Build a Design Storm

Watch this step (5:49) in the overview video. Attaching the result is chapters 7 and 8: inflows at 8:05 and rain-on-grid at 9:35.

A constant rainfall rate is fine for a smoke test and useless for design work. A real event has timing: it builds, peaks, and recedes. Hydrata builds that for you from two ingredients, in the Hydrology panel.

Ingredient	What it supplies	Where it lives
IDF table	The depth. Rainfall intensity for each duration and each return period, in mm/hr	IDF Tables
Temporal pattern	The timing. How a storm's total depth is distributed over its duration	Temporal Patterns
Design storm	The two combined: a rainfall-rate-against-time series the model can ingest	Design Storms

The tutorial storm is a one in one hundred year, three hour event over Dar es Salaam, which works out at about 96 mm of rain in total. Three hours matches the tutorial's Run duration, so the whole storm, peak included, falls inside the Run.

Open Hydrology from the left toolbar. Its rail lists IDF Tables, Temporal Patterns, Design Storms and Hydrographs. You will use all four, in that order.

Step 1: enter the IDF table

Hydrata ships no rainfall statistics for your catchment, so you supply them.

1. Select IDF Tables in the rail and make sure the Input mode is selected
2. Click New Item. A table called IDF Table 01 appears in the Items column on the left
3. Fill in Name:, and put where the numbers came from in Source:. For the tutorial, name it something like Dar es Salaam and cite the World Bank 2017 Msimbazi report in the source
4. The grid has one row per duration in minutes and one column per return period, labelled by Average Recurrence Interval from 0.5yr to 500yr

Typing into the grid

The grid opens inert on purpose, and it tells you why:

Select a duration row and an ARI column, or click a cell, to enable entry.

A cell is editable only when both its duration row and its return-period column are selected. Two display helpers sit above the grid:

• Intensity (mm/hr) or Depth (mm). Intensity is the editable mode; depth is a read-only conversion of it
• Display in hours is ticked by default, so the rows read 1 h to 72 h. Untick it to see the durations in minutes, matching the Prerequisites table

The nineteen default duration rows are exactly the rows of the table on Prerequisites, so that table is a straight type-from reference. Entering the 100yr column alone is enough for the tutorial storm. The other columns only make the curve chart look like a family, and any cell you leave alone stays at zero and is excluded.

Click View IDF curve once at least one intensity is non-zero, to check the shape before you rely on it: the curve should fall smoothly as duration grows. Then click Save.

Step 2: choose a temporal pattern

Select Temporal Patterns in the rail and click New Item. You pick one family, and the page draws its cumulative curve so you can see what you chose:

Family	When to use it
Alternating Block (Default)	Computed from your own table, so it is exact for your rainfall statistics. The default, and the right choice for the tutorial
SCS / NRCS Type I, IA, II, III	Fixed regional curves from US practice, each with its own climate and peak timing
Huff Quartile (auto)	US Midwest empirical curves, quartile chosen from the storm duration
Custom (user-defined)	Author your own dimensionless cumulative curve as a time-fraction and cumulative-percentage table

Alternating Block has no stored curve of its own: it arranges your table's depth blocks around a single peak at combine time. The others are stored dimensionless shapes that get scaled to whatever depth your table gives.

Keep Alternating Block (Default) and click Save.

Step 3: combine them into the storm

1. Select Design Storms in the rail and click New Item
2. The create panel offers two ways to make one, Input or Derive. Pick the second, which builds the storm for you
3. Set the IDF Table picker to the table you saved in step 1
4. Set the Temporal Pattern picker to the pattern you saved in step 2

A grid of return periods against durations appears. Each cell is one candidate storm:

• A tick box means Hydrata can build that storm, and hovering it shows the total rainfall depth
• A cross means your table has no data for that combination, so there is nothing to build from. If you only typed the 100yr column, every other column is crosses, which is expected

The grid labels its rows in hours. Tick the 100yr column at the 3.0 hr row, hover it to confirm the depth reads about 96 mm, then click Derive. Hydrata confirms how many design storms it saved.

Change the table later and Hydrata tells you

A saved design storm carries a badge if the IDF table it came from has changed since, with a Regenerate action next to it. You never have to remember which storms are stale.

Step 4: hydrographs, if water arrives as flow

Rainfall and flow are different quantities and Hydrata keeps them apart:

• A design storm is rainfall, in mm/hr, and it feeds a Rainfall area
• A hydrograph is flow, in cubic metres per second, and it feeds an Inflow line

So a channel entering your domain needs a hydrograph, not a storm. Select Hydrographs in the rail, click New Item, and paste your series in: two tab-separated columns, a timestamp as YYYY-MM-DD HH:mm and a value, one row per line, straight out of a spreadsheet. Hydrata reports what it could not read rather than guessing.

Anchor every series to the same date

A run merges its inputs on absolute time. Give two series different start dates in one model and the later one contributes nothing until its first sample, which is easy to miss in the results. A series that does not overlap the model window at all aborts the Run rather than silently contributing zero. Timestamps are read as wall time in UTC, with no shifting.

The tutorial does not need a hydrograph: the storm falls on the whole domain.

Step 5: attach it to the model

The design storm is now a saved series. It does nothing until an input consumes it.

Rain on grid. Select the Rainfall layer in the Inputs panel, draw a polygon over the area the rain falls on, and set its Data field to Hyetograph / Design Storm instead of Constant. Then pick your storm from the dropdown. For the tutorial, draw one polygon covering the whole domain. Full detail is on Define Rainfall.

Flow at a point. Select the Inflows layer, draw the line, and set its Data field to Hydrograph. Full detail is on Add Inflows.

An attached but empty layer is not an error

If a Rainfall layer is selected by a Scenario but its polygons carry no data, Hydrata says so and the run still goes ahead, contributing no rain at all. A model that produces no flooding is often this, not a solver problem.

Next steps

Next: add inflows, or go straight to define rainfall if the storm is your only input.