Flooding explained: crests, spillways and levees
Townhomes sit in floodwater from the Mississippi River on Monday in Memphis, Tennessee.
May 10th, 2011
09:43 PM ET

Flooding explained: crests, spillways and levees

The swollen Mississippi River on Tuesday was in the process of cresting many feet above flood stage in Memphis, Tennessee, and residents of states to the south are bracing for serious flooding in their communities.

A slow-moving wave of water has been working its way down the river since torrential rains sparked flooding in the Midwest's Ohio and Mississippi river valleys in late April. Hundreds of households were ordered to evacuate in the Memphis area (see pictures), where the river was expected to crest Tuesday around 48 feet - 14 feet above flood stage and less than a foot under the city's record level set in 1937.

The high water is headed for Mississippi and Louisiana, prompting authorities to open one spillway and consider opening another - moves meant to divert some of the water into less populated or unpopulated areas. Up to 5,000 homes will be evacuated in Mississippi, officials there say. In Louisiana, where the river is expected to crest next week, 21 parishes already have issued emergency declarations.

You will hear a lot about crests, spillways and levees over the next couple of weeks. Below you'll find what these mean and what the Army Corps of Engineers and others are doing to mitigate the flooding threat along the Mississippi.

When a river crests

The National Weather Service says a crest is the highest point in a wave. In the case of river flooding, it is the highest stage or level of a flood wave as it passes a particular point. Gauges along the river record the level of the water, and the highest level recorded at each gauge will be the crest for that gauge.

Observers generally know that cresting is occurring when the water level stops rising and becomes stable. However, it's difficult to identify the official crest at the moment that it happens, because during the cresting, small fluctuations happen - water levels can rise a little after they fall a little. The fluctuations are caused by factors such as wind and water currents.

The crest will have passed when the water level continuously decreases. Once this has happened, observers can note what the highest recorded level was. But the cresting of a river at a certain point doesn't mean immediate relief for that area - the water level will lower gradually, meaning flooding can still be a problem days after a crest. On Tuesday, communities in southwestern Illinois and southeastern Missouri still were flooded, days after crests there. In Memphis, although the river was cresting there Tuesday, residents could be dealing with high water levels into June.

Levees meant to keep river in its place

A 2,200-mile system of earthen levees, floodwalls and other controls were built along the Mississippi, Arkansas and Red rivers after a massive flood overwhelmed the southernmost stretch of the Mississippi River in 1927, according to the U.S. Army Corps of Engineers.

The levees guard riverside communities to some extent, but they are rarely tested with the kind of volume and pressure passing through the river now. On Monday in Memphis, the water was moving at 2 million cubic feet per second. At that speed, water would fill a football field at a depth of 44 feet, according to the Army Corps of Engineers.

The system is designed to manage speeds of up to 2,360,000 cubic feet per second at Cairo, Illinois, and up to 3 million cubic feet per second where the Red River meets the Mississippi River in Louisiana. But below the Red River, floodways and spillways (see section below) would be used to divert half that water to other areas in Louisiana so that only up to 1.5 million cubic feet per second flow down the main river channel, according to the Army Corps of Engineers.

Most levees are piles of dirt lined along edges of a river. In theory, vegetation and its roots will keep the dirt stable as water flows by. But levees can fail, in part because moving water can erode, saturate, undermine and destroy them. (Learn more about how levees can fail).

Floodways and spillways

Floodways and spillways are places where the Army Corps of Engineers diverts water from a swollen river. In some cases, this involves intentionally flooding one populated area to prevent or lessen flooding in a more populated area.

On May 2, the Corps intentionally breached a levee near the confluence of the swollen Mississippi and Ohio rivers at the Missouri/Illinois border, sending some of the Mississippi’s water into the New Madrid Floodway - which is 130,000 acres of Missouri farmland.

Missouri officials, wanting to protect the farmers' land, tried unsuccessfully to block the move. The Corps says the move was designed to prevent devastating flooding in Cairo, Illinois, and elsewhere downstream. (See pictures of New Madrid Floodway taken before, after the levee breach.)

The Corps estimates it will take up to two months for the water to recede from the floodway and another month for the land to dry out.

In Norco, Louisiana, the Corps this week opened 72 gates to the Bonnet Carre Spillway, north of New Orleans, diverting millions of gallons of Mississippi River water into Lake Pontchartrain and, eventually, into the Gulf of Mexico.

