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May 10, 2011 12:33 PM by Dave Baker

What Exactly Is Flood Stage??

Every day, we Meteorologists at KATC give you the local river stages.  Most of the time these are noticed by the boaters, fishermen, and hunters along with the tides, and the sunrise and sunset information.  During heavy rain events, more people take note of the rivers, especially the Vermilion River, since it's primarily a local drainage basin.  During hurricanes, people might take note of the Atchafalaya River at Morgan City.  This sometimes helps to measure storm surge.

When flood warnings are issued...we might say, "The river will approach FLOOD STAGE if the rain doesn't stop!"  Kinda scary huh?  But what does "Flood Stage" really mean?

To put it simply, Flood Stage is the level that the water will need to climb to leave the constraints of its banks at a certain location.  Many times this would be a water height where an area would flood if there were no elevation, levee, or floodwall protection.  In most areas, this would be just below this line in order to give proper warning to people in that area.  Like here in Lafayette, the Flood Stage is 10 feet.  This is when a FLOOD WARNING would be put into effect.  But at 10 feet, no homes are flooded.  Only some minor flooding of park areas along the river near the Evangeline Thruway.   In Morgan City, the Flood Stage for the Atchafalaya is 4 feet.  Water is approaching the city dock, but even then, flood gates aren't closed.  In fact the Atchafalaya River at Morgan City was at or above flood stage for more than half of 2010.  Sometimes flood stage can also be referred to as "Action Stage".

The next level is "Minor Flooding", which is where some roads may be covered with water and some buildings would be threatened.  Then we go to "Moderate Flooding".  Buildings may take on water, roadways would likely be closed, and some people would be heading to higher ground.  Evacuations may be necessary at this level.

When you get to "Major Flood", then catastrophic, life threatening water levels are occurring, or is expected.  Flooding is taking place over large areas, homes may be flooded to rooftops, and evacuations of entire cities or areas may be needed.

As an example, in Lafayette, Moderate flooding takes place at 14 feet and major flooding is at 16.  The last time the Vermilion was at Moderate stage was in May of 2004, and we were approaching Major flood levels during Tropical Storm Allison in 2001.  It's been almost 70 years since we've exceeded the 16 foot level.

In Morgan City, the Moderate Stage is 7 feet, and Major Flooding is 12.  The record is 10.5.  There is a flood wall protection system there, so buildings on the river side of the flood wall would take water will be flooded at 9 feet, backwater areas east of Morgan City near Amelia would flood at 12 feet.  The flood walls protect Morgan City up to 21 feet. 

Many locations along the Mississippi River are expected to reach record levels.  Most of the locations are already at Major Flood Stage levels.


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