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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Pumping Up Flood Defenses

By R. L. Mullins Jr., Ph.D., P.E., and John D. Take, P.E., P.Eng., ENV SP

Completed in April 2018, the $731-million Permanent Canal Closures and Pumps project represents the final component of the overall Hurricane and Storm Damage Risk Reduction System that serves the greater New Orleans region. The three massive pumping stations that compose the project are designed to reduce New Orleans's risk of flood surges from Lake Pontchartrain and help the low-lying city maintain its critical drainage features in the event of a major storm.

One of America's most iconic cities, New Orleans enjoys international renown as the home of jazz and Bourbon Street and is famous for its welcoming, open-arms brand of hospitality. Unfortunately, New Orleans often finds itself in the path of hurricanes and tropical storms, making the low-lying city as flood-prone as it is iconic. In 2005, Hurricane Katrina roared through the Gulf of Mexico and overwhelmed the city's flood defenses, inundating nearly 80 percent of New Orleans.

The city took the brunt of the storm, which overall claimed more than 1,800 lives, inflicted more than $100 billion in damages, and displaced hundreds of thousands, many of whom were among the most vulnerable citizens. As a result, Congress directed the U.S. Army Corps of Engineers to overhaul the flood defense system in the greater New Orleans region. The federal government and the state of Louisiana's Coastal Protection and Restoration Authority (CPRA) together contributed more than $14 billion to this effort.

Known as the Hurricane and Storm Damage Risk Reduction System (HSDRRS), the network of levees, floodwalls, floodgates, and pump stations was designed to withstand the surge from a storm that has a 1 percent chance of being equaled or exceeded in a given year, commonly known as a 100-year storm. The final piece of this massive system was the Permanent Canal Closures and Pumps (PCCP) project, a $731-million effort encompassing three enormous pumping stations and gated flood closures that was completed in April 2018.

The city of New Orleans is drained primarily by three main outfall canals that flow north from the city into Lake Pontchartrain: the 13,500 ft long 17th Street Canal, the 11,000 ft long Orleans Avenue Canal, and the 15,000 ft long London Avenue Canal. During Hurricane Katrina, rather than draining the city, the 200 ft wide canals provided conduits for the storm surge from the lake, which rushed into the city and caused well-documented levee and floodwall failures before classic overtopping conditions occurred. The existing storm pumping systems, some of which were dated and in poor operating condition, could not keep up with the interior drainage and significant contributions from levee breaches. Parts of the city became a bathtub behind the levees.

More than a major construction program, the HSDRRS comprised a philosophy and a set of design guidelines that were the result of a top-to-bottom rethinking of the Corps's Gulf Coast levee design approaches, with the changes based on lessons learned from Hurricane Katrina and its aftermath. Along with bolstering the existing storm defenses and repairing levees, the Corps constructed interim control pumping station structures at the mouths of the three outfall canals to serve as surge barriers while enabling the conduits to evacuate drainage from the city. These structures addressed the immediate need to reduce risk to the community while a longer-term solution was planned, developed, and implemented.

Because of their temporary nature, the $400-million interim control pumping station structures were developed to have an intentionally short design life, relying on as many off-the-shelf components as possible. Operational by the start of the 2006 hurricane season, the interim structures had a pumping capacity of 9,200 cfs at 17th Street, 2,200 cfs at Orleans Avenue, and 5,200 cfs at London Avenue. The Corps deemed these capacities sufficient until the PCCP project could be completed. Indeed, the facilities served the community well despite the operational challenge of frequent equipment replacements.

While it was constructing the interim control structures, the Corps began the process of devising the long-term solution-the PCCP project-with the goal of isolating the outfall canals permanently from Lake Pontchartrain and increasing lakefront pumping capacities and resilience. After developing a request for qualifications and then short-listing certain firms, the Corps issued to the firms in fiscal year 2012 a request for proposals to deliver the project by means of a performance-oriented, fast-track, design/build (D/B) approach.

Although the Corps's Military Construction Program has employed D/B for many years, this approach was relatively new for its Civil Works Program. Given the large scale of the undertaking, the D/B approach offered certain advantages, including encouraging innovation and increasing speed of delivery. Rather than providing meticulously detailed plans and specifications dictating how to proceed on the project, the Corps stipulated its project requirements and let the contractors determine how best to address them.

