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A Breakdown of the Possible Morganza Spillway Opening

Posted at 8:28 AM, May 23, 2019
and last updated 2019-05-24 15:08:37-04

It’s time to talk flooding.

Wednesday the Army Corps of Engineers announced that they were considering opening the Morganza Spillway, a result of an abnormally high water season (both in terms of duration and volume of water).

Currently (Thursday, May 23) almost every major river in Louisiana is under a Flood Warning, and three rivers in particular, the Red River, Atchafalaya River, and Mississippi River, are the most concerning.

Map Valid as of May 22, 2019

Since the start of spring, both the Mississippi and Atchafalaya River have begun to rise. This is not uncommon, in fact, it happens every year.

What has been uncommon this year, however, has been how long the rivers have stayed at these flood levels. Typically by now, those levels would have started to recede.

As an example of just how uncommon such a long high water season it has been, the Mississippi gauge at Red River Landing has been above flood stage for 146 consecutive days.

The record for that gauge is 152 consecutive days which was set back during the flood of 1927.

New Orleans has now seen the Bonnet Carre Spillway open twice, the first time this has happened in its history, in order to alleviate some of the pressure on lower sections of the river.

If you’ve driven across either the Basin Bridge or the Atchafalaya River Bridge in Morgan City you will have noticed that it’s not just the Mississippi that’s running high.

The Atchafalaya River has also seen an abnormally high water season and is already causing flooding in portions of St. Martin Parish.

Over the last few days parts of the country have been experiencing violent thunderstorms that have produced massive amounts of rain, and expected to continue to produce massive amounts of rain over the next several days.

This is why there is now concern over the Morganza Spillway.

Map Valid as of May 22, 2019

There are two different river valleys that have seen a tremendous amount of water over the last several days, the Red River Valley and Mississippi River Valley.

The flooding in the Mississippi River Valley speaks for itself but the reason the Red River matters so much is that it actually flows into the Atchafalaya River.

Meaning that the 10″ to 11″ rainfall that northern parts of Oklahoma and southern Kansas received recently will matter to us here in south Louisiana.

Rainfall totals over the last 72 hours, valid May 22, 2019.

This additional water flowing into the Atchafalaya will be another addition to an already swollen river, which now may need to prepare to take on the enormous amount of water produced from the Mississippi should the spillway open.

The decision on whether or not the spillway will open will be made in the coming days, engineers are closely monitoring the current river levels.

Again the Mississippi has been running really high all season long, and a heavy rain event forecast for northern portions of the river has started to raise a few eyebrows.

Forecast totals for the next 10 days.

If the Euro model is correct this will add even more water to an already tenuous situation and could be the final straw that forces the gates to open.

During the announcement Wednesday, officials state it wasn’t the flow rate of the river that would force the gates to open, the usual measurement.

Instead, it would be because the water is already approaching the top of the spillway and threatening to overflow the spillway which would be a disaster.

Since the Morganza has not been designed to operate after the water is spilling over steps would need to be taken to protect i.e. open the gates and lower the water levels.

If this were to occur it would take that water from the Mississippi River and force some of it into the Atchafalaya River Basin, which is the natural spillway for the Mississippi River.

There is a complex web of levees that are in place to protect communities from the Basin flooding, but since the Atchafalaya is already so full those levees would also likely cause some backwater flooding.

Location of the Morganza Spillway
Location of the Old River Control Structure

There are two structures located on the Mississippi that are very important to protect, the Morganza Spillway and the Old River Control Structure (ORCS).

Opening up the Morganza would alleviate the pressure the flood waters are putting on the ORCS, which is the structure that keeps the Mississippi flowing in its current channel.

Historically the Mississippi River has changed course, and during the 1927 flood was trying to take over the Atchafalaya’s path to the Gulf of Mexico.

As a result of the flood and the Army Corps of Engineers projects along the Mississippi River, that never occurred and the river has been flowing in the same direction ever since.

Failure of the ORCS or the Morganza would be devastating so making sure that this round of flooding doesn’t cause structural damage will be crucial.

These will be the factors in play over the coming days as engineers decide whether or not they will open the spillway, and why it will be important to keep a close eye on all the river levels this week.

Governor John Bel Edwards will hold a press conference at the GOSHEP Offices this afternoon at 3:15 pm. The press conference will discuss the current flooding, the potential for opening the Morganza Spillway and its effects on Louisiana.

KATC will be there to cover.

See Daniel’s discussion of the impacts of the opening of the Morganza from GMA