The agency is considering whether to open the Morganza Spillway, which is on the Mississippi north of Baton Rouge, Louisiana. When the spillway is opened, water would be diverted to the Atchafalaya Delta to the west and south of Baton Rouge, and could flood populated areas such as the town of Morgan City, which has 13,000 residents.

soundoff (141 Responses)
  1. ben

    just an fyi, 2 million cubic feet per second is not a 'speed'

    May 14, 2011 at 3:24 pm | Report abuse |
    • xray

      but it is one crapload of water aint it

      May 14, 2011 at 5:58 pm | Report abuse |
  2. Greg

    http://www.reverbnation.com/kingnati

    May 14, 2011 at 3:29 pm | Report abuse |
  3. Bill

    Who needs an explanation. It was Brownie's fault, or Bush's fault, or both. Who needs a technical explanation. Yes, it wasn't Katrina but a poorly constructed levee, but try telling that to the masses.

    May 14, 2011 at 5:17 pm | Report abuse |
    • Rudedog

      Noboy said Bush or Brownie were responsible for the katrina disaster, they were responsible for the disaster that followed. But thanks for apologizing for them, I am sure they appreciate somebody marginalizing their ineptness.

      May 15, 2011 at 5:53 am | Report abuse |
  4. Johnson

    The most advanced country in the world (2011). Really? Look at these horrific pictures of the floods. We look like idiots that are quick to cry socialism. I guess because of socialism we must forget about our infrastructure. We don't invest in Jack-**** anymore. Depending who is in office we become the Nation of "We Can't". We won't even think to re-visit high speed rail until fuel price go into the ozone. China and India are putting more and more cars on the road, they will be after the same oil reserves which increases demand (prices WILL go up) imagine trying to rebuild infrastructure as well as building rail (expensive when oil goes through the roof) BUT, America won't act until it is to late. We are to worried about who gets back into power.

    May 14, 2011 at 7:18 pm | Report abuse |
    • Ret LAEnvEmp

      About 20% of your gasoline, oil, farm chemicals, plastics, fertilizers, salts, chlorides, chemical bases, coatings, paint bases, water treatment chemicals, asphalts for the USA are down here in the Mississippi River basin. Counting the agriculture out put, that feeds the East Coast and the timber industry – why don't we just shut down the paper mills that make toilet paper and see how that grabs you.

      May 15, 2011 at 6:10 pm | Report abuse |
  5. wavejump1100

    its called a flood plain for a reason. if people build on a flood plain guess what? its going to flood a few times a century.

    May 14, 2011 at 7:19 pm | Report abuse |
  6. johnny

    "On Monday in Memphis, the water was moving at 2 million cubic feet per second. At that speed, water would fill a football field at a depth of 44 feet, according to the Army Corps of Engineers."

    Not only is "2 million cubic feet per second" NOT a speed (as pointed out by ben), but water could "fill a football field at a depth of 44 feet" at ANY RATE. You could stick your garden hose in there and it would fill the volume given enough time. A 44 foot high football field covers about 2,534,400 cubic feet; I think what this article is trying to say is that at a rate (not speed) of 2 million cubic feet per second, water would fill a football field to a depth of 44 feet in about 1.5 seconds (using CNN's numbers).

    I find it hard to believe that this information came from the Army Corps of Engineers (which CNN claims as the reference). They were either misquoted, or are not very good engineers.

    May 14, 2011 at 7:21 pm | Report abuse |
    • dogbreath421

      The definition of speed is "The speed v is defined as the magnitude of the velocity v, that is the derivative of the position r with respect to time:

      If s is the length of the path traveled until time t, the speed equals the time derivative of s:

      In the special case where the velocity is constant (that is, constant speed in a straight line) this can be simplified to v=s/t. The average speed over a finite time interval is the total distance traveled divided by the time duration." Therefore the statement of "the water was moving at 2 million cubic feet per second" is a reflection of "speed or rate". If you measure the depth and width of the water (which is being monitored) and determine that x number of cubic feet are passing per second, that would be the speed of the water flow. (Just measured in cubic feet / time) This is an industry standard in commercial transmission of natural gas, water, or any other piped commodity).

      May 14, 2011 at 9:02 pm | Report abuse |
    • johnny

      Velocity is a vector (which has DIRECTION), cubic feet per second is a scalar (and does NOT have direction). Water flowing past a given point will have some speed (and it is true that this is the time derivative of position), but in this case we're talking about a FLUX – which is a rate of water flow through a given area or into a given volume. When considering cubic feet per second, we're talking about how fast a volume fills, not how fast an object (or substance) moves. To put it simply, velocity needs distance, and flux needs area or volume.

      May 14, 2011 at 11:09 pm | Report abuse |
  7. Bob Jones

    G-d hates a lot of the people who live in Tennessee, Mississippi, & Louisiana?

    I wonder what message G-d is trying to send?

    –BJ

    May 14, 2011 at 7:52 pm | Report abuse |
  8. Romny

    Why did they build those new town houses below sea level? It was foolery for those home owners.

    May 14, 2011 at 8:12 pm | Report abuse |
    • VFJ

      The townhouses are in Memphis, well above sea level.

