jlf1961
Posts: 14840
Joined: 6/10/2008
From: Somewhere Texas
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ORIGINAL: Nnanji
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ORIGINAL: jlf1961
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ORIGINAL: Nnanji
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ORIGINAL: jlf1961
Folks, my youngest sister lives in Houston, in a very well off neighborhood.
A half block from her home is an area that is listed on flood insurance forms as an 800 year flood zone, which means it will be flooded once in 800 years. They tied their boat up at the stop sign at the intersection that is under three feet of water when they went to feed their pets today, and then moved all the furniture from the first floor to the second floor.
The amount of rain the area has received is considered a millennial flood, meaning once in a thousand years, or to put in very simple terms, there really aint no place in Houston that will be above water if it continues to rain like this for more than a day longer.
Harvey has set rain fall records for the continental US, and is closing in for the rainfall record for the entire US at 52 inches.
No Atlantic or gulf storm has had this much rainfall.
Oh, when sis left the house today, they took their pets out as well.
I've never seen an 800-year flood calculated. I'd like to see the math. Flood insurance usually asks if you're in or out of the 100-year flood, or in the 100-year zone but mitigated. I wonder why they'd even be interested in an 800-year zone. It seems odd. They calculate a 10-year, 100-year, and 200-year. Next, I've seen once, a 500-year. I wonder why they go to an 800-year instead of a 1000-year. It doesn't make sense.
I have seen flood plains calculated at 50, 100, 150, 200 up to 1500 years in some locations.
And it is not that hard to calculate when a flood deposit was laid down, dig a trench and look at the layers.
The same way they can find traces of ancient earthquakes and tsunamis.
Its more geology than math, since sediments and topsoil layers are relatively constant dependent on the region.
Here is the FEMA guidelines.
And it is not uncommon for such calculations in areas prone to hurricanes.
Well, I looked at your link. It was pretty much what I thought. I'm fairly familiar with the process. Your link says:
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This guide was developed for use by community officials, property owners, developers, surveyors, and engineers who may need to determine Base (100-year) Flood Elevations (BFEs) in special flood hazard areas designated as approximate Zone A on the Federal Emergency Management Agency's Flood Insurance Rate Maps published as part of the National Flood Insurance Program. One of the primary goals of this document is to provide a means of determining BFEs at a minimal cost.
Note the 100-year BFE. FEMA doesn't, that I've ever seen, go beyond that. Although, I have worked on 200-year elevations with the Corp of Engineers and in 1980 the Corp estimated Phoenix had a 500-year event. But that was calculated after the event from observable phenomenon.
So, what you sent me doesn't bear out what you're saying. Although I've not lived and worked in a hurricane zone I'm pretty skeptical that anyone has calculated an 800-year Base Flood Elevation. I don't see that happening from geology since the paths of rivers and streams meander and while you may be able to say that some location was wet at one time, I can't see a hydrologist calling it an 800-year storm event that left the tailings.
But, again, I havent worked in hurricane areas. It just seems very strange to me.
Oh, in your FEMA link you sent it specifically lists hydraulic software, which I've used, to calculate the BFE. It has nothing to do with geology determining BFE on a FIRM map (what you call a FEMA map is actually a FIRM map).
Yes rivers and streams change course over time.
However, every time there is a flood, a sediment layer is laid down, which leaves a geologic record.
That sediment layer is different than the normal soil for that particular zone, and it will contain enough organic matter for carbon dating and other analysis.
For example:
You are digging a trench on a lot that, from all aspects appears to be well above the obvious flood plain, but when you get below the frost line, you discover a flood layer deposit.
Every place I have ever been involved in construction, the footer trench is dug, then the inspectors come out and sign off on it, then you pour the concrete.
So, if the inspector finds a flood layer deposit, he or she, by law has to have the layer examined, because once a flood deposit is discovered, flood insurance needs to be purchased by the home owner to get mortgage.
Once the date is established on when the discovered layer happened, they then have a second, deeper trench dug to see if there are other layers to get an approximate average of how often it happens.
Now, sometimes this backfires. I worked on a house in Colorado that was near, but about ten feet above a creek and we discovered a flood deposit.
According to the number of layers found in the trench, it seemed to have been flooded ever ten years or so, but that seemed to have stopped about sixty years prior to the building of the home.
There was a two month delay while the government tried to figure it out, until one guy figured out that the last time the area was under water coincided with the last known trapping of a few beavers.
There hadnt been any beavers in the county for sixty or so years.
Turned out that the 'flood deposit' layers had been laid down every time there was a beaver dam on the creek.
There are flood deposits in relatively high parts of St. Louis that have not been underwater in the entire history that white men have been in the area, but the tribes native to the region talk of floods that had the eastern third of the state underwater in their oral histories.
Then there is a town in west texas down toward Del Rio that is about six miles from the Pecos river which has not had a major flood in known history. In fact the town is well above the river plain, but situated in a bowl like depression (much like my home town of Abilene) but there was a hurricane back in the 70's that happened to come ashore near Brownsville, then stalled over this nice little dry town.
The town became a 6 foot deep lake.
Because the town was located in what is called a playa, a slight depression in the countryside.
No one in the town had flood insurance because there had never been a flood.
The same year, Abilene experienced a similar situation, and part of the city that had never been underwater in the history of the town was under four feet of water.
But then the coastal areas are different.
While they call it a flood plain, and set times on it like 800 or 1000, it is more accurate to refer to a 800 or 1000 year storm. And they started calculating those types of storms after some major storms in the fifties and sixties, as well as the Galveston storm in the early 1900's.
And when they find a flood deposit where, by all logic, there should not be one, the start all kinds of testing. Those people on the beach front have to worry about storm surge, inland, like Houston, they have to worry about rain, and even when the river or stream course changes, on the coastal plain, the area that river or stream flows through has the same basic elevation.
So, humans come in and build levees and dikes to try and control the stream and river, and 9 out of 10 times, they fail.
Which is why, after Sandy, the States of New Jersey and New York city called in some Dutch hydro-engineers to talk about what to do about it when it happens.
New Orleans did the same thing after Katrina.
And both got the same answer, higher dikes, levees and sea walls, which is a Federal program administered by the US Army Corp of engineers and thus Federally funded, and there aint no money for that.
In the case of New Orleans, the suggestion was to turn the lowest parts of the city into flood water catch basins, which means everyone would have to be relocated and the buildings torn down.
However, in the case of Houston there is another problem, loss of wet lands.
Wet lands are not necessarily swamps, but areas that the natural over flow from rivers and streams go to when something like Harvey happens.
Houston grew so fast during the 70's and 80's that the city fathers got special permission from the department of the interior to develop wetlands and drain marshes.
When they did that, the water had no place to go, except into the neighborhoods and much higher than it had in the past.
Andrew motivated Florida to quite draining the everglades and start putting water back in because the swamps soak up the storm surge. Katrina proved that in Louisiana, and in fact Katrina proved that along its entire impact area.
A storm surge needs to pile up on land, and without salt marshes and wet lands, the storm surge just keeps moving inland.
You dry up wetlands, you create dry land for the storm surge to pile up on.
Of course, that is not quite the case in the keys. The storm surge comes over one shore, flows across the island without getting very deep and goes into ocean on the other side of the island. During Andrew, the southern tip of Florida got a storm surge of 15 feet in some places, but on the northern keys, not 10 miles away from 15 foot storm surges, they got about 6 feet.
Hurricanes and coastal flooding is a totally different animal than what you are thinking of. And 'coastal flooding' can happen as far as 100 miles inland, and that is part of Houston's problem right now.
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RE: Role of the Federal, State, Local governments durin... - 
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