2011 Mississippi River Spring Floods

[Bandwagon King]

_ The problem for the control structures is they won't flow only what they were designed for. They will flow what they will flow based upon the head (elevation difference) between the two rivers along with other factors. Now if the structures are designed to move 620,000 cfs of water, but the head and flow rate of the rivers forces more than that through the structure there is nothing to stop the water. If this happens they are exceeding the design rating of the structure and it could be damaged. I think this actually happened during the 73 flood. Even though the river was no where near the Project Flood flow rate in 73, the increased head between the two rivers (due to the natural deeping of the Atchafalya channel and rise of the Mississippi) caused the flow through the low sill to exceed the capabilites of the structure. Morganza was opened to relieve this pressure (reduce the head) and the structure survived....barely. It was badly undermined, lost one of the inlet retaining walls, etc. This is why the aux. structure was built.

I'm not saying this IS happening, I'm asking how close we are. There are two issues here one is flooding Baton Rouge, New Orleans, etc., the other is stopping the river from changing course into the Atchafalaya. Honestly the second scares me far more than the first, as it would change life as we know it in Louisiana, and would be a terrible blow to the economy of the country at a time we can ill afford it. _

I understand your worries & I would like to more about that occuurence myself. It certainly appears that the worst is over. The lower Atchafalaya is handling everything the upper Atchafalaya is throwing at it & that's a good thing. The Mississippi goes below flood stage on or before May 30th in several places according to the weather channel.That was the original expected crest date. If the flooding is going to happen it will have to happen between now & then.

[Bandwagon King]

_ Ok guys...I've been busy riding all day and this is a lot if info to process on a cell phone as I wait to see if our col. Is coming in my area.

Jumbo,
Not sure where you got 660,000 from but you're close as of the readings this morning.. Not sure if it's available to the public so I'll just say it's less than that but the max so far that I know of In this event was almost there.

King,

As much as stages make sense the determination of gate openings is almost 100% on flow in cfs. Sometimes I'd wish they would go a little more on stage readings, but when they talk about trigger points it's all about cfs.

igeaux.mobi _

Don't levels determine the basis for calculating flow rates to begin with. Not being a math major or engineer, it would seem to me that determining the CSF of a river would be done by calculating the the height by the width by the length of the discharge. I also don't get how a hydrostatic head can be put in there to determine pressure without levels. The bottom line to a lot of people is if the river goes up you open the gates. If it goes down, they should swing the other way. I don't suppose I have to tell you the conspiracy rumors are already starting to boil. It would be a real shame to see gates flying open when the Mississippi is already turning the other way.

[HelmutVII]

_ Have not read it yet-but the Wiki footnotes an information paper from the MRC which seems to explain how they came up with the project design flood over the years

http://www.mvd.usace.army.mil/mrc/mr...fo%20paper.pdf _

The paper does indeed give a brief synopsis of how they came upon the different flow rates to determine what a project design flood is at various stations of the River. However, they still define a Project Design Flood in CFS and not stage.

What one needs to remember is that any given stage at a SINGLE point along a river DOES NOT define what the flow rate is. What defines the flow rate is the DIFFERENCE in stage between two or more stations of a river AND the distance between those stations. It has everything to do with the slope of the hydraulic grade line and the cross sectional area of the channel (that is the cross sectional area of the channel until it gets to areas affected by tides).

It is entirely possible to have a stage of lets say 50.0 at a given point in a river and have, lets say 150,000 cfs flow rate during one event and then have a stage of say 50.5 at the same point and still have the same flow rate of 150,000 cfs. The flow rate at any given point is ENTIRELY dependent on the relative stage of the points upstream AND downstream.

The TOP of the water is literally falling down the river and how fast it falls depends on the relative difference in the top of the water from one point to another. (it's hard to explain the concept because common sense asks how can something fall when it's on the ground).

It took me a long time to fully comprehend this concept, but once I did, it made perfect sense. To this day, I will stand on a drainage structure (especially one that I designed) and just watch the water (even in the rain). Really satisfying to watch it work as designed. It also helps me to understand the nature of the beast.

[HelmutVII]

_ Don't levels determine the basis for calculating flow rates to begin with. Not being a math major or engineer, it would seem to me that determining the CSF of a river would be done by calculating the the height by the width by the length of the discharge.

Mathematically Q (quantity) = V (velocity) x A (area). The problem in large rivers (like the Mississippi) is how do you measure velocity? It can vary substantially from one point to another at any given section. So what you have to do is take velocity measurements at many points to get a velocity profile at a give section. The next thing to do is to determine the area. The problem is scour, how much and how is it affecting the cross section of the river. So what they are doing is making and ESTIMATE of flow rate given an ESTIMATE of the cross sectional area of the channel and using the velocity profile. This is not an exact science and it borders on an art.

If the slope of the hydraulic grade line is high then the velocity is high and will produce a higher flow rate for a given area. If the cross section is scoured then the area is greater and can also produce a higher flow rate (again in those areas not affected by tide ---generally those areas were the bottom of the channel is greater than 0.00 msl)

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[HelmutVII]

_ I also don't get how a hydrostatic head can be put in there to determine pressure without levels.

It's not a hydrostatic head it's a hydrodynamic head because the water is moving. It's not just potential energy it's also kinetic energy and if the water is flowing at a fairly high velocity, kinetic energy can be significant. Put a life vest on and jump in the Mississippi River right now and you will get a very fast lesson on kinetic energy. You would also get a fast lesson in velocity profile. It's all about an energy balance based on Bernoulli's equation.

