Nature as Specifier and Engineering Partner
Major weather events of recent years have forced engineers to change the way they think about managing the nation’s coastlines and floodprone infrastructure.
By Dan Hitchings, P.E., PGMP, M.SAME
The unexpected destruction caused by major weather events like Superstorm Sandy, Hurricane Katrina and Hurricane Isaac has changed the way we think about managing our coastlines and flood-prone infrastructure. Planners and engineers across the globe recognize that traditional coastal protection and recovery approaches may no longer be adequately resilient or sustainable.
With nearly half of the world’s population living in urban areas in increasingly vulnerable deltas, estuaries or coastal zones, engineers are bringing forward new strategies that work with nature and leverage natural ecosystems as powerful lines of defense. In other words, the world has just hired nature as an infrastructure specifier.
Climate change is already altering the water cycle. Modeling shows that floods and droughts are predicted to become increasingly common and more intense as regional and seasonal precipitation patterns change. That means that while sea level rise and climate change may not be noticeable day to day, these forces will continue to impact coastlines and flood-prone regions more dramatically during major storms. We must envision and build for the increased vulnerability of coastal communities.
The need to minimize the risk from future storm surge and flood events has led to more comprehensive solutions that integrate natural features and forces with engineered infrastructure. While no single answer brings absolute protection from all risks, by making the goal “resiliency” instead of “protection,” we can plan for multiple layers of defense to enable communities to better absorb and recover from major weather events and flooding. Knowing what works will bring new innovation forward.
THE ROLE OF HARD ENGINEERING
Historically, the primary approach to flood control has been “resistance,” building large “hard” engineered solutions. These include dams, sea gates, pump stations, barriers and offshore structures. They are designed, primarily, to hold back flood waters and prevent storm surge from inundating developed areas.
Traditional engineering structures are designed and built to withstand events of a specific magnitude. Yet significant damage to these protective structures can still occur, and potentially catastrophic impacts to the protected areas remain a risk when storms of a larger magnitude occur.
Still, these hard engineering methods do have an important role to play as part of a broader strategy that begins with consideration of the natural environment. If designed thoughtfully, building natural systems into a risk reduction plan has the potential to achieve more than flood mitigation: This approach can improve community protection while minimizing erosion and enhancing native habitats.
RISK REDUCTION AND RESILIENCY
Concerns for the environment and the lack of sustainability of some “hard” engineering approaches have resulted in the emergence of new coastal protection strategies that incorporate natural, socioeconomic and political processes.
Rather than seeking to conquer nature, these approaches recognize the need to mitigate the negative impacts of human activities on the natural ecosystem by developing resilient, sustainable programs to reduce the risks of coastal flooding. These include programs such as the EcoShape consortium’s “Building with Nature;” “Working with Nature” as outlined in PIANC’s position paper; and the U.S. Army Corps of Engineers’ “Engineering with Nature” movement.
Central to these approaches is the concept of resilience: the ability of systems to adapt to changing conditions and absorb shocks while maintaining their structure and functionality over the long term. For example, an integrated coastal management framework in Shanghai, China involves controlled inundation of land by setting back sea defenses.
“Soft” engineering solutions integrate natural and engineered systems to achieve coastal protection goals. More resilient, flexible and adaptable than “hard” approaches, soft solutions work by improving natural systems to create sustainable coastlines and waterways—in effect, mitigating flood risks along the coastline.
One way to optimize the combination of hard and soft engineering infrastructure is to create a plan that incorporates multiple layers of defense, in ways that maximize the benefits of natural features and forces. The recently published New York City PlaNYC, A Stronger, More Resilient New York, resulting from Mayor Bloomberg’s Special Initiative for Rebuilding and Resiliency, studied alternatives for flood-risk reduction in Lower Manhattan and other waterfront areas. Using computer models to simulate various scenarios, including theoretical storms and high-tide and sea-level rise far into the future, the studies will help determine the need for flood-risk reduction efforts and identify coastal intervention features to provide the necessary levels of defense— from hard structures (sea walls and surge barriers) to natural measures (wetlands restoration and beach nourishment).
