Project Details

Levee foundation and embankment materials

Levee foundation materials are often not the primary consideration in designing a levee reach or trace; the location is usually based upon reaches needing flood protection. Therefore, the likelihood of a levee being constructed on substandard foundation materials may be a common occurrence. Over 75 percent of the levee failures that occurred during the UMR flood of 1993 occurred at subsurface zones of weakness or at locations where two levee systems were joined. Levee failure during Hurricane Katrina also occurred at locations having low-strength materials in the subsurface. In both cases, these zones of weakness were not visible at the surface. Current subsurface exploration of levee traces is to drill evenly spaced geotechnical borings along the levee trace. Although this exploration work may cost as much $500,000 per mile, the boring spacing may be insufficient in identifying subsurface zones of low strength materials. Continuous geophysical survey data usually complement the boring data and provide information between borings. Wolff (2002) recommended that research is needed to develop better techniques to characterize the top blanket and he suggested that shallow geophysical techniques offer the capability to significantly increase the level of information normally obtained by conventional borings.

The nature and location of levee materials is often a function of the size of the river and its valley. Generally, large river systems such as the Mississippi River, have more complex assortment and distribution of materials. In terms of levee materials, one should consider that there is a three-fold material system (shown in Fig. 1) which includes:

  1. The materials of which the levee is constructed
  2. The top stratum which is the foundation of the levee and the usual source of the levee materials
  3. The substratum which underlies the top stratum

The top stratum (orange in Fig 1) usually consists of fine-grained sediments (silts and clay) which were deposited prior to levee construction during flood events. The substratum materials (yellow in Fig 1) are represented by coarse-grained sediments (sand and gravel) deposited during the lateral migration of the channel across the floodplain. The levee (white in Fig 1) and top stratum materials, being fine-grained, have lower hydraulic conductivity values than the coarse-grained substratum materials and are, therefore, more impermeable to the flow of water. Note that much of the floodplain is overlain by sandy, point bar deposits and that lower lying swales are composed of fine-grained materials. Fine-grained materials may also be found adjacent to the sides of the flood plain and are termed backswamp deposits.

Levee failure modes

A generalized, not-to-scale, levee cross-section is illustrated in Fig. 2 which shows the levee foundation materials and also several possible failure modes which include:

  • Overtopping – The flow of water over the top of the levee causing erosion weakening on both the river and land sides of the levee.
  • Slope failure - A curvilinear failure of the river side of the levee.
  • Throughseepage – The flow of river water through the levee which may result in erosion and/or piping on the land side.
  • Underseepage – The flow of water through either or both the top stratum or substratum which may result in erosion and piping on the ground surface on the land side of the levee.
  • Erosion – Although bank erosion is possible in the situations described above, it also may occur when the location of the levee is too near the bank of the river. Sufficient levee set back distance or bature will result in lower river velocities which, in turn, will result in less susceptibility to erosion on the river side of the levee.
  • Liquefaction – Levees located in seismically active regions and depending upon the nature of the levee and substratum materials may fail by liquefaction produced by earthquake ground motions which decrease the strength of these materials resulting in slope failure.
  • Quick drawdown - In the event the flood stage falls so swiftly that the levee materials fail to adequately drain, the river side materials within the the levee may become susceptible to slope failure.

Levee Data Elements

The following list was developed by the research team and intended to include as much technical and general information for a complete understanding of the levee itself and the environment in which it is located.

Location and General Information

  • Site/reach name
  • Federal or local?
  • Project authorization
  • Levee district
  • Date constructed
  • Regional landuse/cover
  • Seismicity/Liquefaction?*
  • Demographics (agricultural/urban/critical facilities?)
  • Topo map
  • Google Earth view
  • Levee certification*

Construction

  • Age*
  • History and upgrades
  • Engineered, non-engineered, hydraulic fill
  • Presence of sand bags, Hesco Baskets, structures, I-walls, T-walls*

Visual Inspection

  • Condition of levee and reaches (reports by levee district, Corps office)*
  • Vegetation on levee (note position – crest, mid-slope, toe, landside/riverside, distance of
  • veg free zone from toe on riverside and landside
  • Noteworthy maintenance issues
  • Signs of rodent or burrowing animals*

Geometry

  • Height
  • Top elevation (based on topo, lidar, elev. survey, other)
  • Top width
  • Base width
  • River and land side slopes
  • Gradient

Hydraulics

  • Standard or non-standard section
  • Hydraulic analysis
  • Design flood
  • Stage/duration
  • Flood history
  • Top bank width
  • Channel depth (flood stage)
  • Failure history, location and mode*
  • Predominant sediment transported
  • Setback distance (bature)*
  • Presence of river training or erosion control structures
  • Straightening history
  • Man-made intrusions (drains, utilities, structures, etc.)

Levee and Foundations Soils and Borrow Materials

  • Boring logs
  • USDA soil maps
  • Levee soil classification based upon logs or soil map*
  • Top stratum thickness*
  • Top stratum materials*
  • Substratum materials*

Fluvial Geomorphology

  • Physiographic province or subprovince
  • Alluvial valley width upstream and downstream
  • Site/reach landform/depositional environment
  • Cut bank or point bar location.*
  • Straight, meandering (curvature) or braided (intensity) ?
  • Evidence of systemic fluvial instability (local and regional – nick points?)*
  • Rosgen’s classification
  • Migration history (active, moderate, slight, amount)
Possible Ground and Aerial Geophysical Methods and Soil Data

  • Airborne EM
  • EM31
  • GEM
  • Resistivity
  • Seismic – MASW
  • OhomMapper
  • LiDAR