      May 14, 2011 at 10:40 pm | Report abuse |
  9. christian man

    WAKE UP AMERICA WE TAKE GOD OUT OF OUR SCHOOLS AND ARE PUSHING HIM OUT OF THE USA. THIS COUNTRY WAS BUILT FROM THE BELEIF IN GOD AND HE HAS BLESSED US NOW WERE PUSHING HIM OUT . THINK HE IS TELLING US SOMETHING , TIME TO TURN IT AROUND

    May 14, 2011 at 9:00 pm | Report abuse |
    • Rudedog

      God has nothing to do with this, some decided to live in a flood plain.

      May 15, 2011 at 5:47 am | Report abuse |
    • caliguy59

      You're a nut. You'll credit god when something you consider good happens, but you won't blame god when something you consider bad like flooding happens. I remember the people and governor of Louisiana crediting god, when an approaching hurricane rapidly lost strength before hitting the coast. The real reason the hurricane lost strength is because it began to suck in cool, dry air from the northwest. Then, about a year later Katrina hit Louisiana causing a disaster of major proportions. But, no one could or would explain why god intervened on one occasion, but didn't on the other occasion. Of course, the logical explanation is that god, if it exists, doesn't intervene in man's affairs or natural events. But, you wouldn't ever consider that explanation.

      June 5, 2011 at 7:53 pm | Report abuse |
  10. Rob

    They have known for ten days that flooding was expected. I wonder why they did not just partially open the flood gates at that time, rather than just letting the water build up ???

    May 15, 2011 at 12:18 am | Report abuse |
    • Harold

      Opening the floodgate too soon could cause the spillway to fill up before the flood arrives. (a worst case scenario) As long as that section of the river has not reached flood stage no one is in danger so you want to keep them closed until the last minute.

      By waiting you maximize the amount of water that flows down the river without damaging properties along the river and you minimize the amount of water you have to divert to the spillway.

      May 15, 2011 at 8:19 am | Report abuse |
  11. Terry - Indiana

    So when a city building department gives builders a permit to build on a flood plain, that is OK, well maybe. When insurance companies offer "Flood Insurance" that is OK, well maybe. When the Army Corps of Engineers builds Levees in an attempt to redirect the flow of rivers, such as the Mighty Mississippi, that is OK, as long as there are spillways and gates that allow for flooding of low lying areas, called flood plains. So, flooding and damage to homes, apartments, condos, and buildings that were built on flood plains must be OK.

    May 15, 2011 at 6:45 am | Report abuse |
  12. notafish

    We should take a second and think about what is going on here; all this destruction for a bunch of folks who CHOOSE to live below sea-level? At some point we are going to all have to face the fact that living in a city built in an area, albeit not originally, that is below the level of the water trying to get in, is not a very good idea. I understand there is historical and sentimental value to the city, but really?!? Some things we just need to let go of, and think of how to make tomorrow a better place. We need to help everyone, not just some.

    May 15, 2011 at 9:15 am | Report abuse |
    • fishwithoutater

      How about choosing to live on a fault line? Or in tornado alley? Or in a city that frequently gets crippled by snowstorms every year. Applying your logic would include all these things as well.
      Take what you just wrote, cut and paste these 1st lines over the world "sea level"

      How would that read to you? Sorry to point this flawed logic out. But think about it.

      May 15, 2011 at 10:36 am | Report abuse |
    • Scott

      Notafish, you must be a conservative hatefilled moron. Sure places like New Orleans are below sea level, but the vast majority of the affected places are above sea level. It's a RIVER that's flooding, not a sea/ocean. If a river crests 16 ft above flood level, that's 16 feet of flooding. Most major cities are on rivers. The town of Cairo, Ill has an elevation of 316ft.

      Don't be dumb

      May 15, 2011 at 10:38 am | Report abuse |
    • you are an idiot

      Just be honest, you hate black people.

      May 15, 2011 at 10:43 am | Report abuse |
    • LAGirl

      If you were told your house was going to be destroyed due to flooding in a matter of days, would you want to read a post like yours? I think not. We might live in a city below sea-level, but its not a sea thats flooding us.

      May 15, 2011 at 12:20 pm | Report abuse |
    • Wall14

      No, notafishes point, though maybe a little harsh, is well taken. My college geography teacher said living near any body of water is very risky especially if is below sea level. Same with living over a fault line. Indeed, living anywhere has it's geographical risks that residents have to accept and contend with. There's no fooling mother nature. Suggesting this insensitive or racist is unfair.

      May 15, 2011 at 2:02 pm | Report abuse |
    • Robert

      So... People who wish to avoid being branded "stupid" on message boards should not live near rivers, or oceans, or fault lines, or volcanoes, nor anywhere subject to tornadoes, hurricanes, floods, droughts or blizzards. Pray tell where should we live? Do you know of anywhere in United States utterly immune to any geological or meteorological events? Is it big enough to hold all 330 million of us? Humans have been battling nature to survive since humans have existed. It's what we do.