INNER LAYERS OF DEFENSE— CRITICAL INFRASTRUCTURE
Local inner layers help protect critical infrastructure and integrate water management into urban planning. “Hybrid” systems integrate water management and urban planning efforts, including reconstruction and upgrading of infrastructure such as bridges, tunnels, buildings, utilities, waterfronts and transportation networks.
The flood-proofing, for instance, of individual high-value commercial and industrial business and public facilities that is currently underway in New York City is an example of these hybrid systems. Plans are being developed to protect the critically important hospital complexes in Lower Manhattan. Owners of private buildings are taking the initiative to implement flood-proofing measures as well as making the business operations more resilient. Similarly, on Long Island, a Nassau County wastewater treatment plant is developing and implementing plans to protect their facility from future Sandy-magnitude storms. Taking actions of this nature can become part of a publicly coordinated integrated flood protection plan for selected areas of a community.
MIDDLE LAYERS OF DEFENSE—THE TRANSITION ZONE
Middle layer defenses encompass sustainable coastlines and waterways and combine coastal wetlands, marshes, dunes, wide beaches, barrier islands and mangroves with engineered structures such as barriers, beach fortification and multifunctional levees. Using natural water systems, low lying areas can be used as flood capacity to reduce the effects of storm surges in developed areas. Engineering with nature also can reduce construction costs. Small-scale nourishment of beaches and foreshore with new sand allows wind, tides and waves to distribute sand over beaches and dunes.
Globally, recognition of the need to develop new paradigms for coastal protection has led to several noteworthy projects. On the southern coast of Holland, the Dutch are experimenting with a new sustainable strategy for coastal development, called the Delfland Sand Engine. Constructed in 2011, this project goes beyond the Dutch government’s decades-old policy of “dynamic preservation,” which was aimed at compensating for erosion with regular nourishment of beaches and foreshore with sand. Instead, the Sand Engine project uses a proactive strategy of “mega-nourishment,” depositing 20-million-m³ of sand off the coast, ultimately creating a large sand buffer to cope with rising sea level. Involving complex political and policy-making efforts, this project also will provide ecological and recreational benefits. Projects such as the Sand Engine could be applied within the United States in coastal zones where high wave energy results in strong littoral currents, such as the shore of Long Island or New Jersey.
LARGE-SCALE OUTER LAYERS
Outer layer defenses include large engineered solutions, such as sea gates, pump stations and offshore structures. These also use natural barriers to mitigate flooding. Coastal and delta areas across the globe are evaluating solutions to reduce their vulnerabilities to flooding and storm surge. In 2012, the State of Louisiana released its $50 billion Comprehensive Master Plan for a Sustainable Coast, resulting from a detailed study of flood control and hurricane protection alternatives, including storm surge and wave modeling. Detailed studies and modeling are currently underway to evaluate diverting Mississippi River water to improve storm buffering capacity of the coastal wetlands of southern Louisiana.
The post-Katrina measures to rebuild and improve the flood control infrastructure in the New Orleans area created the Greater New Orleans Hurricane and Storm Damage Risk Reduction System, which provides an unprecedented level of risk reduction from once-in-a-100-years storms. The Seabrook Sector Gate Complex performs a critical dual role. When open, the three-story-high, steel-plated gates allow safe waterway navigation between Lake Pontchartrain and New Orleans’ Industrial Canal. When storms approach, the gates are closed, creating a continuous barrier blocking storm surges from the lake from entering the canal. The West Closure Pump Station, the world’s largest pump station, can pump nearly 20,000-ft³/sec of floodwater from the Gulf of Mexico over the closure barrier. In St. Bernard Parish, hard hit when Katrina’s storm surge overtopped existing levees, the Chalmette Loop Levee Floodwall project strengthened and raised the height of 23-mi of levee reaches, using reinforced concrete T-walls with steel foundation piles.
While these traditional structures only provide risk reduction from the once-in-a-100-year storm surge, they are designed to be “resilient” and to survive the once-in-500-year surge without serious damage.
Resiliency is still a new approach, one that continues to evolve. So, too, will our understanding of the full impacts of climate change and potential devastation of extreme weather events.
As we move forward in our planning efforts we have the opportunity to develop integrated solutions and harness the powers and patterns of nature as specifier and engineering partner.