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BobT

Flood crest. How is it measured?

30 posts in this topic

With the recent flooding of the Red River it was noted that the river crest set a new record held for over 100 years. A question that came to my mind is how do they measure this? One source claims that the Army Corps of Engineers has a measuring tool out in the river and basically measures the water depth at a certain point. If this is true, how accurate is that?

In 1897 there probably were no man-made levies built to protect the cities of Fargo and Moorhead or at least what was there was considerably less than today. I don’t know what the land gradient is as one moves away from the river but it is very flat and a crest of 40’ could likely have flooded a rather large area on both sides of the river covering hundreds or even thousands of square miles.

Today on the other hand, we have a network of levies that have been built to contain the river. Now, as the river flow increases, it is forced through a narrow canyon rather than being allowed to spread out. The water column therefore rises faster and moves faster than it would otherwise. It takes far less water to rise to a 40’ depth through a mile-wide gorge than what it would to flood a landscape many miles wider. If the basis for assessment of the crest is a depth measurement alone, isn’t this skewing the data?

The point is that if we are measuring without considering the effect of the levies and sensationalizing this flood based on that data, then the reality is that this year’s situation, although threatening and dangerous, wasn’t anything close to what was experienced in 1897. Squeeze all the water it took to create a 40'deep lake around Fargo-Moorhead in 1897 into a mile-wide funnel and it would have reached a Biblical crest by this method of measuring. In fact, there’s room for doubt that it could be compared to 1997 since the levies were built up even higher since then.

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Hmm, good points on that Bob. Not sure how it's all measured, I'd love to know.

It's tragic no matter how ya slice it though.

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With the recent flooding of the Red River it was noted that the river crest set a new record held for over 100 years. A question that came to my mind is how do they measure this? One source claims that the Army Corps of Engineers has a measuring tool out in the river and basically measures the water depth at a certain point. If this is true, how accurate is that?

In 1897 there probably were no man-made levies built to protect the cities of Fargo and Moorhead or at least what was there was considerably less than today. I don’t know what the land gradient is as one moves away from the river but it is very flat and a crest of 40’ could likely have flooded a rather large area on both sides of the river covering hundreds or even thousands of square miles.

Today on the other hand, we have a network of levies that have been built to contain the river. Now, as the river flow increases, it is forced through a narrow canyon rather than being allowed to spread out. The water column therefore rises faster and moves faster than it would otherwise. It takes far less water to rise to a 40’ depth through a mile-wide gorge than what it would to flood a landscape many miles wider. If the basis for assessment of the crest is a depth measurement alone, isn’t this skewing the data?

The point is that if we are measuring without considering the effect of the levies and sensationalizing this flood based on that data, then the reality is that this year’s situation, although threatening and dangerous, wasn’t anything close to what was experienced in 1897. Squeeze all the water it took to create a 40'deep lake around Fargo-Moorhead in 1897 into a mile-wide funnel and it would have reached a Biblical crest by this method of measuring. In fact, there’s room for doubt that it could be compared to 1997 since the levies were built up even higher since then.

Exactly correct, Bob. Moot points in the context of today's threat to property in Fargo/Moorhead, but correct.

I don't remember if the RR is measured exactly this way, but the flood level in feet is a measure of the river's height alone, though it usually isn't taken from the bottom of the channel (the riverbed changes over time.) Instead, the stage is measured from a benchmark where all the measurements are taken.

Measuring flow is one way to gauge a flood as well, but flow in volume/time is different when dikes push water narrow and high than it is if the river is allowed to spread out for miles. I doubt there's any useful way to compare the total amount of water that moved through the valley in 1897 with today's flood. Too many things have changed to make it an apples-to-apples comparison.

And, again, there's little point in doing it because the result is just a numbers game. What matters is the people and their property.

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They measure it at the buttom of the river in a certain spot. I hear its accurate to around 2 inches. Dont quote me on any of this, just what I heard.

