A few comments on rain and flooding
Probably the most misunderstood concept is that of the "x-year storm". This does not mean a storm that only happens once every x years, and so if it happens a couple of times in a relatively short while it can't be a "x-year storm"
What it really means is a storm that the probability of it occurring in a given year is 1/x. So the odds of a 100 yr storm is 1/100, or 1%. The reason that it's called a 100 year storm is that over the span of hundreds of years, this should average out to once per hundred years. But you can still have them back to back.
Another thing is that there are several different types of 100 yr storm. Using historical data a model of rainfall in a given area is created. Its called an Intensity-Duration-Frequency curve, or IDF. It describes three things
1)how heavily does it rain (intensity)
2)how long does it rain for (duration)
3)how often does this happen (frequency)
Unfortunately our records do not go far enough back in a lot of cases to give accurate data for very large events, as this part of the world has only been settled for 200 yrs, and accurate record keeping is a lot shorter than that. Large events are generally modelled by extrapolating from the smaller events. Below is data for Hamilton from Environment Canada
DURATION 2 YR 5 YR 10 YR 25 YR 50 YR 100 YR
5 MIN 107.5 139.4 160.5 187.2 206.9 226.6
10 MIN 76.2 101.8 118.7 140.1 156.0 171.8
15 MIN 61.7 82.7 96.6 114.2 127.2 140.1
30 MIN 41.4 55.2 64.4 75.9 84.5 93.0
1 H 25.1 35.4 42.3 50.9 57.3 63.7
2 H 14.9 21.5 25.9 31.4 35.5 39.6
6 H 6.4 9.0 10.7 12.9 14.5 16.1
12 H 3.7 5.0 5.9 7.1 7.9 8.8
24 H 2.1 2.9 3.4 4.0 4.5 4.9
What this describes is that for any given year in Hamilton, the odds are 1/100 (1%) that a storm lasting 5 minutes will rain at a rate of 226.6 mm/hr, a storm lasting 10 minutes will rain at a rate of 171.8 mm/hr, and so on. So there are several different types of 100 year storm, depending on how long each one lasts. Notice that these are rates of rainfall, not amount of rainfall. To get the amount, multiply the rate by the duration (226.6 mm/hr * 5/60 hr =18.9 mm)
The amount of runoff created by a rainstorm depends on a lot of different factors.
-intensity of the rainfall (Heavy rainfalls don't have a chance to soak into the ground)
-slope of the terrain (water ponds on flat ground, races away on steep slopes)
-type of ground (asphalt is virtually waterproof, while dirt will soak up water)
-how wet the ground already is
-density of vegetation (tall grass will help hold back water, short grass doesn't)
As was mentioned in today's Spectator, a 50 yr rainstorm can produce a 100 yr flood, depending on if the ground is wet. But comparing old floods to current ones is not that helpful, because we've altered the landscape so much in the last few decades by development. Storms that caused heavy flooding 100 years ago may not cause it today, and vice versa.
Last edited by hamtransithistory; Jul 28, 2009 at 10:26 PM.
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