Do Levees Alone Provide Enough Flood Protection? No, They Do Not.
The simple answer is no, but why not?
Levees can be wonderfully effective, but they need to be built and built correctly. Many areas of our coast can never be inside of levee protection because of their location and the expense associated with building levees there. Levees are expensive and have to be justified economically. Areas of the coast that are too sparsely populated make it difficult to justify the construction of a levee. In addition, many areas with low population are also lower on the coast and more exposed to severe storm surge, meaning levees are even more expensive to build and more prone to fail. So most of our coastal land area is destined to be outside of levee protection, while areas with the greatest population will be behind levees (or within levee protection). For those outside of levee protection, other measures are necessary – such as home elevation or possibly the more drastic prospect of relocation.
Levees can protect the majority of the population within coastal Louisiana by protecting densely populated areas such as cities. However, the levees in Louisiana have limited capacity to survive severe storm conditions. Most of the levees are earthen and subject to erosion if waves are significant. Our coastal landscape, including barrier islands and wetlands, helps dampen the waves and energy of storm surge as water moves inland. However, our coast is deteriorating, and, therefore, provides less protection against storm surge, placing more strain on our levees. It might be said that levees protect dense assets like cities, but our coast protects our levees. More accurately, levees only lower the risk of flooding and are not absolute protection. That’s why elevating homes within levees can also be beneficial.
Credit: Lopez, John A., 2006, The Multiple Lines of Defense Strategy to Sustain Coastal Louisiana, Lake Pontchartrain Basin Foundation, Metairie, LA January 2006.
Finally, levees can be overtopped (leading to modest flooding) or even collapse (leading to catastrophic flooding). Levees are really only intended to protect physical assets like homes and businesses, and only up to a 100-year storm standard (or a storm that has a 1 in 100 chance of happening any given year). They are not really meant to protect people or all assets from all possible storm events. For this reason, evacuation is part of the Multiple Lines of Defense Strategy. Multiple Lines of Defense help protect coastal Louisiana using man-made defenses like levees, non-structural protection (like home elevation), as well as natural lines of defense such as barrier islands, ridges, marsh, land bridges, etc. Multiple Lines of Defense includes evacuation for those outside – and inside – of levees because it is the only method of specifically protecting people. Evacuation is the last resort to protect ourselves and our loved ones and should be taken seriously.
Levees alone are not sufficient to reduce flood risk from hurricane surge, but with a Multiple Lines of Defense approach, we can better ensure their success and sustain some measure of a functioning economy, culture and our beautiful wetlands for the coming decades.
This is part 5 of our ongoing series where our experts will answer 10 fundamental questions with new and updated information, so that reasonable and scientifically-sound decisions can be made about the long-term sustainability of the delta and surrounding ecosystems. View an introduction to this series as well as posts on sediment, vegetation, and diversions.