Levees are usually earthen embankments or concrete floodwalls, which have been designed and constructed to contain, control, or divert the flow of water to reduce the risk of temporary flooding. Vertical concrete floodwalls may be erected in urban areas where there is insufficient land for an earthen levee.
Most of the levees across the country were built in the middle of the last century by federal, state, and local agencies or by private property owners. The average age of levees in the U.S. is 50 years and many are showing their age. While there are newer or reconstructed levees, a large number of levees were built in response to the widespread flooding on the Mississippi River in 1927 and 1937, and in California after catastrophic flooding in 1907 and 1909.
Every state in America and the District of Columbia rely on levees for flood control to reduce risk to homes, businesses, and property. The nationwide network of levees consists of 30,000 documented miles and up to an estimated 100,000 miles of levees, which protect millions of people in cities large and small. Levees are critical to reducing risk to the public and property from devastating floods caused by the rising of rivers during high rain events or from surge and waves during large coastal storm events. With more than half of the U.S. population living within 50 miles of a coast and continued development in flood plains, levees play a critical life safety role. Unfortunately, because this infrastructure often goes unnoticed, citizens are frequently unaware of the risks associated with possible failure of a levee.
According to the U.S. Army Corps of Engineers’ (USACE) National Levee Database (NLD), levees are found in approximately 35% of the nation’s counties, with nearly two-thirds of Americans living in a county with at least one levee. Earthen embankments make up 97% of all the levees in the USACE Levee Safety Program, while floodwalls make up the remaining 3%. The NLD contains 11,900 individual levee systems accounting for the nearly 30,000 miles of documented levees. The USACE maintains authority over 13,700 miles, while other federal, state, or local agencies are responsible for the remaining 15,400 miles in the NLD. Due to the large inventory of levees outside of USACE’s authority, the condition of the nation’s levees is largely unknown, but future efforts are planned to gain a better understanding of the nation’s levees, as authorized in Water Resources Reform and Development Act of 2014.
The USACE has performed engineering inspections and risk assessments to understand the condition and characterize the flood risk associated with levees in their authority. Currently, USACE has completed risk assessment on over 1,200 levee systems out of the 2,500 in the USACE program. The risk assessment shows that of USACE-owned levees, 5% are high to very high risk, 15% moderate risk, and 80% low risk. The assessments are based on several criteria, including possible loading events such as floods, storms, and earthquakes; level of performance; and consequences of failure. Major deficiencies include culverts, seepage – the biggest risk driver – and vegetation. The numbers of high and moderate risk levees are expected to grow as more inspections are performed, raising awareness of their conditions. Currently, less than half of the levees in USACE’s authority have risk assessment and risk characterizations.
Levees function passively or may require active mechanical operations. For example, some levees have gates and pumps, which may require personnel to operate them in times of floods. Levees require regular maintenance and periodic upgrades to retain their level of protection.Back to Levees