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Courtesy of U.S. Army Corps of Engineers
Corps of Engineers Conceptual Design map lays out the configuration of the 100-year Protection System.
For the first time, the U.S. Army Corps of Engineers has a conceptual design for a system to provide a 100-year level of storm surge protection in New Orleans. On February 14, the Corps presented a map detailing design elevations for levees, floodwalls and closure structures to provide the promised 100-year level of protection by 2011.
The Greater New Orleans Hurricane and Storm Damage Risk and Reduction System (HSDRRS) is not the final word on the specifics of location-to-location design and elevations, but rather a big-picture road map that will guide ongoing design and construction. "This is a concept for configuration of the system that can give people a sense of scale about the whole system," says Mike Park, program manager for HSDRRS. "What we build invariably will not be precisely this. This is our best expectation of what the gross configuration will be, but the precise design elements are a work in progress."
Mike Park, project manager, says conceptual plan will give people a sense of the scale of the entire system.
Conceptual designs are in the Independent Environmental Report process now and will likely be finalized by the beginning of 2009, Park says.
Achieving the conceptual design has required "a great deal of rigor, including the modeling of 152 storm events and 64,000 hydrographs," Park says. "The elevations you see here are based on probabilistic analysis of a 1% level of risk each year."
Settlement, subsidence, unreliable benchmarks, sea level rise, insufficient levee and floodwall design all contributed to the failure of the hurricane protection system during Hurricane Katrina in 2005, in New Orleans. The details may be found in the Interagency Performance Evaluation Task Force report. "The previous system was authorized in pieces and parts," says Park, explaining why the map reveals inconsistencies, sometimes lower elevation levees, than those at pre-Katrina authorized levels. "What we are building is a system, not a uniform elevation, but a system that accounts for variability and conditions in the environment," Park says. "We are building for a range and more climate conditions."
One example of this is the conceptual surge barrier at Seabrook Bridge, on the Lake Pontchartrain entrance to the Inner Harbor Navigational Canal.
"What gets built at Seabrook depends on the solution at the MRGO/GIWW," Park says, referring to the funnel-shaped geographic configuration that, during Katrina, invited so much storm surge from Lake Borgne into the Mississippi River Gulf Outlet, the Gulf Intracoastal Waterway and the IHNC. Another conceptual surge barrier to reduce that effect is also outlined on the HSDRRS map.
Other conceptual design features include proposed levee elevations, which may be reduced, depending upon the determination of breakwaters and surge barriers. "In New Orleans East we have under consideration construction of a breakwater at elevation 13 or 13 1/2 ft.," says Reuben Mabry, environmental program manager for Task Force Hope. "The approval for that will be tied to the IER process."
The HSDRRS elevations are being designed to account for subsidence and sea level changes over the next 50 years, Park says. "What we are building will still be up to 100-year levels in 2057." The sustainability of the system could be greater, depending upon coastal features implemented as a result of the separate Louisiana Coastal Protection and Restoration program currently under study.
"What we are building here will not be dependent upon the outcome of LACPR," Park says. "LACPR will make this more robust and could make this a higher level of protection than 100 years."
Proposed levee elevations may also vary according to design, says Nancy Powell, chief of hydraulics and hydrologic in the New Orleans District.
"We've worked up some designs where we could lower elevations by roughing slope for wave hits, using rock instead of grass, for example, or flattening slope so water has to run up a longer slope, or adding breakwaters that will change the wave climate," Powell says.
The HSDRRS conceptual design benefits from not only the ongoing hydrology and soils work since Katrina, but the recently completed vertical settlement study that Congress directed the Corps to develop. The report basically offers Congress an explanation for differences in levee elevations in the pre-Katrina authorized levees, says Gary Brouse, a senior project manager in the Corps' Protection Restoration office. "This report is trying to explain differences because of consolidation, settlement and an incomplete system," Brouse says. The report offers better data on regional subsidence and uses better satellite technology to determine accurate benchmarks, both of which proved beneficial in developing the HSDRRS map. "The benchmarks in the area have been subsiding at different rates throughout the region," Brouse says.
"We thought they were stable and immoveable, but now we know they are not immovable because of regional subsidence. Now, with GPS, we have better technology to establish benchmarks."
Although the report was ordered by Congress to gather particular details about the hurricane protection system in New Orleans, "it's a global issue for all levee systems," says Lt. Col. Murray Starkel, deputy commander and deputy district engineer of the Corps' New Orleans District. "You've got to know where you are, to know how to provide the level of protection we are being asked to provide," he says.
Much of what is in the vertical settlement report was information already learned through the various studies conducted since Katrina, says Col. Jeffrey Bedey, commander of the Corps' Hurricane Protection Office. "We may not have been surprised with the outcome, but we validated and verified some of the assumptions we had."