Does global warming mean we shouldn’t buy this riverside lot?
June 12, 2016 1:06 PM   Subscribe

My husband and I are thinking of buying a particular lot on a river. If we built there, it would be outside of the 100-year flood zone, but not by a ton. Does global warming make that too risky?

We’ve looked a lot, and this land would be tough to match for us, so we don't want to walk away out of an over-abundance of caution -- but we do want to be appropriately cautious.

Here’s the lot with the 100 year flood zone shaded red, and contour lines every 20’. The 100-year zone comes to a bit under the 1860’ contour line. The 500-year flood zone isn’t displayable, but it IS shown in another area of this same river. There, it adds another 15’ in elevation to the 100-year zone. That would wipe out most of the rest of this lot, and definitely all of it that we’d otherwise want to build on. We’d build outside of the 100-year flood zone, but we wouldn't want to be up by the road, away from the creek.

This is Ivy Creek, in Madison County, NC. I'm told it's dam-controlled.

I know that global warming is associated not only with sea levels rising, but also with more frequent extreme storms that lead to more river flooding. But, I have no idea what kind of buffer might be warranted. We hope to live there for decades -- with excellent luck and longevity, let’s say 50 years. How would you decide whether anywhere on this lot is an acceptable risk, and how much further beyond the 100-year zone you had to back off from the creek?
posted by daisyace to Science & Nature (21 answers total)
Wouldn't be, for me. I've seen too many 100-year floods recently. One way to get yourself some data might be to talk to some insurance companies and see what you'd be paying for flood insurance in the various parts of that lot - it might make things clearer.
posted by restless_nomad at 1:12 PM on June 12, 2016 [8 favorites]

(Also, just for clarity, a 100-year flood doesn't mean it'll only happen every 100 years. It actually means a flood with a 1% chance of it happening any given year. So it's totally possible, as places like Houston are discovering, to have three or four 100-year floods in a season.)
posted by restless_nomad at 1:17 PM on June 12, 2016 [13 favorites]

First, understand that "100 year flood" does not mean what most people think it means. It is basically a flood with a 1% chance of occurrence in any given year, so you can, in fact, have 100 year floods two years in a row:

Floods: Recurrence intervals and 100-year floods (USGS)

Second, realize that our records do not really go back that far. The US is less than 300 years old and modern meteorologic records are even younger. The data is sketchier than most people think.
posted by Michele in California at 1:18 PM on June 12, 2016 [7 favorites]

I will add that, historically, rivers meandered. In other words, their course moved around randomly. The idea we have that a river is like a road and stays in one place is a modern concept that we try to artificially enforce by controlling the path of the river and insisting it stay in place. This is not a natural phenomenon at all.

Rivers don't have a lot of respect for human opinions. It takes enormous effort to try to keep forcing rivers to stay put and humans routinely lose that battle. The Salton Sea in Southern California began when we spectacularly lost that battle. To be fair, the indigenous peoples of the area and geology both suggest that the Salton Sea is only the latest incarnation of a lake in that area. But humans did think they could heroically stop the Colorado River from seeking the lowest land in the region when it flooded and the water laughed at their efforts.
posted by Michele in California at 1:36 PM on June 12, 2016 [9 favorites]

And to get another idea of how that 100yr flood can hurt, visit youtube and search for Esopus Creek flood, or the Irene storm flood. My house is somewhere in there. Flood insurance from FEMA paid pitifully and then was raised another $1000./year. Now they say I have to move my house to another location due to their change in the flood maps. My house is only 16 by 24 feet. Judge accordingly.

There are many houses in my area For Sale due to this. It's a 'buyer beware' market. I live in Ulster Co. NY and we do not get the wild weather that can occur in NC.
posted by donaken at 1:37 PM on June 12, 2016 [2 favorites]

If you build a house, consider building on the highest side of the property, and put a path to a impermanent gazebo by the river.
posted by nickggully at 1:39 PM on June 12, 2016 [3 favorites]

I'd be just as worried by the potential for runoff and erosion on the slope. Houses in rural WNC already slide down hills when there's heavy rain; trees come unrooted from saturated ground. You could give yourself a buffer against the creek rising and end up more vulnerable to what gets washed down from higher elevations.
posted by holgate at 2:10 PM on June 12, 2016 [1 favorite]

You might also be interested in this previous ask about coastal property in light of recent level predictions.