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It is based on a benchmark that relates to an elevation above sea level. In Henderson, high water on the Minnesota is reported as feet above sea level. Other sites use the feet above flood stage. I don't know why. If you want to know if your house is threatened you pretty much have to use elevation above sea level. Dikes and roads and developments all have an effect on the way the water moves and backs up. I've been the Mayor of a flood threatened city for almost twenty years so I know more than I really want to know.

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KC, flood stage is to my thinking an outdated term. It's the level at which the river spills over its banks. In that sense, there's a "flood" pretty much every year on pretty much every river in snow country. Boy, did that term generate some arguments in my day at the GF Herald about what is and is not a "flood." I mean, in this day and age, whose property is threatened along any of these rivers when "flood stage" is reached? Pretty much no one's. But, well, word people can be pretty persnickety. shockedshocked

Whether the flood stage is used or sea level is used, it's basically the same benchmark system, because it's either flat out feet above sea level or it's flood stage based on a benchmark based on sea level, and all that's different is the actual number.

In areas where feet above sea level is used, the officials and a lot of homeowners know what number puts their homes at risk. Same is true if flood stage is used (which is how it's done in Fargo/Moorhead and GF/EGF.)

I personally think the measurement system should be normalized over the whole United States, with feet above sea level as the most elegant and universal system because there are fewer calculations involved and because elevations for most things in the country are based on this.

But in either case what happens over time when sea level fluctuates? Is sea level a historical level? It almost has to be, or all those elevation maps and signs across North America would have to change because sea level DOES change over time. And if the global climate change theory is correct (nope, let's not debate THAT again), it'll keep changing, and in larger amounts. I have no idea about how sea level is figured. smilesmile

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Whether they use sea level or flood stage, isn't the proglem the same? When we bottleneck a river, we force it's level to rise even higher even though the amount of water available may not be any more or perahps is even less.

For example, flood stage in 1897 was considerably lower than in 1997 because the river banks were lower. Today, flood stage is higher than it was in 1997 because the levies were raised in 2000 I believe. The thing is, if the levies were not raised since 1997, Fargo-Moorhead would have been flooded this year but possibly not as severly as it was in 1997 simply because the amount of run-off may have been less this year. I don't know this for fact, only speculating because we didn't have as much snow and the moisture content this year I believe was lower. But because the levies were higher, the crest got higher with less water available.

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flood stage in 1897 was considerably lower than in 1997 because the river banks were lower. Today, flood stage is higher than it was in 1997 because the levies were raised in 2000 I believe.

Bob, the river banks and the dikes are not the same thing. Flood stage is the level the river spills over its banks, not the level it spills over the much higher dikes.

I'm just going from memory, but I think flood stage in GF/EGF is 28 feet. The dikes aren't breached until 60 feet.

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I haven't been there but from what you're saying, the dikes are not direclty adjacent to the river banks but set back a ways. I think I understand then what you're saying.

I figured "flood stage" is when an area is in danger of flooding. If the dikes are higher, then from my definition, flood stage would be higher. At any rate, the crest itself is artificially raised by the dikes.

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Well, of course the dikes alter the flood plain giving the water less area to spread out. That's why on the Minnesota private permanent dikes are forbidden. We noticed in the 90s that large areas of young willows actually affected the high water flow and areas that didn't flood previously were affected with essentially the same above sea level measurements. The Red River valley and the Minnesota Valley are entirely different. I would hate to have to deal with what those guys deal with.

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Good discussion.

I hate to nit pick but I don't believe there is/was any sensationalizing "this flood based on that data" going on here. If I'm understanding you correctly.

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I figured "flood stage" is when an area is in danger of flooding. If the dikes are higher, then from my definition, flood stage would be higher.

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Originally Posted By: BobT
I figured "flood stage" is when an area is in danger of flooding. If the dikes are higher, then from my definition, flood stage would be higher.

That makes the most sense to me, too. Why call it flood stage unless people/property are threatened? But that's the way the definition works. Flood stage is when the river spills over its banks. We all have to use the same definitions or we can't have an effective communication.

Originally Posted By: PierBridge
I hate to nit pick but I don't believe there is/was any sensationalizing "this flood based on that data" going on here. If I'm understanding you correctly.

