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What We Know Today About Our Nation’s Levees:
    The U.S. Army Corps of Engineers Levee Portfolio Report

The Levee Safety Program has recently completed the first summary report of the flood risks and benefits associated with levee systems included within The U.S. Army Corps of Engineers (USACE) portfolio of levees. Utilizing the best available information gathered from risk assessments, this report provides valuable information that allows for improved management and investments at a portfolio level, including policy and technical guidance, training, and research and methods development. In addition, this report establishes a baseline that allows for future analysis of portfolio trends in inventory and risks…risks that impact your customers, your loan portfolio, your branches, and your staff.

USACE portfolio levees represent an unknown portion of the total levees in the United States. In the National Levee Database there are roughly an equal number of miles of levees that are outside of the USACE Levee Portfolio as are ones within it.

OVERVIEW OF THE USACE PORTFOLIO: The USACE levee portfolio includes about 2,220 levee systems totaling approximately 14,150 miles in length. Over 1,200 levee sponsors operate and maintain 2,000 of these levee systems, spanning roughly 70% of the length of the entire portfolio, pointing out that effective risk management is unlikely without comprehensive approaches of sponsors, communities, and USACE. To complicate matters further, fifteen percent of levees include multiple segments, which usually mean multiple operations and maintenance authorities. Since performance of the levee is only as good as its “weakest link,” understanding and engagement of all parties within a single system is critical.

Much of What We Value is Behind USACE Levees

  • 4,500 schools
  • 300 Colleges and Universities
  • 34 Major Sport Venues
  • 25% of the National Daily Refining Capacity
  • National Historic Sites (e.g., National Mall)

Breakdown of USACE portfolio levees by entity responsible for operations and maintenance and the percentage of miles of the total portfolio.

Risk assessment results to date, LSAC is an acronym that stands for Levee Safety Action Classification. These classifications have five categories (very low risk to very high risk).

OVERVIEW OF RISK: Thirteen percent of the levees in the portfolio are considered moderate, high, or very high risk – levees that require interim risk reduction measures to reduce risk while longer term and more comprehensive solutions are being pursued. Although this is a relatively small number compared to the overall portfolio, people and property are concentrated behind these higher risk levees. Of the 11 million people that are behind USACE portfolio levees, 86 percent of them live behind moderate, high, or very high-risk levees. Most of these levees have multiple risk drivers. Please note: this information is based on completion of risk assessments for 73 percent of the portfolio.

SOME KEY RISK DRIVERS: The graphic below shows the top levee performance drivers.

  • As you can see, overtopping followed by breach is the top risk driver for levees within the USACE portfolio. The likelihood of overtopping varies considerably across the portfolio – from a 1-in-2 chance to a 1-in-5,000 chance of over overtopping any given year, with a majority around a 1-200 annual chance. USACE continues to work to update data and refine hydrologic and hydraulic (H&H) models to refine these estimates, some of which have high uncertainty.
  • Seepage through or beneath the levee is the second most common performance risk driver impacting 17 percent of the portfolio. This risk driver is impacted by the presence of an estimated 16,000 degrading, undersized, or unreliable pipes and conduits.
  • In addition to risks associated with the levee itself, risk assessments take into consideration how vulnerable a population behind a levee is by assessing preparedness of the community to evacuate those behind a levee if needed. For example, 63 percent of communities have either incomplete or non-existent evacuation plans.

Percentage of levee systems with each levee performance risk driver. One system may have multiple drivers.

COST ESTIMATES TO REDUCE RISK: Initial cost estimates, which are agnostic as to who pays, range from $6.5 billion to $38 billion, with an expected cost of $21 billion. One observation from this data is that a relatively modest cost ($300 million) of improving evacuation effectiveness across all levees is a smart investment. Costs are significantly lower than infrastructure improvements and directly reduce risk to loss of life by getting people out of harm’s way.

Communities behind nearly one quarter of the levees have no evacuation plan.

USACE Levees – Only Part of the Picture. As a Nation, we know little about the condition or risks associated with levees outside the USACE portfolio. As such we do not have a true national look at the risks and benefits levees provide to the nation. The Levee Safety Program is working with
Silver Jackets Silver Jackets teams in states across the country bring together multiple state, federal, and sometimes tribal and local agencies to learn from one another and apply their knowledge to reduce the risk of flooding and other natural disasters in the United States, and enhance response and recovery efforts when such events do occur.
to coordinate with states, tribes, local communities, and private levee owner-operators to conduct a one-time voluntary inspection and risk assessment for all levees in the Nation.

Reprinted and adapted from the FEMA News article of the same name by Brad Arcement, P.E. & Noah D. Vroman, P.E.

If you would like to learn more about the risks we are assuming as a nation, as well as at the state level with our levees, the full report is available Here, and the Executive Summary is available Here.