(I am kind of the loan voice there saying "Live where you want. No land is perfectly safe." but it sounds to me like you aren't really prepared for "normal" realities of riverfront property, much less riverfront property in the face of climate drama.)
posted by Michele in California at 2:13 PM on June 12, 2016

i was poking around on the chilean weather dept website the other day and found simulations for the future climate run for various warming scenarios. i guess something similar (likely much more) exists for the usa. so maybe you could look at the predictions for that area? it's possible that it is going to become drier, rather than wetter, for example.

also, if this is a lot that you plan to build on, it might be worth thinking about how you can mitigate flooding. there may be some simple techniques that reduce damage (for example, maybe you could have a two storey house, where the lower story is mainly garage and work areas). it might be worth talking to an architect.

and how much money is it, relative to what you have / will earn? there's a big difference between losing land that is all that you own, and losing, say 50% of your total worth.
posted by andrewcooke at 2:17 PM on June 12, 2016 [2 favorites]

Talk to your potential neighbors, see how often it actually floods. Less than honest people may be involved in specifying zoning areas to increase property value.
posted by FallowKing at 2:37 PM on June 12, 2016

I'm told it's dam-controlled.

Please investigate what that actually means for your terrain given local practices. I understand some of the flooding in residential suburban Houston has been related to trying to relieve pressure on the dam. At least one major road and one major park were out service for weeks.
posted by beaning at 2:41 PM on June 12, 2016 [1 favorite]

It's all about your comfort level. I grew up on a 160 acre farm that had a mile and a half of creek frontage. Although my mother argued we should build our house next to the road, far from the creek, my father insisted on building the house on a slope close to the creek, but not right on it. As far as aesthetics go, he was right: It was a gorgeous view of the creek and we could hear the soothing sound of a series of seven waterfalls that were about half-a-mile from our house. There were people up and down the creek who built even closer to the water than we did. The old-timers (people who had grown up and lived in the area for over sixty years on average) said there had never been a severe flood. Occasionally it would flood and the creek would turn into a swirling, roaring turbulent mass of raging water that would come several feet up the slope and lap at our backyard. Granted, it was a big backyard. Neighbors downstream would call and ask about the water level to make sure that they didn't need to evacuate. We watched as trees and logs the size of dump trucks shot down the creek like greased lightning. The thirty foot deep fishing hole down from our house turned into a giant whirlpool sucking things down that never came up as we watched.

In 1993 we left to visit my sister in Wisconsin. It was a good thing we did because that was when the famous flood of '93 struck. My brother was in the basement trying to rescue furniture and belongings when the water rose so fast and hard that it was up the slope and into the basement, coming into the garage, traveling down the garage stairs, and bursting through the basement door before anyone knew what was going on. He and my uncle escaped in time, but they had to get a ladder climb on top of a barn because the water basically surrounded our house on both sides as the fields started to flood. They had to wait it out for some time. We lost everything in the basement. We probably would have drowned if we had been there; we would have been too busy to know the water was upon us and, ironically, none of us are great at swimming.

After we came home and found a basement covered in mud and god knows what, one of the old-timers (in his 70s) said he had never seen the creek like that in his entire life. It reminded us of how little control we had and every time it rained like crazy we were much more vigilant about watching the water. I just read an article in Texas Monthly about the people who drowned at Wimberly, Texas, and it made me break out in chills because that could have been us. I loved that creek. It made me who I am. But god, how dangerous it could be.

I don't mean to scare you. I just want you to be safe.
posted by Coyote at the Dog Show at 2:51 PM on June 12, 2016 [10 favorites]

If the lot is downstream but reasonably close to a flood control dam.I wouldn't worry about it and would buy the lot. You really need to know more about the dam location and purpose. None of the other answers here are useful without that information.

Also it's probably gaged. Look for the historical gage records. And keep in mind that the size of a 100 year flood is just a guess based off of some very simple math, it's not particularly accurate as you get fo 100 and 500 year predictions
posted by fshgrl at 3:01 PM on June 12, 2016 [1 favorite]

I don't know if the Ivy River near Marshall, Madison Co, NC is the same stream as your Ivy Creek, but here is that gage. Note that in the past year, the stream that is usually at 2 ft in depth at the gage has gotten above 10 ft twice. They have data for that gage going back to 1934, so a wealth of info, which might be useful given that it's the same county even if it's not the same stream.

