re: Morganza Spillway may or may not open for a 3rd time -- lack of clear info from ACoE

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Here's more reading material for this thread. It's a 3 part series article:


America's Achilles' Heel: the Mississippi River's Old River Control Structure

Escalating Floods Putting Mississippi River’s Old River Control Structure at Risk

If the Old River Control Structure Fails: A Catastrophe With Global Impact

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Every body has already read them. The river is not going to change course. The Morganza and the ORCS would have to fail beyond repair.

I talked to some reliable sauces (2 engineers that work for the ACoE in Vicksburg) and my sauces confirm that the reports of the ORCS or the Morganza being damaged are not true.

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All the operational triggers are based on flow

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Thats not entirely true...

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Structure Opening.

Upon direction from the Chief, Operations Division, the Project Manager will open the structure gates in accordance with the
Operations and Maintenance Manual in the order shown in Tables 1 and 2 (Plates 5-1 and 5-2). This opening sequence was developed by MVN Water Management personnel in collaboration with the ERDC researchers who performed the physical model investigation leading to the improved scour protection in the tailbay. The number of gates to open with each change and their bay numbers will be specified in a joint memorandum prepared by personnel from the Water Management Unit and signed by the Chief, Engineering Division and Chief, Operations Division. Operating instructions will be timed and prepared such that, to the extent possible, the water depth in the floodway does not increase by more than one foot per day for the first three days of operation, in accordance with endangered species constraints detailed in Chapter V of this Water Control Manual. This rate of rise will be assessed at staff gage C-6, located on the Morganza Floodway East Guide Levee / East Atchafalaya Basin Protection Levee at
approximately 30° 43’ 44” N, 91° 39’ 40” W (see Figure 1). Under most flood
conditions, this rate of rise can be achieved by limiting the initial opening to one full bay
per day, which may require opening the structure as much as three days in advance of full
operation.

The structure shall be operated such that the stage on the river side of the structure does not exceed 57 feet NGVD29 (56.7 feet NAVD88 [2004.65]) and the
Mississippi River discharge below the floodway does not exceed 1,500,000 cfs on a
projected rise, based upon a 10 day forecast. The structure may also be operated to minimize flood damages in the lower river reaches, minimize stress in leveed reaches, prevent stages from exceeding the approved flowline (i.e. encroachment on freeboard requirements), and prevent the discharge in the Mississippi River from exceeding 1,250,000 cfs at New Orleans, but will not be operated to increase the total discharge in
the Atchafalaya Basin beyond 1,500,000 cfs unless discharge in the Mississippi River below the structure will also exceed 1,500,000 cfs. A river stage of 57 feet NGVD29 at the structure equates to a Mississippi River discharge of approximately 1,400,000 cfs as of the most recent update of this document.

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PERTINENT DATA
Completion Date 1955
Structure Composition 125 bays, each
containing two steel leaves, each 11.25 feet high by 28.25
feet wide.

Weir Crest Elevation 37.5 feet NGVD29

Top of Gate Elevation 60.0 feet NGVD29

Net Weir Width 3,531.25 feet

Maximum Headwater, Gates Closed 60.0 feet NGVD29

Maximum Headwater, Gates Open 57.0 feet NGVD29

Maximum Tailwater 56.0 feet NGVD29

Design Discharge 600,000 cfs

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USACE Morganza Standing Instructions (USACE pdf)

This is why I am very interested in the 60' elevation marker...

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The confusion you’re seeing is that we have higher stages at lower flows. All the operational triggers are based on flow... since the rating curves are shifted, forecasting the stage has been super important but difficult. 

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This. The sediment trapped on the riverbed has caused a higher river at lower flow rates because the river holds less volume with a higher riverbed than it did in the past.

The levee system's max protection is not only measured by height but flow as well. The 1.25 million cubic feet per second trigger to open Bonnet Carre is due to the levees from New Orleans on down being designed to handle up to that amount of flow. Anything above that & the levees there are at risk of breaching. Same goes for 1.5 million cubic feet per second trigger of Morganza. The levees at Baton Rouge are only rated to handle up to that flow rate.

So the indecision based on crests are understandable. What needs to happen though is due to the changes of the dynamics of the riverbed, the legislation revolving around the trigger to open Morganza needs to be updated accordingly. This is what the engineer in the part 2 of the Tale of 2 Rivers video talks about towards the end of that video. Because of the changes to the riverbed, Morganza becomes at risk of being inundated prior to ever reaching the 1.5 million trigger during a high river & that needs to be accounted for.

What is amazing, is during the 1927 flood the flow rate was almost twice what we are seeing today. Can you imagine how much higher the river would be with 2.5 to 3 million CFS compared to the 1.6 today

The river wasn't as confined as it is today. It would be much worse today.

I agree . Thankfully I don’t think we have ever come close to that flow again . I think the core said 2.6 million but some engineers measured over 3 million at Vicksburg. That’s a lot of water

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Sydney a murray needs to dredge their inflow channel, they are the cause of accretion south of orcs

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