The new model, which will be approximately 90 ft wide and 120 ft long, will be built on a 1:6,000 horizontal scale and a 1:400 vertical scale. © Mougeot Architecture
The new model, nearly as large as an Olympic-size swimming pool, will enable researchers to study sediment diversion projects.
June 18, 2013—Design work is complete and construction will begin later this year on a working model of the lowermost Mississippi River and its delta that will be nearly as large as an Olympic-size swimming pool. When complete, the model will be used for experiments to inform efforts to divert river water and sediment to replenish and help sustain the vanishing coastal wetlands in the region.
The model is being funded and designed by Louisiana’s Coastal Protection and Restoration Authority (CPRA) as a replacement for a smaller model built in 2003. That model provided valuable qualitative insights into the river, according to Clinton Willson, Ph.D., P.E., M.ASCE, a professor of civil and environmental engineering at Louisiana State University.
“We looked at what the impact would be of diverting river water on the overall Mississippi River hydraulics. When you take water out at a particular location, now you have less water moving in the river further down,” Willson says. “We looked at whether it is more efficient to build and operate one or two very large diversions relative to half a dozen small to medium diversions. Larger diversions were more efficient.
“We also saw things that we can point to, and they match up with what we are learning from field studies of the river and numerical modeling studies of the hydrodynamics and sediment transport,” Willson says.
While the 2003 model was created by hand from templates, the new model, which is approximately 90 ft wide and 120 ft long, will be built using special routers that will carve through a foam board of very high density. A router using computer numerical control (CNC) will be guided by detailed computer topographic models of the area developed on the basis of data from the U.S. Army Corps of Engineers and lidar data from several sources. The new model will also encompass a greater area than the older one and will include the key areas of Bonnet Carré, Lake Pontchartrain, and New Orleans.
The model is highly detailed and features scaled replicas of levees and other important features. “It’s a pretty intense data collection effort,” Willson says, adding that the final model utilizes millions of data points. “You take that file that has the x, y, z points and [insert] board number one into the CNC router. It spends several hours routing out to the bathymetry and topography of the first board. You pull that board out, put in the second board, and it routs that one. We’re talking about routing several hundred boards this way,” Willson says.
One of the key engineering challenges in designing the model is balancing the vertical and horizontal scales and the model sediment characteristics to create parameters that allow the model to mimic the river’s resuspension and transport of Mississippi River sand. The new model will be built on a 1:6,000 horizontal scale and thus will be about twice as large as the model built in 2003. The vertical scale will be 1:400.
“Determining the model and sediment specifications was an iterative process,” Willson explains. Scaling the model hydrodynamics follows Froude’s law with a minimum Reynolds number (Re). Once a horizontal and a vertical scale were proposed, the model Re was checked to ensure that the flow would be sufficiently turbulent to mimic the real thing. The model sediment size and density were then determined on the basis of the grain Re and mobility numbers.
“If the Reynolds number in not high enough to maintain that rough turbulence, then you go back through that process again,” Willson says. “All of that is challenging enough to do on a very large-scale model, and we are applying these fundamental principles on a small-scale model.”
The sediment in the model will represent only the sand in the river, not the silt, the mud, or the clay. The sediment particles are a lightweight plastic with a specific gravity of 1.05, just slightly above the specific gravity of water (1.0). That slim difference in specific gravity means the water temperature in the model will have to be carefully maintained at 74°F to preserve the proper balance between water and sediment density. Since the physical model’s riverbed is movable, it will be overbored, and sediment will be placed in the river bottom before an experiment begins.
Water, along with carefully controlled amounts of sediment added in quantities that will depend on seasonal conditions, will be introduced into the river model and will flow through to the modeled Mississippi delta, as well as through any flow and sediment diversion points being tested. The water will be collected, filtered, and stored in a settling tank to further remove sediment before the water is chilled and then reused.
At the scale used, 1 ft represents approximately 1 mi. At that size, approximately one year of river sediment movement can be represented by just 60 minutes of test time. Thus, six to eight years can easily be tested in a single day, Willson says.
The main goal of the model is to help determine the feasibility, as well as the location and size, of large sediment diversion structures that might be constructed along the lower portion of the Mississippi. These structures are being planned and designed as large-scale land-building restoration features.
“As the agency responsible for protecting and sustaining Louisiana’s coastal communities and ecosystem, CPRA feels that large sediment diversions are the keystone feature behind restoring this portion of our state,” said Garret Graves, the chairman of the CPRA, in written comments to Civil Engineering online. “The Mississippi River system naturally created these wetlands ages ago. By building these structures, CPRA, working with the Corps and other partners, intends to use the river to reestablish that land-building process while reducing annual maintenance dredging costs on the navigation channel.”
“What we are really looking at is the ability and efficiency [of] using river water and sediment to sustain a portion of our coastal landscape,” Willson says. “While these river and sediment diversions may take a number of years to build significant land, this approach is most likely the most affordable and sustainable [one], since it is mimicking the way a majority of the southeastern and central Louisiana coast was built.”
The CPRA and Mougeot Architecture, of Baton Rouge, Louisiana, are currently finalizing plans for a new facility that will house the full model. In addition to space for the model, the facility will include a computer laboratory, offices, a conference room, and a state-of-the art exhibit space that will be open to the public and will give visitors an insight into the history of the Mississippi River and into other coastal and wetland issues. Construction of the facility could begin as early as this winter.
The CPRA, through contracts with BCG Engineering & Consulting, Inc., of Metairie, Louisiana, and Alden Research Laboratory, Inc., of Holden, Massachusetts, is concurrently finalizing the design of the model bed. The CNC router has been purchased, and a small test section of the model will be routed and analyzed in the coming months. Routing of the full model bed should begin later this year. The model panels will be assembled once the facility has been constructed. The entire model and the facility are slated to be fully operational by the spring of 2015.
“In addition to the scientific merit associated with this facility, we want to give the public an opportunity to see what’s being planned, what’s being designed, and what’s been accomplished in terms of coastal resilience in Louisiana,” said Graves. “We’re spending hundreds of millions, possibly billions, of dollars annually on coastal restoration and community resilience in this state. But people aren’t always able to visualize what we’re doing and how we’re doing it. This facility and this model will help solve that problem.”