what does 100 year flood mean?
 

What does 100 year flood mean? 3 years ago they said we had a 100 year flood in the area. I say OK- the water will not get this high for another 100 years.
Well now they are saying we are having a 100 year flood now. I don't get it. Are they meaning that this is the highest the water has been in the last 100 years?
I may not be the smartest person in the boat-----but I gat a BOAT!
Thanks

What they mean when they say that is that floods of that magnitude only occur once every 100 years on average. Sometimes you may also hear it referred to as a "spate" as well. You can have 100 year spates, 500 year spates, etc.

You could have them two years in a row, but over time, they only occur on average every hundred years or so. Of course, keep in mind, the definition of what constitutes a 100 year flood changes over time. :)

A 100 year flood is a flood level that has a 1% chance of occuring in any given year. You can have another one next year and it will still be a 100 year flood.

A 50 year flood has a 2% chance.

A 25 year flood has a 4% chance and so on.

Besides, here in this area the 100 year flood occured an hour NORTH of us. We are just experiencing the aftermath. They got 5"-10" of rain. We only got 1"-1.5".

Several years ago we got either 8" or 9" of rain. That's when I lost part of my yard to a mudslide.

Folks drowned north of us because of this recent rain.

Nasty business, water.

In Western WA a 100 year flood seems to mean it happens about every 10 years. :(

I wish we'd have a 100-year flood! We're 5 feet behind on rainfall over the past 3 years.

In many locations, local politicians have exerted pressure on the professionals at FEMA, who supposedly study and map flood zones, to be very conservative in their determinations.

In other words, the upper limit of officailly designated 100 year fllood zones are ususally way to low.

Most places can and will eventually flood to extents that few can imagine.

100 year flood, explained
 

Bonnie,

As a Conservation Commissioner for my town I have had plenty of opportunity to study these laws. I have a 2 part answer to your question.

1) as has been previously stated, a 100yr flood is a statistical term to describe a potential for a certain level of flooding. Just because the statistics show that the probability is 1:100 for a certain level, it doesn't mean that it will be 100 years before seeing it again.

Look at it this way, most things in nature run in cycles. Say you have 10 years of drought, that doesn't mean that your average rainfall has declined. It just means that you will probably have 10 years of wet weather to follow.

You can get multiple 100 year storms in a given season, then you may have a long period without one.

2) A 100 Year Flood is a regulatory term, with regulatory definition and consequences. What I mean is this, the actual level of a 100 year flood is not as important as the impact it has on development.

Someone else mentioned FEMA, but there are also state and local agencies (like my local Conservation Commission) who use these figures, and maps generated from them, to set standards for development along wetlands.

We humans seem to have VERY VERY short memories. A piece of land may go undeveloped for generations because everyone knows it floods every spring. Then a dry spell comes along, and a few years go by with no floods. A developer decides to make a fast buck by building a house on the lot. Sells it to some unsuspecting buyer. What happens next? The weather goes back to normal, and the buyer gets flooded. The regulations are in place to try to prevent this senario.

I know that my explanation does nothing to reduce the inconvenience of the flooding that you have experienced. But I hope it helps you not to take the terminology too seriously.

Russ

Another thing to consider is that with lots of new construction, there is less surface area for water to soak into the soil. New streets and rooftops mean more runoff. What was once a 100 year flood plain is not anymore due to the increased runoff.
Ed

What I don't understand is when they want to spend taxpayers money to build dams, what you hear most is this dam will prevent flooding from now on, But what they don't say is, only upstream of the dam, because downstream will flood as much or more than it did before the dam was built. Eddie

Don't build a house in a 100 year flood plain. Besides the obvious :p , you're likely to be declined homeowners insurance.

Bonnie It is just something they say that sounds more official than "Goose Drownder" For true meaning there is NONE.

How high's the water Papa?

What CJ said....

Plus... to be safe, don't build in a flood plain of any sort. I'd check the records and find out at what level the worst flood was, and build a lot higher.