      May 15, 2011 at 2:43 pm | Report abuse |
    • Lars

      Scott if not for your first sentence, you would have been only pointing out a valid fact. You gave enough information to let us believe that the only difference between you and hin are that you are a liberal hatefilled moron. There are differences in building below sea level. the people that live in the poorer areas of an area of such won't have the wealth to rebuild they will move on and abandon the area. Those that live along the river usually have flood insurance and will rebuild on the same spot unless the cities rezone those areas and buy out the flood plane. San Antonio did this around 2000 after a 750 year flood in one area and a 500 year flood in a spot further North. Now it is being turned in to a park area with biking and hiking trails. Most of the Netherlands is below sealevel however they built a seawall to keep the sea out. It's Ok to disagree, but educate don't iritate. There was no need for your opening comment. Does it really matter if he is conservative or liberal?

      May 15, 2011 at 2:48 pm | Report abuse |
    • notafish

      Ok, i think some people got the wrong idea about where i'm coming from; this is not about race or place, it's about man trying to win a fight with nature. In the past, when an area became dangerous to inhabit, folks would relocate to a safer place. That's not to say that any place is totally risk free, but the risk can be managed with respect for the power of nature. What is going on with the Mississippi could be thought of as an avalanche is extreme slow motion; we know it's not too likely to stop one of those, so we get out of the way. By trying to manage the waterflow of the Mighty Mississippi we are effectively trying to fight that avalanche, and as the events of the last few months have shown us, nature has a way of overwhelming all the best laid plans; it pains me to think of the situation with respects to sensitive infrastructure, and wonder what magnitude earthquake the planners thought would be THE LARGEST EVER....is the infrastructure up to the test? But back to the topic, we need to evaluate whether this is a battle that can be won, the battle against nature; or do we concentrate on making life better for the people that need to be relocated, and be thankful that we won't have to worry about that area being in danger any longer? Seems to me, that one less flood to worry about is better.

      May 15, 2011 at 7:25 pm | Report abuse |
  13. franky

    Nobody cares about africa.if you can't feed ur kids don't have them! Lettem fend for themselves!!!

    May 15, 2011 at 10:37 am | Report abuse |
    • you are an idiot

      Your ignorant nonsense is a tempting solution.

      May 15, 2011 at 10:47 am | Report abuse |
    • c. smythe

      I support franky. I got fixed when I was sure the time had passed for me to father children, however, I DID raise 2 of ANOTHER man's children as my own out of love for them and their mother . . . sometimes good s#it happens!

      May 15, 2011 at 2:03 pm | Report abuse |
  14. you are an idiot

    This article is another CNN embarrassment.

    "You will hear a lot about crests, spillways and levees over the next couple of weeks."

    Oh yeah?

    And I suppose river "crests" might be when the water gets really high?

    May 15, 2011 at 10:41 am | Report abuse |
    • Robert

      Read the story and you will know. If you still don't understand, google it and do your own research. CNN is not your mother.

      May 15, 2011 at 2:45 pm | Report abuse |
  15. CTYank

    Little/no discussion of two very important matters:

    1) Channelizing a river with levees elevates the crest for downstream areas. Used to be that the Missouri and the Mississippi could overflow into huge floodplains, and deposit silt there. No longer.

    2) The Mississippi is about due to shift to another delta. The current one extends way out into the Gulf, with all the sediment that's been deposited there, and the one down the Atchafalaya is much shorter. The Old River Control Structure, which controls the flow down the Atchafalaya has an increasingly difficult task in flood conditions like these. When the river shifts to a new delta, New Orleans/Baton Rouge/etc. will have to adjust to being backwaters, so this will be a big problem. Big changes for ocean shipping. Big changes for deposition of silt to the liquid land of the bayou country.

    All the rest is just buying time, seems to me. The big river will have its way.

    May 15, 2011 at 11:02 am | Report abuse |
    • Robert

      Water management is never "done". It is an ongoing, difficult and expensive process, but it is possible. The current levee system was built in 1937. There has been little done since than basic maintenance and emergency repair. The Corps needs to redesign the whole system and Congress better be prepared to write a big check. If we decide it's too much bother to protect our own country, we may as well fold up our flags and tell the rest of the world we resign as a nation.

      May 15, 2011 at 2:49 pm | Report abuse |
    • S101

      During large events like this, it is not unusual for natural neighboring river to join and then get back within their respective banks. The Mississippi wants to go the way of the Atchafalaya and if they don't divert within the levees of Atchafalaya as they are doing now, there's a pretty high likelihood of the Mississippi and Atchafalaya joining together (uncontrolled...).

      May 15, 2011 at 4:12 pm | Report abuse |
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