^Map of the IHNC Lake Borgne Surge Barrier – Image Courtesy of the U.S. Army Corps of Engineers at http://www.mvn.usace.army.mil/Portals/56/docs/PAO/FactSheets/IHNC-LakeBorgneSurgeBarrier.pdf
A Monumental Engineering Marvel
Located in the vicinity of and across the confluence of the Gulf Intracoastal Waterway (GIWW) and the Mississippi River Gulf Outlet (MRGO), is the 1.8 mile long Inner Harbor Navigation Canal Lake Borgne Surge Barrier (IHNC-LBSB).
Built in response to the carnage unleashed by Hurricane Katrina, the purpose of this barrier, lying about 12 miles east of downtown New Orleans, is to protect parts of New Orleans from the deleterious effects of a storm surge.
IHNC-LBSB works in tandem with the Seabrook Floodgate Complex located at the northern end of the IHNC near Lake Pontchartrain. The $1.1 billion IHNC-LBSB is part of the $14.45 billion Hurricane and Storm Damage Risk Reduction System (HSDRRS) for southeast Louisiana.
Designers have made it capable of minimizing the brunt of a 100-year, 1% chance annual occurrence event. The intended protected areas are New Orleans East, Gentilly, Metro New Orleans, St. Bernard Parish, and the Ninth Ward.
U.S. Army Corps of Engineers awarded the contract for the IHNC-LBSB to Shaw Environmental & Infrastructure in 2008. The IHNC-LBSB is the largest design-built civil works project in the history of the U.S. Army Corps of Engineers.
In order to achieve the stated objective of 100-year level of risk reduction by 2011, builders started construction even as engineers were finalizing the design of some of its elements.
Authorities put the barrier to use for the first time on August 29, 2012 – the seventh anniversary of Hurricane Katrina – to shield the city from the menace of Hurricane Isaac. The contractors completed all major construction by June 2013.
New Orleans: In the Eye of Hurricane Katrina
Few disasters leave behind an imprint as potent and as permanent as Hurricane Katrina. Utter the word and you will see the faces of many a resident of the Gulf Coast states of Louisiana, Florida, and Texas. New Orleans, Louisiana was particularly hard hit.
Currently, the third most intense landfalling tropical cyclone in the US, the storm left behind 1,245 dead and destroyed property worth $108 billion. Not for nothing does it inspire such dread.
On August 29, 2005, Hurricane Katrina hit the Gulf Coast. In New Orleans, storm surges (large waves that cause coastal flooding) reached a mammoth 9 meters destroying everything that dared cross their path. This, in addition to the 20-30 cm rainfall the storm brought along.
Levee failure i.e. the breakdown of the flood protection barriers was the prime reason for why the storm unleashed mammoth catastrophe. This is because the failure exposed people, houses, and beaches to the full vigor of the storm.
A combination of complicated, interconnected processes, natural and man-made, makes the rate of coastal erosion and wetland loss in Louisiana’s delta plain the highest in the world. The state has lost 4,900 square kilometers of wetlands since 1900.
With an average elevation of six feet below sea level, New Orleans was the worst hit. While the levees along the Mississippi River were robust, those made to restrain Lake Borgne and Lake Pontchartrain were not.
So bad was the surrender of the levees that by 9 am on August 29, low lying areas such as the Ninth Ward and St. Bernard Parish were totally inundated and inhabitants had to seek refuge on rooftops and in attics. The city was in dire need of a sturdy storm surge protection system.
Climate Change @ Coastal Regions
Although opinions on whether Hurricane Katrina was a symptom of Global Warming and Climate Change are divided, there is ample room to believe so. This is because only a sea surface at or above 820F (27.780C) can form and sustain a tropical cyclone.
Humans have designed and built coastal protection structures for over a century. And these have stood the test of time because the design assumptions have largely maintained their relevance.
Rising global temperatures are now conspiring to out date these design parameters. And the effect is such that these structures are shaking down till their very foundation, literally and figuratively. Future developments will not continue with past trends or present events.
Over the past three decades and particularly in the last ten years, global mean sea-level rise (GSLR) has picked up and so has the intensity of tropical storms, storm surges, coastal flooding, high tide nuisance flooding, and the destructive capacity of waves.
Then, there are the extremes of precipitation and temperatures. The former includes excessive and torrential rainfall, river floods, and droughts. We feel the latter through increasingly sweltering summers and progressively biting winters.
