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Message
re: Morganza Spillway may or may not open for a 3rd time -- lack of clear info from ACoE
Posted on 2/25/19 at 4:03 pm to TDsngumbo
Posted on 2/25/19 at 4:03 pm to TDsngumbo
quote:
Eventually the river will either overtop the levees, break the levees, or destroy the ORCS. Or all of the above.
This is all true, and not talked about enough, it is all inevitable. I will add an important note: a primary reason why it is happening is the riverbed itself is rising as sediment deposits. This is what causes all Alluvial rivers to meander and change course. Sediment deposits on the riverbed causing the entire column of water to be higher year over year.
Unless we either 1.) dredge more or 2.) build levees higher and higher and higher, the Mississippi River will eventually overtop the barriers we currently have.
This post was edited on 2/25/19 at 4:04 pm
2
Posted on 2/25/19 at 4:05 pm to WizardSleeve
quote:
WizardSleeve
I don't disagree. But hasn't the Corps been able to make the Atchafalaya self scour?
Posted on 2/25/19 at 4:08 pm to JudgeHolden
Good point. I think there is a settling velocity that keeps sediment of a certain density suspended, but I'm not an expert. I am fairly certain the Mississippi riverbed is in fact rising because it isn't flowing fast enough to not deposit sediment. The Atchafalaya could be different.
https://www.nola.com/environment/2017/12/rising_river_bottom_could_swit.html
https://www.nola.com/environment/2017/12/rising_river_bottom_could_swit.html
quote:
What's leading to the potential river bed switch is the filling of the Mississippi's river bed with sediment along an area just below the Old River Control Structure. That section of the river bed has risen more than 30 feet in the past 20 years, leaving less room for the water carried by the flow to the Gulf. During a major flood, it will act like a stopper, causing water to back up and rise higher than the present Old River Control structures.
This post was edited on 2/25/19 at 4:11 pm
Posted on 2/25/19 at 4:26 pm to WizardSleeve
quote:
I am fairly certain the Mississippi riverbed is in fact rising because it isn't flowing fast enough to not deposit sediment. The Atchafalaya could be different
The other thing is that IIRC the Corps does not dredge above BR.
If the river is silting above BR I am not sure how they would know as theres most likely less eyes on it.
Posted on 2/25/19 at 4:31 pm to TDsngumbo
quote:
More development = more runoff into the rivers that drain into the Mississippi.
Thats actually a good point.
Q=CiA
If you keep raising that runoff coefficient enough through development, you will raise runoff.
Posted on 2/25/19 at 4:35 pm to WizardSleeve
quote:
What's leading to the potential river bed switch is the filling of the Mississippi's river bed with sediment along an area just below the Old River Control Structure. That section of the river bed has risen more than 30 feet in the past 20 years, leaving less room for the water carried by the flow to the Gulf
Why wouldn’t they dredge this?
Posted on 2/25/19 at 4:40 pm to WizardSleeve
quote:
That section of the river bed has risen more than 30 feet in the past 20 years,
That isn't good--so now its risen over 33 feet in the last 23 years. The USACE is on top of this matter so no worries at all they are professionals.
Posted on 2/25/19 at 4:40 pm to Y.A. Tittle
quote:
I would imagine the increased flow it would create on the lower Mississippi would overwhelm the existing levee system big time.
Flow would increase by about 43%.
Posted on 2/25/19 at 5:11 pm to TDsngumbo
quote:
destroy the ORCS.
And to the question of why not dredge below the ORCS, it's probably just not feasible. We're not talking about keeping a hundred foot wide ship channel clear. From the ORCS to the Gulf outlet is like 200 miles of half mile wide river. If you only need to dredge 1 foot a year from the bottom that's over 100 million cubic yards per year.
To put that into perspective, imagine a plot of land two miles long and one mile wide.
Now, build a 5 story building covering that entire piece of land. That building has a volume of 100 million cubic yards. Even a tenth of that would be daunting.
Do it every year.
We're not talking about the sediment carried by Dawson's Creek here. It's the Mississippi River. It carries mountain ranges to the sea.
This post was edited on 2/25/19 at 5:40 pm
Posted on 2/25/19 at 6:16 pm to TigerstuckinMS
Yeah disposal is a huge issue, as is simply the cost of maintenance dredging
Posted on 2/25/19 at 6:26 pm to JudgeHolden
quote:
But hasn't the Corps been able to make the Atchafalaya self scour?
