Modern engineering for New Orleans18 January 2013
Tetra Tech INCA’s work on the Lake Borgne Surge Barrier in New Orleans has earned it a prestigious engineering award, as well as restoring confidence in the US city’s flood defences six years after they were destroyed by Hurricane Katrina. Dale Miller, Angela DeSoto and Jane Geisler give more information about the construction of this gated structure.
Following four years of intensive effort, Louisiana's Inner Harbour Navigation Canal (IHNC) Lake Borgne Surge Barrier is complete. The 3048m long surge barrier is the central feature of the world's largest Hurricane and Storm Damage Risk Reduction System (HSDRRS), as well as the largest design-build civil works project in the history of the US Army Corps of Engineers (USACE). Since its conception, the project has received worldwide attention and considerable acclaim from within the engineering industry. As New Orleans successfully rebuilds itself in the wake of Hurricane Katrina, the knowledge gained and innovative methods developed during the design of the surge barrier are serving to inform others across the globe.
Hurricane Katrina and the need for change
In late August 2005, Hurricane Katrina descended on the Gulf, battering the Louisiana and Mississippi coastlines. The resultant storm surge - the highest ever recorded in North America at 8.5m - overwhelmed the city's existing flood control systems and was responsible for 90% of the damage in the greater New Orleans area. Along with other floodwall and levee breaches in the area, the force of the surge collapsed a 1219m long section of the IHNC, a central waterway connecting the Mississippi River with Lake Borgne and the Gulf of Mexico.
Over 233,098km2 were affected and many of New Orleans' historic neighbourhoods submerged under 4.5m of water. Southeast Louisiana was left in a state of crisis, with thousands displaced by the loss of homes and livelihoods.
In the year that followed, USACE moved to implement the US$14.4B HSDRRS. The previous hurricane protection system had existed in name only. The collection of projects constituting the system were left in various stages of completion and provided inconsistent levels of protection throughout the greater New Orleans area. A newer, integrated solution was long overdue.
One proposed alternative to the surge barrier involved constructing a breakwater system on the western edge of Lake Borgne. This was considered an incomplete solution, as the system would need to be augmented by other structures. A second alternative involved raising the existing levees and floodwalls along the Gulf Intercoastal Waterway (GIWW) and the IHNC to the 100-year design elevations. But the negative socioeconomic impacts would be great; hundreds of homes and businesses would have to be relocated due to the levees' increased footprint.
A third alternative, representing the no-action alternative, called for the restoration of the levees and floodwalls along the IHNC to pre-Katrina authorized heights. This was also rejected in favour of the surge barrier as the no-action alternative did not fulfil the project's goal of providing risk reduction for 100-year storms.
The US$1.1B IHNC Lake Borgne Surge Barrier consists of a 2.9km, 7.9m high barrier wall, three gates, and complete floodwall closure of the Mississippi River Gulf Outlet (MRGO). Locating the surge barrier at the confluence of the MRGO and the GIWW moved New Orleans' first line of defence 19.3km from the city centre, eliminating the need to upgrade 48.3km of levees and floodwall. This solution took advantage of the water storage capabilities of the existing geometry, allowing for storage of 6.41Mm3of water. The surge barrier is the only feature in the HSDRRS designed with this provision. Because there is no established industry standard methodology for calculating the magnitude of wave downfall forces, the project team was required to develop new methodologies, consulting with pre-eminent experts from around the world. Wave forces on the barrier wall and sector gate were measured on physical models to successfully verify that the theoretical loads determined by the designers would address wave impact, wave downfall and plunging forces.
The necessity of maintaining an active navigation channel during construction resulted in the design of two gate structures at the GIWW -- a sector gate that would allow marine traffic to pass through the barrier and a bypass gate to be used during construction of the sector gate. Design of these gates was staggered so the bypass gate could be constructed first, while still allowing unobstructed passage through the northern part of the channel.
The hydraulically-operated, buoyant steel sector gate closes a 45.7m wide navigation channel. Most of the gate's weight - more than 675 tons per leaf - is concentrated at the extreme end of its 27.4m radius where the heavy skin plate assembly is located, along with a bridge designed to support an HS-10 truck. The long span requires perimeter support to relieve the load on the hinge at the monolith, which was achieved by providing buoyant tanks behind the skin plate. These tanks form an integral part of the gate to minimise weight and were designed to allow for fabrication in a separate facility prior to insertion into the gate structure. The tanks are ballasted to control contact with the gate sill and have minimal maintenance concerns.
The second 45.7m wide channel at GIWW is regulated by a concrete barge swing gate. In the event of a storm, the 5714-ton structure can be closed, ballasted to a seated position, and locked into place atop four closure pins. Both the barge gate abutment and the sector gate abutment were designed not only for the significant loads the structure might sustain during a 100-year storm, but also for impact loads from passing barges. As of 2012, thousands of barge tows have safely passed without incident.
