I don't actually know what underlies the Middle Devonian brachiopods of my childhood, but I might get to find out soon. Iowa City is experiencing its second "500 year" flood in 15 years, and Coralville Lake has overtopped its dam... again. And the river hasn't crested yet. Fortunately, my family is on high ground, and playing host to some friends who've been evacuated from the flood zone.
Buildings in Iowa City must have floors that are at least 1 foot (or 1.4 of your Earth football diameters) above a designated "100 year" flood elevation. These elevations are determined by FEMA; climate records and projections are fed into hydrologic models to determine water elevations during freakishly wet years. Whether we're just having a statistical freakout, or whether the amount of rain that FEMA expects to occur in a 100-year flood needs adjusting... are there hydrologists or climatologists in the audience? This is beyond my expertise and I'm not in the mood to wade through the literature tonight.
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A 500-year flood doesn't necessarily have to happen once every 500 years. You could have three 500-year floods in a row and then nothing for 1500 years and it would still be a 500-year flood.
Or it's because of global warming.
The reason for that is that every year is a new test of the 1 in 500 chance. That means it is possible for this type of flood to happen several years in a row.
For a better analysis you'd need to have the weather data of the last 30 years or so (and preferably longer).
I have no expertise in hydrology, but I do want to say that's a fantastic photo you've got up there. Smooth roaring power :)
Erik and Who Cares have it right; I work in a flood determination business, so thought I'd pop in to clarify. 100-year flood and 500-year flood designations are somewhat misleading. Technically, what they represent is a 1% chance per year of major flooding (which, on the average, happens once in 100 years), and a 0.2% chance per year (or average once per 500 years). Hydrological data is based on evidence of prior flooding, average rainfall, river conditions, development and drainage, etc. The rule of thumb in this industry is that, over the life of a standard 40-year mortgage, an average home has about a 50% chance of sustaining flood damage.
Caveat: I'm not a hydrological engineer nor a climatologist, nor did I stay in a Holiday Inn Express last night; I work with GIS data (so take my opinion for what it's worth :)). But I would think it's obvious that climactic change might invalidate prior data on which the current flood zones are based, or force a change in future predictions.
"Buildings in Iowa City must have floors that are at least 1 foot (or 1.4 of your Earth football diameters) above a designated "100 year" flood elevation."
Is that any floor, or just the ground floor?
And what do you mean by MY Earth football diameters?
Apart from the discharge that results from a given rainfall (and soil moisture &c.), development on a floodplain can affect flooding and its effects. The effects may be quite different upstream and downstream.
First imagine a football-shaped earth. Now imagine the tectonics of such a planet ...
Multiple random surface-focused events of low intensity, with periodic high-intensity narrowly focused surface impacts around the equatorial region, causing a high degree of global acceleration and axis-over-axis tumbling?