BY GEORGE L. SILLS, PE
The word “levee” is derived from the French word “lever,” which means “to raise.” The word levee was brought into the English language through its use in connection with the Louisiana Purchase.
It is believed that the first use of levees for flood protection occurred more than 3,000 years ago in Egypt. Approximately 600 miles of levees were built along the left bank of the Nile River.
Currently, two of the largest levee systems in the world are constructed along the Mississippi and the Sacramento Rivers. Millions of people and billions of dollars of commerce and property are protected by these systems.
The U.S. Army Corps of Engineers (USACE) is responsible for thousands of miles of levee systems that protect millions of citizens across the United States. During the 1860s, particularly on large rivers such as the Mississippi River, floods impaired commerce, destroyed property, and cost lives. In 1879, Congress created the Mississippi River Commission (MRC) to ensure that the best advice from the military and civilian communities would be heard on the subject of improving the Mississippi River for navigation and flood control. After much debate, the Commission decided to rely principally on levees to protect the lower Mississippi Valley.
Levees across the world are generally constructed of soil-in most instances, from the types of soils adjacent to the levee. Levees are generally designed and constructed with the intent of providing flood protection to lowlands for a short duration during typical high water, seasonal flooding, or high-tide events. Basically, a levee is an area where the soil has been placed and compacted to form a barrier to protect against high water. The soil for the levee is generally taken from shallow pits, termed “borrow pits,” adjacent to the levee. Unlike a dam, a levee has water on it only during high river stages. Also, a dam’s location is generally set on a site that provides a favorable foundation, whereas levee sites are determined by flood control needs, regardless of the suitability of foundation conditions. Levees generally have grass on their slopes to protect the soils from erosion during flood events. In areas where the levee must be constructed across a small stream, drainage structures or pumping plants are constructed to provide drainage from the protected side to the river side during normal conditions. During higher river events, these drainage structures must be closed, and the only way to remove water from the protected side to the river side is by pumping.
In some cases, floodwalls are constructed in urbanized areas where wide earthen levee footprints are not practicable because of property or land-use concerns. Floodwalls serve the same purpose as a levee. The two most common types of floodwalls are the T-type and the I-type. T-type walls are concrete walls or stems constructed on a base slab that forms an inverted “T.” The structural members are fully reinforced to resist the loads. On firm foundations, these walls can be constructed on the natural foundation. Over softer soils, they are generally constructed on pile foundations. I-type walls generally consist of driven sheet piles capped by a concrete wall. These walls are generally used when the maximum height is less than eight to 10 feet.
Levee systems are only as strong as their weakest point and, consequently, are generally constructed to fairly rigid standards based on their foundation conditions, which is a function of the local geology and land use. However, even with these standards, the levees will seep or leak water when subjected to a long-duration flood. Hydraulic structures, such as river levees, embankments, and dams, are often built on pervious soils typical of alluvial valleys. The difference between the maximum water level in the river and the tailwater is referred to as the head (H). The change in head-per-unit seepage path length is referred to as the “hydraulic gradient” (i). This gradient forces water to flow under the structure and toward the downstream toe of the structure, as shown in Figure 1.
This seepage flow may emerge vertically at the landside toe of the structure. Where foundation conditions are uniform, the greatest pressure and flow will be concentrated at the landside levee toe, the point where the levee slope meets the natural ground.1 In some instances, the water may seep out gently, leaving the landside ground surface soft and spongy but not endangering the levee. Where critical combinations of water levels, soil types, and foundation stratigraphy (layering) are present, water pressures below the surface or velocity of the water at the surface may be sufficient to move the soil near the levee toe. This condition produces a “sand boil,” or volcano-like cone of sand (photo 1), built from foundation sands carried to the surface by the upward-flowing water. The movement of foundation soils to the ground surface is a condition referred to as “piping.”
 A sand boil. (Photo courtesy of U.S. Army Corps of Engineers.) |
Once a sand boil has developed, it may stabilize itself naturally or it can be stabilized during flood fighting by adding a ring of sandbags around the boil to a height sufficient to cause the boil to seep clear water. Adding a ring of sandbags around the boil artificially raises the head on the boil and decreases its flow rate. Placing sandbags around boils is a standard flood-fighting technique to prevent piping and possible levee failure. If the sand boil is not stabilized and continues to erode the subsoil, it will create cavities in the levee foundation that may lead to settlement, overtopping, or sudden collapse of the levee. Seepage in the soil beneath structures can also sometimes lead to catastrophic uncontrolled subsurface erosion. This erosion is sometimes referred to as a “blowout.” Any time this condition is discovered, it must be addressed to ensure the integrity of the system.
Failures of levees/floodwalls are principally caused by overtopping, surface erosion, internal erosion from piping, or slides within the levee embankment or the foundation. Phenomena related to all of these potential failure modes must be carefully monitored during a flood event. Structures are best monitored by visual inspection or walking along the toe of the levee during high-water events. Four-wheelers or other slow-moving off-road vehicles can help with the visual inspection during flood events. Firefighters patrolling levees during a flood should be observant of the following:
- Structures about to overtop;
- Heavy quantities of seepage emerging from the structure (especially if high up on the levee);
- Sand boils, especially those moving material from the foundation, often visible as sand or muddy discharge;
- Erosion along the structure; and
- Lateral movement of the structure.
If overtopping of a levee or other flood-control structure is a possibility during a flood event, people should be evacuated from the impacted area immediately. Also, if a sand boil that has been ringed suddenly starts flowing greater quantities of flow and this flow has a color change (indicating movement of foundation soils), the area should be evacuated because this could be an indication that the levee/floodwall is about to fail.
Levees are a safe form of flood protection. However, during each flood event, they must be closely watched and a contingency plan must be in place to react to any situation that arises during the flood so that protection for people and property is maintained.
Additionally, it is equally important for public safety to maintain the levee right-of-ways. They must be clear of woody vegetation, animal burrowing, and excessive livestock grazing and monitored for loss of levee height because of vehicular traffic or other agricultural activities and encroachment onto the right-of-way by adverse land-use activities. Public safety during flooding also requires that the area within the flooded portion of the floodplain have sufficient capacity to move the floodwaters rapidly through the flood-management system. Environmental concerns and endangered habitats often result in public pressures to reduce the floodplain capacity to rapidly route water through the area between the levees. These issues can be resolved to ensure that public safety is maintained while ensuring healthful environmental management policies.
Permission to publish was granted by Director, Geotechnical and Structures Laboratory.
Reference
1. U.S. Army Corps of Engineers. >Recommendations for Seepage Design Criteria, Evaluation and Design Practices, report prepared for the Sacramento District, July 15, 2003.