The Corps awarded the D/B contract to PCCP Constructors, a joint venture consisting of Kiewit Louisiana Co.-a subsidiary of the Kiewit Corp., of Omaha, Nebraska-as the lead, along with Traylor Bros. Inc., of Evansville, Indiana, and the M.R. Pittman Group LLC, of St. Rose, Louisiana. As the lead architect and engineer, Stantec Inc., headquartered in Edmonton, Alberta, Canada, was assisted by a group of specialty consultants, including Fugro Consultants LLP, a global engineering firm that has its U.S. headquarters in Houston, which completed geotechnical designs, and PND Engineers Inc., headquartered in Anchorage, Alaska, which was responsible for the design of temporary works.

The Corps served as the lead agency for the project in partnership with the CPRA and also involved such local interests as the Sewerage & Water Board of New Orleans and the Southeastern Louisiana Flood Protection Authority-East. In this way, the team brought citizen and stakeholder interests to the table and served the interests of cost-sharing partners.

Among its key project requirements, the request for proposals stipulated the pumping capacities for the main components of the PCCP project: 12,500 cfs for the 17th Street pumping station, 2,700 cfs for the Orleans Avenue pumping station, and 9,000 cfs for the London Avenue pumping station. As a result, the pumping stations constructed as part of the PCCP project would have significantly larger capacities compared with those of the interim control pumping station structures they were to replace. In fact, the combined PCCP project facilities would amount to one of the largest drainage pumping systems in the world.

Additionally, the new permanent stations also had to protect against storm surge to an elevation of 18 ft above sea level, as measured by the National Geodetic Vertical Datum, and enable personnel to operate safely for up to five days without access to the electrical grid or other supporting utilities. The project team had to plan for vital components, such as pump station walls, to achieve a design life of 100 years. The Corps also required that the pump stations be designed in such a manner that they could be adapted efficiently in the future to accommodate an additional set of design criteria conditions primarily related to increased flows and deepened outfall canal profiles.

At the outset, PCCP Constructors and Stantec made a strategic decision to create a single solution for the three sites. This approach entailed designing the major storm pumps first, and then continuing outward to the rest of the station. In this way, the team could standardize pump and generator sizes and approach site layout and building configuration similarly while remaining sensitive to individual neighborhood aesthetics. The approach offered significant advantages for procurement, logistics, construction, and long-term operation and maintenance. Standardized equipment simplified supply chain issues for the contractor, enhanced field crew learning, and improved the cost-effectiveness of operator training, maintenance, and the purchase and storage of replacements and spare parts throughout the project's life cycle.

The PCCP project was delivered in a transparent, collaborative manner. Key representatives of the Corps, the CPRA, PCCP Constructors, and Stantec's design team were colocated in New Orleans during design and construction to build a cohesive project delivery team, facilitate communication, and quickly address issues as they arose. Stantec also used a distributed design delivery model that included some 500 team members from more than 50 offices across North America, in addition to 16 specialty subconsultants.

To illustrate the single three-site approach, the figure on page 54 shows an aerial photograph of the 17th Street station with significant components labeled. These components are typical of the single solution. All stations were designed for the hydrostatic water, boat-impact, wave, 200 mph hurricane wind, and unbalanced loads that the stability analysis indicated would be necessary for each structure.

Because of the soft soils in Southeast Louisiana, H piles and pipe piles were needed to support the station foundations, which were up to 120 ft deep. Overall, 48 lineal mi of piling was placed to support the three stations. Sheet pile walls were also installed up to 60 ft deep below all foundations to cut off any potential seepage from Lake Pontchartrain to the canal side. The buildings' lake-facing walls are integrated with the foundation system as part of the surge barrier, and, as noted above, can resist significant unbalanced loads during times of elevated lake levels.

The T walls facing the lake are connected to the concrete bypass gate structure, which moves flows by gravity out of the canal and into the lake during nonstorm events. The bypass gate structure consists of a concrete substructure and superstructure, which includes slots for typical 14 by 15 ft steel gates. Three means of positive gate closure-hydraulic actuators, manual (by means of gravity), and handheld powered actuators-were provided, and all gates can be closed rapidly within 30 minutes. When closed, the gates form part of the overall surge barrier, which incorporates the adjoining pump station, auxiliary/control building, and generator building in addition to the levees and concrete T walls. This overall line of protection isolates the canal from Lake Pontchartrain, effectively reducing risk to the city. Cast-in-place concrete T walls tie the generator building back into the existing line of protection, and 24 ft wide steel gates form the closure structure at the entrance to the pump station complex where it connects to the roadway.