I do know that area is sometimes subject to tropical storms. The fall of 2004 was a pretty devastating time in that area for tropical storms, so if you can find out what happened on your property then, that might be a good test case for future severe storms.

I agree with fshgrl that you should investigate the dam, find out its purpose and also how old it is and how well maintained it is. Some of the disaster last fall in Columbia, SC was because of small, old dams in residential areas that had not been maintained. On the one hand, the dam could help control floods. On the other hand, its failure could be devastating.
posted by hydropsyche at 4:06 PM on June 12, 2016 [2 favorites]

USGS also has a flood mapping tool that will allow you to look at previous flood events in your area on a map and model more extreme ones. On phone, sorry, no link.
posted by momus_window at 4:22 PM on June 12, 2016

nd keep in mind that the size of a 100 year flood is just a guess based off of some very simple math, it's not particularly accurate as you get fo 100 and 500 year predictions

This is true, and flood maps are often wildly innacurate on top of this. Sometimes they just follow some contour lines, sometimes they show water going uphill, and sometimes they leave out low-lying areas that routinely flood. Most floodmaps are fine for big-picture analysis, but you wouldn't want to use them for edge case decisions.

And you need to know about the dam management -- is there a flow above which they start releasing water, or are there proposed changes in how flows are managed? Have there been upstream or downstream changes (like houses and infrastructure, say) since the flood maps were drawn?

There really isn't much of an answer that anyone here can give, other than in generalities. What matters are the exact specifics, and probably some expert interpretation.
posted by Dip Flash at 4:44 PM on June 12, 2016 [1 favorite]

In terms of global climate change, plan for weather to be more severe. Do you plan to build? Is there a good building site where you can have solar? Can you have trees that will shade you in summer, reducing A/C needs? What's the environmental impact of your home?
posted by theora55 at 5:56 PM on June 12, 2016

Thanks for the useful input so far -- experiences and data alike. I'd be interested in the flood modeling tool momus_window mentions, if anyone can link to it. And yes, I think the Ivy River and Ivy Creek are the same thing, or maybe one term is used before a particular fork and the other after.
posted by daisyace at 6:26 PM on June 12, 2016

Are you in the area at all right now? Or shopping from out of state? I would be wary of purchasing land in a flood zone in a rural area when you don't know the area well.

How recently was the flood map updated? Because other development in the area can mean the flood zone maps are outdated.

Also, why is it tough to match this lot? Because of size or price or both? If it seems like a better deal than other things you're finding, maybe it's not such a great deal.

Finally... when you buy a piece of land in a rural-ish area and plan to drive into Mars Hill, or, more likely, Asheville regularly, please remember that you are contributing to the very global warming you are concerned will flood your house.
posted by bluedaisy at 7:31 PM on June 12, 2016 [1 favorite]

You want the link in the box in the upper right to open the mapping tool. If you're still interested in the property, it might be worth trying to find out more about flood control measures / if there are upgrades planned, maybe call the dam to start?
posted by momus_window at 9:46 PM on June 12, 2016

Thank you for all the help. We're going to wring everything we can from the available data, and look into whether there are building techniques that mitigate the risk. In starting that process, I already noticed that the flood zone comes right up to the 1860' line on the lots surrounding this one, and there's no reason it would be any lower on this one in real life. I'm going to dig into that discrepancy. I'm also thinking that even if the feared flood never came, I wouldn't want to be anxious every time the weathermen were waxing hyperbolic about a coming storm. So, unless we find reasons to be more confident, we'll walk away.

To answer some specifics that came up...
-I do understand what 100-year flood means, and I agree that people often don't.
-I was posting from the area, and now I'm back home crunching the data.
-It's tough to match the lot for us because the combination of criteria we're seeking (including significant waterfront) is rare in our target area.
-We will be building small (750-1000') and green, with consideration of heating/cooling and car use.
posted by daisyace at 8:07 AM on June 15, 2016

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