This article discusses measures being taken by the U.S. Army Corps of Engineers to protect New Orleans from future flood events. The U.S. Army Corps of Engineers is redesigning and upgrading the 350-mile network of linked levees, floodwalls, gates, and pumps in the New Orleans area. The Corps of Engineers’ plan for New Orleans flood control involves upgrading some existing structures such as levees and floodwalls and adding some new risk-reduction features, two of which are movable gates. The proposed Bayou Bienvenue Gate will sit near the junction of the Gulf Intracoastal Waterway and the Mississippi River Gulf Outlet to keep floodwater away from the southern end of the Industrial Canal. The Seabrook Gate will span the northern end of the Industrial Canal where it meets Lake Pontchartrain. By preventing storm surges from entering the Industrial Canal, these gates will protect a densely populated residential and commercial area. However, the presence of the new gates will change day-to-day current patterns in ways that could impact existing structures and use.
In early May, New Orleans residents—along with other residents along the southern Mississippi—held their breaths in anticipation of record flooding.
By mid-May, the opening of the Morganza Floodway in New Orleans and the ratcheting up of the Bonnet Carré Spillway flow to Lake Pontchartrain was keeping water levels in New Orleans right at the Mississippi's official 17-foot flood stage.
The looming 2011 flooding threat brought back ever strong memories of the devastation wreaked in August 2005 by Hurricane Katrina. It was the great cost of that event, in both human and structural terms, that pushed the subject of flood protection squarely into the national consciousness and made the U.S. government consider changes to water management strategies.
Particularly in New Orleans, a major overhaul in water management was clearly required. Within the year, the U.S. Army Corps of Engineers had taken steps to temporarily improve operation of the city's three storm-drainage canals where levees and floodwalls were breached in numerous places during Katrina.
But more permanent steps were required, including new designs for flood gates at key sites in the city's waterways. This is the story of one engineering firm's work for the Corps of Engineers, and its methods of gaining insight into the fluid dynamics of a design.
In New Orleans, a region sitting almost entirely below sea level, Katrina's rains unleashed a triple threat, filling Lake Pontchartrain to the north, swelling the Mississippi River as it twisted through the heart of the city, and inundating canals specifically built to drain stormwater back to the lake.
At the same time, wind and waves surged across Lake Borgne on the eastern city-edge, sending stormwater into the natural funnel formed by the junction of the Gulf Intracoastal Waterway and the channel known as the Inner Harbor Navigational Canal.
Though earthen levees topped with concrete floodwalls lined the banks of the various canals, breaches to these levees occurred for a number of reasons. In some cases, water eroded levee walls from below; in others, the surge simply was greater than the floodwalls. Levee walls gave way in the Inner Harbor Navigation Canal—also called the Industrial Canal—a primary route used for barge traffic between Lake Pontchartrain and the river, allowing floodwaters to inundate portions of St. Bernard's Parish.
To protect New Orleans from future events of equal scale or even worse than Katrina, the U.S. Army Corps of Engineers—which is responsible for the New Orleans levee system—is redesigning and upgrading the 350-mile network of linked levees, floodwalls, gates, and pumps in the New Orleans area.
The Corps of Engineers’ plan for New Orleans flood control involves upgrading some existing structures such as levees and floodwalls and adding some new risk-reduction features, two of which are movable gates. The proposed Bayou Bienvenue Gate will sit near the junction of the Gulf Intracoastal Waterway and the Mississippi River Gulf Outlet to keep floodwater away from the southern end of the Industrial Canal.
The floods that surged through the lower Mississippi valley this spring were some of the largest on record, comparable to the great floods in 1927, 1937, and 1973. Although the proposed Seabrook Gate in New Orleans would not come into play—it will protect the city from Lake Pontchartrain— other engineering projects built over the last century have been employed.
As in 1927, the 2011 flood was the result of record-breaking rains across the South and Midwest coupled with the melting of heavy winter snows. But engineers now have the infrastructure in place and an understanding of the river's dynamics to avoid the most disastrous outcomes, such as the inundation of Memphis or New Orleans. Because of the flood control structures—levees, floodgates, and spillways that were designed to contain the waters from several severe storms simultaneously—the U.S. Army Corps of Engineers estimates that it can guide more than 2.7 million cubic feet per second of flow safely to the Gulf of Mexico.
