[QUOTE=Snarky_Kong]
Keep in mind that there is a fairly large amount of uncertainty about what exactly those storms consist of.
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So true. We depend on measured data yet there is error inherent in the measuring. The accuracy of the data improves as the period of record lengthens and the number of gages increases.
Related interesting factoid: areas downwind of large urban centers frequently experience larger rainfalls than would be expected.
Also, the 1%-chance rainfall may not necessarily produce the 1%-chance flood. Many other factors come into play such as antecedent moisture condition, % of impervious areas in the watershed, changes to storage areas in the watershed, changes to channel capacity, etc. etc.
[QUOTE=Snarky_Kong]
Is this because the warm air from the city pushes the moist air down and condenses the moisture?
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Alas, I have not followed the literature on it, so I don’t know what has been theorized about the mechanism causing it. I agree that the warm city air has something to do with it. I would be tickled if someone else had time to find out.
I’m also vaguely remembering some place in the Southeastern U.S. which has abnormally high rainfalls (nothing to do with an urban center). I’ll ask around and see if I can come up with the location.
[QUOTE=NinetyWt]
The 1% chance flood is that which has 1% chance of being equalled or exceeded within any given year. Formerly referred to as a 100-year flood because we expect it to occur at least once within a given 100-year period of time. I don’t like to use ‘100-year’ because people tend to think that it can “only” happen once in a 100-year period; and that’s untrue. For example, it could happen once a week every week from now till Kingdom Come (unlikely but possible).
Divide the number 100 by the % chance to get the recurrence interval. 100/1 is 100; that’s the 100-year. 100/0.5 is 200; that’s the 200-year. The 50-year is a 2% chance flood. Not as elegant as the Ideal Gas Law eh?
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Ugh, don’t remind me! Although the MatLab code did end up coming out very pretty, if I must say so myself.
I was curious about the nomenclature, because I’m working with some airplane crash data right now where the expected frequencies are in the range of 10^-4 to 10^-7, depending on the type of plane (carrier/taxi/military, etc.). Our old tables described them as once in 100 operating years to once in 10,000, but IIRC we’ve gone away from that definition and towards the one you and Snarky have described.
I looked over the site to refresh my memory (It’s been ages since I red McCullouch’s book, or visited the actual dam), and was surprised to find the website say that the dam was actually lowered, rather than raised the dam to allow a road to go over it; this made it easier for water to flow over the top. The addition of the fish weir didn’t help (it trapped debris rather than letting it go over the spillway), and other changes made the dam less stable.
[QUOTE=CalMeacham]
I looked over the site to refresh my memory (It’s been ages since I red McCullouch’s book, or visited the actual dam), and was surprised to find the website say that the dam was actually lowered, rather than raised the dam to allow a road to go over it; this made it easier for water to flow over the top. The addition of the fish weir didn’t help (it trapped debris rather than letting it go over the spillway), and other changes made the dam less stable.
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Oops, I got that backwards didn’t I? I was remembering that the elevation of the auxillary was higher compared to the top of dam. That is, of course, because of said lowering. My apologies for the mistake.
We have a large flood control dam system in my area and the dams are designed as a 2 stage system. Stage one is a fixed flow outlet at river level. It is designed to flow only enough water that the downstream levies can handle. The 2nd stage is an overflow that is designed to protect the earthen dam from damage and thus prevent a catastrophic dam rupture. Example
