June 17, 2011
What Does the Future Look Like - Avoiding Unavoidable Disasters (Part 3)
by Shalva Marjanishvili & Brian Katz
In my earlier blogs I have attempted to question the current approach to risk, reliability and safety, by discussing how disasters become catastrophes. In this third part of the blog series, we attempt to discuss why disasters are unavoidable in the near future and what can be done.
The current state of building and infrastructure engineering defaults to a definition of "system failure" rooted in the magnitude of a catastrophic event (i.e., too big of an earthquake, hurricane, etc). The result is a system that is tailored to resolve foreseeable stability or protection problems up to a maximum hazard cap… "performance based design". However, observed damage in response to recent natural disasters makes me question whether adequate performance is being achieved. Let’s consider Hurricane Katrina, which was 1 in 400 year storm event. 12-ft levee walls protecting the city of New Orleans were no match for the 14-ft high wall of flood water that threatened the city. It is no surprise that the walls failed and the city quickly flooded, trapping thousands of people. To understand why this happened we need to examine the design process starting from conceptual risk assessment all the way to the development of the construction documents and subsequent levee wall construction.
Similar to other large scale projects, the first step in the design and construction of the New Orleans levees was a risk assessment. This study carefully weighed the risk associated with credible storm threats against the cost of engineering solutions to mitigate associated damage. The final product was an agreed 12-ft levee wall height to protect against a range of expected severe storm and hurricane events. From here, standard design procedures were pursued to develop the explicit system performance criteria and corresponding construction specifications, which serve as a legal document dictating construction requirements. Finally, contractors were able to bid on the project and, based on guidance provided by the project construction specifications, conduct their own design of the levee walls. In theory, 12-ft high wall should be able to “hold” up to 12-ft of water. Furthermore, for the levee design to be economical, a 12-ft wall has to retain exactly 12-ft of water and not an inch more. Following this line of rationale, the New Orleans levee walls were designed to hold exactly 12-ft of water. When Katrina threatened the city with a 14-ft high wall of flood water, the levees were overburdened and failed allowing the entire 14-ft of water to rush into the city.
The correct approach should have pursued a levee wall and foundation system design that precluded the failure of the entire wall no matter what. This concept is commonly known as a “balanced design” and entails engineering of a system to achieve a specified performance independent of the experience hazard/threat. In the case of New Orleans, this design approach translates into the design of 12-ft levee wall that is able to withstand loading corresponding to 12-ft depth of water regardless of the total height of storm flood water. The result is a wall that holds back 12-ft of water and allows the excess to spill over. Although the finished product would not have the appearance of a mitigation solution catered to a specific threat/hazard, the levee wall (in this case) would be uniquely tuned to balance "acceptable performance", cost of recovery from catastrophic damage, and cost of construction. As engineering professionals this should always be the goal… safe, conscientious, and economical design.
We would have hopped to have learned a lesson from Katrina and employed the proper design concepts when rebuilding levees. However, the recent Mississippi River flooding showed otherwise. Championing the philosophy of "bigger is always better", some states along the river had walls that were too high or too strong in comparison with the walls protecting other states from flood waters. The result was a discontinuous system that pushed flood waters to find and take advantage of the "weak links" along the river. Consequently, US Army Corps of Engineers had to step in and use explosives to intentionally destroy some walls, causing flooding of farmlands in order to protect heavily populated areas. The sad part is that all of this could have been prevented with proper design practice, which should have treated the design of all flood walls along the Mississippi River as a single system. This design approach could have identified “fuse like” locations where flooding would have been permissible to relieve swelling Mississippi River.
Most engineers divide disasters into two categories: Natural and Man-Made. We would argue that all disasters are perpetuated by people. With the development of technology, construction of very large and complex structures has been made possible through finite element models. Because the user is able to control the output by manipulating model inputs, the result is often an unrealistic representation of structure performance in response to a hazard scenario. These large models are often used to perform risk evaluation of the project. In our next blog, we will show that there is fatal flaw inherent in this type of modeling.