About IDF Curves

From a submission by Yasemin Parkinson, December 10, 2009

Here in the Pacific Northwest it has been raining almost every day this month. The rain gauge station closest to my house has recorded over 550 mm (22 inches) for November (2009), which is considerably above average.

We have had several storms during this period, but this has resulted in very few flooding issues. After one of the storms passed through last week, my neighbor and I were talking. He commented about how much it had rained the previous night (the local rain gauge measured 27 mm). I agreed with him that it did seem like a good one, but later that afternoon I did a quick analysis using FlowWorks’ Intensity- Duration- Frequency Analysis tool and it turned out that the storm wasn’t “significant”.

So, what constitutes a significant rainfall event and how did I use the IDF analysis tool to quickly determine the severity of the storm?

The Basics: What is an IDF curve?

Here’s the simple answer: An IDF curve tells you how rare a given rain storm is. Take the storm from the other night. It is intuitive to understand the idea of how many mm or inches of rain falls. It’s not quite as easy to say how often that storm might occur? Every 1 year? 2 years? Maybe it was a 1-in-10 year event? An IDF curves helps to quantify that.

This is oversimplified. Read on for a more detailed explanation.

The official definition of an IDF curve

An Intensity-Duration-Frequency curve (IDF Curve) is a graphical representation of the probability that a given average rainfall intensity event will occur.

Rainfall Intensity (mm/hour), Rainfall Duration (how many hours it rained at that intensity) and Rainfall Frequency (how often that level of a rain storm repeats itself) are the parameters that make up the axes of the graph of IDF curve. An IDF curve is created with long term rainfall records. You need a lot of data. The more data you have, the more accurate your curve will be.

How to interpret an IDF curve

Rainfall intensity in the IDF Curve is the average rainfall depth that falls per specific time duration. Simplified, high rainfall intensity indicates that it’s raining really hard and low intensity that it’s raining lightly. Typically the rainfall intensity is stated in mm/hour in Canada and in inches/hour in the United States.

In the above graph, the y-axis shows the rainfall Intensity in mm/hour while the x-axis shows the rainfall Duration. This is the “I” and “D” in the IDF.

The nearly parallel lines on the IDF Curve represent probability, or Frequency (“F”). Thus the 10-year line would represent rainfall events that have a probability of occurring once every 10 years. Another way to put it is that the probability of a 10-year magnitude storm (or greater) occurring in any given year is 1/10 or 10%, and of a 50-year storm occurring 1/50 or 2%. This is a statistical analysis of past data, not a prediction of actual storms.

Each plotted line in the graph represents rainfall events with the same probability of occurrence, in a range of durations (durations are shown on the x-axis). Thus moving along the 10-year line shows the probability of a 10-year storm with different durations – a 10-year 30-min storm, a 10-year 2-hour storm, or a 10-year 12-hour storm.

The last line on the curve is the actual rainfall measured during the recent event based on the data collected from the local rain gauge. In this case it falls below all the parallel lines. Where this line crosses any of the parallel probability lines, it would represent the actual Intensity, Duration, and Frequency of the storm.

Where you can find IDF curves

IDF curves are available for many locations in Canada, produced by the Environment (formerly Atmospheric Environment Service). They are updated periodically and are available for free download from their FTP site.

A different system of illustrating rainfall statistics exists in the United States. Areas of same statistical rainfall depths are mapped for specific return periods and storm durations. Their iso-hyetal maps are called TP40s (Technical Paper No. 40) and can be found for free download on the US National Weather Service. These links are great starting points for understanding rainfall behavior in different areas of the US or Canada. Local or specialized agencies may have their own rainfall statistics that more closely emulate local conditions and experience, or particular uses for the rainfall information. Examples of these agencies include state/provincial transportation departments, municipalities, environmental protection agencies, etc.

FlowWorks and IDF curve

FlowWorks can provide you with a quick severity analysis for a storm event.

Did the rain that fell yesterday meet or exceed the 10-year storm? Do you need to report to your City Council the possible reasons why flooding occurred during today’s big storm event? FlowWorks has a tool that can easily answer those questions. By linking your rainfall station to FlowWorks and using the Rainfall Analysis Tool you can plot the storm event as it occurs on the station’s IDF Curve in a matter of seconds.

Unlike going through this exercise using a spreadsheet and finding it to be cumbersome and time consuming, the analysis can be redone instantly as new data appears in the database.

You can even “watch” a storm event as it rolls in from the comfort of your desk and send out operations crews as a storm event hits a particular severity rating to areas where you know may have issues.

The analysis can also be easily done on all the historical data in your database. The graph below shows that rainfall that fell near the District of North Vancouver’s rain gauge August 13, 2009 exceeded the 1 in 10-year event for short duration storms (5-15 minutes) and exceeded the 2-year event for a 2-hour storm.

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