I think Bob's point (one that I agree with to a degree, if I understand him right) is that the media gloms onto the simplest way to describe an event when they need a headline or a lead-in. That can feel sensationalistic. Reading further into a news story or a TV or radio broadcast/telecast will generally yield more information, but the words "record flood" definitely have a certain amount of impact. And, well, the media is always an attractive victim when criticism is handed out. Just like the National Weather Service.

And I agree that the volume of water moving through the Valley this season may not be near as much as what went through in 1897 (we'll never know for sure), and that altering the Valley landscape and dikes has forced less water to climb higher. We pointed that out a number of times in print in GF in 1997. No doubt the point will be made by the F/M media sooner or later, possibly in a deeper retrospective piece after the flood has passed.

And 1897 can take care of itself. Today, we are taking care of today's business in Fargo/Moorhead -- and taking care of each other. That's the important part. Just my opinion.

I think it needs to be said that Humans view floods as a disaster and well they are for us. The rest of the flora and fauna on a flood plain actually thrive because of the regular inundation. The flood plain is a unique environment that's under appreciated. I guess what I am saying is, it's a "flood" when ever the river goes over it's banks. Civilization was born on the flood plains of great rivers and we are just not going to move away from them. I have lived within a few hundred yards of the Minnesota all my life and I could not stand to leave the valley. Henderson is fortunate to have a permanent levy that protects us to the 500 year event.

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I wonder what the future impact will be when mankind looks back at how we prevented the rivers from regularly or periodically flooding as nature intended.

We complain about farmers and how they alter the landscape through tiling, ditching, and such but how willing are we to admit that cities also alter the landscape for their own benefit as well?

The Red River and Mississippi river valleys contain some of the worlds best agricultural land simply because of the fact that the rivers flood the area and deposit the sediment. We've been changing things in such a way that we disrupt this process. Wonder what the impact would prove to be?

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The red river valley has great soil because it used to be the bottom of a lake.

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The Red River and Mississippi river valleys contain some of the worlds best agricultural land simply because of the fact that the rivers flood the area and deposit the sediment. We've been changing things in such a way that we disrupt this process. Wonder what the impact would prove to be?

I've often thought the same thing. City or country, we change everything we touch. Well, all things change what they touch, but we do it on a scale that moves faster and is much larger than our understanding of the results.

The Inuit have a word for white people (I forget the word) that essentially means "the people who change nature." Nature writer Barry Lopez reports that they use the word with a combination of awe and fear.

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We complain about farmers and how they alter the landscape through tiling, ditching, and such but how willing are we to admit that cities also alter the landscape for their own benefit as well?

Very good point. Look at all the runoff created by concrete and asphalt. I think it really is a balancing act that must be continually modified, on farmers side and urban side. I think it is getting better on both fronts, but of course some of us/you want it to go faster or slower, as it does affect us all in the pocketbook.

Makes me think of the discussions I had back in biology courses, about what really is "natural" - my point was that "mankind" is natural, so what how can what we (humans) do really be called "unnatural" and so hated?... I.e. a beaver dam changes things the same as a human dam, only thing is that we nowadays are "trying" to lessen the impact - though that is not always good either, but at least we are trying.

That Inuit word is really appropriate IMO.

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I suppose one could take my question a step further and ponder what future generations will learn about the impact we had just by being alive. In order to continue our existance we need shelter, water, food, and I guess clothing….yup…some of us definitely need clothing, which is evident when I look in a mirror. No matter where we set up shop or how we secure our food and other raw materials needed to fulfill those most basic needs, we will impact our environment one way or another not to mention the impact we have as we do things to improve our way of life.

Out of need we’ve built many of our cities along water sources including rivers and therefore we need to take steps to protect them. In order to grow the food we need it is necessary for us to till the soil, fertilize, and control competition. To obtain meat we need to kill animals. These things don’t come without a price especially if we do them with reckless abandon.

The point is, change is inevitable. We're here and we will cause change just as any other creature. In fact, change will occur with our without the existance of life. We change things to our benefit, a beaver changes things for its benefit, trees change things for their benefit, etc. It's a fact of life with one difference. We are capable of analysing the effect of the changes we incur whereas other living things are not.

