SHOULD I TRUST THAT LEVEE?

J. David Rogers, Ph.D., P.E., R.G., C.E.G.
Karl F. Hasselmann Chair in Geological Engineering
Department of Geological Sciences & Engineering
Missouri University of Science & Technology
129 McNutt Hall, 1400 N. Bishop Ave.
Rolla, MO 65409-0230

We’ve all been there; glued to our TV sets watching the roving reporter in their raincoat, standing by some levee that’s flooded in the past, asking: Will it happen again?

Each time it begins to rain with intensity it seems the media begins using roving reporters to broadcast “flood watch” segments. We all know the spots, usually near levees or bridges, where flooding has occurred in the past. Along the Mississippi River near Cape Girardeau, MO or the Sacramento/San Joaquin Delta in northern California, we are given a glimpse of levee patrols, the occasional sand boil, and sandbagging operations that seem weak and ineffective when compared to the scale of the high flow onslaught. Interspersed with these images of the here-and-now are re-runs of past flood damage, usually in the same locations, just to remind the viewers of what’s happened before. Combined with pessimistic weather predictions, these are the images used to advertise the news when it’s raining hard.

WHAT ARE LEVEES ?

Rivers naturally prone to peak flood flows during spring runoff develop natural overflow levees, deposited when the low flow channel overtops its channel (see Figure 1). Natural flow levees are typically comprised of more coarse grained sediment, like sand, while the flood plain (Figure 2) is usually covered in extremely fine silt. 75% of all the sediment deposited upon the continent is overbank silt, deposited upon well-defined flood plains.

Unfortunately, we have concentrated much of our societal infrastructure upon these flood plains, and have thereby placed such property in peril anytime the rivers begin to reach flood stage. In their natural state rivers tend to flow onto their respective flood plains during peak runoff periods, generally during the late spring months in North America. However, sustained precipitation can cause flooding throughout the summer, depending on antecedent soil moisture levels.   During the Lower Missouri River Flood of 1993 the river remained at flood stage for over 6 months.

Figure 1 - Natural levees are formed along the margins of the low flow channel whenever

a river overflows its banks, usually during spring runoff.

Figure 2 - Maximal loading of a flood protection levee in Mississippi during the flood of 1973. 
Note school bus for scale and the seepage along the landside toe of the embankment.

HOW ARE LEVEES DESIGNED ?

In the 19th Century most towns were sited next to the major rivers so that river-born commerce could access the community. Levees, or dikes comprised of earth, were soon built upon the river’s natural overflow levees as a means to provide protection from flood flows (Figure 2). In the Sacramento River Basin these levees began to become more and more numerous following the record 1862 floods. In order to finance flood control levees individual “reclamation districts” began to be formulated in the mid-1880s. The State became more involved in flood control following disastrous flooding of the lower Sacramento Valley in 1907 and 1909, which destroyed most of the Corps of Engineers built flood/debris catchment structures built a few years before.

Local reclamation districts and the Army Corps have since joined forces to gradually heighten and extend great systems of levees along those rivers normally prone to flooding, taking thousands of acres of flood plain lowlands and “reclaiming” them for agriculture. Major sequences of Federal-aided levee construction occurred in funding "sequences," in the 1930's and again, in the late 1950's thru early 60's, providing the system of levees we see today. Following the disastrous 1927 and 1937 floods along the lower Mississippi, the Corps began to design levees for so-called “project floods”. The project flood is an acronym given to that level of flow felt most cost effective to handle, given the constraints of risk, development within the floodable area, and the consequences of failure. For most of their work the Corps ended up choosing project floods that were about half of the “probable maximum flood”, or “PMF”. The PMF is the maximum flood that could conceivably ever occur (under natural conditions), say once in a thousand years. A PMF scenario usually involves the arrival of warm tropical moisture (precipitation) over a near-record snow melt, causing both rainfall and stored moisture to come tumbling down the channels in one combined mass.

Figure 3 - Generalized geologic cross section through the west bank of the Mississippi River near Arkansas City, AR. Overbank deposits on the flood plain create

a relatively impervious cap above the sands and gravels lying below.  The deeper coarse grained materials were deposited by the river and are much more

pervious.   Borrow pits excavted into the silt to construct the levees can serve as dangerous percolation conduits during high flow.

Figure 4 - Sand boils occur when upward-percolating seepage forces exceed the effective confining
pressure of the soil cap.  Boils can occur as conical spouts (shown at right) or linear ridges (shown at left)
along the landside of the levee, and are usualy noticed prior to seepage-related failures.

Figure 5 - Sand bags are commonly employed around recognized boils during floods to increase
pressure head on the exit point by ponding water over the point of discharge.  This serves to reduce
the excess pressure head and diminish hydraulic piping of fines from the levee foundation.

CAN LEVEES BE DEPENDED UPON FOR FLOOD CONTROL ?

