Wednesday, August 06, 2008

Down Came the Rain

I was all excited about the rain about 2 weeks ago when I posted this hydrograph from a local creek - you'll note that the stage (water surface elevation) nearly reaches 2 m. This after some clearly serious rain.



Here is a longer view of the water surface elevationin the same creek. Even though yesterdays rain didn't seem that heavy - clearly there was a lot of water dumped in this watershed - because the level peaked at over 2.5 m - which is a big deal for a little creek.

I almost didn't bother to go out because the rain didn't seem that great - in fact I was surprised at the amount of water streaming down my driveway given that it didn't seem to be raining that hard. I mean it was raining hard - just not the kind of sheets of rain that I would call a downpour. So, here is my little water resource lesson. Rain events are often plotted on an IDF curve - Intensity (how hard it rains in mm/unit time), Duration (the length of time that it rains) and Frequency (the statistical likiness of it raining that hard for that long - usually in terms of a return period). So, water resource engineers often talk about a 5 year storm or a 100 years storm when planning city works - particularly those related to storm water management and flood policy. So, a short light rain may occur several times a year, where a storm that dumps 50 mm/hr for 45 minutes might only happen - once every so many years. If I were going to build a big, expensive highway bridge - I might be interested in making sure that even a catastrophic storm event (the dreaded 100 yr storm say) would not cause my highway to fall into the river, on the other hand - I probably don't care if my baseball diamond gets a little soggy every spring - so I'm not going to spend a fortune to keep it dry. So that is the basics. What I don't know, and I wish I could find it - is how much rain fell here yesterday - because I'm going to say that it was a whole lot more than 11.2 mm. Hamilton did record almost 50 mm - but its a long way away and no one else has a value for yesterday yet. I think if I wasn't leaving - I might just install a rain gage for my own curiousity. There is a lot more rain on the way it looks like - which means less time spent watering the garden and more for blogging and packing....

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