Back on topic.

The sensationalizing that I was referring to was not done by members on this site but rather by others. Stating that this was the highest crest in over 100 years is a bit deceiving when one factors in the dikes and levies that force the river upward. It’s like blowing up a balloon to a one foot diameter, squeezing it, measuring a diameter of 1-1/2 feet at it’s widest point, and then claiming it was 1-1/2 feet in diameter. It still conatained the same volume of air. Please understand that I’m not down-playing the fact that the situation was a very serious threat. It was and it was unfortunate but we sure do like to over dramatize things sometimes.

If we want sensational, consider what it must have been like when the river reached a 40’ crest before there were the dikes and levies we have today. I wonder how high those dikes would have to have been to protect the city against that? Another 20’ perhaps? Now that’s sensational.

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Is it time to build a flood control system ?

Something similar to what Winnipeg built.

http://en.wikipedia.org/wiki/Winnipeg_Floodway

I've heard some Fargo/Moorehead officials talk about it, but complain it would uproot too many people. Seems these 100 yr. floods they have every 10 yrs. could do a pretty good job of that.

I don't know the area, but it sounds like a worthwill project to spend some "stimulus" money, and create some jobs.

That's what the stimulus money is all about right ??

Creating Jobs ??

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We also make change when we see the adverse conditions from our previous changes and not really fully realizing the total impact both positive or negativly until 20 or more years later. While it is a very bad situation up there there also has created a lot of good, good will, community strength, and more knowledge.

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I've heard of the possibility of diverting/pumping river water into farm fields and slowly letting the water back into the river. Apparently someone has calculated the amount of acres needed for the water. It would be much cheaper to pay farmers for lost production every so many years than pay for other types of flood control, especially a diversion channel.

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I've heard estimates of $2 billion for creating a diversion around Fargo Moorhead.

$2 Billion could restore a lot of wetlands in the watershed (while also having a lot of other benefits besides flood reduction).

I don't know what it would take to stop people from building new homes next to the river, but that would be a good start. Buying out homes would be helpful, making the imediate floodplain into a "green area" would be another good idea. Replace the most dangered areas with parks and golf courses.

I think they'd be missing on a lot of oppurtunities if they decide to just focus on a diversion.

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I've heard estimates of $2 billion for creating a diversion around Fargo Moorhead.

$2 Billion could restore a lot of wetlands in the watershed (while also having a lot of other benefits besides flood reduction).

I don't know what it would take to stop people from building new homes next to the river, but that would be a good start. Buying out homes would be helpful, making the imediate floodplain into a "green area" would be another good idea. Replace the most dangered areas with parks and golf courses.

I think they'd be missing on a lot of oppurtunities if they decide to just focus on a diversion.

Exactly!

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I don't know what it would take to stop people from building new homes next to the river, but that would be a good start. Buying out homes would be helpful, making the imediate floodplain into a "green area" would be another good idea. Replace the most dangered areas with parks and golf courses.

That's what they did in GF/EGF. Complete neighborhoods were bought out and converted to parks/greenways. Both the houses I lived in growing up were close to the river and are now long gone, the land part of those parks/greenways. Of course, it's easier in one way to buy people out once their homes have already been flooded than to do it as preventive measure. Costs a ton of money to do in either case.

I like the flood diversion as well, but the price tag usually dissuades politicians because of the controversy such huge-dollar projects generate.

And it's logical to argue for diversions or preventive buyouts by saying it's no more expensive to prevent a problem than fight a catastrophic flood, but we as a country seem a lot more willing to help people after disaster strikes than we are to spend the big bucks to prevent the disasters in the first place. And as has already been stated, floods are not natural disasters, they are simply natural. What makes it a disaster is people/businesses in the flood zone. Remove the homes/businesses from the flood zones or channel water away and presto! no disaster.

Also, in widening the green "no-occupy zone" along the rivers, you widen the funnel BobT has been talking about, allowing more water to pass between the dikes without squeezing it and raising it as severely as had been happening. That means either lower dikes are possible (lowering costs) or that higher dikes would protect even more than they had in the past.

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