Using Rainfall IDF Analysis

Rainfall IDF (Intensity-Duration-Frequency) Analysis is useful when you want to compare rainfall data to historical values. Like all reports in FlowWorks, data for IDF analysis is updated as frequently as new data is received – for IDF analysis, this enables users to watch as a storm develops in real time.

Setting up a Rainfall IDF Analysis

These directions walk you through the process of creating an IDF Analysis.

  1. Log in to your FlowWorks account.

  2. Hover your cursor over , found in the Main Toolbar. A drop-down menu appears.

  3. Select . The Rainfall IDF Analysis setup page loads.
  4. Select the Rainfall Site that you wish to analyze; then (if applicable) select a Rainfall Channel.

  5. Next, select the source for the IDF Curve from the drop-down list. Usually one will be automatically selected when you choose a Rainfall Site; you can change this to another IDF Curve source as you wish, or leave it as is.

  6. Set a date range for your graph. This can be done by customizing the date range, by using the Start Date and End Date calendars to select the desired dates. You can optionally hone-in your date range further by setting the hour and minute for the range (found to the right of the date selectors).

  7. Click . Your IDF graph will appear in a separate window, showing an IDF plot for the station you've selected.

Note: You must configure your web-browser to accept popup windows from FlowWorks, in order to view the IDF graph output. If your output does not show in a separate tab in your web-browser, ensure that your browser settings have popup windows enabled.

Interpreting the Results

Below is an example output graph that was generated using the IDF Analysis graphing tool:

The colored lines on the graph represent the historical IDF information for the site. This information is obtained from either the National Oceanic and Atmospheric Administration, or Environment Canada. You can also supply your own rainfall data if you have more detailed local records.

The red line shows the data for the storm you previously selected (known as the ‘current period’), overlaid on top of the historical values. If the red line crosses over the historical values for any given rainfall duration, it indicates that a historically significant value has been exceeded by the current period at that duration.

The above graph is an example of the current event exceeding a two-year historical line (pink line), between the 4-hour and 12-hour duration. This graph indicates that for that particular time duration, the current event was a two-year historical event.

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