A third gate - the vertical lift gate at Bayou Bienvenue - closes a 17m wide opening where the barrier wall intersects the bayou. When in the raised position, the 132-ton gate provides 10.6m of clearance from water at an elevation of +0.3m. The gate is powered by generators, with limited shore power to run auxiliary systems. An independent vehicular lift bridge system, located on the barrier's protected side, allows operations and maintenance vehicles to cross Bayou Bienvenue without lowering the gate. The gate was designed with the ability to be closed manually should the need arise.
The barrier wall itself is made up of 167cm diameter, 43.8m long spun cast piles; 45cm closure piles; 91.4cm steel batter piles (to resist lateral movement); both precast and cast-inplace deck sections; and a parapet wall. In addition to withstanding hurricane loads, the entire structure, including all three gates, is designed to resist seismic events.
Collaboration and complications
The project's massive scale of involvement required the close collaboration of over 250 design engineers across the US and Europe. Shaw E&I was awarded the surge barrier contract in April 2008, with Tetra Tech INCA leading the design effort as part of a joint venture with Ben C. Gerwick. USACE members were embedded within the design team throughout. Company responsibilities included:
- Shaw, acting as prime, was responsible for the barge gate abutment design, the bulkhead storage site, civil engineering and dredging.
- Tetra Tech INCA provided design project management services and value engineering services, as well as designs for the GIWW sector gate, the structural monoliths and foundations of the Bayou Bienvenue lift gate, and barge gate approach walls.
- Ben C Gerwick developed designs for the barrier wall, GIWW monoliths, GIWW north and centre approach walls, and structural aspects of the barge.
- Waldemar S Nelson designed the Bayou Bienvenue lift gate.
- USACE's Engineer Research and Development Centre (ERDC) provided navigation simulation and physical modelling services.
- AECOM conducted the project's extensive hydraulic physical modelling and analysis.
- Eustis Engineering Services and Ardaman Associates performed geotechnical services.
- Linfield, Hunter & Junius, in addition to providing architectural services, designed the T-wall, safe house and access roads.
Dozens of companies, many of which are Louisiana firms, were subcontracted to perform construction services. These included Shaw Global Energy Services which fabricated the Bayou Bienvenue vertical lift gate, GIWW sector gate leaves and steel piles. Construction began in December 2008.
Complications caused by the accelerated schedule, procurement requirements and the project site location posed serious challenges but did not impede progress. Due to the aggressive design-build schedule, the project was substantially complete in only four years, as opposed to the 15 years considered standard for a project of this magnitude under the traditional design-bid-build process. Nearly 380 workers were employed on site at the height of construction, with multiple crews working shifts over 20 hour days, seven days a week.
Because of the fast-paced nature of this schedule, decisions regarding materials procurement and fabrication had to be made early to maximise project efficiency. Using a request for fabrication (RFF) procedure allowed engineers to develop contingency strategies to ensure that the piles, pre-cast concrete, reinforcing and other materials could be used in the final design, providing mitigation measures to cost-effectively increase the capacity of the early order materials when the design was complete. This strategy was key in maintaining the rapid pace of the construction schedule while allowing adequate time for design.
In addition to time constraints, the project team's methods were heavily influenced by location. The majority of the surge barrier spans the Golden Triangle Marsh, a sensitive marsh environment accessible only by boat. The soils are alluvial deposits with very poor structural capacity. Furthermore, the size of the waves, storm surge, and wind loads applied in the project's conditions are the largest in the Metropolitan New Orleans system. Developing 100-year-level risk reduction compatible with and considerate of the marsh's ecosystem was a concern at the forefront of the design effort. The project team investigated various alternatives in compliance with the National Environmental Policy Act (NEPA) and incorporated systems and techniques to help minimise environmental impact while providing additional social and economic benefits to the area.
Recovery and recognition
Despite obstacles, 100-year-level risk reduction was attained on 24 May 2011, surpassing USACE's goal for completion by the 2011 hurricane season. The public reaction has been one of great relief. Media interest in the project has helped to inform local, national, and international audiences, restoring a sense of security for both residents and visitors to the city. People are returning to destroyed areas and New Orleans is fast recovering.
Earlier this year, Tetra Tech INCA's effort on the IHNC Lake Borgne Surge Barrier was awarded the American Council of Engineering Companies' (ACEC) prestigious Grand Conceptor Award for outstanding engineering accomplishment. The award - ACEC's top honour - has attracted even more attention to a project already hailed as a model of USACE and industry partnership and a modern marvel of engineering.
The IHNC Lake Borgne Surge Barrier is an example of what can be accomplished when the most innovative science, technology and engineering meet dedication and collaboration. Six years after Hurricane Katrina, the city of New Orleans is better protected than at any time in history and as vibrant as ever.
Dale Miller, Regional Vice President, Tetra Tech. Email: [email protected]
Angela DeSoto Duncan, Director of Civil Works, Tetra Tech. Email: [email protected]
Jane Geisler, Contributing Editor, Tetra Tech. Email: [email protected]