|+ Click to enlarge.|
|Defense. Commission calls for linking railroad embankments to create basins, closing waterways and sinking massive pumps at canal entrances.|
Even as plans to build interim gates and bypass pumps at the mouths of three big stormwater outfall canals in New Orleans advance, the U.S. Army Corps of Engineers is testing a new concept to block the canals with racks of ship propellers to jet water from the city in huge volumes, holding back storm surges in the process.
Responding to a Corps request for proposals, local engineering and architectural firm Waldemar S. Nelson and Co. submitted the proposal, which Nelson would perform under a 30-day indefinite delivery/indefinite quantity contract with the Corps. The RFP sought designs with high-capacity pumps that could prevent the canals from back-flooding without damming them.
The company submitted a design that uses a row of 8-ft-dia bow-thruster propellers like those used to hold oil and gas rigs in position when drilling in deep water. The Corps is testing design viability with a 24-in. valve thruster at Alden Research Laboratory in Holden, Mass.
"It was a no-brainer for me," says Charles Nelson, company president. He says the firm is familiar with the technology from its oil and gas work. "You ask me what’s the biggest propeller I can think of and I say a bow thruster," he says.
Nelson had not presented a cost estimate to the Corps by press time, but a subcommittee of the Bring New Orleans Back Commission, a task force formed by New Orleans Mayor C. Ray Nagin (D), included the plan among six recommendations presented Jan. 18 for augmenting the Corps’ restoration of the city’s storm defenses. Most of the suggestions included time and cost estimates. That plan estimated the barge/bow thruster pump installation at the 17th Street Canal could be built in six months for $10 million to $20 million. Similar installations at Orleans and London canals could be built for a total of $15 million to $30 million, the committee reported.
Other infrastructure modifications proposed to the committee include modifying railroad beds that cross the city to form a secondary system of interior levees and create floodwater containment areas; building jetties across the outfall canals to protect them from surge; closing the Lake Pontchartrain entrance to the Inner Harbor Navigation Canal with a dam and building weirs and storm locks on the Intracoastal Waterway and the Mississippi River to Gulf Outlet channel to block storm surge from funneling into the city from open waters to the east.
"This isn’t a wish list," says John Koerner, chairman of the levees subcommittee. "These are feasible and affordable solutions."
For the 17th Street Canal, Nelson would mount six bow thrusters, with a capacity of 1,350 cu ft per second against a 6-ft head, onto a barge that would be sunk near the mouth of the canal. Diesel generators and electric pump motors, also typical in offshore applications, would provide dedicated power.
The structure would be braced on either side by thrust blocks—dolphin-like structures supported by numerous batter piles. "Once in place, water could rise up to 15 ft above sea level before it would overtop the structure," Nelson says. Lake Pontchartrain’s levels rose to 11 ft during Hurricane Katrina, he says.
Structures proposed for the other two canals would differ because the components must be trucked in to bypass fixed bridges, Nelson says. They would each use eight or 10 smaller thrusters on pontoons, centered in the canals with concrete culvert and sluice gates on either side, he says.
The thruster’s output would nearly equal that of the New Orleans Sewerage & Water Board’s installed pumps, Nelson says. Those are located deep in the city to drain stormwater and throw it into the heads of the canals. Nelson says the design currently being advanced by the Corps would deliver a significantly smaller capacity than his proposal.
"What the Corps has proposed so far is only 1,000 cfs, and 3,000 [cfs] by the end of summer," Nelson says. He says his firm’s design would deliver an estimated 8,500 to 10,000 cfs at 17th Street, 3,000 cfs at Orleans and 8,000 cfs at London. Capacity of the current pumping stations is 10,520 cfs at 17th Street; 2,690 at Orleans and 7,980 at London, Nelson says. "With our concept, 90 to 95% of the time we could pump all the water," he says.
The Corps is testing the concept. For now, plans to build operating gates and shore-based pump farms are moving forward. "If this [proposal] is viable, we have the gates where we can retrofit to put those in," says Fred Young, project manager for the Orleans east bank basin portion of the Corps’ Task Force Guardian, which has a mission to restore the protection system to pre-Katrina levels by June. Walter Baumy, deputy program manager, says the deadline makes it critical that the Corps move ahead with current plans. "Otherwise, we wouldn’t get constructed what we have in hand," Baumy says. "Our timeline is really compressed."
The bow-thruster idea is "interesting and very fast moving," but many questions remain, he says. "There are no ratings on the differential heads, and there are no studies for these because they usually aren’t used for that," he says. The Corps also would like to learn more about the capacity of the pumps and the thrust force against the blocks.
"It’s not just a Corps of Engineers decision," Baumy adds. "We have to have a lot of local input on this, including the [sewerage] board, and the levee district."
Tests on the thruster pump should be completed in three to four weeks, Young says.
While Task Force Guardian rushes to meet its June deadline to restore the Federal levee protection system, the New Orleans District of the Corps is "picking up the longer term mission of how to build up a higher level of protection," says Walter Baumy, deputy program manager for Task Force Guardian. "They are looking at how we raise the system to meet the demands of a category 5 storm."
The levee subcommittee of the Bring New Orleans Back Commission, hopes its suggestions will help. The committee consulted with and received input from numerous sources, including construction industry experts, members of the dock board, the Corps and the Lake Pontchartrain Basin Foundation.
The first component of the committee involves using a secondary systems of protection, primarily raised railroad beds throughout the city. The rail system would act as internal floodwalls to create containment areas, isolating floodwaters, Koerner said. "We unfortunately have undermined all of our railroad tracks with undercutting for overpasses," he added. "We could fix that with overpasses, flood gates or earthen berms. We could also raise the railroad tracks by two feet." Cost and time estimates depend upon which methods are chosen to seal the rail system.
Immediate installation of jetties across outfall canals to protect pumping stations from a direct hit is the second component of the plan. "Jefferson Parish paid $5.5 million for two jetties, so we think for less than $20 million we could add jetties to each [Orleans, London, and 17th Street] canal." The written plan best estimate states "$15 million - $25 million."
The third component suggests something the Corps is already considering: construction of pump stations at the mouths of the canals at Lake Pontchartrain, rather than at the present locations about two miles inland. However, the committee proposed the use of the barge/bow thruster pump design, instead of the gate concept currently supported by the Corps. The barge-mounted system would have a capacity almost equal to that of the Sewerage & Water Board�s designated inland stations, unlike the 17th Street gate design currently being let for bid by the Corps, which will have a much smaller pumping capacity.