Earthen Levees
They are good at channeling and holding back water under relatively constant hydrostatic pressure where the height of the water is not rapidly changing. An example would be a river at flood stage. The soil and water reach equilibrium.
One of the dangers is a rapid fall of water against a levee system. A rapid fall destabilizes the soil. The factor of safety is reduced. A subsequent rise could cause levee failure, such as back-to-back storms.
- Hurricanes Katrina and Rita
- Hurricanes Gustave and Ike
If a clay levee is properly built tall enough, wide enough, and given enough time to compact, there likely won’t be a failure. However, The cost of properly building clay levees tall enough and wide enough is ENORMOUS due to the shear size of the structure. Earth levees that are expected to withstand powerful tidal surge must be very wide. In many cases a levee with an elevation in excess of 20 feet must be 500ft-750ft wide. The levee itself might only be 120ft-150ft wide, however, expansive rinforcing berms (150ft-250ft) must be put on both sides of the levee. This helps to keep it stable and prevent global failure. All of this extra clay and the expropriation of property is expensive.
I-Wall Levees
I-walls do not work because they rely on the passive pressure of the soil, which is the horizontal pressure of the soil. The passive pressure of the soft soils in the New Orleans area is not very strong.
The force of the storm surge is exerted on the top of the wall and the bottom is not supported by the soft soils.
In addition, the height to which an I-wall can be built is limited, and over topping scours out the soil supporting the backside
Seepage is also a major problem.
T-Wall Levees
PROS
• Very strong
• Will absolutely work
CONS
• Very expensive
• Not cost effective
• A breach below the T-Wall is difficult to repair during an emergency situation.
Since the wall is above the force couple, the taller the wall becomes the longer the lever surge forces have to apply pressure to. In order to resist overturning the base must be extremely heavy (4′ to 5′ thick) and H-piles must be extremely deep. In many cases they are driven to a depth of 120′. Because they are driven in at an angle they must be considerably longer than 120′ to reach that depth
The Army Corp of Engineers estimated cost is $14,000 to $19,000 per linear foot. That comes to $75 million to $100 million per mile.
There are hundreds of miles of levees that cannot be built because short tracks of t-wall levees have sucked up all of the funding.