State Developing New Physical River Model

By Morgan Crutcher

In early February, state of Louisiana officials met with the public to talk about itsplans for a new small-scale physical model (SSPM) of the lower Mississippi River.

Photo by Jim Zietz, LSU University Relations
The old model, to be replaced with the more modern version. Photo by Jim Zietz, LSU OCUR

The current model was first designed in 2002 and fully functional by 2004.  The construction at that time consisted of Styrofoam cross-sections glued together then attached onto plywood cells to create a grid totaling twenty-four by twenty-eight feet.  The total cost was about $840,000.  This particular model taught us some important things, like possible future dredging scenarios in the river, and methods that could affect those scenarios. Through using the model, researchers determined that in 100 years approximately 70% of sand in the river will need to be dredged out as a result of sea level rise.  But modelers also learned that large diversions could perform well, and in fact decrease dredging downriver after 50 years of operation.

Though the old model has yielded valuable research, its construction is starting to wear out over time. Leaking, warping and rotting of the plywood base have resulted in 11 of 24 panels needing replacement as of February 2011.

The project team working on the new model consists of members of the CPRA, the consulting firm BCG-Dewberry, and a senior advisory panel including experts from around the world.  The new model will be more than four times larger than the current model, separated into four by eight foot panels, and housed in a building constructed solely for this purpose.  It will also represent a larger segment of the river than before, starting at Donaldsonville, and ending beyond Head of Passes.  The team is considering eschewing an angular grid pattern for a more organic patterning along the river.  With the technology of today, cross-sections are gone.  As Project Engineer Rudy Simoneaux says, “We enter the CAD data and press go!”  A router then cuts the channel into each Styrofoam panel.

The size and construction of the new model will allow researchers to examine phenomena beyond the capability of the previous model.  Surface tension of the water in the old model was one factor that prevented the modeling of the West Bay Diversion, for example.  That factor will now largely be overcome in the newer, larger model.  The team is considering cutting the channel deeper to provide space for bed load, so that bed load transport down river can be measured.  They estimate that they will now be able to see turbidity, how water moving through the channel picks up sand and moves it upward through the water column.  Purchasing a router will allow them to create multiple versions of any cell on the grid allowing for constant innovation and adaptation.  The team is gathering all available data on hydrology and sediment to be used to best calibrate the model.  Given the size of the new model, the team is hoping to automate activities such as water levels and sediment input as much as possible.  Other considerations in building this new model include the type of synthetic sediment used, water recycling through the system, and temperature control.

This photograph displays the section of the river which will be replicated in the new model. The old model only replicates the section in the red box.

The design phase will be complete by October of this year, while they estimate another year for construction.  After that, a two-year verification process will go into effect to make sure the model replicates past realities of the river.  Project engineers will run the model making sure that hydrograph data from real-life gauges on the river match what they see on the model.  By then, the next update of the master plan will be one year away.

It is important to note that these physical models are qualitative and just one part of a three-prong approach to modeling the river, which also includes physical observations of the river itself in addition to computer modeling.   The small-scale model does not tell us what will happen at river mile 4.53, for example.  What it does show is trends, including flow lines, scouring and shoaling.  In all, the new model will be a powerful tool for modeling the impacts of major diversions individually and cumulatively as well as the consequences of maintaining the status quo.  This model will aid in forecasting the need for future dredging, examine methods for deep river sediment storage, and anticipate the impact of sea level rise.

Old SSPM

New SSPM

Area

24 x 48 feet (3,526 square miles)

90 X 120 feet (13,945 square miles)

Horizontal Scale

1:12,000

1:6,000

Vertical Scale

1:500

1:400

Cost

$840,000

$4,198,200 (includes facility)

Run time

One hour on model = Two years

One hour on model = One year

 

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