Levees and flooding
Levees are engineered structures designed to prevent flooding by acting as barriers near rivers and coastal areas. Commonly seen along major waterways like the Mississippi and in cities such as New Orleans, levees are typically constructed from earth and reinforced with vegetation to combat erosion. However, they can be vulnerable to breaches, which occur when water overpowers the structure, leading to flooding of adjacent areas. In the United States, levee systems are maintained by various agencies, including the US Army Corps of Engineers and local levee boards, which are often responsible for their safety and upkeep.
The significance of levees is heightened by climate change, which is contributing to rising sea levels and increased precipitation, exacerbating the risk of flooding. Historical data indicates a notable rise in extreme weather events, posing challenges to existing levee designs and safety standards. Moreover, studies reveal that marginalized communities often bear the brunt of levee failures, highlighting the need for equitable flood management practices. As climate models predict more severe weather patterns, there is an urgent call for reassessing levee safety protocols and exploring alternative flood control solutions to protect vulnerable populations.
Subject Terms
Levees and flooding
Definition
Levees are embankments or engineered structures near bodies of water that are designed to prevent flooding of the land behind them. Permanent levees are used along rivers, such as the Mississippi and Sacramento Rivers, and in coastal areas, such as New Orleans, Louisiana. Once constructed, levees are planted with vegetation such as grass or willows on the river side to protect them from erosion. Concrete abutments may also be used on the river side of a levee to protect the structure from strong currents. A breach occurs when a section of levee is washed away, letting water onto the adjacent land. On large floodplains, levees are built in a series, stepping back from the river, to provide an extra measure of protection against a breach.
In the United States, levee systems along coasts and rivers are built and maintained by a variety of state and federal agencies, including the US Army Corps of Engineers (USACE), state and tribal levee boards, and private groups. For example, in Missouri, a group of private landowners who own a majority of the land in a wetland or another region where flooding occurs may create their own levee district, obligating themselves to pay taxes to finance the building and maintenance of levees.
The National Levee Safety Act (NLSA) of 2007 contains several provisions related to the oversight of levee safety and maintenance, including for nonfederal (state and private) levees. One provision of the NLSA authorizes the USACE to develop a plan to create a national levee safety program and to inventory and inspect all federal levees. Nonfederal levees can be added to the USACE inspection and inventory program at the request of local levee boards, but the maintenance and safety of those levees remains the responsibility of the local board. The NLSA also authorized the creation of a National Levee Database (NLD) and created a National Committee on Levee Safety (NCLS), which consists of representatives from USACE, the Federal Emergency Management Administration (FEMA), and state, regional, and tribal levee boards, and which was in turn charged with creating a national levee safety program.
The NCLS completed its draft reports and presented its recommendations to Congress in January 2009, and the Water Resources Reform and Development Act, which authorized the USACE to implement the recommendations, was signed into law in June 2014. However, a report by the US Government Accountability Office (GAO), released in July 2016, found that while the USACE had been working on completing the NLD, neither it nor FEMA had "taken [any] action on the remaining key national levee-safety-related activities for which they are responsible under the act," which included submitting reports on the state of US levees and the effectiveness of the levee safety initiative, on the "advisability and feasibility" of including dams along with levees in a joint safety program, and on any potential legal liabilities of engineering levee projects.
Significance for Climate Change
The US levee system is a large, complex patchwork of many different structures that have been built by a variety of organizations over many years. Some of the oldest levees in the United States were built more than 150 years ago. Most levees are earthen structures, built with a tapered profile, at a width-to-height ratio of seven to one. Thus, a maximum height must be determined by the time construction begins, based on analysis of historical data regarding maximum local flood heights and storm surges, and the recurrence interval of these events.
Results of climate models, notably those used by the Intergovernmental Panel on Climate Change (IPCC), predict that future increased global temperatures will lead to higher sea levels, increased rates of precipitation, and a greater potential for large hurricanes. A report released by Environment Texas in 2007 concluded that weather data from 1948 to 2006 demonstrated that extreme precipitation events had increased by 24 percent. Together, such weather events have the potential to lead to higher flood levels and storm surges. For example, during the Upper Mississippi Valley flood of June 2008, peak flows were approximately 1.8 meters above those previously recorded. The storm surge from Hurricane Katrina on August 29, 2005, was 7.6 meters. A report by scientists at the University of California, Berkeley, concluded that flooding in New Orleans during Hurricane Katrina was a result of the levees not being built high enough; however, this conclusion has been disputed by the USACE and independent investigators. And in August 2017, Hurricane Harvey intensified over the Gulf of Mexico until the very moment it made landfall on Texas's Gulf Coast—an unprecedented event, as it was the first time in thirty years of record-keeping that a storm west of Florida had intensified, rather than weakened, within twelve hours before landfall. Scientists at the US National Center for Atmospheric Research (NCAR) suggested this could be a result of sea-surface temperatures in the Gulf increasing to between 2.7 and 7.2 degrees Fahrenheit above average. Typically, a storm the size of Harvey would begin to pull in cold water from deeper in the ocean, which would then lessen the energy of the storm; instead, the warmer water allowed it to continue to grow in size and strength until it hit land. The result was a total of thirty-three trillion gallons of precipitation falling on Texas, Louisiana, Tennessee, and Kentucky, the majority of it concentrated on the Houston area, and a resulting "thousand-year flood"—that is, a flood of such magnitude that it has a 0.1 percent chance (1 / 1000, or 0.001) of occurring in any given year—the first ever in recorded US history.
Following Hurricane Katrina, the United States Congress passed the Water Resources Development Act of 2007, which included under Title IX the provisions of the National Levee Safety Act. The direct effects of global warming on levee safety have not been considered in initial plans for levee safety standards. A further concern for levee safety in coastal Louisiana, and in other cities located on deltas, is the combined effects of ground subsidence and projected sea-level rise. One potential solution is to replace levees with other flood-control technologies, such as the movable water gates employed in Japan, England, and the Netherlands.
In 2023, the levee system for the Pajaro River in California was breached by floodwaters, forcing the evacuation of thousands of people. That same year, a study carried out by United Nations University (UNU) found that ethnic minorities and poor communities throughout the United States were disproportionately at risk of levee failure. These risks have increased due to severe weather patterns resulting from global climate change. The UNU urged the United States to consider the socioeconomic and demographic disparities in housing and levee failure risk when planning to mitigate the effects of climate change. It also called for a fundamental reform in the country's flood-management practices.
Bibliography
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"UNU Study Uncovers the Injustice Implications of Aging Levees in the United States." UNU, 5 Sept. 2023, unu.edu/press-release/unu-study-uncovers-injustice-implications-aging-levees-united-states. Accessed 17 Dec. 2024.