Lots of folks figured my home would flood when the river got out... before digging the foundation, I asked my granpa how high the river got when it flooded back in the 40's (the worst {or best, if you're a flood aficionado} flood on record)... figured out that elevation and added 15'. My guesstimating's worked good so far... have regular floods, the worst so far has gotten to within 150 yds from my home, but 18' lower in elevation... My new home will be 40' higher than the worst flood, and 15' higher than the proposed Lake Carthage.

Back to the OP...
why risk living or building in a floodplain for any reason...? The earth's climate is changing, and average flooding data will be skewed.... 100 yr floods might turn into ten year floods.

If you must buy flat land, get flood insurance.

It is all GWB's fault

:)

Be careful what you wish for! Right after we bought our farm here in MS there was a 100 year flood. Two teenage girls tried to drive across the flooded road, got washed off and drowned in our front field.

Wish for rain, but not for a flood!

You've got that backward, unless the dam breaks.

Mother Nature doesn't listen to weather reports or read insurance plat maps.

If you build in a flood plain, you aren't right bright. :shrug:

True, Mount Whitney has not flooded in a long time.

However everywhere has been underwater at some point in history.

Some places even suffer with flash floods. How smart is that?

Means its the worst flood they have seen until the next one which they will then call a 500 year flood unit theres one thats worse .
We've had 6 100 year floods in the last 40 years and one 500 year flood :rolleyes:
On Our place a 100 year flood means its knee to waist deep on the highest point of the property, a 500 year flood means its bellybutton to chest deep

thats like saying if you build in tornado ally you arent too bright
Or an earth quake area,or a hurricane Zone, along a faultline,coastline, forest (cant forget fires ) or anywhere lightning might strike
If you build in a flood plane (which actually covers most all farm land near any stream) and within 20 miles + of any river . You build accordingly.
a basement is out of the question . as is a bermed or hay bail home.
You look and see what the high water marks are and build two feet higher than the highest projected level. Or you buy a ton of flood insurance and build low and cheap :rolleyes:

Hurricane Floyd, and Fran,show me a picture of one flooded house from Raleigh to the mountains. Thats what most folk are led to believe. I have a friend that bought a lot and built a house on Kerr lake, he had a signed paper with deed, saying the water will never be allowed to rise over normal level more than six inches. The only way for that to happen is if the water is rising close to that six inches, the gates on the dam are opened to allow enough water out to honor that promise no matter if downstream was already to the max. A picture of a flooded house on Kerr Lake would work also. Yes its true that what I said should be backwards, but its not. Show me the picture and I'll admit I was wrong. Eddie

Dams work on the idea that you permanently flood the area behind them and make a lake....this in turn regulates the water down stream. How ever when the acumulation exceedes the amount that the dam can hold it won't matter what is happening downstream they will let it flood....much better than a dam failure. They still have some control over the water being released. One way they can control it is if they know a lot of rain is coming they will lower the normal level of the lake so they don't have to release as much when the rains are filling the rivers.

Not that easy. When you look at what a typical 100 year flood plain encompasses, there's little land that isn't in it.

As well the previously noted political/legal methods used to define it. It's not all science, not by a long shot.

Being in a 100 year floodplain of far from being assured of flooding.

[QUOTE=uncle Will in In.]Bonnie It is just something they say that sounds more official than "Goose Drownder" For true meaning there is NONE. /QUOTE]


Thanks guys: I like UNCLE WILL'S BEST. There are a lot of people under water between me and Uncle Will. Lord help them, cause I wouldn't know where to start. The river as it is now is about 1000 yards from me and still in the flood plain. My property is half in and half out of the flood plain so I am lucky--I am almost sure the water won't get in the house.

prince george bc
 

Rite now prince george is having a 200 yr flood. And its been up and down since b4 christmas. ice builds up and the water has no place to go. this area is flood prone and every yr it floods but this yr it is prolonged and no end in site. sawmills flooded..homes flooded and folks just evacuate. The ice blocks the 'channels' and the water has to go somewhere.

Short answer: All Bets Are Off.

