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Rain Storms and Return Periods.

There is a rainfall warning on for today with 40-50mm of rain forecast in the next 24 hours. There is a modest amount of snow on the ground here in Castlegar, more up in Rossland, so there would be some snow melt occurring as well. If it comes, this is probably the biggest daily rainfall we will have had this year. Looking at the climate averages on the Environment Canada website, the data to 2000 shows few extreme rainfalls greater than 40mm in any month for Castlegar. One phrase I hear way too much of around here is reference to a “1 in a 100 year storm”. Since moving here in 2007, we’ve had a couple of good ones, each time I’ve heard someone tag it as the “1 in a 100”. When we talk about 1 in a… we are describing a return period, or a probability…

The likelihood or probability of an event with a specified intensity and duration, is called the return period or frequency. The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location. The term 1 in 10 year storm describes a rainfall event which is rare and is only likely to occur once every 10 years, so it has a 10 percent likelihood any given year. The rainfall will be greater and the flooding will be worse than the worst storm expected in any single year. The term 1 in 100 year storm describes a rainfall event which is extremely rare and which will occur with a likelihood of only once in a century, so has a 1 percent likelihood in any given year. The rainfall will be extreme and flooding to be worse than a 1 in 10 year event. As with all probability events, it is possible to have multiple “1 in 100 Year Storms” in a single year

Source: Precipitation (meteorology) – Wikipedia, the free encyclopedia So, while the probability of a 1 in a 100 year storm is 1% per year, this doesn’t negate the possibility that you could have one of these storm each year for a couple of years, or even two in one year. Unfortunately for most people who don’t understand this, every storm that leaves a bit of debris or flooding is a 1 in a 100. For Castlegar, we have the following graph that describes design storm intensities for different durations. This is called an Intensity-Duration-Frequency Curve (IDF) and is often used to size stormwater systems. based on a design storm (say 10 year) and the size of the catchment it is serving which influences how long a storm would have the greatest impact on the system. This is called the Time of Concentration, which is a whole other topic in itself.

IDF Curve for Castlegar


See the USGS page on 100 year floods here
. Will the rain come? I hope not, but from the above graph it would have to rain steadily at 5mm an hour for 10 hours for it to be the 100 year  10 hour storm. That’s a lot of rain.

Mike Thomas

Mike Thomas P.Eng. ENV SP, is the author of UrbanWorkbench.com and Director of Engineering at the City of Revelstoke in the Interior of British Columbia, Canada. If I post something here that you find helpful as you navigate the world of engineering, planning and building communities, that’s wonderful. But when push comes to shove: This is my personal blog. The views expressed on these pages are mine alone and not those of my employer.