Being added onto every few decades, levees are not engineered with the same precision, care, cost and engineering as other hydraulic structures, such as dams. In addition, levees extend for countless miles along major river channels, traversing a heterogeneous assemblage of foundation conditions: crossing old channels, sloughs, and oxbows perturbing any flood plain. The only constant of levees is the inconsistency of their foundations. Because borrow pits and gravel quarries are generally situated next to levees, a situation arises where seepage pressures are introduced into the underlying flood plain beneath the levee, and the duration of flooding dictates how long and how far elevated seepage pressures will percolate beneath such embankments. This situation is sketched in Figures 3, 4 and 5.

It only takes one break at a single discrete point to flood the lowland protected by an entire system of levees. Given the variability of the levee foundations, seepage-induced breaks are inevitable, especially in areas subject to localized fluctuations of the water table, such as occurs when agricultural irrigation practices change, or fields are left fallow (without summertime watering). In the delta area the underlying soils are also consolidating each year, causing differential settlement of the levees they support.

Longer duration floods tend to spawn greater hazard to levees than short-term peak flows, as the longer the waterside of the levees is inundated, the further flood-induced seepage can travel, beneath the levee, and increases the possibility of destructive boils being formed on the landside of the levee. It is for this reason that reclamation districts, responsible for levee maintenance, employ crest patrols during flood stage, to continuously inspect the landside toe of the levees for onerous levels of seepage. Such patrols have limited ability to make immediate response, that usually consisting or placing sandbag walls around boils to increase confining (hydrostatic) pressure at the exit (boil) point (Figure 4).

Judicious placement of sandbags around boils can dispel disaster, but not in every circumstance (Figure 5). In a growing number of cases studied by the Corps of Engineers, short-term, semi-explosive decay of levee integrity has occurred, due to excessive straining of any foundation materials subject to an upward-flowing seepage front, usually emanating from an old, buried channel (lying beneath the levee). In these cases, routine patrols with sandbags may be insufficient to dispel a disaster.

Levees can also fall victim to to undertow scour, a common problem in sandy materials just downstream of channel bends, where helical underflow can undermine the levee, if insufficient armoring or well-graded filter blankets are lacking.  Another common problem with extended flooding are slope failures, such as that shown in Figure 6.  Slope failures can be triggered by excessive seepage presures along old (buried) channels, seepage through adjacent borrow pits, bearing failure caused by localized channel scour, or from any number of site-specific falws, such as excessive settlement or desiccation.

Figure 6 - Levee failures can also be caused by massive slope failures, such as that shown
here, on the river side of the Marchland Levee in Louisiana, which occured in 1983. 

WHAT DOES THE FUTURE HOLD ?

Flooding is a natural peril that has been exacerbated by wholesale development in California upon natural flood plains. California obviates much of this risk with a complex system of flood control infrastructure, such as check dams, debris basins, multi-use flood control reservoirs, lined channels and storm runoff collection systems in urbanized watersheds.

In order to safeguard development on the flood plains, we have attempted to control nature through catchment of debris and safe conveyance of the flood waters to the ocean. This flood infrastructure requires extensive maintenance in order to be effective. Any number of factors could combine to create inadequate flood control at any given time. Some of these factors include: back-to-back storms, warm rainfall on snow pack; inadequate design storage of reservoirs or channels due to lack of siltation maintenance; volcanic erruptions clogging channels; or the failure of any particular piece of the “chain of flow”. As our flood infrastructure ages, we can expect more failures of the system to occur. For instance, the loss of the radial gate on Folsom Dam in July 1995 was a failure mode never seen before, tied to the age of the structure (built in 1955). It is difficult to assess risk for causes of occurrence which have yet to have ever occurred, but we can expect more.

The passage of the National Flood Protection Act of 1968 created a federally-funded system of insurance for property owners living in defined flood plains, and set rates according to relative risk of inundation. As the levees held and insurance became universally available, these agricultural tracts have slowly been giving over to urbanization, putting more people at flood risk each year. Still, floods are the only natural peril currently under a federally-mandated system of insurance, adjudicated by the Federal Emergency Management Agency (FEMA). A problem which continues to plague the NFIP is updating flood plains with increasing development, adjusting rates with the changing risk of inundation, and determining regional value indexes tied to inflation and cost of repair, which vary considerably depending on the area.

Levee or not, floods, like other natural perils, will always be with us.

J. David Rogers is the Karl F. Hasselmann Missouri Chair in Geological Engineering at the Missouri University of Science & Technology. He received his B.S. in geology from the California State Polytechnic University in 1976, M.S. in civil engineering from U.C. Berkeley (1979) and Ph.D. in geological and geotechnical engineering from U.C. Berkeley in 1982. Between 1979-2001 he owned three different geotechnical consulting firms and from 1994-2001 he served on the faculty of the Department of Civil & Environmental Engineering at the University of California at Berkeley. His career has focused on natural disasters associated with earth movement, floods and earthquakes. The author of over 100 articles, he is a recipient of the Rock Mechanics Award of the National Research Council, the E.B. Burwell Award of the Geological Society of America, and the R.H. Jahns Distinguished Lecturer Award of the Association of Engineering Geologists and Geological Society of America, for his contributions to geoforensics, with particular emphasis on the failure of dams and levees. He can be reached at rogersda@mst.edu

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E-mail Dr. J David Rogers at rogersda@mst.edu.