What is a ten year storm?
Over the past few weeks, I've written a lot about "ten year storms," but I haven't really explained what that means.
The term's explanation can be found in National Weather Service Technical Paper 40, found here. You'll need the DjVu Browser Plug-in to view the paper. Realize I'm no expert in this stuff, so you may want to read the entire paper for further perspective.
A "ten year" storm refers to the frequency at which a particular amount of rainfall in a given duration (from 30 minutes to 24 hours) is expected to "return," on average. There are also "one year storms," "two year storms," and of course, "hundred year storms."
TP-40 has a long introductory section about how the data was collected and organized, but the meat of the paper is the frequency-duration maps.
You can browse through the various maps in TP-40 to see for yourself what the various storms are supposed to drop here in New Orleans. For example, the 12/21/06 storm, which dropped about 6 or 7 inches in about 12 hours. This would appear to qualify it as a ten year storm over the 12 hour duration.
For reference, here are the rainfall amounts for various durations at the ten year frequency. Many drainage systems - due to economics - are designed for ten year storms.
Ten year storms in New Orleans:
30 minutes: 2.6 to 2.8 inches
1 hour: 3.2 to 3.6 inches
2 hour: 4 to 4.5 inches
3 hour: 4.5 to 5 inches
6 hour: 6 to 6.5 inches
12 hour: 7 to 8 inches
24 hour: approx 9 inches
If we receive rainfall over any of these amounts in each of these duration periods, then we're into a storm bigger than the typical drainage system can handle. Of course, there are local variations in drainage capacities; the typical quote from local officials is that their system can handle an inch in the first hour and half an inch each hour after that. Like I said, I'm no expert, so I don't know how to explain the discrepancy.
But I can speculate. I bet the inch an hour comes from the 6 hour duration and the 1/2 inch an hour comes from the 12 hour duration. If one were to design for the "instantaneous" ten-year shot of rain (3.2 to 3.6 inches in a single hour), the system would probably be so big as to be unaffordable, and possibly wouldn't work very well for smaller storms.
The term's explanation can be found in National Weather Service Technical Paper 40, found here. You'll need the DjVu Browser Plug-in to view the paper. Realize I'm no expert in this stuff, so you may want to read the entire paper for further perspective.
A "ten year" storm refers to the frequency at which a particular amount of rainfall in a given duration (from 30 minutes to 24 hours) is expected to "return," on average. There are also "one year storms," "two year storms," and of course, "hundred year storms."
TP-40 has a long introductory section about how the data was collected and organized, but the meat of the paper is the frequency-duration maps.
You can browse through the various maps in TP-40 to see for yourself what the various storms are supposed to drop here in New Orleans. For example, the 12/21/06 storm, which dropped about 6 or 7 inches in about 12 hours. This would appear to qualify it as a ten year storm over the 12 hour duration.
For reference, here are the rainfall amounts for various durations at the ten year frequency. Many drainage systems - due to economics - are designed for ten year storms.
Ten year storms in New Orleans:
30 minutes: 2.6 to 2.8 inches
1 hour: 3.2 to 3.6 inches
2 hour: 4 to 4.5 inches
3 hour: 4.5 to 5 inches
6 hour: 6 to 6.5 inches
12 hour: 7 to 8 inches
24 hour: approx 9 inches
If we receive rainfall over any of these amounts in each of these duration periods, then we're into a storm bigger than the typical drainage system can handle. Of course, there are local variations in drainage capacities; the typical quote from local officials is that their system can handle an inch in the first hour and half an inch each hour after that. Like I said, I'm no expert, so I don't know how to explain the discrepancy.
But I can speculate. I bet the inch an hour comes from the 6 hour duration and the 1/2 inch an hour comes from the 12 hour duration. If one were to design for the "instantaneous" ten-year shot of rain (3.2 to 3.6 inches in a single hour), the system would probably be so big as to be unaffordable, and possibly wouldn't work very well for smaller storms.
posted by mcbrid35 at
11:19 PM
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