Two of the most important flood control structures are the Morganza Spillway and the Bonnet Carré Spillway. The Morganza Spillway, completed in 1954, is a 1,200-meter-long concrete barrier situated between the Mississippi River and a floodway leading to the Atchafalaya Basin. The structure sits on an embankment and so is dry under normal conditions. When the Mississippi River floods and overtops its banks, the water rises along the length of the spillway as if it were a normal levee. But if the Corps of Engineers determines that the floodwaters pose a serious threat to areas downstream—at a flow of 1.5 million cubic feet per second and rising—the Corps takes steps to divert some of the flow through the spillway by opening some of 125 gates. When all the gates are open, as much as 600,000 cubic feet per second can pass through the spillway toward the Atchafalaya River.
Before this spring's flood, the Morganza Spillway gates had been opened only once, in 1973. In May, the Corps decided to open 31 gates to relieve pressure on levees around Baton Rouge. As a result, however, hundreds of square miles of land along the Atchafalaya River was expected to be inundated.
The other major floodgate structure is the Bonnet Carré Spillway, which is situated about 15 miles upriver from the New Orleans city limits. The structure, which dates to the 1930s, is a mile and a half long and its 350 floodgates can divert as much as 250,000 cubic feet per second of water. In May, all 350 bays were opened for just the sixth time.
The Seabrook Gate will span the northern end of the Industrial Canal where it meets Lake Pontchartrain.
By preventing storm surges from entering the Industrial Canal, these gates will protect a densely populated residential and commercial area. However, the presence of the new gates will change day-to-day current patterns in ways that could impact existing structures and use.
These massive gates will usually be open for boat traffic but can shut when needed to hold back rising waters. stormwater, channeled from the city into the canal, will flow around the Bienvenue Gate via bypass pipes out into the lake.
The Army Corps of Engineers tapped design and engineering firm Arcadis of Highlands Ranch, Colo., as the prime consultant to minimize the potential for scourt at locations near the Seabrook Gate. The company has designed flood-control structures in St. Petersburg, Venice, and the Netherlands.
Two constraints were central to the design process. First, The gate had to allow for the safe navigation of boat traffic into and out of the Industrial Canal.
Next, flow patterns that resulted from the gate couldn’t cause scour, or erosion, in the vicinity of two existing structures: a 100-year-old drawbridge founded on wooden pilings and the more modern Ted Hickey highway bridge spanning the canal at the junction with Lake Pontchartrain.
Significant erosion in the form of scour holes—holes scoured from the soil by water—was already present in the project area and the Corps wanted to avoid any more erosion.
Ideally, the structure across the canal would consist of static walls on either side of the gate and would accommodate the largest expected ship or barge. Arcadis designers immediately found, however, that blocking half the width of the canal, as they initially proposed, would create currents considered too fast for safe navigation.
By maximizing the span of the opening across the full width of the canal, designers could solve the problem, but the gate and its surrounds would be so large that its construction would necessitate claiming large areas of real estate on either side of the canal, which was not an option.
Nor could the designers narrow or reconfigure the canal itself. As a result, the accepted approach included a central sector gate, shaped somewhat like two pie-slices, paired with vertical lift gates on each side.
The Arcadis design and engineering team conducted computational fluid dynamic analyses of flow through the project area, with myself, a CFD expert, as the lead. The team used Flow-3D software from Flow Science of Santa Fe., N.M., for CFD analysis.
The modeling approach used to carry out this work probably saved about six months and reduced costs by several hundred thousand dollars, the Arcadis team estimates.
Even with years of experience, Arcadis engineers wanted assurance that the proposed flood-control structure would function properly before completing additional mechanical and electrical design work.
The company has previously fast-tracked the design of hydraulic structures using CFD simulations to predict flows. An expert in open-channel hydraulics, Richardson was able to simulate and analyze expected flows through the Seabrook gate for a variety of conditions—such as daily and extreme flow—downstream of the structure.
The team used the simulated results to improve the gate's design. They verified the operation of the final design based on results of scalemodel testing done at the Army Corps’ Engineering Research and Development Center in Vicksburg, Miss.
The Corps then used those results to address some navigational concerns.
The biggest challenge to be overcome was time. To meet the schedule imposed by the Corps of Engineers, the modeling work had to be completed quickly and reliably.
The engineering team carried out about 24 simulations in total. In less than four months’ work, the Arcadis team arrived at what would become the final Seabrook Gate design using the CFD software system.
The quality and productivity of the work dumbfounded even the most ardent skeptics.
The Corps of Engineers has used the simulation results to educate the public about the planned construction as well as to provide quantitative data used to develop plans and specifications for the gate.
Construction of the Seabrook Gate is progressing well and is on schedule for completion in June 2011.