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. Bigfatshady

    This is obama's fault

    May 15, 2011 at 11:38 am | Report abuse |
  2. Dug

    I sincerely respect what enviromentelist try to accomplish but whats happening is a result of the levee system every Engineer including myself knows that if there were no levees the flooding would not be occuring. There was 10 times as much water in 1937 because it reached this stage including millions of acres of land it was allowed to flood because of the lack of levees so for people to blame this on global warning is being naive.

    May 15, 2011 at 12:04 pm | Report abuse |
  3. B lo me

    @Scott. You are a dum a$$. You may continue inbreeding now.

    May 15, 2011 at 12:42 pm | Report abuse |
  4. B lo me

    You too lagirl. Your logic is the definition of insanity. Just plain dum.

    May 15, 2011 at 12:45 pm | Report abuse |
  5. Art

    How does this relate to flood insurance? If someone does not have flood insurance, and gets flooded because the Army Corps of Engineers decides to open a floodgate, will FEMA or someone else cover the damage? If I paid taxes to build a levee in my area, and it would have protected me, except that someone consciously decided to breach the levee to protect people not in my community – should that be my cost to bear?

    May 15, 2011 at 12:49 pm | Report abuse |
    • R. Scott Grimes

      Because it encourages building in flood plain areas and beachfronts. Another Federal monstrosity. Good report during Katrina, John Stossel admitted he had a beachfront home wiped out 3 times from flooding, but the feds kept paying the bill.

      May 15, 2011 at 12:57 pm | Report abuse |
    • S101

      Without flood insurance, you'll have to wait for a declaration of federal emergency to qualify for funds.

      May 15, 2011 at 4:20 pm | Report abuse |
    • Art

      Scott,

      I have a slightly different take: if it is a known flood plain, with no protection, I agree with you. Shame on the homeowner for building on a known flood plain, double shame if you did not have flood insurance. However when a homebuilder has paid taxes to have a levee system built to protect from flooding,and indeed the levee system could protect you from the flood, but you get intentionally flooded because other people downstream did not have adequate protection, it is i different story. Now, a homeowner who took reasonable and adequate measures to protect their home is being intentionally flooded to protect people downstream who did not. Here, I cannot find fault with the homeowner for making a stupid decision, and I think it should fall under some sort of emminent domain protection, where the feds have decided to buy your property to use for the greater good.

      So I'm curious what the government policy is toward these people who would not have had a problem, had the feds not decided to intentionally flood them out.

      May 15, 2011 at 10:46 pm | Report abuse |
  6. Ian

    its not hard to pick out the liberals just look for hate

    May 15, 2011 at 12:50 pm | Report abuse |
    • KG

      Really Ian, liberals look for hate??? And conservatives, including the tea packet, are a bunch of friendly loving 'all inclusive' folks right

      May 15, 2011 at 1:41 pm | Report abuse |
  7. Rick

    Clearly levees contribute to the pervasiveness of the flooding by defeating the natural flood plain. But they do generally protect the cities. The Mississippi does alternate between its current path through New Orleans and the Atchafalaya every few hundred years and it is over due to switch. That will make NO no longer a River Port, but probably still a seaport, since ships don't come in through the delta any more now.

    May 15, 2011 at 12:52 pm | Report abuse |
  8. R. Scott Grimes

    This is how man screws up with nature. The levees are the WHOLE PROBLEM! They bottle up a river instead of letting nature take its course! Then when this happens, its a catastrophe in the making. Without the levees, the flooding would have been minute, spread out, and more importantly, without the levees, people would not have been dumb enough to believe in their false security and build a life in a flood zone! When is mankind going to learn?

    May 15, 2011 at 12:52 pm | Report abuse |
  9. Rick

    Yes,our levee and land fill policies, mostly 100 years ago, have encouraged occupation of fllo0d plains that in the past were agricultural. In retrospect it seems that these were incorrect policies, but hard to undo. Didn't someone say a long time ago, to paraphrase, seek higher ground? At least for domicile!

    May 15, 2011 at 1:07 pm | Report abuse |
  10. Todd B. Pigott

    Today, in Abita Springs, LA, It is such a beautiful day, Sunny, 68°, Light Breeze, and Crystal Clear Blue Skies. Just across the State, people are loosing EVERYTHING. A Tragic Sacrifice so that THOUSANDS more will be saved. My heart goes out to those who, either personally or through family and friends, have made this sacrifice for so many. Baton Rouge, New Orleans, and countless other community might be spared because of their loss. I believe we ALL owe them a debt of graditude for their sacrifice. May God Bless them ALL.

    May 15, 2011 at 1:27 pm | Report abuse |
  11. Brandon

    2 millon ft. per sec. is a speed if you put that water down a 1 ft by 1 ft chanel a point w millon ft away woult reach the end in 1 second. and that 44 ft deep football feild is 2 million cubic ft. filling in 1 sec.

    May 15, 2011 at 2:38 pm | Report abuse |
    • Ret LAEnvEmp

      2,000,000 cubic feet per second is not a speed; it is a measure of flow or volume.

      May 15, 2011 at 5:13 pm | Report abuse |
  12. Realist

    If you think how people are nowadays maybe we should change our ways and stop worrying about who n whats fault it is and look at reality that we shouldnt take advantage of the place inhabit

    May 15, 2011 at 2:40 pm | Report abuse |
    • Ret LAEnvEmp

      Very few people live in the Morganza/Atchafalaya spillway. Morgan City now has 20 foot levees/sea walls around it. Morgan City had 10 foot levees/sea walls in 1973 when the spillway was opened; it did not flood. Water got almost to the top of the levees/sea walls in 1973 but it did not flood . So, Morgan City raised the levees and sea walls to 20 feet in 1975. Flooding ain't gonna happen in that town.

      May 15, 2011 at 5:41 pm | Report abuse |
  13. bob

    It's still raining up north. This ain't over yet.

    May 15, 2011 at 2:40 pm | Report abuse |
  14. Martin

    It seems that you all must have some horrible lives to be filled with such HATE

    May 15, 2011 at 2:58 pm | Report abuse |
  15. Brandon

    If it wasnt for the leves these flood plans would flood yearly. if the leve gave way in north lousisana water would reach monroe likly shuting down about a 100 mile strech of i20. the leves arnt just about flooding, they keep the river on track for indusrys that rely on the river like ports and power plants.

    May 15, 2011 at 3:03 pm | Report abuse |
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