Cast in approximately 50 ft long sections, each concrete T wall monolith has a width ranging from 10 to 17 ft, a 4 ft deep base slab, and a 2.5 ft thick cantilevered stem wall ranging in height from 10 to 12 ft. All T wall monoliths are founded on closely spaced battered steel H piles that extend from the bases to approximate depths of 100 ft with seepage cutoff sheet piles extending to depths of 50 ft.

Each bypass gate structure has a 40 ft wide, 4 ft deep base slab supported on battered 24 in. diameter steel pipe piles that extend from the base to a depth of approximately 100 ft, with seepage sheet piles extending to a depth of 50 ft. The 2.5 ft thick, 17 ft high bypass walls extend from the base slabs and support 3.5 ft thick, 9 ft high flood protection walls that run along the length of the bypass structure and connect to the pump station and T walls at either end.

The pump station structures each include a 125 ft wide, 6 ft thick foundation slab located approximately 40 ft below grade. The foundation slab is supported on 24 and 30 in. diameter steel pipe piles extending to depths ranging from 70 to 100 ft, with seepage sheet piles extending to depths of approximately 20 ft. Ranging in height from 50 to 60 ft, the pump station foundation walls have a thickness of 24 to 42 in. and, together with the main floor slabs, support the rigid steel frame building superstructure.

Each adjoining auxiliary/control and generator building has a 90 ft wide, 3 to 5 ft thick slab supported on 24 in. diameter steel pipe piles to an approximate depth of 100 ft, with seepage sheet piles extending to an approximate depth of 60 ft. Steel-frame superstructures are supported by the foundation slabs.

Normally, the stations are operated from the auxiliary/control building, where operators can live for five days with no assistance from the outside world. However, the stations can be controlled remotely, if necessary.

OPEN CELL cofferdams, a proprietary product of PND Engineers, were used to provide temporary dewatering areas for construction activities and formed part of the permanent works, serving as the retaining walls on the pump station side of the canal. Unlike a conventional closed cofferdam that uses struts or similar interior features, the OPEN CELL cofferdam employs steel sheet piles that require no internal bracing, maximizing construction workspace. A generous amount of riprap was employed for erosion control both on the banks and in the canal.

The storm pumps are the heart of each drainage station. The Patterson Pump Co., of Toccoa, Georgia, designed and supplied the 1,800 and 900 cfs equipment used for the PCCP project. To put the project in perspective, the combined pumping capacity of the three stations could fill an Olympic-sized swimming pool in fewer than 4 seconds or the Mercedes-Benz Superdome, home of the National Football League's New Orleans Saints, in fewer than 90 minutes. Although the pumps normally are powered by the local electric service, a fleet of Caterpillar generators is available to provide backup power. Having a total capacity of 78 MW, including redundant capacity, these diesel-powered generators could power a city of about 50,000 people for five days with the fuel stored on-site.

Informing this design was exhaustive three-dimensional computational fluid dynamics (CFD) modeling. Although a powerful tool used to study and predict the movement of fluids, until recently, CFD has been applied most commonly in the automotive and aviation industries.

The Stantec hydraulics team successfully used CFD to model dozens of water and flow scenarios. For example, the team modeled how fast water is pumped into the lake during a storm event, where water will go when pumps start under normal conditions, and what happens when a wave from Lake Pontchartrain meets the barrier walls or rotating pump machinery.

Armed with intricate knowledge of how water is expected to flow and behave at the outfalls, the project team moved to the next crucial test of the design: building a physical model. The innovation of seamlessly combining 1-D, 2-D, and 3-D CFD computer modeling with 1:16 and 1:10 physical scale modeling provided insight into how the design would perform. Together, these models accurately portrayed equipment performance and the resulting conditions at the outfalls. The exhaustive modeling and testing protocol of this complex system also helped the D/B team meet the demanding project delivery schedule.

Construction was completed on schedule, accounting for time extensions associated with change orders. PCCP Constructors turned the project over to the Corps in April 2018, in time for the June 1 start of the hurricane season. The Corps paid the up-front costs of the project, and the CPRA entered into a cost-sharing arrangement under which it agreed to repay a portion of the cost over time. The Corps will decommission the interim control pumping station structures under a separate contract.

The station complexes can be used during hurricane events, tropical storms, and other severe weather-related emergencies, as needed. When the pump capacity is not required, flows leave through the bypass gates and enter the lake via gravity.