If that is possible, the the doomsday scenarios people have predicted about Morgan City becoming the next Atlantis are way over blown.
So a controlled switch could redirect more flow down to Morgan City with out major issues. This could be the start of something new for coastal restoration.
There are still other problems if it goes fully down the Atchfalaya like ensuring shipping needs of the existing river bed.
Posted on 2/25/19 at 6:58 pm to WizardSleeve
I’ve seen the Corps open ORCS about halfway in relatively low water to flush debris and driftwood from the channel to the Atchafalaya. I bet one of the consequences of that is a sandbar down river of the structure.
Maybe if Congress changed the rule about how much has to go down the Atchafalaya, the Corps could shut the structure completely for a while and scour the Mississippi.
Maybe if Congress changed the rule about how much has to go down the Atchafalaya, the Corps could shut the structure completely for a while and scour the Mississippi.
Posted on 2/25/19 at 7:09 pm to JudgeHolden
When the guy who was updating all the backflow flooding drops in then I will get worried. If anyone wants to go find his name I can’t recall?
LINK /
LINK /
Posted on 2/25/19 at 7:09 pm to SATNIGHTS
Supernovasky
That dude was awesome.
That dude was awesome.
Posted on 2/25/19 at 7:13 pm to SATNIGHTS
They are expecting the Mississippi to take a shite on every river city up and down the River this spring into Summer.
They were talking about it locally, they expect really bad flooding, possibly nearing or on for record numbers.
They were talking about it locally, they expect really bad flooding, possibly nearing or on for record numbers.
Posted on 2/25/19 at 7:16 pm to JudgeHolden
quote:
Maybe if Congress changed the rule about how much has to go down the Atchafalaya, the Corps could shut the structure completely for a while and scour the Mississippi
I would imagine ACE would shite bricks if ORCS had to hold back 100% of the flow.
Posted on 2/25/19 at 7:19 pm to JudgeHolden
quote:
Maybe if Congress changed the rule about how much has to go down the Atchafalaya, the Corps could shut the structure completely for a while and scour the Mississippi.
The problem is this is open channel flow, not pipe flow.
By increasing the discharge you would also be increasing the head going downstream- i.e. raising the elevation of the river, not increasing the velocity.
Posted on 2/25/19 at 7:20 pm to TigerstuckinMS
quote:
And to the question of why not dredge below the ORCS, it's probably just not feasible. We're not talking about keeping a hundred foot wide ship channel clear. From the ORCS to the Gulf outlet is like 200 miles of half mile wide river. If you only need to dredge 1 foot a year from the bottom that's over 100 million cubic yards per year.
If only there was some way we could use all that dredged material... you know... like to help build new marsh or something...
Take some of that BP money / offshore royalty money, and put some people to work doing this. No, you won't get it all... but you might be able to at least keep pace. And you don't need to do the whole river... maybe a 10-20 mile stretch south of ORCS would be a start.
Posted on 2/25/19 at 7:28 pm to udtiger
quote:
would imagine ACE would shite bricks if ORCS had to hold back 100% of the flow.
Not during lower water. In high water, yes. But they have to send a third or so down the Atchafalaya no matter what the level.
Posted on 2/25/19 at 7:31 pm to LSUFanHouston
quote:
No, you won't get it all... but you might be able to at least keep pace
LSU put out a study last year basically saying that more sediment is lost to coastal erosion than what is being pumped down the Miss. The reason is all of the various water control structures along the Tennessee and Ohio rivers that keep all of that silt.
If you took those down you would have a start.
quote:
From this comprehensive study, we’ve determined that the Mississippi River Delta has entered a stage of decline. The outlets of the Mississippi River also known as the Bird’s Foot Delta have been prograding, or spreading, naturally for hundreds of years, but that has now stopped. The underwater portions of the delta are now retreating like the land loss occurring in our landscape,” said study co-author Sam Bentley, professor and the Billy and Ann Harrison Chair in Sedimentary Geology, LSU Department of Geology and Geophysics.
Recent studies have shown that the construction of levees for flood protection has led to extensive land loss. The levees disconnect the river and an estimated 210 million tons of sediment that would naturally flow down to the delta and build the wetlands and the seafloor.
LSU
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