Understanding Levees, Gradients and Factors of Safety:
Challenges Facing the Industry from a Geotechnical Point of View
The ability of levees to "do their job" effectively has generated a lot of interest, and some confusion lately. At the center of the discussion is the capability of levee systems to withstand a "100-year flood" event or a flood which has a one percent chance of occurring in any given year.
Increased interest in this subject was triggered by a post-Hurricane Katrina study conducted by the U.S. Army Corps of Engineers (Corps) on the frequency of flooding and stage (elevation) of flooding for the Mississippi and Missouri Rivers. One of the results of the Corps' re-evaluation of flood stages and frequency on the rivers is that the elevation of the 100-year event has risen in some cases, especially on the Missouri River. Even for levee districts on the Mississippi, where 100-year elevations have not changed significantly, there are consequences that emanate from changes in the design criteria for gradients.
Lending to the confusion is that some of these levees have already been subjected to events greater than the 100-year flood event and have performed adequately. This fact causes some people to mistakenly assume that nothing needs to be done to improve a levee's ability to protect an area.
The post-Hurricane Katrina study prompted the Federal Emergency Management Agency (FEMA) to re-map areas affected by flooding for the purposes of assessing "more accurate" levels of flood protection insurance requirements. Because these areas can change, all levees must now be recertified at the 100-year frequency to obtain flood insurance. As FEMA's technical partner with respect to levees, the Corps was not comfortable recertifying some levees without extensive reanalysis; hence, the current effort by some levee districts to pursue private levee certification.
Former Regulations and Design Requirements
To better understand the current challenges facing those charged with deeming levees "safe," a look at past regulations is necessary. Many levees were designed and constructed 50 or more years ago using different requirements for levee stability in regards to underseepage than what is required today. More specifically, design requirements have changed regarding gradients at the landside toe, where a levee meets level ground.
Gradients directly relate to how safe a levee is, and can be described in several ways. The most basic method assigns a number relating the ratio of diminishing water pressure (head) with respect to distance. In this case, we are interested in the piezometric head at the landside toe of the levee. The piezometric head is the elevation the water would rise to under its own influence. Therefore, a gradient is the difference in river elevation and piezometric head divided by the distance from the river to the landside toe. More simply stated, as the river surface rises, the piezometric head and resulting uplift pressure at the toe of the levee also increase. A graphic representation of piezometric head is shown as Figure 1.
Figure 1: Piezometric head is affected by water pressure and distance (modified from Corps of Engineers, Fig 67, Investigation of Underseepage and its Control Lower Mississippi River Levees, TM 3-424, Vol. 1, October 1956.)
Gradients can also be described as the piezometric head at the landside toe divided by the thickness of the landside blanket, which is a soil layer that inhibits the upward flow of water. When these gradients approach 1.0, flood water can loosen foundation soils and carry them to the surface in the form of sand boils (Figure 2), which greatly undermine the level of protection a levee can provide over time.
Figure 2: Sand boils occur when gradients approach 1.0 (courtesy of USACE)
Sand boils can begin at gradients of 0.4 to 0.5, and become very severe at gradients of 0.7 and 0.8. Therefore, it is desirable to keep gradients at the toe of levees at 0.5 or lower. Figures 3a and b graphically present the mechanics of underseepage and the formation of a sand boil.
Figure 3a: Levee under normal conditions. | |
Figure 3b: Potential for underseepage due to high water. |
In the past, a distinction has been made between urban levees and "agricultural" levees, those often found in rural areas, and many times on private property. Gradients in excess of 0.5 were allowed at levee toes within "agricultural" levees; today, no distinction is made between urban and agricultural levees, and toe gradients are limited to 0.5 regardless of the levee type.
In cases where gradients exceed 0.5 at the levee toe, remedial seepage measures are required, such as relief wells or seepage berms. The Corps is currently re-evaluating design criteria specifying required gradients for seepage berms.
Factors of Safety
Since there are many unknowns that can't be accounted for in design, engineers use "Factors of Safety" to try to account for the variability that could affect a levee's effectiveness. This is especially applicable to geotechnical engineering, where highly variable soils are the norm. Part of the concept of a Factor of Safety is that there is a limiting condition where the Factor of Safety is 1.0. In the case of levee underseepage, the threshold of unsatisfactory performance (or a Factor of Safety of 1.0) is a gradient of 0.8. Since design requires that gradients at the landside levee toe be limited to 0.5, this results in an equivalent Factor of Safety of 1.6.
In many levee districts on the Mississippi that were once considered agricultural, a landside toe gradient in excess of 0.5 but less than 0.8 is no longer acceptable, even though there may be enough height to prevent overtopping. These areas must be reanalyzed at the revised 100-year elevations to determine whether the underseepage Factor of Safety meets the new 1.6 criteria.
In some levee districts on the Missouri River that were once considered agricultural, there is the possibility that the levees will have to be raised and/or improved with underseepage remediation measures to meet the 1.6 Factor of Safety criteria.
SCI is currently involved with assisting a number of clients in their efforts to assess what steps are required in light of many new regulations affecting levee systems. Of particular relevance in this area is SCI's ability to assist a client in evaluating the status of levee systems as well as performing the analyses required for certification. If you would like more detailed information on these services, please contact Mark Harms, P.E. at 618-206-3002, mharms@sciengineering.com, Dr. Moe Dirnberger, P.E., R.G. at 618-624-6969, mdirnberger@sciengineering.com , or Tom Casey, P.E., at 618-206-3045, tcasey@sciengineering.com.
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