The stations can be adapted to possible future conditions without major civil or site reconstruction or the need to change such components as the climber screens, intakes and discharges, or superstructures/bridge cranes. Some pump components, including the impellers and motors, would have to be up-sized. However, these changes can be accomplished one pump at a time without taking the station off-line. Spare pump bays afford the necessary space for future dry weather pumps should the local sponsor want to make these upgrades. If future conditions warrant, the bypass gates can be readily decommissioned by placing reinforced concrete in the existing bulkhead gate slots. Making these changes would result in an almost 15 percent increase in pumping capacity.

Thankfully, the infrastructure added as part of the PCCP project had not been tested by a major storm event as of January. Commissioning was uniformly successful, and the team is confident the system will perform as designed. Client and community feedback have been very positive.

In September 2018, the entire team was thrilled when the PCCP project was featured on the History Channel's Project Impossible, a show that celebrates the ingenuity and excitement of engineering. A professional film crew captured not only the engineering complexity involved but the importance of this infrastructure to members of the community, some of whom live just yards from the outfall canals.

The single three-site solution approach, as established in the original scope of work, was another factor that proved critical to success. These early concepts established an efficient design delivery process. More importantly, the approach enabled timely procurement, construction, and delivery of such large equipment as pumps, generators, and bridge cranes. PCCP Constructors set up supply chain agreements during the bidding stage to lock in key subcontractors and equipment delivery schedules to support overall project delivery.

The station designs included resiliency measures to facilitate rapid recovery after a major storm. For example, during Hurricane Katrina, as levees were overtopped, splash pads or similar structures were not present to prevent the protective works from being undermined. The splash pad is typically a section of riprap designed to protect the foundation of the T wall from being undermined by erosion from waves overtopping the wall. For the PCCP project, the splash pad consists of varying depths of riprap-typically 2 ft or more-and extends up to 25 ft out from the base of the T wall foundation. Depending on the location of the wall, water either runs back into the canal eventually or flows to a drainage inlet to be pumped into the lake. Water overtopping the project floodwalls from Lake Pontchartrain is directed back to the main storm pumps for transmission back to the lake. These types of considerations were included in the design of the PCCP project.

Finally, the project team realized the importance of setting realistic expectations among the public and stakeholders. Although an integral part of the overall effort to reduce risk to the greater New Orleans region from storm surges, the PCCP project cannot by itself solve the area's flooding problems. However, communicating this distinction to the public is not easy. Often the public will misunderstand the concept of a 100-year surge event, thinking that such an outcome will occur once a century.

Engineers know that the 100-year storm surge is a probabilistic construct and that it is possible, though unlikely, that multiple 100-year or larger storms could occur within a single year. The PCCP project was designed to provide a measure of risk reduction up to and including the flow rate criteria and prescribed storm surge lake levels. As such, the project will perform a long-term critical mission for this vulnerable community.

Reducing a community's risk from flood damage is difficult in any environment. It is even more challenging in Southeast Louisiana, with its soft soils, history of hurricanes, and potential for increasingly worse storms, in addition to the need to respect historical and aesthetic considerations. Building on lessons learned by the Corps after Hurricane Katrina and applying the most modern design techniques and construction approaches, PCCP Constructors and Stantec developed a three-in-one solution, designing from the pump out. This approach provided the requested level of risk reduction for much of the city while accounting for structure longevity, operator safety, transparency with surrounding businesses and homeowners during construction, resilient recovery after storm events, and compatibility with neighborhood aesthetics. During this D/B project, team partners advanced the current state of flood reduction design and deployed a highly effective distributed resource model for design delivery. These results are already being applied to other projects across North America.

PROJECT CREDITS

Owners U.S. Army Corps of Engineers, Louisiana's Coastal Protection and Restoration Authority, and the Southeastern Louisiana Flood Protection Authority-East
Contractor PCCP Constructors, a joint venture comprising the Kiewit Louisiana Co., a subsidiary of the Kiewit Corp., Omaha, Nebraska; Traylor Bros. Inc., of Evansville, Indiana; and the M.R. Pittman Group LLC, of St. Rose, Louisiana
Lead architect and engineer Stantec Consulting Services Inc., Edmonton, Alberta, Canada
Geotechnical consultant Fugro Consultants LLP, Houston
Temporary works design PND Engineers Inc., Anchorage, Alaska
Physical hydraulic modeling Northwest Hydraulic Consultants, Edmonton, Alberta, Canada

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