Problem

How can I generate a time-depth storm event using IDF curve data? For example so that IDF data can be used with the Unit Hydrograph Runoff Method.

Solution

The Storm Events Dialog has a feature to convert IDF to Time-Depth. This feature uses the Alternating Block Method. Details of this method are documented in the book Stormwater Conveyance Modeling and Design (page 97). This method results in a hyetograph consistent with the IDF curves for a particular location.

1. You will need to first export the IDF curve to a storm event group library before you can select it. See this article for details: Modifying and using Engineering Library Data

2. Create a new Time-Depth storm event by right clicking on the folder and choosing, New > Time-Depth.

3. Click on the new time-depth curve that you created and look to the right side of the window under the Storm Event Input tab. Click on the drop down arrow next to the new button and choose "Add return event from IDF..."

4. This will open up a dialog box where you can choose the IDF curve if already created or choose from the Engineering Library. Apart from this, parameters such as minimum time duration, number of steps and the total storm duration have to be specified.

By clicking on the ellipsis (...) the required IDF curve can be chosen from the Engineering Library

This will create the time-depth storm event from the IDF curve specified.

However, the “Depth” column would show as “N/A” for this data as opposed to using dimensionless curves where the depth field is populated. This is because, the depth is calculated by multiplying with the dimensionless new depth. When creating time-depth storm event by using IDF data, this value is not computed currently; however a defect (# 1041898) for this has been filed to remedy this in a future version.

See Also

How can I apply a rainfall depth or intensity to a dimensionless curve to create a storm event?