Flood maps made easy

[WillandHelen]

Hey,

If you are looking at buying land or building check emapsplus.com to see if your state is listed. If so then you can get property info and 100yr flood zones.

[adubar]

Something that came to my mind especially with the current local flooding are the 100 yr flood calculations. They probably should only be used as a preliminary reference. Local archives and people's memories, along with the topography and geology will tell you much more.

If I were to be looking at land in a flood plain, I would look at the geology and ask would it EVER be possible for my house to be under water, given the natural disposition of the land. If the answer is yes, I would look elsewhere.

The 100 yr. flood stats are based on probability of a specific type of event or events. They do not and cannot be used to forecast anything outside the bounds of the limits chosen to do the math.



-A

[WillandHelen]

It's funny that you mention that. I am designing a detention basin for a valley that is flooding houses regularly outside of the FEMA flood area. Any local could have told you that the area floods, but FEMA and the developers thought different. That being said, often times the locals will not be very accurate either. Like you said, if you want to do it right, ask everyone and look up everything. This is not a decision to take lightly. I am building near a flood zone. The hard thing to do is to gauge how much you are actually outside a 100yr event. Some maps have 500 yr events deliniated (we use them to place landfills). As crappy as the maps can be, you are probably pretty darn safe if you are out of the 500 yr flood. If these maps are not available, ask locals and check the archives. But also, go to where the 100 yr flood is closest to you. Can you tell how much higher you are by a topo map? Maybe just by sight (hopefully you are tilting your head up!) How much flood storage stands between your elevation and the flood elevation. I am a few feet higher than the 100yr elevation next to me, but so is about half the county, so I feel safer than if I was the only and next place more water could go. Plus, my grandma says it never floods, and I trust her more than FEMA!

[adubar]

You know, even with that much information actually out there I'm always amazed when local authorities still issue building permits in areas that actually have a very high probablity of flooding, historically (geological record or even modern record).

Case in point, about 20 years ago, a Sacramento Valley developer got the city of Sacramento to allow him to build McMansions ($9 million plus) in an area that is part of the diversion system intended to be used to save LA from a catestrophic flood (now the 100 yr projections are thrown out as you must rely on a government worker to make a decision that could flood you--and they DO make plenty of mistakes).

The homes all sold and now the home owners are suing the state of California to move the diversion gates to the poor section of town!!!!

I'm waiting for the day that the real big flood comes a knocking on their doors. I don't think $9 million can displace 60 feet of water!

You can bet that a buyer looking for realestate there today will be blissfully unaware of the potential liability.

-A

[WillandHelen]

You got that right. People don't check these things. One look at a flood map and you see the peril that so many subdivisions are in.

Another thing, If you live in an area that has alot of new develpoment it could cause the flood maps to be drastically out of date.

[jruz]

Sorry, dragging an old topic back up...

My inlaws have property near Leavenworth, WA. It has reached the 500 year flood stage numerous times in their ownership...just more food for thought...

(luckily no damage to their place, it's right on the Wenatchee River)

Jim

[adubar]

jruz,

That illustrates the point exactly.

The calculations that are used satisfy design, insurance and regulatory requirements. In a sense they are somewhat arbitrary. Often times, they are calculations that fly in the face of reason and intuition.

Still, you see people bemoning events be they geological or atmospheric because they have not seen them happen in the last ten years, 100 years or 500!!!

Back to my main point, look and see if the land you are purchasing could EVER be flooded due to natural or man made reasons. If so, try to look elsewhere.

-A