Extreme weather events and the natural disasters they bring along are more frequent now than ever before – particularly along low lying coastal zones and vulnerable deltas. New Orleans, for example.
Life, limb, and coastal infrastructure are crumbling before such a combined onslaught like a pack of featherweight cards. The situation calls for a radical change in design approach that must strike a delicate balance.
Balance, because although the destructive potential of such incidents is high, their probability is low to medium at best. At worst, this possibility is precarious. The benefit-to-cost ratio is not immediately apparent.
And while the aforementioned calamitous effects are global, local fallouts vary in accordance with the following local factors:
- Oceanographic: ocean circulation, wind patterns etc.
- Geophysical: faulting, land subsidence etc.
- Anthropogenic: coastal settlements and structures, wetland reclamation, and extraction of oil, water, gas etc.
Storms and global mean sea level rise are the two most potent destroyers of coastal, low lying, and delta regions. After being relatively stable for thousands of years, global mean sea levels jumped by an average of 1.2 mm/year in the 20th century.
For the 21st century, this statistic has already spurted to 3.2 mm/year. By 2100, the global mean sea level rise (GSLR) will be anywhere between 0.2 to 2 meters. Specific examples of higher-than-average rise will further underline the severity of the issue:
- Louisiana Delta: 4-10 mm/year
- New York City: 4 mm/year
- Nile Delta in Egypt: 2-5 mm/year
Cutting down carbon emissions to sustainable levels is of course the long term solution to this annoying complication. In the meantime we have to implement region-specific solutions that offer optimal protection levels within the desired time frames.
This requires us to design structures that while incorporating cutting edge cum most compatible technologies, work well in combination with the natural coastal processes and landforms such as barrier islands and wetlands.
Plus, we have to leave room for handling uncertainties. And we have to plan beyond the usual 50 years, for climate change is here to stay, perhaps for centuries. Already, we can see the effects redoubling their ruinous prowess even as they spread around the globe like wild fire.
Design & Construction Details
IHNC-LBSB is a part of the $14.45 billion Hurricane and Storm Damage Risk Reduction System (HSDRRS) for Southeast Louisiana. With five parishes, the HSDRRS includes:
- 350 miles of levees and floodwalls
- 3 canal closure structures
- 73 non-federal pumping stations
- 4 gated outlets
Designers started IHNC-LBSB with meticulous modeling of wind, rainfall, surge, and waves for 152 artificial storms chosen from among those that have hit southeast Louisiana over the past several decades.
They then interpolated the results from this limited storm set to a larger set in order to forecast the full spectrum of possible storm conditions and to conduct recurrence analysis.
Such analysis gave them the design height of 8 meters, enough to minimize the force of a 100-year, 1% chance annual occurrence event and provide resilience to a 500-year, 0.2% chance annual occurrence event.
At an average elevation of 25 to 26 feet (about 8 meters) above sea level, the IHNC-LBSB structure consists of:
- Composite Floodwall of 1.8 miles long
- Gates, the major ones being:
- Bypass Barge Gate and Flood Control Sector Gate, each of 150 feet width at the GIWW
- Vertical Lift Gate at Bayou Bienvenue of 56 feet width
These three gates lower the intensity of a storm surge coming in from Lake Borgne or the Gulf of Mexico
Another gate viz. the Seabrook Floodgate is a part of the aforementioned Seabrook Floodgate Complex and holds at bay any storm surges entering the IHNC from Lake Pontchartrain
At the GIWW, the Flood Control Sector Gate is the primary passageway for shallow draft navigation. The Bypass Barge Gate serves as a temporary channel for the same purpose
The Vertical Lift Gate at the Bayou Bienvenue offers a passage to and from Lake Borgne for recreational and commercial fishing vessels
There is another gate – the Bayou Bienvenue Sector Gate at the southern end – experts believe, the use of this gate can substantially upgrade the protective capacity of the IHNC-LBSB
- Floodwall Tie-Ins to the:
- New Orleans East Risk Reduction System at the North End
- Bernard Risk Reduction System at the South End
- Approach Walls at the GIWW Sector Gate and the Bayou Bienvenue Gate protect the structure from possible boat collisions
Engineers deposited the organic material dredged from the waterways in the wetland habitats in the vicinity in order to improve the environmental conditions therein.