[cjr3888]

I'll have my life vest on in the Mississippi this weekend patrolling around Angola, but I sure as hell ain't jumping in the water. I'll make sure to take pictures to post on here.


igeaux.mobi

[biged05]

Ok this is going to be a long one and I'm typing on my phone also hopefully I don't Repeat anything b/c I know other posts are coming in.

King,

The determination of how many gates to open is based on flow. Is flow and stage related? Yes but it's all about flow. Let's see if I can show you why in my opinion. A river is dynamic we all know that it changes in width and depth therefore the height needed to pass the water (flowline) changes. We are actually looking at some sections of river now where it appears the river bottom may actually be rising and it's not the usual shoaling in. We use flow as indicated by the project flood diagram. Here is one example of why flow is used. When we opened morganza the head on the gates were very high...let's say 20' just for discussion. Now when they opened the first gate there was about 10,000 cfs flowing thru it. Then a second was open and it was about the same thru that one. Now we've opened more gates the head has been reduced with water levels on the opposite side getting higher, therefore the flow thru each gate has been reduced. So what they do is take a measurement upstream and one downstream of the structure to make sure the flow does not exceed 1.5 mil cfs. If they did not open any more gates as the water levels on each side equalized then we would force more than 1.5 mil cfs thru the system below morganza and yes in turn make the water levels rise. But it is based on a total cfs. I hope that part is a little more clear now. Can we adjust downstream levels with the gates or in the case of bonne carre bays? Yes they are doing that now. They are trying to keep the river at the carrolton gate around 17'. So they have opened more bays there to allow less flow past new Orleans so the coast guard can keep the channel open. Can it handle the 1.25 mil south of bonne carre? Yes but to keep everything open and to try and affect industry the least we are passing more thru bonnet carre.

Now as far as Morgan city and calumet are concerned, they are not keeping up. How do I know the? 1) I've been watching water levels in the basin for a while now and today watched the water along the west basin levee again. 2) it can't handle water that is coming in but has not reached there yet. The water from morganza as of this morning had not made it to I-10 yet. So the river has yet to be affected by the water from morganza, much less the river being affected in Morgan city and calumet. Jumbo's thinking does not sound off. I don't really have the time to look over all the charts and information we have out there and make better points, just too damn busy with the water itself. I'll be honest and say from the new predictions and other things I'm seeing the crest predictions may be a little high, but I'm not going on just the charts your looking at. Let's hope that's true but I wouldn't count on it. The next few days will start to tell the story.

I know I've forgot some of the things that I wanted to say, but my mind is running in too many directions right now.

igeaux.mobi

[biged05]

Yep, just as I figured. Helmut took the lead on all the technical stuff and did such a better job than me lol . But that's ok I'm absorbing it all in too!

igeaux.mobi

[Bandwagon King]

_ The paper does indeed give a brief synopsis of how they came upon the different flow rates to determine what a project design flood is at various stations of the River. However, they still define a Project Design Flood in CFS and not stage.

What one needs to remember is that any given stage at a SINGLE point along a river DOES NOT define what the flow rate is. What defines the flow rate is the DIFFERENCE in stage between two or more stations of a river AND the distance between those stations. It has everything to do with the slope of the hydraulic grade line and the cross sectional area of the channel (that is the cross sectional area of the channel until it gets to areas affected by tides).

It is entirely possible to have a stage of lets say 50.0 at a given point in a river and have, lets say 150,000 cfs flow rate during one event and then have a stage of say 50.5 at the same point and still have the same flow rate of 150,000 cfs. The flow rate at any given point is ENTIRELY dependent on the relative stage of the points upstream AND downstream.

The TOP of the water is literally falling down the river and how fast it falls depends on the relative difference in the top of the water from one point to another. (it's hard to explain the concept because common sense asks how can something fall when it's on the ground).

It took me a long time to fully comprehend this concept, but once I did, it made perfect sense. To this day, I will stand on a drainage structure (especially one that I designed) and just watch the water (even in the rain). Really satisfying to watch it work as designed. It also helps me to understand the nature of the beast. _

Once again, thanks for this reference, & I really do appreciate the engineering involved in the study & the design of this massive project as well as the hard work that was done to make our flood control system what it is today. I do see that flow rate estimates are absolutely important to the design of these structures as well as the projected river crests & flood stage warning systems.

However, flow rate estimates are just that, estimates. While I find it pretty wild to know that there are people that can figure these rates out & computer models that can warn people weeks ahead of time of possible danger & give them time to make proper preparations, I also understand it is impossible to take every variable into account that causes the river to flood at one stage in the river & not another. I know that by reading the information on the link you just sent me, the NWS & the Corps are bound by moral authroity to give highest probable & worst case scenarios. In this particular instance we are having an anomally of river flooding through country that is in a drought situation. this is making a huge difference in the amount of water being discharged into the GOM & I would assume making it very difficult to calculate rover flow rate accurately. That is why sooner or later someone is going to have to say hey guess what, the water on the Mississippi is going down up & down the river. I don't think it's going to flood after all.

Call me crazy, but this is one of those times the government has actually done something right & the results are better than expected.

[biged05]

Man, I wouldn't say it's not going to flood. It may not go outside the levees into protected areas down here, but this is definitely a flood. LOL

igeaux.mobi

[Bandwagon King]

_ Man, I wouldn't say it's not going to flood. It may not go outside the levees into protected areas down here, but this is definitely a flood. LOL

igeaux.mobi _

I didn't mean to make it sound trivial, but I am talking about the local area of the lower Mississippi/ Atchafalaya. The river cresting at Vicksburg & Natchez several days early & 1-2' lower is huge for us.