If you build in a flood plain, better have a flotation barge for a foundation or build an artificial hill or on stilts.
If you live in tornado alley, better live in a soddie bunker.
If you live in a hurricane target area, better not build on a barrier island, or below sea level, and better have a sturdy house.

We all know that cookie cutter developments are tangible evidence of evil. This is because they pay no attention to the land under and around them. The land will not be ignored.

100 year flood
 

Y'all talk as if everyone is buying land and building their own houses.

I bet the vast majority of homes bought are already built, and unless you grew up in the same area where you are buying you haven't a clue, except for the maps.

How many buyers go digging through the records to see what the highest water level was, historically?

The developers are counting on buyers getting emotional, and losing logic.

In the entire NE United States, there is damned little buildable land that is not floodplain. That's just how it is. You'd better hope that someone is holding the developers accountable for where the houses are being put.

For those of you who build you own house on family land, yes you should know where the flooding will occur.

As far as flooding from dams, it can go either way depending on control of the flood gates. We had a manmade lake in town, and if there was a warning of flooding the town manager would lower the lake level in anticipation. Then the rains were allowed to refill the lake. Basically, the same amount of water flows through the dam, but you hope to make it take longer to minimize flood damage.

Russ

my family and i always lived in the 100 year flood plane. My parents house is on 12' stilts and so is my aunt's. The rest of us had the house up on a higher, built up area of the property. I never remember anyone flooding where water came into their house but would come up some. That was years ago. NOW is a different story. My place flooded 3 years in a row with water right up to the house. My mom's house which I also happened to be living in at the time when they were away got water to a few inches under their 12' deck. My aunt lost her house and was rescued by the coast guard from her porch (12 feet in the air). then she rebuilt in the same spot!!! The flooding keeps getting worse and worse. We all had flood insurance and home owner's insurance, but the last 2 years my parents and aunts premiums have gone through the ROOF. they really have no business living there at this point but will never move of their own free will. They live on the river and that's just who they are. I can't convince them otherwise. for myself, I moved away and will never live in a flood plain again.

I coulda swore that the NE US had some hills, indeed some mountains!

Yes, it is easier to build cities and developments on flat land... Easy don't make it wise.

Anyone buying or building in a floodplain, without checking on the history of the flooding, is living the American Dream.... someone else will take care of any problems.... There are consequences to making bad mistakes... Just last week, thousands of people were stranded in the Rockies during a blizzard/avalanche scenario... most had zero food, clothing, or survival gear in their vehicle.
Some day, after a thousand people die in such an event, everyone will learn, and be prepared. As it was, they all survived, and were hauled into shelters.

I'm against federally supported flood insurance... if you want to build next to a river, or on the seashore... take the risk, or buy private insurance... oh wait! No insurance company is silly enough to offer flood insurance (I think? Am I wrong?) If it's stupid for private enterprise, why does the govt. do it?

Yes indeed, and much of it is in the 100 year floodplain. If a creek comes down that mountainside, it's in the 100 year floodplain. Generally they just take that creek and declare all the land around it that is less than 20 feet above it to be in the 100 year floodplain. Do that to every creek, brook, stream and spring, and you've got a whole lot of land considered a floodplain.

Is it realistic? Not at all. But that's frequently how it's done.

Then there's the grandfathering of old deeds. Old deeds aren't in a floodplain by virtue of their age.

Tex, in addition to what Foxtrapper said, note that I said "buildable land". Most developers would rather build on flat floodplains than deal with slopes.

Our town does have code regarding restrictions on slopes as well as floodplains. Will they eventually learn to build on extreme slopes like California allows, maybe. But for now the biggest regulatory issue remains wetlands and floodplains. You don't often see houses on extreme slopes in the NE. The majority of the population is in the river valleys.

We haven't even touched on the issue of waste in this discussion. Getting floodplains to perc for a septic system can be a nightmare, and expensive. We have a pond in town with a swimming beach that has to be closed several weeks each summer due to e-coli. The pond is surrounded by old "camp" houses that have been converted to year round residences. The septic systems can't handle to usage, but the properties are grandfathered.

Russ