Construction started with the cutting of deep portions in the channels viz. the GIWW and the MRGO and filling them with rock and sand to serve as the foundation for the floodwall.
While building any kind of structure – wall, building, road etc. – you have to transfer the structure’s weight on to the hard bedrock at or below the ground level. In the final analysis, only Mother Earth bears all the weight.
Marshes and swamps are inundated by water and make such load transfer a civil engineer’s nightmare. You have to therefore make specialist arrangements for laying the foundation.
Then again, you have to provide for the effect of high tides that lift up and wash away sediments at the base of any structure that intends to hold seawater back.
A composite structure, the floodwall consists of a combination of:
- 66-inch circular, reinforced, spun cast, concrete piles driven deep down vertically into the ground
Each of these concrete piles weighs 96 tons and is filled with reinforced concrete. The IHNC-LBSB employs 1,271 such piles and this forms the framework of the vertical face of the floodwall
- Grout was placed between each of these circular concrete piles along with two smaller 18-inch closure piles to fill the gap
- 36-inch steel batter piles support the circular concrete piles. Designers placed these batter piles on the protected side of the wall (the western side). These piles provide additional stability
Personnel drove them into the ground at an angle of 2 horizontal to 3 vertical. Measuring 288 feet long, these are installed in two sections – lower 158 foot driven and upper 130 foot section fitted on top and welded in place
- A precast concrete cap at the top joins the circular concrete piles with the steel batter piles. This cap provides a roadway for operations and maintenance crews
- A parapet wall on the exposed (eastern) flank of the floodwall tops the concrete cap. The wall is similar to the ramparts of forts and castles – the upper edge is not at a continuous elevation but with vertical drops
Such a structure breaks down the incoming surge into smaller waves and thereby lowers its detrimental capacity
Building of the gates started with driving piles deep into the mud. The piles support the foundation. Next, they placed a large cofferdam around the piles and inserted water conveyance pipes to enable the flow of tides when construction was in progress.
Cofferdams are temporary, watertight structures built inside water bodies. You can pump out the water after building a cofferdam and use the dry space for construction or fabrication operations.
Personnel employed the tremie concrete method to place concrete below water level using pipes. This seals the bottom of the foundation. Next, they removed water from the inside of the cofferdam, cut the protruding tips of the piles, and poured in the floodgate foundation.
This was followed by building of the concrete superstructure including control rooms and the steel frameworks to support the gates and provide access bridges. After building the approach walls, they dismantled the water conveyance pipes and removed the cofferdam.
Apart from erecting this mega structure, engineers also tested building pavements with pervious concrete. These allow water to percolate instead of flooding the streets. Forget about hurricanes, even ten minutes of rainfall floods the streets of New Orleans.
City officials and green-builder Make It Right Foundation experimented with this idea. A porous material with an underlying layer for capturing water and oily pollutants, pervious concrete has about 18-20% void spaces.
Rebuilt houses stand on higher, more slender foundations. Their width is low to reduce the surface area for the surge to act upon. This is similar to shotgun houses with maximum 12 feet (usually) width. Such houses are longer with rooms behind one another.
S Jeffress Williams & Nabil Ismail in their article Climate Change, Coastal Vulnerability, and the Need for Adaption Alternatives: Planning and Design Examples from Egypt and the USA posted in the Journal of Marine Science and Engineering make some valuable recommendations.
Their focus is on extending the design’s relevance to a 500-year event. The authors opine that the design height should have been 10 meters as opposed to the present 8 meters. But as this is no longer an option of convenience, they suggest two alternatives:
- Increasing the storage capacity of water that overtops the IHNC-LBSB barrier. This however presents a danger in that if a vessel left inside loses its mooring, it may collide directly with the IHNC floodwall
- Allowing water to flow through the Bayou Bienvenue Sector Gate (not to be confused with the Bayou Bienvenue Vertical Lift Gate) and enter the Central Wetlands will amplify the storage volume of IHNC while not raising water levels
Global Warming is here to stay and so is the rise in sea levels as are their baneful repercussions. We have to live with them and fight them, day in and day out. Structures such as the IHNC-LBSB will be more common in the near future.
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- S Jeffress Williams & Nabil Ismail in their article Climate Change, Coastal Vulnerability, and the Need for Adaption Alternatives: Planning and Design Examples from Egypt and the USA posted in the Journal of Marine Science and Engineering