No easy task to prepare flood protection system if hurricane approaches
19th August 2013 · 0 Comments
By Bob Marshall
As the heart of hurricane season approaches, residents protected by the new Hurricane and Storm Damage Risk Reduction System can be excused for feeling a little confident. After all, last year while Hurricane Isaac dealt pain and suffering to communities outside the system, those inside endured only power outages and some downed trees.
So when Mother Nature throws the next haymaker at New Orleans, it’s just a matter of filling the generator, storing some ice, and sitting back while that $14.5 billion, 130-mile ring of levees and floodwalls does everything else, right?
If it were only that simple.
It would have been ideal if this system had a single purpose like the wall around a medieval castle: a solid ring with one gate that would be slammed shut to protect lives when a rampaging horde approaches.
Instead, the U.S. Army Corps of Engineers had to balance public safety with two other concerns: shipping traffic and urban rain drainage.
The result is a system that isn’t a solid wall, but a chain of linked sections pierced by more than 250 openings, varying in size from drain pipes to mammoth navigation gates.
Anyone can see which gates are open or closed by using the Levee Information Management System, a software program created by the Southeast Louisiana Flood Protection Authority-East.
All of them must be closed as a storm approaches the region. Even that isn’t simple, because those closures must follow a complicated procedure spanning 96 hours and involving personnel from more than 10 local, state and federal agencies, as well as the maritime industry. And because the plan must account for changes in tide and wind predictions, it has twists and turns that can make it look like a Rube Goldberg creation.
It all results in a complicated dance in which a misstep could result in parts of the city getting very wet.
“It is carefully choreographed,” said Bob Turner, the engineer who is the regional director of the Southeast Louisiana Flood Protection Authority-East.
“It’s somewhat unique in that the things you have to do in order to mitigate flood risk don’t always have strictly to do with flood control.”
Closures Must Accommodate Shipping Industry
The largest and most challenging part of storm prep deals with the shipping industry. This key part of the area’s economy also creates one of its greatest vulnerabilities to hurricanes. Most of that occurs at the intersection of the Gulf Intracoastal Waterway, the Industrial Canal and the now-decommissioned Mississippi River-Gulf Outlet. Together, this region on the eastern flank of the city is called the Inner Harbor Navigation Canal, which is managed by the U.S. Coast Guard.
These deep canals and their levee systems can trap and channel storm surge. That’s what happened during Hurricane Katrina, when the canals and levees created a virtual funnel that guided water into the Industrial Canal, eventually collapsing its floodwalls, drowning hundreds of people in the Lower 9th Ward.
If public safety were the only consideration, the entire area would be permanently blocked, members of the Southeast Louisiana Flood Protection Authority-East have said. But the Gulf waterway and Industrial Canal are critical to commerce in New Orleans and the Gulf. So engineers had to design surge protection that included these big openings.
That was accomplished through the most expensive features of the system: the Lake Borgne Surge Barrier and its shipping gates at the Gulf waterway and Bayou Bienvenue, and the shipping gate on the Seabrook Canal where it empties into Lake Pontchartrain.
But closing the gates on these shipping lanes doesn’t account for all the industry-related risks. Canal floodwalls are not designed to withstand a collision with a multi-ton ship, so vessels must leave before the storm arrives — yet another complicating factor.
When a storm approaches, the flood-protection authority has daily conference calls with the state Coastal Protection and Restoration Authority, the corps, the Coast Guard, the Governor’s Office of Homeland Security and Emergency Preparedness, the National Weather Service, the Port of New Orleans and shipping companies.
“None of these decisions are made in a vacuum,” Turner said. “We have to monitor predictions for wind speed and direction, tide strengths, storm track and other crucial information. It all bears on the decisions we have to make.”
The choices begin when the National Weather Service predicts that a storm will make landfall in the area in 96 hours.
According to the Flood Protection Authority, here’s the process:
96 Hours From Landfall
1. Crews begin closing gates and valves most vulnerable to rising water, and they certify that gates normally closed are secure.
At the same time, crews are sent to begin closing the bypass barge gate on the Gulf waterway at the Lake Borgne Surge Barrier. The early start here is necessary because of the gate’s complex design.
First, the main Gulf waterway gate must be closed so crews and equipment can cross over to the bypass gate. However, if the current is greater than one foot per second, the motor used to close the barge gate may not be strong enough. So a crew must first close the Seabrook gate, about 10 miles away at Lake Pontchartrain, to reduce current at the barge gate.
Once the current is reduced and crews have reached the barge gate, the main gate on the Gulf waterway is reopened to allow shipping to resume.
It takes about nine hours to close the barge gate. After that, the main gate must be closed again to allow the crew and equipment to exit. Then Seabrook and the main gate on the Gulf waterway can be reopened — for the time being.
2. The Bayou Dupre and Bayou Bienvenue gates at the MR-GO (Mississippi River-Gulf Outlet) must be closed when water in the Central Wetlands reaches one foot. All pumping stations in St. Bernard Parish and one in New Orleans discharge to the wetlands, so managers must save enough capacity to keep rainwater from flooding streets.
3. The shipping gate across Caernarvon Canal adjacent to the Mississippi River diversion must be closed when the water reaches two feet and is rising.
4. The new Bayou Bienvenue Lift Gate on the Lake Borgne Surge Barrier must be closed at some point after the barge gate is closed.
36 Hours From Landfall
5. The Coast Guard begins clearing vessels from the Inner Harbor Navigation Canal area if one of two conditions is met:
• A storm’s impacts are about 36 hours out.
• It’s 24 hours before wind speeds are forecast to reach 35 mph.
The major evacuation route for most ships inside the canal system is into the river, through the locks on the Industrial Canal. The drawbridges across the Industrial Canal must be raised so the ships aren’t trapped inside the canals, where they could knock a hole in a floodwall. But engineers say some of the bridges cannot be safely raised once the wind reaches 35 mph.
The process of raising and lowering the drawbridges is complicated by an evacuation order, if there is one.
6. The main gate on the Gulf waterway and the gates at Seabrook are the last major openings in the system to be closed.
Because Lake Borgne is closer to the Gulf, it experiences storm surge earlier than Lake Pontchartrain. So the corps closes the main gate on the Gulf waterway first.
7. It takes about two hours for the water level in the Inner Harbor Navigational Canal to fall to a level equal to Lake Pontchartrain. When that happens, the Seabrook gates are closed.
The corps will begin this process when the water level inside the Inner Harbor Navigational Canal is between 1.5 and 3 feet and rising.
If a surge is predicted up the Mississippi River, gates in the Mississippi River levee that allow access to the port and ferry landings are closed.
Storms Are Unpredictable
The schedule for closings was developed over months of meetings between the agencies involved. But they all know nature can quickly throw a wrench into those carefully considered plans.
For example, what happens if a storm develops just off the Louisiana coast and is forecast to strike in 48 hours? It’s happened before. Could the barge gate be closed without the 96-hour head start engineers say is required? Could the winds exceed 35 mph before the drawbridges are raised, trapping ships in the Inner Harbor?
Those are the kinds of scenarios Turner said keep him awake at night.
“We may get a prediction of when one trigger point is going to be reached — say the wind at 35 mph — and that may happen sooner than predicted,” he said. “So we have to be ready for those contingencies.
“It’s not a simple system.”
But so far, it has worked.
This article originally published in the August 19, 2013 print edition of The Louisiana Weekly newspaper.