.6 | 8.4 | |
| 1440 | 4.1 | 4.8 | 6.1 | 6.8 | |
| 2880 | 2.5 | 2.8 | 3.7 | 4.0 | extrapolated |
| 4320 | 1.8 | 2.1 | 2.7 | 3.0 | extrapolated |
Rows marked extrapolated sit outside the curve's fitted range of 1 h to 24 h. They use the same equation and are fine for a tutorial; they also keep the alternating-block interpolation well formed.
Cells you leave untouched stay at zero and are excluded from the curve. A cell is editable only when both its duration row and its return-period column are selected.
Once the table is saved you pick a temporal pattern and build the storm from it. Build a Design Storm is the step that walks through all of it, in the app. See Define Rainfall for attaching the result to your model, and Rainfall inputs for what Hydrata does with an intensity series.